File: Lecture 7 Coastal Migration and Aquatic Resources
Populations of H. erectus were the first human emigrants from Africa. Traveling a route that must have been much the same as that hypothesized for moderns, Lecture 6, they reached the islands of Indonesia (review the map in lecture 5) where fossil crania have been found at Trinil, Ngandong, Sangiran, and Perning near the city of Mojokerto. A group of investigators led by O. Frank Huffman and Yahdi Zaim have just undertaken a major multi-disciplinary project to properly describe the Mojokerto site where the partial skull of a juvenile was found. In the latest publication of this project the authors describe a paleolandscape similar to a modern landscape from Mojokerto pictured above.
The preliminary analysis of the fauna and flora in the beds at the site indicate an environment very much similar to that pictured above where mangrove vegetation, grassland and montane forest provided a variety of resources including small and large bovids, Asian elephants, deer, and turtles.
Bantengs (wild oxen) would have come to gaze on the grassy plains between the water and the montane forest inhabited by large cats, monkeys, and pigs. The oxen shared their grazing ground with muntjaks, antelope, rhinos and hippos, some of which foraged on smaller shrubs. Preliminary analysis of fossil teeth from the site indicate that the grasses on these grazing grounds were primarily C4 tropical grasses. The analysis of phytoliths in the sediments reveal two types of grasses, open-land taxa from the subfamilies Panicoidae and Arundinoidae and the temperate climate subfamily Pooideae which probably drifted in from montane habitats. Notice from the opening photograph in this lecture the habitat diversity within a short distance from shore line to ridge line. Early humans might have varied their foraging strategies with the seasons depending upon which resources were most abundant in each habitat. Notice also that the presence of large herbivores near the shore doesn’t rule out big game hunting in habitats characterized by aquatic resources that women and children could collect.
At other locations in Java where crania of H. erectus have been found, a succession of dry and wet climatic regimes can be documented in the fossil pollen record. The stratigraphy of the Sangiran dome, near where Sangiran 17 was found, documents a record from 2.6 M. A to 0.2 M. A. (Semah et al. 2003 ). The sediments at the Plio-Pleistocene boundary indicate the retreat of the seas and emergent land bridges between mainland Southeast Asia and the Indonesian islands. At all times, the habitat of East Java was characterized by high diversity with the hominid fossils dating to 800,000 years associated with pollen documenting a drier climate but tropical rain forest taxa still present. “In the area outside Sangiran, an Indonesian-French team has worked in the Southern Mountains of Java documenting a long sequence of occupation and adaptation to wet tropical environments from the late middle Pleistocene to the Holocene (Semah et al. 2003 p. 161).” The second wave of human emigrants from Africa, H. sapiens, surely passed along these shores as they reached southern Australia by at least 40,000 years ago. The use of coastal, marine, and estuary resources by sapiens in Southeast Asia has been little considered as most of the focus in Asian archaeology has been on H. erectus in China and later populations transitioning from hunting-gathering strategies to farming. Erlandson (2001) attributes this neglect of coastal and aquatic adaptations to two factors, the widespread perception that hominids did not adapt to aquatic habitats before around 15,000 years ago and the obsessive attention to the idea that male-dominated big game hunting explained the origin of tool use, the formation of the nuclear family, and in its most recent incarnation the evolution of large brains. Erlandson labels one set of ideas about aquatic resources the “Gates of Hell” model as theorists proposed that humans only began to use lower quality aquatic resources when forced into marginal habitats by declining returns from hunting and/or by density dependent population growth, that is when they were forced from more desirable habitats. In the next section we are going to ask if foraging returns derived from one habitat can be used to guide inferences about returns in another time and place. This issue is especially relevant to the claims that evidence for the use of aquatic resources is evidence for habitat stress and decline of higher ranked resources. Last resort scenarios ignore the evidence for the nutritional value of aquatic resources, the cultural complexity of societies subsisting on those resources, high human population densities in some of the most widely exploited riverine and coastal habitats, and the emerging archaeological data suggesting early adoption of aquatic resources. Instead proponents, of the last resort hypothesis, cite the small size of many of the marine resources such as mollusks, crabs, sea urchins, barnacles, and shrimp while ignoring the giant clams available on the reefs of the southern oceans, the abundance of individuals in extensive mollusk beds that characterize rocky coastlines north and south, the large number of stranded fish that can be acquired in drying pools, and nesting sea turtles as a source of eggs and meat. Since many of the aquatic resources are sessile and predictable in time and space little search and no pursuit is involved in their harvest and interactions with other predators are rare. Further shell fish gathering requires no prior tool production, maintenance or preparation. Shellfish in particular are characterized by low variance, high density, and ease of collection, the very qualities that meet the daily nutritional requirements of growing children who can gather many of these resources for themselves. Erlandson cites a study by Jones and Richman (1995) showing that “mussel beds produce one of the highest rates of biomass production on earth (Erlandson 2001 p 294).” The supposed low quality of marine resources is derived from an estimate by Bailey (1978) “that 156,800 cockles were required to provide the caloric yield of one red deer (Erlandson 2001 p. 294).” Erlandson questions the accuracy of these estimates and later notes the analysis by Lindstrom (1996) of returns from the Truckee River fishery that are higher than return rates calculated by Simms (1987) for terrestrial Great Basin habitats. Shell fish are low calorie foods so would rank low in foraging models based on net energetic return but they have high nutritional value as sources of protein, calcium, and omega fatty acids. Ordinarily I would not post information from commercial sources but the chart comes from a USDA handbook and is a useful comparison of beef and shellfish. Later in the semester we will review arguments over the value of marine resources in the diet when we consider the research from the Okinawan centenarian study. When you read the assigned pages in Erlandson pay attention to arguments over aquatic resources, the ambiguity of the archaeological record, the theoretical prejudices of the investigators, the nature of the resource in question, and the problem of changes in sea level with high interglacial levels destroying whatever archaeological evidence there might have been from periods of lower seas. Remember from Walter et al. (2000), Lecture 6, that the Pleistocene reef terrace at Eritrea was preserved by tectonic activity that uplifted a portion of the former shallow water reef. Notice, from Table 1 page 306 Erlandson (2001), that the sites with evidence of use of aquatic strategies span the time from 2.3M to 16.5K, a considerable time frame for the occupation of “marginal” habitats. Instead of regarding these habitats and strategies as marginal, new evidence and new hypotheses call for serious consideration of aquatic resources as central to the evolution of human dietary strategies. Often the most obvious fact of human subsistence is unmentioned because it is so taken for granted. Erlandson calls our attention to the fact that freshwater for drinking is the “single most important aquatic resource for humans (Erlandson 2001 p. 293).” Even as our earliest ancestors exploited the resources of the woodlands and grasslands, they must have stayed close to sources of water. The important archaeological sites of Africa are all in riverine or lacustrine environments. It is hard to imagine wild foragers ignoring the resources in and and around water sources especially when those resources could be captured and processed with very simple tools. Where simple tools were not adequate to the job, early sapiens were capable of making more sophisticated tools as evidenced by the hafted bone point technology from Katanda. Although I have emphasized coastal strategies in this lecture we know that early anatomically modern humans did move into other ecological niches as shown by the occupation of Niah Cave in Borneo. The Deep Skull, tentatively dated to the time period between 43,000 – 40,000 B.P., has been found in late Pleistocene sediments of the cave. Niah cave is one of a series of caverns in sheer-sided limestone walls that rise nearly 400 m above the lowland rain Though the evidence from Niah is scanty, starch grains from palm pith and deep-rooted species of yams have been identified from the site. At other sites in Melanesia, starch grains of elephant yams, taro, ginger, and swamp taro have been identified. Some of these taros require cooking, drying or leaching to remove toxic calcium oxalate crystals but the pith of certain palms can be eating raw. These palms have high energetic yields but some processing costs as the palms have to be felled and the fiber pounded or otherwise extracted from the trunk. Evidence from sites in Southeast Asia and the evidence we will discuss in Lecture 9 from the Ache demonstrates that forest environments provided rich habitats for human foragers. Recall from Lecture 6 that the islands of Indonesia were accessible by land during glacial periods when water was locked up in northern hemisphere ice shields.
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File: Lecture 8 Optimal Foraging Theory
The lectures to date have focused on the evolution of Plio-Pleistocene diets inferred from the archaeological evidence and from paleoecology informed by modern food preferences through an implicit assumption that the environment is populated with nutritious plants and animals to be captured by individual hominids ranging over that landscape. Most of you know from other classes and from your own diets that foragers are selective in their choices. The simple coincidence of resource and forager in the same habitat tells us little about the diets of the forager. To be fair we have looked at circumstantial evidence for use of some resources such as cut marks on bones, opened and cracked shells, processed fish bodies, special tools for capturing specific animals, and isotope ratios but all of that evidence provides only a coarse grained picture of early diets. Archaeologists interested in refining their assessments of past foraging strategies have applied models from optimal foraging theory (OFT) to help them understand the past. These models were originally developed by biologists interested in explaining the specific resource choices they observed in their study populations. They were subsequently used by evolutionary anthropologists to study diet choices in modern foragers subsisting on wild resources. Most of you know that the faculty and students from the University of Utah departments of biology and anthropology were very active in developing and applying these models.
How can the information from OFT be reconciled with the Grandmother Hypothesis for the evolution of human life histories from O’Connell et al. 1999 (Lecture 5)? The fundamental premise of OFT is sound as we expect natural selection to favor individuals that allocate energy to tasks that promote somatic growth, maintenance and reproduction. In any habitat those genotypes better at allocation decisions are expected to leave the most descendants, that is have the highest fitness, yet the expectations of the diet breadth models are often violated in real populations. All models assume a generic forager but the early work with the Ache demonstrated that men routinely ignore the highest ranked resource in the forest, palm starch, in favor of hunting. This falsification of the model was as important as the confirmation of most of the predictions for other resources because it raised the question of why men hunt which led through many populations and many papers to important ideas about the role of costly signaling in male-male competition and linked sexual selection theory and foraging theory. But sexual selection on males is not the only factor determining diet choice as risk and uncertainty influence forager choices. The idea that the value of a resource item is determined by the costs of search and handling changed the way investigators ranked prey items in the foragers environment. Many resources are energy dense but have high search and handling costs. It is not the absolute value, to a forager, of the resource but the net energy gain to the consumer after the energetic costs of pursuit and handling are debited. In most investigations of actual foragers net energy gains are calculated by debiting post encounter costs against the currency of the gain, usually calculated in caloric return per unit of time spent in pursuit, capture and handling.In the simple version of the optimal diet model resources are included in the diet based on the tradeoff between handling upon encounter and the option of continuing to search for other items. The decision is based on the mean rate of return for that particular habitat and the suite of resources within it. Be aware that post encounter returns are always much larger than returns reported with search time included unless search time is nil. The inclusion of processing costs in the equation reflects the net gain to the forager from choosing any resource and influences forager preferences for certain resources over others. Richard Lee’s failure to include processing time in the cost of mongongo nuts seriously under estimated the contribution of women’s work to family energy budgets and led to the characterization of the Dobe !Kung as the the original affluent society, meeting their subsistence needs with few hours of work. I have assigned two short readings, Sih and Milton (1985) and Hawkes and O’Connell (1985) debating the the use of OFT to study human resource choices and specifically the ranking of mongongo nuts in !Kung diets. These two papers will give you a sense of the early arguments over the application of OFT models to human foragers and the value and problem of simplifying assumptions. As you have already read in Erlandson, the failure to report search time for resources leads to the over estimation of returns from resources such as collared peccaries and makes comparisons of post encounter returns of resources, such as red deer and mussels from two very different habitats, meaningless. Note in all of the lectures that follow I have tried to maintain the notation, calories, cal/hr, Cal/hr, or Kcal/hr used in the assigned papers. The notation is confusing, but the link provides a simple explanation of the differences. For our purposes we assume that investigators are referring to Kcal when they use cal, Cal or calories to indicate the energetic gains from any resource.
Winterhalder, Lu and Tucker (1999 p. 302-303) distinguish between risk and uncertainty by defining risk as “unpredictable variation in the outcome of a behavior, with consequences for an organism’s fitness or utility.” Further “with risk the probability distribution of outcomes is in some sense known to the organism, but stochasticity makes any particular outcome unpredictable.” “Uncertainty refers to incomplete knowledge of outcome probabilities. Uncertainty can be overcome by acquiring information about an environment; risk cannot.” They make the point that risk as used in behavioral ecology and economics does not “mean exposure to danger.” The authors point out that simple foraging models assume the forager always experiences the average conditions of its habitat where decisions have predictable outcomes. This assumption is central in all debates over the role of big game hunting in the evolution of the human diet. When we examine Hadza foraging strategies we will consider the high variance in game capture and contrast that to figures for the average amount of meat in the diet. In contrast to deterministic models, risk sensitive models assume organisms adjust to a range of possibilities arrayed along a probabilistic distribution. In addition risk-sensitive analysis specifies the relationship between the outcome and its fitness value resulting in a sigmoid curve. To the left of the inflection point of the curve value rises with increasing resources but at an accelerating marginal rate when the resources are scarce and at a decelerating marginal rate when they are abundant. In simple language, with too little food “added increments have high value and with too much, added increments count for little (Winterhalder et al. 1999 p 304).” The work of Curaco and his colleagues, with yellow-eyed juncos, showed that the utility function for the birds takes the sigmoid form. There is no reason to suspect that hominids are any less responsive to variable outcomes. If you are interested in further exploration of these ideas the figures presented in the Winterhalder et al. (1999) paper will give you a much better sense of the issues. I have posted the paper under also recommended for those of you interested in a more detailed explanation. Since value is measured as a rate, usually in calories gained per unit time, the sigmoid value function is time sensitive and this sensitivity varies with respect to the physiology of the organism and the character of the resource. Think back to Lecture 1 and the link to the Moran Eye center Web vision site with the graphic demonstrating the daily rebuilding of the pigment containing discs of the rods and cones. Stores of vitamin A must be sufficient to support the daily demands of this process. Winterhalder et al. (1999) make the same point about water acquisition. The preference for a reward of k liters/hr and equal probabilities of 0 or 2k liters/hr will depend upon whether the result will be suffered for hours, days or weeks before choice and outcome are reiterated. Responses of the organism to risk are determined by design by natural selection over the long time frame and the physiological state of the organism over shorter time frames. In general females are expected to be more risk adverse than males and organisms in negative energy balance are predicted to be more risk prone. The latter prediction was confirmed in experiments with yellow-eyed juncos whose foraging choices were observed under different temperature regimes that influenced energy balance. Given two foraging options with constant or variable rewards, birds in negative energy balance favored the high variance reward, presumably in the attempt to gain a reward sufficient to change their energy balance. The excitement generated by these results was tempered by a number of experiments that failed to replicate the results. Winterhalder et al. (1999 p. 323) conclude; “These studies suggest that the expected energy budget rule may apply only rarely to hominids, nonhominid primates, and modern humans which are omnivorous and relatively large species.” They might have qualified their statement to include only adult humans as anthropologists and archaeologists, with few exceptions (cf. O’Connell et al. 1999), have ignored the consequences of short periods of negative energy balance on the growth and development of neonates, infants, and juveniles. Conversely, resistance to short-term perturbations in energy balance may be an advantage of large body size that contributed to the success and expansion of H. erectus. We need to be careful here not to attribute the evolution of large body size to a resistance to short term negative energy balance. Such resistance can be a consequence of large body size but the causal arrow on the evolution of large body size runs from mortality risks in the environment of the species. Following Charnov’s (1993) mammalian life history model, large body size is a consequence of more time to grow larger because lower average adult mortality, from extrinsic causes, favors delaying the change from somatic to reproductive allocation. Organisms that experience low average adult mortality rates, grow longer, mature later and have longer lifespans. These are the derived traits that distinguish us from our nearest primate relatives. These arguments were reviewed by O’Connell et al. (1999).
The last assigned reading for this lecture is a paper by Elston and Zeanah in which they use models from behavioral ecology to understand the Holocene transition evident in the archaeological record of the Great Basin. Most of you know that our department has been in the forefront of extending foraging theory to hunter-gatherers but you may not be aware of the innovative ways the models are being applied to solve longstanding problems in Great Basin archaeology. The paper by Elston and Zeanah is a good example of this work as well as providing a link, in this course, from ancestral populations known only from the archaeological record and the next set of papers from more recent foragers. The opening graphic of this lecture, from the work of Edward S. Curtis, is intended to emphasize the importance of Great Basin studies to applications of optimal foraging theory to human foragers and to highlight the importance of ethnographic analogs to this work. The record of human occupation in the basin can be divided into two distinct time frames labeled Prearchaic for the period from 10,500-8000 BP and Archaic for later occupation. The early Holocene (EH) climate experienced by Prearchaic populations was cooler and wetter than climates after about 7800 BP. “The relatively cool, even EH climate, abundant surface water and complex steppe vegetation created productive habitats for a rich biota of fish, waterfowl and mammals (Elston and Zeanah 2002 p. 107).” The tool assemblages from this time period have an array of projectile points and flaked tools indicating a hunting economy but the coprolites recovered from the sites indicate a diet that included many of the resources of the lakes and marshes plus seeds and small animals, quite unlike a diet predicted from the graph of energy yield posted above. Basin occupants in this time period had access to a variety of resources within short travel distances as basins with lakes and marshes were separated by ridges with brushy steppes and juniper woodland from mid-elevation to ridge tops providing high-quality habitat for larger game animals. Moreover the scatter, size and character of the archaeological sites from this time period indicate low population density and high mobility of foraging groups. An attempt to use the diet breadth model to explain the evidence for the addition of seeds to the diet failed as the model developed by Simms predicted that women should have by-passed seed under all conditions, including the absence of large game, a prediction contrary to the clear evidence for seed use. A second model analyzed the distribution of soil types in the basin to develop a patch choice model. The simulations based on the distribution of lowland and upland habitats predicted that women should bypass seeds in favor of other plant resources and small game and men should have targeted only large and medium size game. This model fails to explain the evidence for small animal consumption found in the coprolites. Finally a risk sensitivity analysis returned the prediction that risk prone foragers should favor the high variance option “preferentially pursuing large game over smaller game and foraging in habitats where encounters with large game were most likely (Elston and Zeanah 2002 p. 115)” while risk sensitive foragers should prefer lower-ranked resources characterized by low variance in acquisition. From this model, Pinson (1999, cited in Elston and Zeanah 2002) hypothesized that Prearchaic foragers avoided starvation risk by pursuing small game. That is they were risk adverse. Elston and Zeanah (2002) note the following problems in Pinson’s analysis; 1) the model ignores the possibility that Prearchaic foragers might have entered basins where foraging returns did not meet daily requirements at which point the risk of starvation should have prompted the risk prone strategy of choosing the high variance option of large mammal hunting, 2) if Prearchaic foragers were risk adverse they would be expected to forgo migration from known basin habitats to new, unknown locations, and 3) if EH environments were as patchy as predicted by Pinson, the Prearchaic archaeological record should more nearly correspond to the Archaic record when climate deterioration reduced the density of high ranked prey. Increasing search time would have been a consequence under both conditions. Zeanah et al. (1999) used some of the techniques from Pinson’s analysis of Carson Desert foraging in simulations of optimal foraging in Railroad Valley but in the latter analysis “more detailed regional palaeoenvironmental data, better information on the productive capacity of modern soil types and improved GIS (Geographic Information System) capabilities permitted a more finely tuned reconstruction of prehistoric foraging landscapes than was feasible in the Carson Desert (Elston and Zeanah (2002 p. 117).” The Railroad Valley simulations considered male and female foragers randomly encountering resources across spatial and seasonal variation. From the early Holocene (EH)to middle Holocene (MH) hunting returns for men diminished by as much as 75% in all seasons. By contrast women’s returns varied much less across the transition, increasing somewhat in autumn as pinyons entered the ecosystem. As habitats dried out in the MH, wetland resources disappeared as options for women allowing lower ranked small seeds to enter the diet in late winter-early spring. The authors conclude that the highly productive ecosystems of the EH and the low population numbers encouraged high mobility that allowed men and women to forage in the most productive habitats for their resource choices. As the climate changed and women’s wet land resources declined, Great Basin foragers faced the risk of starvation in certain seasons. In the face of this risk women collected small seeds in seasons when they were abundant and stored them for the short season. Seed caches changed the pattern of mobility for both men and women, encouraged residential bases, lower investment in tools for large game hunting and more investment in tools such as grinding stones for seed processing. Bright, Ugan and Hunsaker (2002) predicted increasing investment in processing tools to reduce handling cost as lower ranked resources enter the diet in response to a decline in the density of higher ranked resources. The change in grinding stone technology, through time, in the Little Boulder Basin area of north-central Nevada supports that prediction. These innovative approaches linking optimal foraging theory, resource changes through time, and changing investments in technology increase our confidence that ancestral diet choices can be studied and understood. In lecture 9 we will look at how foraging models, applied to resource choices in modern hunter-gathers, have improved our understanding of human diet choices.
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File: Lecture 9 The Ache: Broadleaf Evergreen Foragers
The Ache are foragers of the subtropical, broadleaf, evergreen forest of Paraguay. I chose this example because Ache foraging has been intensively studied by the Utah Ache Project and as a comparison to semiarid African strategies. The warm, wet summer regime of the Ache forests are similar to habitats widely distributed from South America to Indonesia between 20 degrees north and south of the equator in regions with annual average precipitation varying from 1600 mm to 2000 mm. Temperature regimes in these habitats are characterized by warm summers and warm to cool winters with January maximums in the Ache region of around 40 degrees centigrade and July minimums of about -3 degrees centigrade. Early anatomically modern humans moving south toward Australia and north into China, Viet Nam, and Thailand must have encountered these habitats and foraged in them. The early Pleistocene archaeological record of upland Southeast Asia is little explored but the Ache can serve as a reasonable analog of human foraging strategies in humid subtropical ecosystems. Keep in mind that the distribution of these ecosystems, through time, was determined by northern hemisphere glacial cycles. On the Koppen climate classification map pictured below the Ache habitat is shown in green, warm to hot wet summer, cool dry winter.
The required reading for this lecture, Hawkes, Hill, and O’Connell (1982) is one of the earliest reports of the use of OFT to analyze resource choices of humans foraging on wild food. Even though the preliminary data in this paper has been revised and corrected in subsequent papers, I have assigned this early paper because it provides a brief history of the arguments, in anthropology, over human diets and because of the broad picture it provides of Ache foraging. The northern Ache were full-time hunter-gatherers until mid-1970 but now live at settlements sponsored by a Catholic mission. At the time the research was conducted by the University of Utah Ache project, they still engaged in foraging expeditions and lived on wild resources during their time away from camp. Hawkes et al. (1982,) contrast the views of Richard Lee, developed from his field experience with the !Kung, to those of Marvin Harris developed in a series of debates with Napoleon Chagnon over the causes of violence among the Yanomamo. Harris argued that the Yanomamo fought over scarce protein resources while Lee’s tally of foods eaten daily in !Kung camps convinced him that plant foods were more important than meat. Harris argued that meat was nutritionally superior to plants and would only be replaced by plants when large game animals had been depleted by hunting pressure. The important themes that would subsequently dominate debate over foraging strategies in modern hunter-gatherers are set out in Hawkes et al. (1982 p. 380). “Lee argues that plant foods are favored because they are abundant, reliable, and readily located, and therefore more efficiently exploited than are animal foods. Plants are said to be low-risk/high-return resources, while animals are high-risk/low-return resources. Animals are taken in spite of the inefficiencies involved because of the taste appeal of the meat and the prestige that accrues to successful hunters.” Hawkes et al. (1982) use two models from OFT, the optimal diet model and the patch choice model, to predict the resources the Ache should gather if they are maximizing net energy return. You were introduced to the optimal diet model by Elston and Zenah (2002), lecture 8, so you should know that the forager is assumed to make a decision about whether to take a resource upon encounter or to continue searching. Resources are ranked based on the return in calories over the post encounter processing times including both pursuit, for mobile prey, and processing to turn the resource into food. The forager is predicted to take only those resources that give a return rate equal to or higher than the average rate for resource in the optimal diet. Recall the Hollings Curve which showed that search and handling time are distinct elements of the problem, the search time is determined by the density of prey on the landscape but the net returns from that prey are determined by the handling time. It is the net return, after handling, that determines the rank of the prey in the diet. In the simple form of the model high ranked prey should always be taken when encountered. Hawkes, Hill and O’Connell make a very important point on page 388: “Note that the resource rankings of this model say nothing about the quantitative importance of a resource to optimal foragers. High-ranked items may be so rarely encountered that they represent only a very small portion of the diet; low-ranked items in the optimal set may be encountered with sufficient frequency to contribute the bulk.” Keep this in mind when we get to the disputes over the importance of various foods for contemporary human nutrition. Three additional points, 1) when the cost of search is added to the cost of any resource, its value to the forager, in absolute terms, may decline, 2) all optimal diet models are specific to a habitat and time period, and 3) the optimal diet model assumes a fine-grained environment where resources are encountered at random. Foraging trips with the Ache showed that the environment is coarse-grained or patchy with many resources clumped. On foraging trips the Ache often stop to harvest in some resource patches but not others. The authors suggest that the “distribution of tools” (Hawkes et al. 1982 p. 391) might account for the decision but discard that suggestion because the Ache nearly always take oranges and honey, even though both are harvested with the use of axes, but often pass palm fiber. The patch choice model predicts that foragers use patches that produce the best return when travel time, search within the patch, and handling time are considered. Notice that return figures for patchy resources include the time for searching within the patch. If both animals and oranges are considered as patches the average energetic gain return for hunting, including search is about 1115 Cal. per hunter-hour while the return for oranges is 4438 Cal per forager-hour and the return for honey is 3231 Cal/hr. Palm larvae has a similarly high return rate of 1849 Cal per forager-hour but involves the risk of high variability across logs. Returns calculated for fishing patches were similarly variable, ranging from > 2000 Cal per forager-hour to about 733 Cal per forager hour leading to two questions, when should foragers exploit palms and when to take fish. The average return rate in calories per hour of sixteen resources taken by the Ache are listed in Table 3, page 389. Notice that collared peccaries are a very high ranked resource but no search time is factored into the estimate of return.
Subsequent analyses of the data from five years of research on Ache resource choices reported in Hill et al. (1987) highlighted both the value and problem of using optimal foraging theory for analysis of human foraging choices. First detailed records showed that a theory for generic foragers failed to capture the different strategies of males and females. Among the Ache, men achieve significantly higher returns, 1253 Cal/hr, than women, 1087 Cal/hr. These figures are averaged over all resources taken by Ache men and women and include both search and handling costs. Before processing the rates are more nearly the same, 1339 Cal/hr for men and 1221 Cal/hr for women. The differences in rates indicate the high cost of processing palm fiber. Men often by pass palms but when they do take them they gain higher rates because they process the fiber faster than women. Consistent with the predictions of OFT models only one of the 26 resources taken by the Ache gave returns, after encounter, lower than the overall mean. That resource was palm larvae, mostly taken by women but sometimes also collected by men. Contrary to the predictions of the theory that foragers should preferentially target high return resources, returns from adult male hunting, 1349 Cal/hr, were lower than the caloric return, 2630 Cal/hr that could be obtained from palm starch and hearts (Hill et al. 1986). The observations that men gain higher returns than women, on average, changes if returns are compared for days when camp is not moved. On days when the group is moving women carry babies, pets, household belongings, and any meat the men catch. Men keep hunting after turning over meat they have already caught. Because of the opportunity cost of processing, most Ache processing work is completed in camp at the end of the day. When the Ache women are moving they pass many resources they might otherwise gather so their return rates on camp move days is very low. By contrast when they stay in one camp for several days, the women achieve a return on gathering of 2804 calories per hour compared to a return rate for men, on these days, of 1344. These figures include all time spent in food acquisition and processing. These results raised the question of why men hunt and suggested two alternative hypotheses, 1) caloric return per unit of time is not the only currency by which foraging strategies should be judged and 2) fitness payoffs for hunting are broader than meeting subsistence requirements. The first hypothesis proposes, in agreement with Harris, that meat is more highly valued than plant food because of high protein and lipid content. You can evaluate this hypothesis by recalling the human RDA for lipids and protein in the diet as reported by Conklin-Brittain et al., lecture 2 and the values of lipids and proteins that can be acquired from plant foods. When you open the link search on RDA to compare values in chimpanzee diets to RDA for humans. Also have a look at the figures and ask if annual averages reported in the charts are misleading. Monthly mean lipid levels in the diets are much more variable than monthly mean protein levels. Even though the annual mean lipid level in the chimpanzee diet seems sufficient to meet the RDA for humans the high variance may not provide adequate daily lipid consumption. The second hypothesis, suggested but not fully developed in this paper, is that men opt for a high variance strategy that promises a large payoff when successful. The payoffs for hunting include not only the calories, proteins, and lipids gained from the meat but mating benefits for good hunters. This last payoff is more fully developed in the papers on Hadza hunting. The work with the Ache demonstrated that human foragers differ, by age and sex, in the amount of time they spend foraging, the resources they take, and the amount of time they spend processing. The resources a forager takes on any day is not only a function of the costs of pursuit and handling but of the foragers state and the time frame over which the subsistence needs exist. Lower ranked fruit that can easily be gathered and eaten while moving will often be collected to satisfy short term needs while large game and palm starch are taken to satisfy needs over several days. In all cases the ranking of resources is habitat specific and even in the relatively stable habitat of the lowland deciduous forest can vary by season. The Ache only hunt and dig armadillos in the late wet season when the animals are fat and the average return is 3948 cal/hr compared to a return of 1220 cal/hr in the early wet season. The emphasis on highly ranked resources might leave the impression that the Ache diet is very narrow but the list of wild plants and animals taken by the Ache suggests otherwise. Have a look at Table 2 from Hill et al. (1987), below to get some idea of the variety in the diet. Remember larva are the only resource that reduces the average return, over all resources included in the diet. The optimal diet model predicts that the Ache should never take bamboo larva but ethnographic observations indicate larva are often collected. Do the Ache take larva because they value the high fat content of larva? This question is still the subject of debate so no clear answer but exceptions to the predictions of OFT do not diminish the usefulness of the model. Instead they open new windows into human foraging strategies.
The Ache are exceptional, among studied hunter-gatherers, in the number of calories they consume daily, 3610, and in the amount of meat they eat, 80% of calories from meat. Notice in Table 2, the difference in return rates depending upon the included costs. The return for White-tipped peccary is 5323 calories/hour if time spent tracking is debited but as high as 8755 calories per hour if only post encounter rates are considered. Similarly Ache men obtain a much higher return for 9-banded Armadillos they encounter on the surface compared to those they dig up. The variation in these rates makes it nearly impossible to compare return rates across foragers if the ethnographers don’t clearly distinguish between the methods of calculating rates. Keep these problems in mind as we survey foraging strategies across habitats.
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File: Lecture 10 Semiarid African Strategies
In this lecture we focus on the foraging strategies of the !Kung and Hadza hunter-gatherers of the semiarid region of Africa. The work of Lorna and John Marshall and Richard Lee made the Kalahari San the archetype for hunter-gatherers and the evolution of human foraging strategies. Their work in Southern Africa was complemented by the early work of James Woodburn among the Hadza of Tanzania and the detailed work of James O’Connell and Kristin Hawkes among the same group. As a consequence of these efforts we know more about human foraging in semiarid environments than in almost any other habitat. You viewed the map of present climatic regimes in the last lecture. Go back and have a look at the distribution of semiarid lands in Africa, the bright yellow and three shades of orange areas. The !Kung San live in the Kalahari desert of southwestern Botswana and eastern Namibia and the Hadza occupy the area around Lake Eyasi just south of the equator in Tanzania. Both areas are characterized by seasonal rainfall and shortages of surface water in the dry season. In these semiarid, seasonal rainfall climates, precipitation is highly variable from year to year and across the region in any one rainy season. In the Kalahari, over eleven years, precipitation varied from a low of 252.1 mm in 1962 to high of 788.9 mm the next year. Temperatures in the Kalahari vary from a high of 40 degrees centigrade in November-December to a low of nearly -10 degrees centigrade in July and August. The climatic regime of the two regions is very similar but the topography couldn’t be more different. These topographic differences have important implications for differences in foraging strategies of !Kung and Hadza children. In the lecture on children’s strategies we will consider those differences. This is a long lecture but the reading assignment is not excessive and I promise you a short Lecture 11.
The widely read early work and films from Lorna and John Marshall and later the careful measures of !Kung foraging returns by Richard Lee established the !Kung as archetypal human hunter-gatherers even though their habitat represents a very small percent of the habitats occupied by early H. sapiens. The notion that early humans evolved in an ecological niche similar to those occupied by the !Kung and Hadza focused the attention of researchers on big game hunting as the principle dietary strategy of humans to the exclusion of other foraging strategies. You were introduced to this problem in the reading from Erlandson (2001), lecture 7. Review it here so that you can put the dietary strategies of semi-arid lands in the context of variable human responses to the opportunities presented over the long migration from Africa. Once Richard Lee measured the actual food brought into a !Kung camp, in the Dobe area of the Kalahari, he came to the conclusion that plant foods represented the principal component of the diet. The Dobe !Kung men do hunt but the returns are highly variable. In spite of the variable nature of returns from meat, it is a highly desired resource. When a large game animal comes into camp everyone eats meat but on the many days when there is no meat in camp, men, women and children do not go hungry. They enjoy other foods gathered from the sandy plains of their homeland. I have assigned chapter 2 of Jiro Tanaka’s 1980 report on the !Kung San of the central Kalahari. You can download the reading from Marriott Course reserve. Tanaka’s field work was accomplished in the sixteen months he spent in the field between 1966 and 1968, sixteen months from April 1971 to August 1972 and two more months in 1974. In his introduction to San, Hunter-Gatherers of the Kalahari, Tanaka makes the point that desert is a misnomer for this region as it is an expansive, monotonous, grassland dotted with open woodland trees and shrubs characterized by a summer rain season from December to March. Tanaka gives you a study of diet in an area where there are no mongongo nuts, the most important dietary staple in the area where Richard Lee worked. Richard Lee made the San famous through his measurements of the number of hours spent gathering food. Since he didn’t count processing times his estimates left the impression that the !Kung could satisfy their dietary needs in no more than 2-3 days per week. As you know from reading the Hawkes and O’Connell (1982) response to Sih and Milton (1982) Lecture 8, mongongo nuts have particularly high processing costs. The principal foods of the central Kalahari San are listed in Table 8. Even though Tanaka did not use the framework of OFT to study resources gathered, the first eleven foods listed in Table 8 constitute the important resources in the diet. Notice the importance of melons and roots, that store water, for a people who have no access to surface water through much of the year. Tanaka attributes the ability of the San to survive in this very habitat to the technology of the digging stick which allows humans to access deeply buried tubers that cannot be gathered by any other primate, including savannah baboons who live on shallow rooted species in other habitats but need access to surface water so are unable to colonize the central Kalahari. Four of the important resources are shown in the next table. I was unable to find a good picture of Bauhinia beans. The shrub is a beautiful ornamental of the nursery trade so all of the online images feature the flowers and not the pods and beans.
Consistent with OFT, Tanaka (p 59) reports that beans of Bauhinia petersiana are the “central food item for several months” making “up nearly the whole of the San’s diet” in months when they are ripe and water is available. “Even though this is the period of greatest abundance and variety of food types during the whole year, people often totally ignore other foods.” This latter is somewhat surprising as the beans must have high carrying and processing costs since the inedible pod accounts for 75% of the whole weight so approximately 5 kg of pods and beans are carried back to camp to yield 1 kg of edible beans. Table 12 in Tanaka gives nutrient analysis of the important plant foods and Table 13 provides an estimate of the daily caloric intake per person. The figures are not intended to be totaled but to show what might be gained if 5 kg of Kan melon were eaten each day. On page 74, Tanaka gives an estimate of 2,000 kcal for the per capita yield of food taken during the study period and finds that return consistent with the caloric needs of individuals of the size and weight of adults in the population. Compare the protein content of the dried beans of T. esculentum and B. petersiana and dried Terfezia to the protein composition in animal foods, Table 12 and Table 14, Tanaka. We can’t be sure the units reported are the same but if we assume g/100 grams in each case the plant foods are comparable in protein gain to the animal foods. T. esculentum, marama bean, is being developed as a commercial crop in arid regions of Australia and Texas as it “is an excellent source of good quality protein and compares well with other protein foods including soybeans. Its oil is rich in mono- and di-unsaturated fatty acids and contains no cholesterol. It is also a good source of calcium, iron, zinc, phosphate, magnesium, B vitamins and folate (quoted from the web abstract). ” The number of game animals taken during the study period is shown in Table 11. Giraffes are the largest animals hunted by the San but they are rarely captured in the very dry habitat of the Central Kalahari. More commonly captured are the gemsbok, eland and kudu each weighing 200 to 300 kg. When the amount of meat taken is averaged over all camps, Tanaka estimates that a group of 50 San kill and share approximately 5,600 kg of game in a year or about 112 kg per person or .30 kg per day. From his description we know that meat comes into camp rarely so averages are hardly adequate to estimate the amount of meat eaten. When a large animal is captured people eat nothing but meat until the animal is finished and then go without meat for many days. On page 67, Tanaka describes the sharing pattern for game, large game animals weighing 100 to 300 kg are shared among all the members of the camp, smaller animals such as duiker and steenbok are distributed over a smaller circle of families and still smaller game, birds, springhares and hares are “usually consumed within the family.”
The !Kung Bushmen who live in the Dobe area of the Kalahari were studied by Richard Lee from October 1963 to January 1965. Dobe is in the northwest corner of the Kalahari where there are some permanent waterholes. During the dry season, May to October, groups cluster around the permanent water holes, moving out each morning to collect resources within about a 6 km radius of the camp. In the summer rain season between December and April they scatter across the landscape visiting the pools of water that collect from December through April. The population is more widely dispersed at this time of year. In the season of the rains the women gather fruits, berries, melons, and leafy greens. In the dry season they gather roots, bulbs and resins. Remember from Lecture 2, that chimpanzees add resins to their diets in the dry season. Mongongo nuts are gathered year around as the nuts that fall to the ground in one season are preserved by the very hard shell surrounding the kernel. Richard Lee (1968 p 33) makes the point that “food is a constant but distance required to reach food is a variable, it is short in the summer, fall and early winter and reaches its maximum in the spring” that is before the rains begin. Recall the debate from the Sih-Milton/Hawkes-O’Connell exchange, assigned in lecture 8, over the high processing cost of mongongo nuts. That cost, when only time is considered, needs to be reviewed in the context of other resources that might be gathered in each season and against the background of high variability in game capture. It is almost certain than no dry season resources provide a higher return, otherwise the women would not pay the opportunity cost of processing mongongo. Lee reports the nuts account for 50% of the vegetable diet by weight with the average daily consumption of 300 nuts yielding about 1260 calories and 56 grams of protein. Lee (1968 p 33) estimates that 7.5 ounces of nuts “contains the caloric equivalent of 2.5 pounds of cooked rice and the protein equivalent of 14 ounces of lean beef.”
The figures in this table, copied from Richard Lee (1968) What Hunters Do for a Living, or, How to Make Out on Scarce Resources In Man the Hunter eds. R. B. Lee and I De Vore. pp 30-48. Chicago: Aldine, show that mongongo nuts are an important source of both protein and calories in the diet of the Dobe !Kung. Even though vegetable foods comprise from 60-80 per cent of the total diet by weight, meat is still highly valued and widely sought. During the period charted in Table 5 !Kung hunters brought 410 lbs of meat into camp. Lee assumed that meat was shared equally by all consumers in camp, during that month between 23 and 40 individuals, distributing 410 lbs over an average of 31.8 persons per day over the 28 days between July 6 and August 2 gives an average of just over 7 ounces per person per day. In Table 5 Lee reports the weight in grams (230) which is just over 8 ounces. Beware of these average reports. Looking at the large animal prey pictured above this might be one medium sized animal. Just after the animal is brought in people eat much more meat per day than 8 ounces and when the carcass is fully consumed they go without meat for many days. However, since the !Kung hunt small game as well, the total figure may represent many small animals. San Bushmen may hunt singly or go out in groups but once an animal has been spotted and hit the hunters return to camp to rest. Since the poison used on the arrows is slow acting, the wounded animal will wonder over the landscape for some time before it dies. The !Kung men know they will have a long tracking job, the next day, before they find the dying animal. I have copied a few pages from Tanaka describing the hunt. The copy I have posted for you is missing the figures of hunting equipment but the description gives you a good sense of the difficulty of procuring a gemsbok that has only been hit in the leg. Since the amount of poison on an arrow is small and the poison is slow acting the wounded animal may wonder a long distance before it dies. Notice that the !Kung men of the central Kalahari also scavenge prey from other carnivores and take many small species. According the Tanaka’s description, the internal organs and the ribs are cooked and eaten at the spot of the kill and the bones are smashed to get at the marrow. The uneaten portions are cut up for transport back to camp. The San consume the entire animals except for hooves, horns, bones and stomach contents.
Can you reconcile the information in Table 2 with the graph in Figure 1? The Table shows that the Hadza are most successful at scavenging large game in the late dry season but the Figure shows that large herbivore biomass increases with annual rainfall. Two points should be taken from this observation 1) large game animals may have been relatively more abundant at the Plio/Pleistocene boundary when the genus Homo diverged from the australopithecines so more scavenging opportunities and more opportunities to eat meat, but 2) game animals, predators and humans are more dispersed over the landscape in the wet season and more concentrated around the water holes in the dry season. By now you should expect human diets to vary with the seasons. Recall Stewart’s hypothesis, from Lecture 5, for seasonal dependence on fish easily taken from drying ponds. No matter if fish or mammalian flesh provide the meat component of the diet when no meat comes into camp people still need to eat.
The women walk from 4 to 60 mins. to the tuber patch where they spend the day collecting. Within the patch they stop about mid-day to build a fire and cook some of the tubers they have collected. In the afternoon they continue collecting, stopping again to cook and eat before they gather up the tubers they are taking back to camp for the evening meal. About 39% of the tubers gathered are eaten in the field. During the wet season women add berries to their foraging schedule, spending about half their foraging time in berry patches. Women gain very high returns, measured in grams per hour in the berry patches but they spend about 34% of the foraging time traveling to the patches and 23% walking within the patch between bushes. I estimated the daily gain for berries based on 6 hours of foraging in the wet season at 26% of 6 hours times a gain of 4017 cal/hr to be 6268 cal. for a days work picking berries. In the dry season women spend 4 hours foraging for tubers. On those trips 38% of the time is spend digging for an average of 1.52 hours digging tubers that provide a return of 2000 kcal/hr or 3040 kcal/day. The foraging hours reported here are for child-bearing women. In general senior, post-cycling women forage 22-52 % longer than women of child-bearing age. When you consider the “take home calories” remember that the women and children over 6 that accompany them have been eating in the field so their need for daily subsistence from the take home “bag” is less than that for individuals who were not in the patch. You can see from these estimates that changes in the distance to tuber or berry patches could have considerable effects on the returns that women are able to gain from their daily foraging round. In general the travel distance to berry patches is higher than the distance to tubers because tubers are more densely distributed in the environment. These estimates are reported by Hawkes, O’Connell and Blurton Jones (1989) for observations October to November 1985 and March to April 1986 ( you can find the reference in O’Connell et al. 1999).
How many calories do foragers need? How many hours should they work for a net energetic gain? Was Marshall Sahlins right, needing little they meet their needs in a few hours of work and have many hours of leisure? If we remove our western lens and think about foraging through the lens of OFT we get a new perspective. The theory predicts that foragers should target resources that provide a net energetic return after the costs of pursuit and handling are considered. We have already learned that search time is not debited from the gross return, however all researchers know that walking time is energetically expensive. As a consequence of occupying a niche different from that of our nearest relatives we range more widely in the food quest but because we are bipeds we may be more efficient over large ranges. Leonard and Robertson (1997) modeled the energetic cost of walking for a generalized quaduped compared to a biped and found that male and female bipeds are more efficient over all speeds from 2.4 to 6.0 km/hr. They estimate that the evolution of obligate bipedality produced an energetic savings of 40 to 47% for male and female hominids compared to large bodied primates. Since modern hunter-gathers have an average range of 13.10 km/d compared to 1.77 km/d for large bodied apes an increase in locomotor efficiency is particularly important to occupation of the human foraging niche. However the cost of search is related not only to distance traveled but to other variables such as temperature, humidity, topography, and load carried. Nursing infants are a particularly important component of the female forager’s load which also includes food transported back to camp where other dependent children are waiting to be fed. Unlike chimpanzees, human males carry tools, tool stone and large portions of meat. Could it be that foragers allocate their efforts to maximize calorie gain while minimizing energetic expenditure. Are foraging strategies a tradeoff between energetic returns from the resources in their environment and the energetic cost of searching for, handling and processing those resources? Are there seasons of the year and periods of the day when it pays a forager to conserve energy by not foraging? In the extreme temperatures of the Kalahari resting in the shade during the hottest part of the day may be a very efficient strategy. Lee reports that Dobe !Kung women work an average of 2-3 days per week but Hawkes and O’Connell (1985) point out that Lee ignores the cost of processing mongongo nuts, 5 hours of cracking and pounding to produce 1 kg of nutmeat. Lee reports per capita consumption of 210 grams of mongongo nuts so a women could produce enough nuts for approximately 4 days of consumption if she only cracked for herself. However she cracks for her family dependents as well. If we compare the work effort of the foragers we have studied so far we find differences in their foraging patterns and the number of hours worked. Tanaka (Table 16) reports large variation in male work effort, hours out of camp searching for food, among the !Kung he studied, 9.35 to 4.15 hours but less variation in women’s effort; 4.25 to 1.50. The Ache move camp almost daily, walking during the day, processing food, cooking, visiting and resting during the evening and night. The !Kung gather resources during the morning hours, unless they are tracking game, and process food, rest in the shade, and visit during the mid-day and evening hours. The Hadza also begin gathering early, stopping at mid-day and sometimes gathering late in the afternoon but often spending no more than 6 hours in the field. Hadza men are more similar to Ache men in the habit of going out to hunt every day. Variation in work effort and foraging returns across habitats would make a good term paper topic.
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File: Lecture 11 Meriam Aquatic Foragers
Mer island is located in the Torres Strait on the northern end of the Great Barrier Reef 142 km southeast of Papua New Guinea. With this lecture we revisit the sub-humid tropics for a study of foraging strategies of a population surrounded by the sea. The island is fringed by reef, the light blue-green color center left and zigzagging toward the center of the photo to the right of the island. The foragers of Mer practice a subsistence strategy that includes marine foraging, growing of yams, manioc, and bananas as well as keeping of pigs and chickens and subsidies from the Australian government. The gardens and domesticated animals provide a small fraction of the diet as most carbohydrates are now purchased from a small store on the island and most of the meat in the diet comes from marine resources. Three distinct strategies are found in the marine niche; hand-line fishing and netting on the nearshore at high tide, shell fish collecting and spear fishing on the reef flat at low tide, and deep and shallow water fishing, lobster collecting and turtle hunting offshore, and hunting and collecting turtles in the nesting season. This lecture has two purposes; 1) to introduce sub-tropical marine collecting strategies, and 2) to consider an important body of ideas advanced to explain widely noted falsifications of the predictions of optimal foraging strategies. Those falsifications were first noted in the finding that Ache men rarely collect palm starch in spite of the high caloric returns provided by that resource (Hill et al. 1987). The evidence that men pursue resources characterized by high variance and wide sharing, once captured, was included in the discussion by Elston and Zenah (2002), developed by O’Connell et al. (1999) in the review of the grandmother hypothesis as an explanation for the evolution of the genus Homo, and evidenced in the hunting strategies of the Kalahari San Bushmen and the Hadza big game hunters. You were prepared for these ideas by the four hypotheses advanced by Mitani and Watts (2001) to explain hunting by chimpanzee males. By now you should suspect that male foraging is about more than satisfying personal nutritional needs. Everywhere humans forage, men and women target different resources. The resources taken by women are characterized by low day to day variance in acquisition rate, by small package size and by limited distribution within the family. By contrast men target resources characterized by high variance in acquisition, large package size, and distribution to all in the vicinity of the catch. The fact that men do not discriminate between their wives and offspring and the families of other men provides strong evidence that male strategies are not about parental investment or offspring provisioning. The argument that male hunting can best be explained as sexual selection for male-male competition is set out in the assigned papers for this lecture.
Notice that women allocate very little time to spearing, 9% of their time on the reef compared to 76% of the time shell fish collecting. Because of the high cost of transporting the shells of the very large clams they collect, women process as they collect. Women collect only three species, Hippopus hippopus, Tridacna spp. and Lambis lambis, the spider conch which is very much smaller than the conchs found in tropical Atlantic waters. The largest tridacna clams can reach 140 cm, over 4 feet on the longest dimension.
Women forage on the dry reef carrying a bucket, knife, hammer and a small spear. As they collect they cut out the meat from the shell and drop it in the bucket using the hammer to crack conch shells and the spear for balance, although they do stop to spear small fish or octopus trapped in the tide pools. When the man and woman return home from the reef he shares his fish with the neighbors and she cooks her share of the fish and the resources she has collected from the reef for the family dinner. Over a sample of 44 women, female forages brought in 1962 grams/per bout, on average, while spearfishermen (n = 36) brought in 356 grams. Study Table 1 (Bliege Bird, Smith and Bird 2001) where they report returns from the ebb tide reef flats according to activity. Reef collecting of large clams gives the highest return in calories, proteins and fat. Even though men and women spend the same time on the reef, their energetic gains are quite different. Figure 1, in this same reading, illustrates the gains achieved by forages targeting shellfish compared to gains from spearfishing. Pay attention to the kcal scale on each panel. Although the curves seem comparable the gains for spearfishing vary from 0 to 1700 kcal while those from shellfish collecting vary from 0 to 5500 kcal for just over 140 mins of foraging time. Notice also the high variance in returns on panel A. The extreme outlier of just under 1800 kcal in 25 minutes of time is not representative of the average gain reflected in the regression through the mean. The returns plotted in panel B cluster more tightly around the mean. This evidence lends support to the hypothesis that spearfishing is a costly signal of male talent. Keep this example in mind when we get to the lecture on children’s foraging and the argument that male foraging strategies explain the evolution of long juvenile periods in ancestral H. sapiens. If foragers process as they go, they leave no record of past meals. Think about how we derive information on ancient diets. The archaeological record consists of remains left at central processing spots or middens, kill sites, camp sites, and dumping sites. The record is over-weighted with resources with durable discard. When foragers process as they forage, inedible portions are scattered across the landscape. Absence from the archaeological record never implies absence from the diet of ancestral humans. You can begin to appreciate the importance of ethnographic evidence drawn from foragers who still collect wild foods. The power of the simple OFT models helped ethnographers understand the evidence they collected. Even where modern foragers practice a mixed subsistence, including cash purchases of food, a great deal has been learned about the energetic costs of wild foods from the use of OFT models to understand foraging strategies for wild resources. On their small, 2.8 by 1.7 km, bounded world the Meriam practice a seasonal round. As noted above, March to early Throughout the year men take turtles in the open water. There is a rich description of this type of hunting in the required reading for this lecture. You might be surprised that a reputation for a good hunter falls on the leader of the boat who directs the driver, makes decisions about pursuit and directs the chase rather than on the jumper who actually goes into the water and wrestles the turtle. Turtle hunting is open to all men between the ages of 16-47 but participation varies. “Thus 44.5% of the 90 Meriam males ages 16-47 hunted at least once in the study period, but the 3 most active participants (3.3% of males) were over 5 times more likely to engage in a turtle hunt than the average Meriam male in this age range (Bliege Bird et al. 2001 p 14).” These same 3 men were named as the best turtle hunters in a series of interviews with other island residents. Hunting in open water is more costly than hunting on the beaches. Leaders of open water hunts are older and more experienced while young men who are jumpers in open water hunts are active participants in nesting season captures perhaps as a way of working toward hunt leadership.
In all discussion of optimal foraging across environments and in the same environment across methods we need to exercise care to make certain we are comparing net returns calculated in the same way. If search costs are debited, as they are in Fig. 7, the returns will be much lower than returns reported as post encounter return rates, the method of calculating returns using the classical optimal foraging models.
One of the features that distinguishes humans from other primates may well be male-male competition in the arena of provisioning others. Proponents of costly signaling hypotheses to explain the evolution of men’s work propose that human males signal their quality as mates and allies and their danger as competitors by providing high variance, high return resources that involve skill in acquisition and some personal risk. The caloric returns from the two types of signaling explored in this population are vastly different. The return from spearfishing is insignificant over the total collected resources but turtle hunting in the open water provides a very high caloric return shared over many individuals in very public feasts. In the first case, the spearfishermen signal the personal physical qualities that make them successful hunters of small mobile prey; hand-eye coordination, patience, and endurance. Their successes on the reef provide few caloric benefits to observers, but provide useful information to potential competitors and allies. Those who attend to the signal receive honest information about the quality of the man because the task is too difficult to be accomplished by those of lesser talent. In the second case, turtle hunting in the open water, observers gain not only information but valuable calories and opportunities for communal feasting. Pay attention to the predictions Bliege Bird et al. (2001) derive from the costly signaling hypotheses and how they are tested. I have also assigned Hawkes and Bliege Bird because they review Zahavi’s handicap principle, link it to the analysis of chimpanzee hunting by Mitani and Watts (Lecture 2) and to Veblen’s classic analysis of conspicuous consumption. Finally, Hawkes and Bliege Bird (2002) argue that reciprocal altruism fails as an explanation of most human sharing patterns because pair-wise exchanges for large meat packages have not been demonstrated. These are important arguments and you should be clear about them before the second exam. If you are not familiar with modern arguments linking conspicuous consumption and sexual selection in humans, have a look at Geoffrey Miller’s prize winning essay Waste is Good. This essay should be especially interesting to those of you majoring in psychology or marketing.
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File: Lecture 12 Juvenile Foraging
Unlike other primate females, human mothers have overlapping dependents, a new child is born before the previous child becomes independent. A number of hypotheses have been developed to explain who feeds a women and her child when her foraging returns are constrained by child care. As illustrated in this photo and those in previous lectures, women take their nursing infants with them but here you see a !Kung women from the Dobe area carrying two of her dependents plus the food she has just gathered. In the early versions of the man the hunter hypotheses, investigators imagined that women with dependent children remained in a central place caring for children while men provisioned the family with large game. A more recent hypothesis links the evolution of long juvenile dependencies to food sharing between older female kin and young dependents (see O’Connell et al 1999). In environments of low extrinsic mortality young animals can risk devoting more time to personal growth, growing longer and larger before maturing. In the mosaic habitats of the Plio-Pleistocene boundary mothers and children were able to move into new habitats because long-lived grandmothers helped provision weaned infants. In turn the benefits of provisioning increased longevity genes in these populations, further reducing average adult mortalities and promoting later age of maturity. It is late age of maturity that provides long juvenile periods. The consequence of selection for longevity may well be long learning periods but as always we should be careful to separate cause and consequence in evolutionary puzzles. The authors assigned in this lecture review these arguments over and over. Be sure you understand the different hypotheses because they are all related to the occupation of new habitats, the provisioning of juveniles, the evolution of the human diet, and the evolution of longevity in our lineage. By now you have ample evidence, from ethnographic studies, demonstrating that the man the hunter scenario under-estimates the work of women who forage for themselves and their children even when diets are supplemented by widely shared resources provided by hunting men. The second proposition of the hunting scenario, that human children are dependent until they become adults is true for the !Kung and some South American subsistence horticulturists but quantitative studies of other foragers demonstrate that young children and juveniles provide some of their own resources. Differences across habitats and subsistence strategies are indicators that juvenile strategies might be structured by ecology and physical development and not by learned capabilities. The differences, across habitats, bear upon the modern version of the man the hunter proposal, the embodied capital hypothesis, which proposes that the long juvenile period in humans evolved in response to the value of a long period of learning for mastering adult foraging skills. Once individuals, males in this particular hypothesis, master hunting skills they are able to provision mates and juvenile dependents. The evidence that children can be effective foragers at a young age challenges this proposition. The various explanations for the evolution of our unique life histories are well rehearsed in the assigned readings for this lecture.
Hadza children as young as 5 years old forage near camp for resources to supplement their diets. In certain seasons, children are left at home when mothers, carrying only nursing infants, go to dig tubers. Detailed observations reported in Blurton Jones, Hawkes and O’Connell (1989) showed that infants older than 2 1/2 years rarely accompany mothers to tuber foraging patches. Children left in camp forage on tubers growing near the surface, the honey of stingless bees, and baobab fruits. The latter fruits have high processing costs as the pods need to be cracked and the pith pounded into a dry powder. This powder is moistened with water or eaten dry with water. The seeds can be cracked and the kernels eaten raw. In spite of high processing costs, children aged 5-10 years can gain 629 cal/h from baobab, while those 10-15 get about 1014 cal/h. However children do not allocate many minutes to foraging so it would be a mistake to imagine that Hadza children entirely support themselves. Honey is mostly taken by boys between the ages of 12-15 if one can get an axe to chop out the nest. Honey yields about 339 cal/h but boys who accompany women on foraging trips often acquire 650-1350 calories on these trips. Blurton Jones et al. (1989 p 380) estimate that “If children spent just 2 hours per day foraging for baobab, or (for older boys with access to an axe) honey, they would acquire around 800-1000 calories, almost half the calories they need.” In a subsequent field season, the same research team completed time allocation studies to determine how mothers change their own foraging patterns in response to opportunities presented by resources they can gather with their children (Hawkes, O’Connell and Blurton Jones 1995). Contrary to the expectation from optimal foraging theory that mothers choose foraging patches that optimize their own return rates, they found that women, in certain seasons, choose to forage in berry patches with their children instead of traveling to patches to collect tubers. Mothers maximize team foraging rates by focusing on resources that children can gather with adults. “If higher rates of food acquisition are advantageous to women because with higher rates they can feed children more, or feed more children, then when children are active foragers themselves a woman’s children will consume food at a higher rate if she chooses the strategy that maximizes the team rate she and her children earn collectively, even if the rate she earns herself is less than the maximum possible (Hawkes et al.1995 p 695 emphasis original).” The team rates achieved for mothers and children depend upon the number of children and the time allocated to foraging. The more children a mother has the higher the team rate. If the team forages for longer than 556 mins, berries are the best choice. If a women wants to maximize her own return rate collecting tubers she needs to forage for an additional 2.5 hours. Over the mean length of observed trips, women and children do best focusing on berries. The foraging strategies of mothers and kids might have been influenced by the scarcity of baobab fruit during the late dry season of 1988 when the time allocation studies were completed. In spite of children’s abilities to forage for themselves around the Hadza camp, they still depend upon mothers. The time allocation data show a significant difference between foraging time for children with co-resident mothers and those without. Notice on Table 2, copied from Hawkes et al. (1995), that an 8 year old girl with no mother or father in camp spent more time foraging than any other child. In a sample of 20 juveniles, aged 3.5 to 17 years old, 9 had no coresident mother in camp.
The caloric returns children achieve from their own foraging efforts depend upon the resources they target. The two types of tubers, Makalita and //ekwa, gathered by children yield 73 to 85 Cal/100 g and children over the ages of 7 and 8 can gather 598 and 314 g/hr, respectively, of these two resources gaining about 200 calories for 29 mins of digging Makalita. When children accompany mothers to the berry patch they can gain from 964 to 2,223 Cal/hr depending upon which berry, Salvadora persica or Cordia sp. they are picking. Children gain about 50-70% of the adult rate picking S. persica berries, Table 6 (Hawkes et al. 1995). Since Cordia berries were not fully ripe until the end of the observation period, rates by age were not available for the berries yielding the higher caloric return. There are two important take home points from the Hadza field work on children’s foraging. 1) Hadza children’s foraging choices are consistent with optimal foraging theory predictions, near camp children generally choose to forage on the tubers with the highest caloric return just as they do in the berry patch. Before Cordia berries ripened children picked berries giving a lower return but once the Cordia ripened they ignored S. persica berries even though they were still available. 2) Mothers adjust their foraging strategies to include resources their children can gather. In berry picking season mothers forego tuber picking, where they earn high returns, to travel long distances to berry patches in order to maximize the team rates they can gain with their children. The stashing rate shown in Table 6 is defined as the “total weight of fruit accumulated for transport per hour in the resource patch. They represent the minimum rate that would have been taken had foragers eaten none of the berries they picked (Hawkes et al. 1995 p 693).” Actual picking rates are a combination of eating and stashing rates. The last column is an attempt to measure the amount of time spent picking, the stashing rate and consumption rates are added and divided by the tin measured rate. The results show that women are the most steady pickers and men, boys and children the least effective. Men may have spent more time in the berry patch in this particular field season as “hunting was poor(only eight large animals taken over 43 days of direct observation)(Hawkes et al. 1995 p 689.).” Men pick at very high rates but they also consume at a high rate so their stashing rate is lower than that of women and girls.
The foraging efforts of Hadza youngsters stand in sharp contrast to those of Dobe !Kung children who do almost no foraging. Instead !Kung children contribute to their own subsistence by cracking mongongo nuts carried home from distant patches by their mothers. Hawkes, O’Connell and Blurton Jones (1995) attribute the difference to the scarcity, in the Kalahari, of near camp resources that children can gather and the high, dry season temperatures and lack of water that preclude taking children on long walks to the nut groves. Instead !Kung mothers and kids do better if toddlers and older youngsters stay in camp and crack nuts that mothers have carried home. Recall the topographic differences between Hadza country and the Kalahari with no prominent landmarks to guide children from foraging sites to home camps. Blurton Jones et al. (1989) suggested that the harsh dry season temperatures keep !Kung children from foraging with mothers and the danger of getting lost in the flat environment restrain children from foraging alone near camp.
At both of their field sites, Mer Island and the Western Desert of Australia, Douglas and Rebecca Bliege Bird conducted quantitative studies of juvenile foragers to determine if optimal foraging theory could explain choices made by children. At both sites they found that children take resources that give them high rates of caloric return as predicted by theory but their foraging returns are constrained because the rate at which they encounter resources is determined by size and walking speed rather than age or experience. Their evidence indicates that physical abilities rather than learned capabilities determine juvenile foraging strategies. On Mer Island, mixed-age groups of children, independent of adults, forage on the reef flats after school or on weekends. On these trips they collect some of the same resources collected by adults but they encounter them at a lower rate so their overall returns, in the patch, are not reduced by stopping to take smaller shellfish such as black lipped conch and small top-shell. Post encounter processing costs of the large shellfish are higher for children because they do not have the upper body strength to cut out and remove edible flesh from the largest shells. These differences, between adults and children in physical strength rather than skill, explain why children take resources ignored by adults. Further Bliege Bird and Bird, in Children on the reef (2002), suggest that children are very efficient at extracting meat from small shells because they can more easily reach in the small valve openings with their small fingers to pull out the meat. Mer children also fish, from the beach, a foraging strategy requiring some skill. Across the Torres Strait islands, Meriam kids are known for their skill in hand-line beach fishing. Children begin as young as five years old and quickly master the technique. After an initial steep rise in efficiency there is little effect of age on efficiency. However, at every age there are large differences in individual skill and efficiency such that most of the variation in this task is explained by individual differences with one 62 year old man out-ranking all others in return rate. Figure 1 taken from Bliege Bird and Bird (2002b) illustrates the scatter and the large disparity when a very good older fisherman (the open circle top right) is included in the analysis. Residuals of overall return rates are plotted on the y axis to remove the effects of varying sample numbers across foragers. In spite of the title, return rates are calculated from both small-hook and large-hook fishing methods. Men and children of both sexes choose large-hook fishing and women choose small-hook. One difference between children and adults is that children usually have fewer choices of line size. From the figure, we might conclude that only the most skilled children choose to hand-line fish but without knowing the proportion of children who choose to fish it would be impossible to say the sample is self selected by only the best children.
Martu (sometimes Mardu) children of Northwest Australia are often left at home when mothers hunt goanna lizards in the sand hill flats but these children are active forages in patches distinct from those targeted by their mothers. Bird and Bliege Bird (2005) argue that size determines the The analyses, of Martu children’s foraging success, was designed to tease apart the factors which influence foraging choices of juveniles. The regression plots show that height has a larger effect on return rates than age (compare Figs 6.2 and 6.3). The proponents of the embodied capital hypothesis for the evolution of human life histories propose that the long human juvenile period evolved to provide a long period of subsidized dependence during which children forego supporting themselves in favor of learning the foraging skills that are later cashed out as productive adults. In this scenario juvenile dependency is subsidized by paternal support of wives and children. Bird and Bliege Bird show youngsters can be effective foragers using the same tools and techniques as their mothers but foraging in different patches. Since within patch encounter rates constrain juvenile choices, size not experience makes a difference to children’s effectiveness. These results are important because they undercut the proposition that long juvenile periods, in humans, evolved in response to the value of learning adult foraging skills. Instead they support the proposition that long juvenile periods evolved in response to the benefits of growing larger before maturing. The question remains, why don’t children forage longer hours? The work effort of the 8 year old Hadza girl with no co-resident parents suggests children are capable of longer work hours. It is possible that children allocate energy to fogging effort while still conserving energy for growth. Humans are determinate growers which means we reach skeletal maturation and hence most of our adult height about the time of sexual maturity. Bliege Bird and Bird (2002a) suggest that foraging efficiency increases after puberty because both males and females have greater opportunity to enhance fitness gains from foraging efficiency. During the thousands of years spent subsisting on wild resources, males excelled in male-male competition through costly signaling of hunting ability and providing resources for public consumption. In this same niche, females promoted their own reproductive success by gathering low variance resources to support their dependent children and grandchildren. Foraging efficiency increases at puberty in response to natural selection on females and sexual selection on males.
In Growing Up Mikea, Tucker and Young (2005) review the various arguments for the long juvenile period in human life histories and report detailed time allocation studies of Mikea tuber foraging. They emphasize what you have learned earlier in this lecture, the cost of children varies across habitats. In the dry woodland forest of Madagascar, where the Mikea live there are few dangers for children foraging in the woodland, no poisonous snakes or large predators and plenty of shade to moderate temperatures. Even though the Mikea practice a mixed subsistence economy that includes subsistence farming and herding, trade, and cash labor they still collect wild resources. Figure 7.2 shows that children spend much more time in leisure activities than other age classes but by adolescence young people spend as much time working as do married adults. Notice also that adult males and females allocate nearly the same proportion of daylight hours to food production. Among children, boys allocate nearly twice the time to food production as do girls but these figures change when allocation to tuber foraging is measured. Adolescent girls devote more time than any other age group to this task (Figure 7.3). Still across all foragers net acquisition rates increase with age and men achieve significantly higher rates than women but Table 7.2 shows that daily returns for men are low because they spend 1/3 the time that women spend foraging for tubers. In the early dry season, Mikea children actively forage with adults and mothers, who suffer no decline in foraging efficiency when accompanied by children. Notice in Figure 7.8 that adult females forage in groups with children and adolescents more frequently than do adult males. In certain seasons adults and children forage in the same patch but when patches near home are depleted in the largest tubers adults range further to exploit new patches while children stay in the older patches harvesting smaller tubers that lie close to the surface. The ovy tuber, collected by the Mikea, grows in sandy soil so digging and collecting is relatively easy. The ease of digging ovy combined with its higher energy density produces a net return rate much higher than the tubers collected by the Hadza. Have a look at Table 7.2 and notice that children gain 505 to 537 kcal/hr gathering ovy but they gather only 35-41 minutes/day. The same question arises over and over for children, why don’t they devote more hours to foraging? Tucker and Young suggest that children are neither rate-maximizers or time-minimizers. Since they are provisioned by adults they are also not energy limited. Instead they forage for the physical and mental challenge and for the social opportunities afforded by mixed age play groups. However the ability of Hadza, Merriam, Martu, and Mikea children to collect resources for themselves suggests that they might be able to support themselves if adult provisioning failed. As illustrated from the detailed Hadza data, children often have no co-resident parents. In such cases orphans must rely on near-relatives, neighbors and their own ingenuity. We expect natural selection to favor strategies, in young mammals, that help them survive maternal accidents. To expect youngsters to forage on the same resources as adults is unreasonable given their size and physical strength. The studies reviewed in this lecture demonstrate that children can be capable foragers depending upon the opportunities in their environment but over all environments, given adult support, they choose to allocate more time to leisure.
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October 14, 2006
Lectures 7-12
As rivers flowed off the highlands they formed deltas characterized by mangrove swamps with several species of trees including Nypa fruticans palms and the sugar palm Arenga, with flowers that produce a sugary juice, and shrubs of Passiflora with edible passion fruit, and edible climbing ferns, Stenochlaena palustris. The trunks of the Nypa are submerged so the fruit, which is edible when it is immature, would have been accessible to terrestrial hominids. The delta facies contain shells of edible mollusks including oyster shells but at low density and scattered distribution. Crocodile and turtle fossils have been reported from the same sediments at other places in the Perning district. To date only one fish fossil has been identified. The photos of bantengs and nypa palm were taken from a web site of photographs of Ujung Kulon National Park but unfortunately the web site is no long available. 
forest. The present climate in the vicinity of the cave is tropical super wet with mean annual rainfall greater than 3000 mm and rare or very short seasonal dry periods. The Pleistocene climate may have been more seasonal but changes in rainfall and temperature regimes at this site, through time, have not been resolved. At the time of Pleistocene occupation, the cave may have been about 30 km from the sea. Floral and faunal elements recovered from the late Pleistocene levels suggest that “humans were foraging in a mosaic of closed forest, scrub, grassland, swamp, and freshwater lakes and rivers (Crangrook 2000:83, cited in Barton 2005:57).” Barton, along with others interested in drawing as much information as possible from the archaeological record, has been involved in the development of a new line of evidence from archaeological assemblages, the identification of starch grains through the comparison of grains found in sediments and on tools with grains from modern starches.
Unlike the !Kung, the Hadza focus exclusively on large game animals. Over 256 days of observation in the years from 1985 through 1989, the Hadza killed or scavenged 72 large animals for an average of one animal every 3.6 days. But considering there were 6 hunters in camp during the wet season and 10 in the dry season, individual hunter success rates were not high. The average of 1 large animal every 4 days seems to guarantee that Hadza individuals eat meat every day but averages obscure the high variance in acquisition. The researchers calculate the chance of failure for any one hunter on any given day as 97%. They compare this to a success rate for the Dobe !Kung of one animal every four man-days and for the Ache, 2-3 successful days out of every 4. In the case of both the !Kung and Ache the success rate reported here includes all the small game added to the diet. Each Hadza hunter maximizes his mean rate of return by focusing on large game but at the cost of high variance, the probability of failure on any day. Remember from earlier discussions that one of the problems with the simple OFT models is they ignore variance in acquisition. The only measure in the OFT models is the rate of return, in calories per hour, for that resource after the time cost of handling and processing is debited. Remember also that the high variance in individual hunter acquisition doesn’t necessarily translate into high variance in individual consumption because a large animal taken by any hunter is shared over all the individuals in the camp as well as others who come from longer distances when the “bush radio” indicates there is meat in the camp. The chance of consuming meat, in a camp with 6 hunters, is about once a week. For a large animal, such as a zebra, the meat might last 3 days increasing the probability of eating meat. In the picture below a Hadza hunter carries a portion of meat that was butchered at the kill or scavenging site. 
I have assigned, O’Connell, Hawkes and Blurton Jones (1988) for an argument using the ethnographic example of Hadza scavenging to assess the probability that ancestral Homo might have regularly eaten meat even though the weapons they had were much less effective than the weapons of the modern Hazda. In the final exam you should be able to integrate the ideas and information from O’Connell et al. (1999), Lecture 5, with the information in this lecture’s reading assignment. In the 1988 paper the researchers report a subset of the observations I have summarized above. In the period from 1985 through 1986 the Hadza took 54 medium/large animals of which 14% by carcass weight were acquired by scavenging. Table 2 shows the high variability of success in both hunting and scavenging. The Late dry season, Sept.- Oct. 1985 was the most successful of the four seasons tallied. During that period Hadza hunters brought 28 carcases into camp compared to 13 in the late dry, Aug.-Oct. 1986. Pay attention to “day-to-day availability” reported in column one, mid-page 359. Note the variation from one season to the next. Average income from scavenging was very high in in the early dry season 1986, 245 g per camp resident per day, but all the gain came from one animal, consumed in 3-4 days, leaving consumers many days without meat in spite of high average income from scavenging. 
As reviewed in O’Connell et al. (1999) berries and tubers provide the dependable daily nutritional requirements of the Hadza. Hadza children are effective foragers on berries, baobab fruit and some shallow rooted tubers but they do not have the strength to gather the deeply rooted tubers that provide the highest returns for adult women. This photo of a senior Hadza women illustrates the strength needed to gain access to these resources. Hadza women, with unweaned babies and children over the age of 6, leave camp in early morning to gather. They are usually accompanied by armed teenage boys and perhaps an adult male as protection against pastoralists who range through the same area. Men and boys who accompany women on these trips sometimes take honey from wild bees. Honey is a valued resource with high caloric return. Hadza men can achieve a mean daily return rate of .78 kg per man-day by spending an average of 41 mins a day in honey collecting. 
Both men and women forage in the shallow water of the reef flat in the 2 to 4 hour low tide periods from March to end of September. Men walk to the reef edge and fish with spears catching a variety of small and large fish. The largest fish they take is giant trevally, pictured on the left, which can grow to 
1.7 m, but they also take a variety of very small fish such as spine foot, sweet lip, and mullet about 38 cm, 45 cm and 78 cm, respectively. The catch for the day is displayed openly as the men leave the reef at the end of the fishing bout. If you can imagine these small fish, pictured in order below, flashing through the coral and the distorted optical perspective from the surface of the water to the shallow depths where fish school and hide, flash and dart you can get a sense of the skill needed to take these fish with wooden spears. In the methods section of the assigned reading, Bliege Bird, Smith and Bird (2001), the authors note that the spearfishing forager ignores other prey types as he travels across the reef. During the low tide men spend 63% of their time spearing and 31% of their time shell fish collecting. This time allocation bears no relationship to the potential returns from the prey, 292 plus or minus 135 kcal/h for spearing and 1492 plus or minus 173 kcal for shell fish collecting.
October the Meriam forage on reef flats at ebb-tide. In October through April they switch to collecting nesting turtles and eggs. Everyone participates in turtle collection during this season as the turtles come onto the sandy beaches to dig nests and lay their eggs. The foragers go out at night or early morning and wait on the beach for turtle to come ashore. When a turtle is spotted, it is flipped over, the flippers tied back and the turtle hauled home by boat. Nesting season collections are undertaken mostly for household provisioning but raw meat from the butchered turtle is always shared out with neighbors. In some cases nesting turtles will be taken for pre-arranged feasts. 
Hunting Returns from turtle collecting and hunting are reported in Table 2. The after-sharing returns during the nesting season are comparable to the returns from reef shellfish collecting. Notice the large variation between return rates from collecting in the nesting season and hunting in the hunting season, 21,875 kcal/h vs. 4922 kcal/hr. The very large difference is caused by the inclusion of search time and energy expended in travel. The energetic costs of travel in the hunt are calculated from the conversion of the cost of fuel for the boats into calories of meat that could have been purchased at the store. When per capita returns are reported after-sharing, open water hunting results in a net loss to the hunters because of the time and fuel costs of the activity. Notice that hunting in the hunting season is more costly than hunting in the nesting season. “In this season, hunting is much easier and hunters take on fewer costs: turtles are found on nearby reefs waiting to crawl onto the beaches to lay eggs at night, the tradewinds have largely ceased, and in between monsoon storms, the water is clear, calm and visibility is excellent, allowing hunters to dog turtles more closely, to lose fewer and to more finely discern size and sex (Bliege Bird et al. 2001 p 14).”
Men often hunt on open water for other resources such as tuna, mackerel and large marine sea mammals. This picture of men displaying their spanish mackerel catch comes from Rebecca Bliege Bird’s home page at Stanford. This looks like an impressive catch but you can see, in Fig. 7 below, how large fish rank in return compared to other resources. The returns graphed in Fig. 7 represent after-sharing returns. The bar represents mean returns with the long line to the right of the bar representing one standard deviation. The longer the line the higher the variance in return. In the case of large pelagic fish, the standard deviation indicates that men may achieve a positive or negative 5500 Net E/hr return per hour. Remember a negative return results from an unsuccessful hunt as the cost of fuel, calculated in purchasing power of store-bought meat, is debited from the return to quantify the costs of search. Only returns from netting sardine, 11,008 kcal/h before sharing, are reported in the assigned reading (p 14) but the figure from Bliege Bird and Bird (1997) illustrates returns from the most important resources. Both men and women use casting nets to intercept schools of sardine along the perimeter of the island and hand-line fishing using hook, line and bait to catch reef carnivores, herbivores and surf fish from the tidal margin. On this chart the returns from sardine netting are lower than those reported above but I assume acquisition costs are not debited from the figure of 11,008 kcal/hr.
rate at which foragers encounter prey in the two patches. Adults are able to move at faster speeds across the sand flats encountering goanna at about .90 items/hour searching whereas children would encounter prey in this same patch at .68 items/hour because of their slower walking speeds. Larger children gain nearly the same caloric gain foraging on the sand plain as in rocky outcrops but smaller children do much better on the rocky outcrops, 448 kcal/hour compared to 385 kcal/hour on the sand plain. The authors suggest children forage in patches that minimize time and effort expended. “Although the gross foraging return rates are the same, foraging in sandhills as compared to rocky outcrops requires 2.3 times greater time investment and 2.2 times more walking as does rocky outcrop foraging for only 1.3 times as many calories per hour (Bird and Bliege Bird 2005 p 142).” The young girls in this picture, copyright Rebecca Bird from her home page at Stanford, are holding a small lizard they have dug from a rocky outcrop burrow. 2 Comments »
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