Evolutionary Social Science: the Behavioral Ecology Approach Evolutionary Social Science: the Behavioral Ecology Approach

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Evolutionary Social Science: The Behavioral Ecology Approach Evolutionary Social Science: The Behavioral Ecology Approach

Evolutionary social science applies theory and method developed in evolutionary biology, anthropology, psychology and economics to understand adaptive variation in human behavior, particularly social behavior. Hypotheses and models about resource use, mating and parenting strategies, and cooperation and competition are derived from evolutionary theory, and empirically tested to understand how humans adapt to their diverse natural and social environments.

Eric Alden Smith, Professor, Bruce Winterhalder, Professor, Department of Anthropology, Department of Anthropology, University of Washington University of North Carolina

Introduction ed recording of behavioral observations in their im- There are several approaches in the evolutionary so- mediate socio-environmental context, often in small cial sciences (Smith et al, 2001), the most promi- communities), whereas evolutionary psychology is nent ones being human behavioral ecology (HBE), more reliant on data collection via surveys or labo- evolutionary psychology, and cultural evolution (or ratory experiments. Dual transmission theory has dual transmission) theory. All these approaches not yet developed a significant body of empirical combine theory and methods from a number of dif- research. ferent academic disciplines. From evolutionary bi- This review will concentrate on summarizing ology they draw mathematical or graphical models theory and findings in HBE, with limited reference anchored in basic principles of evolution by neo- to research in the other two primary traditions of Darwinian natural selection. From neoclassical eco- evolutionary social science. nomics they adopts concepts and analytical tech- Behavioral ecology is a branch of the larger field niques such as optimisation, marginal value analy- of evolutionary ecology, the study of evolution and sis, and game theory. Various research methods are adaptive design in ecological context. Evolutionary drawn from the conventional social sciences (an- ecology emerged as a distinct field in the 1960s, thropology, economics, psychology, etc.) as well as and includes topics ranging from the structural and from animal behavior. In empirical research, HBE behavioral traits of organisms to the organization of tends to emphasize ethnographic methods (extend- ecological communities. The first textbooks on be-

SAMFUNDSØKONOMEN NR. 4 – 2002 13 Evolutionary Social Science: The Behavioral Ecology Approach havioral ecology appeared in late 1970s and early notypic adaptation (see Glossary) to diverse social 1980s, and there is now a voluminous literature, in- and ecological conditions. The research strategy fo- cluding monograph series, specialized journals, and cuses on understanding variation in behavior that a number of field-defining volumes edited by Krebs occurs within short time spans (less than a lifetime), and Davies (e.g., 1997, and earlier). or that accumulates over a few generations through Researchers in anthropology use the theory de- cultural evolution, rather than genetic evolution over veloped in animal behavioral ecology as a tool to many generations. HBE researchers test predictions understand human behavior, considering humans as about the match between environmental conditions one more (though admittedly unique) evolved or payoffs and behavioral variation; less attention species. The HBE approach began to emerge slowly has been paid to the developmental or learning in the 1970s, and accelerated rapidly in the 1990s mechanisms that create or maintain this match. (Winterhalder and Smith 2000). Since it incorporat- The link between phenotypic or cultural adapta- ed material and methods from the much older tradi- tion and genetic evolution is provided by positing tion of ecological anthropology, and paid attention that the former is guided by »decision rules.« These to the roles of intentionality, social complexity and decision rules are presumed to be pan-human cog- cultural evolution, the development of HBE re- nitive adaptations that have evolved by natural se- quired that it adapt and extend biological approach- lection in order to generate behavioral variation that es to better fit human materials and problems is sensitive to environmental context. In the lan- (Smith and Winterhalder 1992). We will summarize guage of game theory, decision rules are usually some fundamental theoretical principles that char- conditional strategies (see Glossary) that take the acterize HBE (and behavioral ecology generally), general form »In context X, adopt one behavioral and then describe selective HBE research problems, tactic; in context Y, switch to the other tactic,« and grouped into several topical categories. so on. For example, in many cultures, it is common for some men to have multiple wives simultaneous- Theoretical Framework ly, a practice termed »polygyny.« The polygyny The adaptationist program (Mayr 1983) in contem- threshold model (Figure 1) hypothesizes that female porary evolutionary biology proposes that natural mate choice follows the evolved decision rule »If selection has designed organisms to respond to lo- the bachelor suitor has at least half the resources of cal social and environmental conditions in fitness- an already-married suitor, accept his offer; other- enhancing ways. From this starting point, behav- wise, become the second wife of the married suit- ioral ecologists formulate and test formal models or.« Behavioral variation arises as individuals incorporating several components. These generally match their conditional strategies to their particular include an adaptive goal (which the strategy under socioecological settings. consideration is designed to optimize), a currency In common with many scientific fields, HBE re- (for measuring the relevant costs and benefits – see search is strongly theory-driven. The research strat- Glossary), a set of constraints (characterizing the egy is built around simple and general mathematical social and environmental context in simplified form models of particular phenomena. Any given model – see Glossary) and a decision set (the range of be- is designed to answer a particular set of questions – havioral options considered). Different evolutionary e.g., What is the optimal set of prey to harvest? goals may require different optimisation methods – How much should a parent invest in male versus fe- deterministic, stochastic, or dynamic methods, or male offspring? Models are used to generate hy- game-theoretic analysis. potheses that can then be tested empirically, and the Because they focus on behavior, and particularly results of these tests indicate whether the model ap- social behavior with a strong cultural component, pears to correctly capture essential features of the human behavioral ecologists must analyze a much phenomenon being investigated, needs significant more labile and causally complex set of phenomena modification, or should be discarded. As an area of than an evolutionist studying, for example, primate research develops, sets of related models are linked anatomy or the foraging behavior of birds. HBE together to form a body of theory covering a rela- generally attempts to explain such complex patterns tively broad domain. of cultural and behavioral variation as forms of phe- A ideal HBE explanation combines models of

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Figure 1 ruling out strategies that increase longevity without increasing number of surviving descendants or other genetic kin). In order to achieve generality, most HBE models are designed to be as simple as possible. They aim to capture the essential features of an adaptive prob- lem, and thus analyze complex socioecological phenomenon in a relatively reductionist fashion. HBE models are thus caricatures of reality intended to be heuristic tools, rather than realistic descriptions of the cognitive or ontogenetic processes that produce human behavior. This sacrifice of realism is made in order to obtain the compensating benefits of increased generality, analytical tractability, and clear empirical appraisal. More specifically, HBE research often assumes that the details of genetic, phylogenetic, and cognitive mechanisms do not, to a first approximation, seriously constrain human adaptive responses to ecological variation. This strategic shortcut, known The polygyny threshold model. When female fitness is at least as »the phenotypic gambit« (Grafen 1984), is taken partially a function of the resources controlled by her mate, fe- because it makes it much easier to build and test males may benefit reproductively by mating polygynously general (widely applicable) models, focused on with males who control greater amounts of resources. In the adaptive design. In this regard, it is broadly similar hypothetical example graphed here, a female who became the to the »complete information« assumption often third wife of a male controlling C resources would obtain hig- made in economic decision models. A model of for- her fitness [W(c )] than if she were to be the married mono- 3 aging strategy that was designed specifically for gamously to a male controlling A resources or the second wife of a male controlling B resources, but would have higher fit- bats, for example, might be made more accurate ness if married monogamously to a poor male controlling A and realistic by incorporating model representations than being the third wife of a male controlling B. of echolocation and the specific nutrient composi- tion of insect prey, but it would then be of little use for understanding foraging behavior in birds, bees, or wolves. The phenotypic gambit may of course be wrong in any particular case; for instance, humans may, as some evolutionary psychologists claim, be circumstance and models of mechanism (Winter- too easily attracted to sugar and fats for our own halder 1997). Models of circumstance ask how so- good because in our ancestral environments these cioecological factors shape the costs and benefits nutrients were rare and of high adaptive value. Ig- associated with alternative behavioral strategies in a noring such evolved biases might lead to erroneous given domain; for example, high amounts of varia- interpretations of the adaptive value of diets in tion in male resources should favor polygyny. Mod- modern populations. On the other hand, humans els of mechanism attempt to specify how natural se- seem to have the cognitive and cultural abilities lection, or a variant such as sexual, kin, or cultural needed to rather quickly produce adaptive respons- selection, will act on these costs and benefits. By es to many novel environmental conditions (includ- combining these two elements, the HBE approach ing, in the present example, dieting regimes, gym- avoids some of the problems associated with func- nasiums filled with exercise equipment, and nutri- tionalist explanation in the social sciences. In par- tional and medical knowledge for dealing with the ticular, neo-Darwinian theory identifies a restricted threats posed by overeating). Given the present state set of units, costs, and benefits that will play a sig- of our knowledge about human behavioral adapta- nificant role in evolutionary processes (for example, tion, it is probably too early to draw any firm con-

SAMFUNDSØKONOMEN NR. 4 – 2002 15 Evolutionary Social Science: The Behavioral Ecology Approach clusions about the overall validity of the phenotypic ing extensive field data collected among a variety of gambit. hunter-gatherer populations, from deserts to jungles to coastal areas, and from tropical to arctic climes Empirical Research (reviews in Kaplan and Hill 1992; Winterhalder and Production Smith 2000). The theory is relatively successful in HBE research can be grouped into three broad topi- explaining observed patterns of prey choice and cal areas: production, distribution, and reproduc- patch use, though it appears that meat is more high- tion. Analyses of production – resource acquisition ly valued than the caloric equivalent in plants, and behavior – draw from »optimal foraging theory,« a that men and women have different decision rules family of models initially developed by biologists (see below). that borrows heavily from neoclassical economics. The optimal foraging framework is increasingly Optimal foraging models address resource selec- being used by archaeologists to study shifts in sub- tion, time allocation, and movement between differ- sistence patterns in prehistoric populations in re- ent habitat sectors or »patches.« By far the most sponse to such factors as technological change, cli- popular has been the prey choice model (PCM), matic fluctuations, prey depletion due to overhar- which nicely exemplifies the HBE research strate- vesting, and human population growth. It has illu- gy. minated why children harvest different resources PCM predictions test our assumption that for- than adults in the same society (one important rea- agers have the goal of choosing the set of available son seems to be their smaller size and strength, prey types that, under given environmental condi- which limits the rate at which they encounter and tions, yields the maximum value per unit foraging harvest resources), why foragers do or do not con- time. Because it can be readily measured and is serve resource species, and why some resources are quite general, the currency usually used is the net processed at the harvest or kill site and others trans- energy acquisition rate (i.e., calories acquired mi- ported whole back to camp, and why foragers in nus calories expended, per person-hour). Net acqui- various parts of the world have independently en- sition rate is appropriate if foragers are time-limited gaged in a process of plant and animal domestica- (i.e., gain more from freeing time for other activi- tion leading to agricultural production systems (re- ties than from harvesting additional resources), en- view in Winterhalder and Smith 2000). ergy-limited (i.e., gain more from additional units Given the universal and recurrent short-term of harvest than from reduced foraging time), or face need for metabolic energy, it is reasonable to as- foraging conditions that expose them to hazard lev- sume that foraging strategies which maximize the els greater than those they experience when not for- net acquisition rate of energy while foraging have aging (e.g., predation, higher risk of injury, or cli- higher fitness, at least within broad limits. We mate stress). Thus, contrary to common intuition, should expect selection to favour cognitive mecha- energy return rate may be adaptively important even nisms and culturally-inherited rules of thumb that if food is relatively abundant, as long as there is an produce behaviors keyed to this goal. However, opportunity cost associated with the time or risk ex- most optimal foraging models are general enough posure involved in foraging. that the currency could be any rate measure of re- The PCM predicts that diet breadth will shrink source value – protein capture, raw-material value, as high-ranking prey become more abundant, but monetary return, or prestige. For instance, recent that increased abundance of any resources lying applications of the PCM have examined the circum- outside of the optimal set will not cause it to be har- stances under which sexual selection might favour vested. (As an analogy, think of piece-work; a per- different currencies for males and females (Bliege son paid by the piece will only make time-consum- Bird 1999). Some applications have even used in- ing pieces that pay less per hour worked if she can- formation return rate as the currency in very mod- not get enough orders for higher-paying pieces, and ern contexts: library scientist Pamela Sandstrom the abundance of low-return pieces she could sell (1994) finds that foraging theory provides useful should have no influence on her decision.) These guidelines for optimising searches by library pa- and other predictions, as well as those derived from trons, while computer scientists Pirolli and Card other optimal foraging models, have been tested us- (1995) argue that foraging models are applicable to

SAMFUNDSØKONOMEN NR. 4 – 2002 16 Evolutionary Social Science: The Behavioral Ecology Approach analysis and design of effective information-gather- not possessing a resource packet benefit more by ing tools for the world-wide web and other large- taking portions than the holder can benefit by de- scale, patchy informational databases. fending them. Voluntary resource sharing is usually modeled in terms of the delayed, cost-benefit calcu- Distribution lus of reciprocal altruism. For example, if resource Foraging models concern themselves with the harvest is unpredictable and relatively unsynchro- short-term production decisions of individuals. nised, harvesters might benefit by pooling the catch However, for humans and their hominid ancestors, and thereby minimizing subsistence risk (variance the harvest and/or consumption of resources gener- in resource consumption). A very different type of ally occurs in a social group, a context adding a explanation invokes costly signaling: by successful- host of theoretical and empirical challenges. ly harvesting and then distributing difficult-to-cap- Cooperative subsistence efforts may offer several ture resources, individuals may reliably signal their advantages: increased per capita resource harvest skill or other socially-valued qualities, benefiting rate, reduced variation in harvest rates, reduced themselves as well as potential allies, mates, or losses to competitors, and increased vigilance in competitors who gain useful information about the predator detection. However, group foraging can al- provider as well as food (Smith and Bliege Bird so increase local resource competition and deple- 2000; Gintis et al. 2001; Hawkes and Bliege Bird tion. Even where cooperation is beneficial, model- 2002). ing has shown that optimal group size may be un- There are several empirical studies assessing the stable due to conflicts of interest between existing explanatory power of one or more of the resource members and potential joiners. (For example, even transfer models. Collectively, these studies indicate if parties of two offer the highest per-capita return that transfer behaviors are much more diverse and rate in deer hunting, I might selfishly wish to be- context-specific than has been appreciated in the come the third group member, thus driving down standard ethnographic literature. These studies also the returns of the first two members, if the per-capi- suggest that transfer behaviors are probably multi- ta returns in a group of three are higher than those I causal in origin, the result of several selective pres- can get from solitary foraging.) Once groups form, sures whose relative importance depends on the sit- they provide the context for complex social dynam- uation (Winterhalder 1997). ics, including competition and conflict over labour contributions and division of the harvest, any of Reproduction which may affect production in ways not anticipat- While classical sociobiology analyzed reproductive ed by simple foraging models. behavior in terms of factors inherent in sexual re- The conditions favouring different kinds of reproduction, such as genetic relatedness and gamete source transfers have been the focus of considerable asymmetry, HBE analyzes variation in reproductive research in HBE. Unlike most other primates, hu- behavior as a function of local ecological context man foragers and agriculturalists often harvest re- (Borgerhoff Mulder 1992). In contrast to evolution- sources of sufficient »package size« (e.g., large ary psychology, HBE posits that this variation in- game) or in sufficient bulk (e.g., an agricultural volves phenotypic tracking of current circum- crop) that some combination of transfer to those stances, rather than the playback of relatively without the resource and storage for later use is »hard-wired« species-, sex-, or age-specific behav- likely. There are a variety of models to study this ioral routines that were adaptive in our remote evo- (Winterhalder 1997), each making somewhat differ- lutionary history. Nevertheless, HBE approaches ent assumptions about the socioecological circum- overlap considerably with these other two traditions stances specified (e.g., group size, information flow, and with certain versions of cultural evolution , as frequency of interactions among the individuals in- well as with less explicitly Darwinian fields such as volved, the nature of the resource), and the evolu- demography and reproductive ecology. tionary mechanism invoked (e.g., individual, kin, The central concept in HBE studies of reproduc- sexual, group or cultural selection). Simple individ- tive strategies is the principle of allocation (Hill ual-level selection will generate transfer by scroun- 1993): any effort (time, energy, resources) allocated ging (also known as »tolerated theft«) when those to one domain (for example, enhancing personal

SAMFUNDSØKONOMEN NR. 4 – 2002 17 Evolutionary Social Science: The Behavioral Ecology Approach survival and maintenance) cannot be allocated to 1999) but has only recently begun to be studied in another domain (for example, reproduction). Mat- detail (M. Borgerhoff Mulder, personal communica- ing and parenting together constitute reproductive tion) effort, and models often assume that effort allocated Whatever form the mating system takes, human to mating cannot be allocated to parenting (and vice offspring require extensive and extended parental versa). Thus, the principle of allocation can be used care. This »parental investment« (Trivers 1972) be- to define a set of key tradeoffs that are amenable to gins with gestation and, among humans, can contin- study with optimisation models. ue beyond the parent’s death (via bestowing of land, The distribution of key resources strongly shapes wealth, and other forms of inheritance). HBE analy- the behavior of males and females, generally ses ask how the amount and timing of such invest- through different routes. If some males can monop- ment might vary according to social and environ- olize resources necessary for female survival and mental constraints. Most research is concerned with reproduction, they can use this control to attract one of three categories: birth spacing, differential mates, or to compete with other males for social investment in offspring (by sex or expected repro- dominance. As we mentioned above, polygyny and ductive value), and interactions between mating and increased variance in male mating success is the parenting. predicted result. Male resource control coupled Since parental time and resources are finite, with female mate choice is the basis for the polygy- higher fertility rates should result in less parental ny threshold model (Figure 1). The outcome pre- investment per offspring and may eventually reduce dicted by the simplest versions of this model is an total reproductive success. This insight provided the »ideal free distribution,« in which the number of basis of the optimal clutch-size model first devel- mates per male will match the resources each male oped by behavioral ecologists to study reproduction can offer, and female fitness will be equal across in nesting birds, but easily generalized to apply to mateships. any species with parental investment, including hu- The polygyny threshold model has received mans (Figure 2.a). This model predicts that beyond broad support in empirical tests among a variety of a certain point, increased fertility (larger clutches, human societies, though with various qualifications or shorter interbirth intervals) will result in reduced (Borgerhoff Mulder 1992; Winterhalder and Smith overall parental reproductive success. 2000). The male-controlled resources may be politi- Nicholas Blurton Jones (1986) used this ap- cal rather than economic. Coercion by mates or by proach to show that among the !Kung San hunter- relatives may severely constrain female choice. Fe- gatherers of southern Africa, interbirth intervals males mated polygynously may face reduced repro- much shorter than the actual mode of 4 years result- ductive success due to competition with co-wives, ed in increased offspring mortality, sufficient to though this may be compensated in the next genera- cause a net loss in expected reproductive success. tion if the sons of polygynously-married women One attempt to replicate the !Kung results among have increased chances of inheriting wealth and Ache foragers of Paraguay failed, possibly because mating polygynously themselves. Ache offspring mortality is less sensitive to varia- Polygyny has been a common form of marriage tion in interbirth interval (Hill and Hurtado 1996; in the great majority of societies in the ethnographic but see Strassmann and Gillespie 2002). and historical records (just consult the Old Testa- Parental investment affects a child’s health, sur- ment for examples). Even in societies where vival and future mating success, and thus the par- monogamy is legally prescribed, extramarital mat- ents’ inclusive fitness. Parental fitness payoffs de- ing and remarriage biased towards wealthier or pend on three sets of variables: (1) the genealogical more powerful males creates a situation of effective relatedness between parent (or other caregiver) and polygyny. Human behavioral ecologists have also offspring, (2) the effect of investment on the expect- analyzed monogamous systems, especially those in- ed reproductive value of the offspring (as well as volving social stratification and dowry, as well as present and future siblings), and (3) the effect of in- the rare but intriguing polyandrous case. »Serial vestment on the caregiver’s own reproductive value. polyandry,« in which women remarry to find better Sets (2) and 3) are more directly affected by ecolog- mates, is presumably much more common (Hrdy ical variables, and hence are at the center of HBE

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Figure 2 analyses (Voland 1998; various authors in Cronk et al. 2000). HBE analyses of broader life history topics center on such categories as the timing of growth and maturation, sub-adult and adult dispersal strategies, the onset of reproduction, the timing of reproductive events (e.g., birth spacing, weaning, menopause), mortality patterns, and senescence. Most such research thus far has focused on four topics: (1) links between production and reproduction, (2) reproductive effort and maturation, (3) menopause and extended human lifespan, and (4) evolutionary analysis of the so-called demographic transition (re- duction in fertility and family size with moderniza- tion). The first three of these topics are given exem- plary treatment in an extended case study by Hill and Hurtado (1996; see also Hawkes et al. 1997; Hill and Kaplan 1999), while (4) is reviewed by Borgerhoff Mulder (1998).

Conclusions Evolutionary social science applies contemporary Darwinian theory and ethnographic and ethological methods to understand the evolutionary origins and adaptive variation in human behavior, particularly social behavior. One approach within evolutionary social science, human behavioral ecology (HBE), combines elements from a number of different academic disciplines, including anthropology, economics, animal behavior, and evolutionary biology. In HBE, formal models anchored in basic principles of evolution by natural selection are used to derive hypotheses concerning how humans adapt to past or present natural and social environments. Predictions are then tested with ethnographic, historical, and ar- A graphical model of optimal fertility rate. Solid lines repre- chaeological data. HBE research assumes that hu- sent fertility rate (births per unit time), while dashed lines man decision-making is guided by evolved »deci- represent mortality as a function of fertility rate, the latter sion rules« or conditional strategies, but focuses curving upwards to represent the effect of increasing compe- primarily on behavioral variation and adaptive con- tition among offspring for parental investment. The net differ- sequences, rather than on the underlying cognitive ence between these two curves is the number of surviving mechanisms. offspring, with a local peak at a. The model assumes that se- While much has been learned in the relatively lection favours maximizing a given the constraints a parent short time (circa two decades) in which HBE re- faces, and hence favours an optimal fertility rate Fo. Graph (a) considers a single parent. Graph (b) models two parents with search has been carried out, the field is in its infan- different constraints and hence different offspring mortality cy, and many fundamental issues remain unre- curves as a function of fertility (parental investment). A poor- solved. These unresolved issues include the degree ly-endowed parent suffers higher offspring mortality at a giv- to which contemporary behavioral variation (particularly that in industrialized societies) is adaptive, en fertility rate, and hence a lower optimal fertility rate Fp than a more highly-endowed parent with optimal fertility rate the role played by cultural inheritance, and the evolutionary mechanisms (e.g., kin selection, group se- Fh.

SAMFUNDSØKONOMEN NR. 4 – 2002 19 Evolutionary Social Science: The Behavioral Ecology Approach lection, costly signaling, reciprocal altruism, etc.) References Bliege Bird RL (1999) Cooperation and conflict: The behav- and selective environments that have shaped various ioral ecology of the sexual division of labour. Evolutionary aspects of human nature. New and promising Anthropology 8: 65-75. methodological developments such as risk-sensitive models, dynamic programming, and agent-based Blurton Jones N (1986) Bushman birth spacing: a test for op- models are receiving greater attention, as is the timal interbirth intervals. Ethology and Sociobiology 7: 91- more mechanistic question of how individuals actu- 105. ally achieve the adaptive responses predicted and Borgerhoff Mulder M (1992) Reproductive decisions. In: often found in HBE research. Smith EA and Winterhalder B (eds.) Evolutionary Ecology and Human Behavior, pp. 339-374. Hawthorne, NY: Aldine de Gruyter.

Borgerhoff Mulder M (1998) The demographic transition: Are we any closer to an evolutionary explanation? Trends in Ecology and Evolution 13: 266-270.

Glossary of Terms

Conditional strategy: In game theory, a rule that specifies responding in one of several ways, depending on environmental cues (e.g., if it is a cloudy morning, pack an umbrella) or on the player’s current state (if I am bad at math, don’t major in economics). Currency: The units in which costs and benefits of different options are defined theoretically or measured empirically; while evolutionary theory specifies inclusive fitness as the ultimate currency, both decision-makers and analysts often use proxi- mate currencies (such as mating success, food harvest rate, etc.). Constraint: Any factors which determine the possible options, costs, benefits, and abilities of individual decision-makers; a key element in any optimisation or game-theoretical analysis. Decision rule: A pan-human (universal) adaptation that evolved by natural selection and guides behavioral variation to adapt it to environmental context; for example, a predisposition that makes one prefer to behave in ways that locally increase food acquisition rate or social status. Decisions rules are often modelled as conditional strategies. Inclusive fitness: Net rate of gene replication realized through both direct descendants (offspring) and through the effect of one’s actions on the reproductive success of close genetic kin. Marginal value: Additional value (fitness, food capture, wealth, etc.) obtained through an additional unit of investment (of time, effort, energy, wealth, etc.) Parental investment: Any expenditure that enhances the expected reproductive success of a given offspring while reducing the parent’s ability to invest in other existing or future offspring. Phenotypic adaptation: Actions or changes in condition (e.g., tanning) that take place within an individual’s lifetime and without genetic change, but which enhance that individual’s ability to survive and reproduce. Polygyny: A form of polygamy involving marriage or sustained sexual coresidence of one man and two or more women si- multaneously; a less common form of polygamy is polyandry (simultaneous marriage or mating of one woman and two or more men). Reciprocal altruism: A social relationship wherein a donor pays a cost to help a beneficiary who profits disproportionately (e.g., sharing one’s catch with an unsuccessful forager), and is repaid in the future when fortunes are reversed, generating a net fitness gain to both parties. Reproductive effort: Any actions or changes in condition (e.g., ovulation) that directly increase the probability of reproduction; subdivided into mating effort and parental investment, and distinguished from somatic effort. Reproductive success: Some measure of the number of surviving offspring (e.g., all offspring surviving to maturity). Scrounging (tolerated theft): Extraction of resources from a producer by a non-producer (other than a dependant) through begging, coercion, or some other means of imposing costs on the producer.

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Hill K (1993) Life history theory and evolutionary anthropol- Strassmann BI, Gillespie B (2002) Life-history theory, fertili- ogy. Evolutionary Anthropology 2: 78-88. ty and reproductive success in humans. Proceedings of the Royal Society of London, Series B 269:553-562. Hill K and Hurtado AM (1996) Ache Life History: The Ecolo- gy and Demography of a Foraging People. Hawthorne, NY: Trivers RL (1972) Parental investment and sexual selection. Aldine de Gruyter. In Sexual Selection and the Descent of Man, 1971-1971,ed. B.G. Campbell, pp. 136-79. Chicago: Aldine. Hill, Kim and Hillard Kaplan (1999) Life history traits in humans: theory and empirical studies. Annual Review of Anthro- Voland E (1998) Evolutionary ecology of human reproduc- pology 28:397-430. tion. Annual Review of Anthropology 27: 347-374.

Hrdy S (1999) Mother Nature: A History of Mothers, Infants Winterhalder B (1997) Gifts given, gifts taken: The behav- and Natural Selection. New York: Pantheon Books. ioral ecology of nonmarket, intragroup exchange. Journal of Archaeological Research 5: 121-168. Kaplan H and Hill K (1992) The evolutionary ecology of food acquisition. In: Smith EA, Winterhalder B (eds.) Evolu- Winterhalder B and Smith EA (2000) Analyzing adaptive tionary Ecology and Human Behavior, pp. 167-201. strategies: Human behavioral ecology at twenty-five. Evolu- Hawthorne, NY: Aldine de Gruyter. tionary Anthropology 9: 51-72.

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