FORAGING STRATEGIES The behavioral ecological study of foraging strat- egies has been centered primarily on a family of related models known
HUMAN BEHAVIORAL ECOLOGY 31 collectively as optimal foraging theory. Many different optimal foraging models have been designed to deal with such aspects of forager decision- making as what they should eat (diet breadth or prey choice), where they should forage (patch choice), where they should live (habitat or settlement choice), with whom they should forage (group size), and how long they should forage (time allocation). Most of these models share a number of features, including a theoretical basis in the theory of natural selection, the use of proxy currencies (usually calories) in place of fitness to measure the success rate of different foraging strategies, and the assumption that selection favors maximal net rate of return to time spent foraging.
The early 1980s saw the publication of optimal foraging analyses of hunting group size among the Inuit (157) and diet breadth and patch choice among the Cree (188), Alyawara (144), Ache (104), and Siona-Secoya, Ye'kwana, and Yanomamo (92). Because those studies have been reviewed and criticized elsewhere (34, 44, 121, 158), here I focus instead on more recent, less familiar studies that have gone beyond the traditional assumptions and limitations of optimal foraging theory.
One of the advantages of working with clear-cut, almost procrustean models such as optimal foraging theory is that it is easy to see when peoples' behavior does not fit the theory. Such failures of the theory can be especially enlightening. The foraging decisions of the Bari of Venezuela are a case in point (12). The productivity of Bari fishing and hunting varies with monthly rainfall patterns, and, in general, the Bari spend more time fishing when this activity is more productive than hunting, and more time hunting when the reverse is true. The puzzle is that they never abandon hunting entirely, even in months when the return rate from fishing is several times better. The problem is not with the foragers, but with the simple initial model, which considered only gross seasonal variations in resource availability. Because the Bari environment is temporally fine-grained, day-to-day variations in the availabil- ity of good fishing sites must also be considered. The Bari themselves say that they hunt a lot simply because of these variations: Even in the dry season when fishing returns are highest, daily variations frequently lower the prob- able return rates for fishing and make hunting a more attractive option. And, finally, some men prefer to hunt rather than fish because they happen to be better at it, suggesting the need to incorporate individual differences in foraging skills into future studies.
Many optimal foraging models may also be too simple in their reliance on calories as a proxy currency for fitness. In some cases, calories may be easy to obtain, and other, scarcer nutrients may be more important determinants of foraging patterns. For example, three different groups of South American foragers, the Cuiva (Hiwi) of Venezuela, Ache of Paraguay, and Yora (Yaminahua) of Peru, sometimes forgo plant foods with high caloric returns in favor of meat, suggesting that a desire for proteins and fats may at times
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outweigh the need to forage efficiently for calories alone (110). This problem has been modeled mathematically using both linear programming (13) and indifference curves (110, 121). To test the utility of the latter method, Hill (110) derived indifference curves for meat and carbohydrates from the observed preferences of three groups of South American foragers and used them to predict the exchange values of meat and carbohydrates among the Mbuti. The results were mixed. While the Mbuti data agree fairly well with predictions based on the behavior of the Ache and Cuiva, the Mbuti obtained a significantly larger proportion of their calories from meat (30%) than predicted by Yora indifference curves (17%). Although this is an enlightening first step toward the incorporation of nutrients into optimal foraging studies, the indifference-curve method is essentially inductive, and by itself can do little more than provide post hoc descriptions of observed behavior. We now need predictions of indifference curves based not solely on observations but on an understanding of the relationship between diet and fitness in different environments (110; see also 121). Behavioral ecologists have also studied many other aspects of the strategies of human foragers, but space does not allow a full treatment of them here. The most notable are the idea of the original affluence of foragers (see 44), egalitarianism (41, 185), spatial organization (see 46), sex differences in foraging strategies (see 112, 121), the effects of different foraging technologies (8, 188, 111), predator-prey relationships (189), reproductive and social aspects of foraging (7, 112), and the overkill hypothesis for Pleistocene extinctions (see 188).
BEYOND FORAGING Although foraging has dominated the behavioral eco- logical study of resource acquisition, there is no a priori reason why a similar approach could not be taken to other types of resource acquisition, such as horticulture and pastoralism. Hames (90) compared time allocated to garden- ing, hunting, and fishing among several groups of Amazonian Indians, arguing that Amazonian horticulturalists may face trade-offs between settle- ment stability, which may increase yields from gardens but deplete local game populations, and settlement mobility, which may increase yields from hunting but decrease the efficiency of gardening. Hames also suggests that since gathering appears to be less efficient than hunting in Amazonia, horticulture may have been pioneered by women as a way to increase their productivity (see also 123).
Pastoralism also provides many opportunities for the expansion of be- havioral ecological studies of somatic effort. R. Dyson-Hudson (70), for example, has developed an ecological explanation for the flexibility of social and residential patterns among the Ngisonyoka Turkana of Kenya, and De Boer & Prins (62) have used optimal foraging models to study decision- making by cattle herders in Burkina Faso. Models of predator-prey relations
HUMAN BEHAVIORAL ECOLOGY 33 may also provide insights into herd management, and field studies of both wild ungulates and modem human foragers may help us to understand the process of domestication.