Human Behavioral Ecology at Twenty-Five

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Analyzing Adaptive Strategies: Human Behavioral Ecology at Twenty-Five

BRUCE WINTERHALDER AND ERIC ALDEN SMITH

INTRODUCTION cesses of resource competition and tionally termed “behavioral ecology.” transfers, now supplement and enrich Behavioral analyses have been an in- Human behavioral ecology (HBE) those of resource production. Demo- tegral element of evolutionary ecology began in the mid-1970s with the ap- graphic and life history analyses have from the beginning, treating topics plication of optimal foraging models begun to show how ecological factors such as foraging strategies,27 mating to hunter-gatherer decisions concern- of production and distribution relate systems,30 and spatial organization ing resource selection and land use. In to those of mortality, fertility, and life and competition.32 The first textbooks the 25 years since, the field has devel- course. on behavioral ecology appeared in oped and successfully adapted evolu- Besides providing a comprehensive late 1970s33 and early 1980s.34 There tionary ecology theory and methods view of the field, we hope to demon- now is a voluminous literature, in- to a wide range of topics important to strate that HBE has established itself cluding monograph series, dedicated archaeology and to anthropology gen- as a progressive research tradition,1 a journals (e.g., Evolutionary Ecology, erally. In this review we examine the Behavioral Ecology and Sociobiol- basic theory and its extensions to chil- question we take up in our conclu- and dren’s foraging, conservation biology, sion. We note that this review supple- ogy), and a widely read, state-of-the- 35–38 demographic transitions, domestica- ments others. Two edited collections art series of volumes, each edition tion and agricultural origins, the evo- summarized the state of HBE in the with a new set of papers. 2 3 lution of menopause, field processing early 1980s and early 1990s; shorter Human behavioral ecology applies 4–6 and central place foraging, life his- reviews have appeared as well. A evolutionary ecology models and con- tory, male-female division of labor, pending collection carries us to the cepts to the study of human behav- 7 mating tactics and fertility decisions, late 1990s. Reviews of HBE general- ioral diversity. HBE began in the mid- 8,9 and resource intensification. Work on ly, as well as life history and demog- 1970s with a small set of interpretive 10–12 13 39–41 resource acquisition continues, but raphy, maturation, primate life papers and independent disserta- 14 has been extended from foragers to span and litter size, mating strate- tion projects. Initially centered on for- 15,16 17,18 pastoral and agricultural production gies, reproductive ecology, re- aging theory and hunter-gatherer systems. Studies of resource distribu- source transfers,19 resource conserva- studies, HBE has expanded over the tion, and especially intragroup pro- tion,20 and division of labor21 have last 25 years to encompass diverse appeared previously in this Journal. topics and subsistence systems (Fig. Evolutionary ecology studies “evo- 1). Although a second generation of lution and adaptive design in ecologi- HBE researchers is now in academic cal context.”22 As a distinct field, evo- positions, the field is young enough Bruce Winterhalder is Professor of Anthro- pology and Chair of the Curriculum in Ecol- lutionary ecology emerged in the that its initiators remain the majority ogy at University of North Carolina, Chapel 1960s with the work of Charnov and of those publishing in it. Hill. He currently is working on a book- 23 24,25 26 length analysis of the microecological Orians, Hutchinson, Lack, HBE’s early goals were to set the bases of the hunter-gatherer mode of pro- MacArthur and Pianka,27 MacArthur cultural ecology of Steward,42 partic- duction. E-mail: [email protected] and Levins,28 MacArthur,29 Ori- ularly as developed in his hunter-gath- Eric Alden Smith is Professor of Anthro- pology and Director of the Graduate Pro- ans,30,31 and others. Textbooks on evo- erer work43 and as represented in later gram in Environmental Anthropology at lutionary ecology appeared in the studies such as those by Lee,44 on a University of Washington, Seattle. He is currently engaged in studies of marine 1970s, covering topics from the struc- sounder theoretical footing by allying foraging, reproductive strategies, and so- tural and behavioral traits of organ- it to emerging, neo-Darwinian ap- cial status among Torres Strait Islanders isms to the organization of ecological proaches to behavior. From the start, in collaboration with Rebecca Bliege Bird and Douglas W. Bird. E-mail: communities. Evolutionary ecology the proposed alliance was a somewhat [email protected] shares fuzzy boundaries with evolu- wary and selective one. To varying de- tionary genetics, community ecology, grees, there were attempts to distance animal behavior, and decision theory. this anthropological effort (some- Key words: behavioral ecology; foraging strate- 2 gies; human adaptation; human evolution; hu- When applied to the analysis of behav- times termed “socioecology” ) from man life history; reproductive strategies ior, evolutionary ecology is conven- the sociobiology of Wilson,45 and 52 Evolutionary Anthropology ARTICLES

particularist tradition of anthropology.70 HBE assumes that complex socioecological phenomenon are most fruitfully studied in a reductionist rather than holistic fashion. However, the methods of HBE and anthropology overlap in a common concern with ethnography: the extended recording of behavioral observations in their immediate socioenvironmental context, often in small communities in remote and materi- ally impoverished conditions, supple- mented with data collected by survey, interview, or archival research. Here the uncluttered predictions of simple models meet the messy reality of ﬁeld- work and participant observation. Al-

HBE is an anomaly within sociocultural anthropology due to its hypothetico-deductive research strategy and its neo-Darwinian Figure 1. A tabulation of theoretical sources. HBE HBE publications, by topic and decade. The topical derives testable divisions correspond to sections of this article, with hypotheses from “Other” representing publi- mathematical or cations that cross these boundaries (e.g., review graphical models articles) or do not fit neatly in any of them. As indi- anchored in basic cated, the graph is divided principles of evolution into ethnographic, archaeological, and com- by natural selection. bined tabulations. The count is based on a combination of the authors’ personal bibliographic databases, plus a survey of though HBE is more committed to selected journals and recent review articles (we quantitative methods that reliably thank Geoff Kushnick for document observable behavior than is bibliographic assistance). sociocultural anthropology generally, it is rare that key variables can be controlled, samples randomized, or more recently from the approach sources. HBE derives testable hypoth- replication achieved. HBE research- known as evolutionary psychology eses from mathematical or graphical ers share field methods with their so- (see Box 1). models anchored in basic principles ciocultural colleagues, but within a of evolution by natural selection. Em- theoretical and epistemological phasizing generality, most HBE mod- framework that is alien to that field. THEORY AND METHOD IN HBE els strive to be as simple as possible. A complete HBE explanation com- HBE is an anomaly within sociocul- They seek to capture the essential fea- bines models of circumstance and tural anthropology due to its hypo- tures of an adaptive problem, and ne- models of mechanism.71 Models of cir- thetico-deductive research strategy glect to some degree the myriad ancil- cumstance ask, “How do socioenvi- and its neo-Darwinian theoretical lary variables of concern in the more ronmental factors shape the costs and ARTICLES Evolutionary Anthropology 53

Box 1. HBE Compared to Some Closely Related Fieldsa

Behavioral ecology Evolutionary psychology Dual inheritance theory Explanatory focus Behavioral strategies Psychological mechanisms Cultural evolution Key constraints Ecological, material Cognitive, genetic Structural, information Temporal scale of Short-term Long-term (genetic) Medium-term (cultural) adaptive change (phenotypic) Expected current Highest Lowest Intermediate adaptiveness Hypothesis generation Optimality and ESS Informal inference Population-level models models Primary hypothesis-testing Quantitative Survey, laboratory Mathematical modeling methods ethnographic experiment and simulation observation Favored topics Subsistence, Mate choice, sex differences Cooperation, reproductive maladaptation strategies a Modified from Smith.46 HBE is only one of several distinct approaches to evolutionary analysis of human behavior that have developed in recent years; other prominent ones include evolutionary psychology and dual inheritance theory. Here we offer a brief comparison with these two fields.46–49 Evolutionary psychology (EP) aims to uncover the evolved “psychological mechanisms that underpin human...behavior,” and “the selective forces that shaped those mechanisms”50 (see reviews51–54). EP posits that human behavior is guided by specialized cognitive modules rather than “general purpose” mechanisms that work across multiple behavioral domains. It argues that these modules evolved in the EEA (environment of evolutionary adaptedness), and are designed to produce very specific outcomes that may no longer be adaptive in modern (post-paleolithic) environments. Based on these assumptions, some evolutionary psychologists55–57 have actively criticized behavioral ecologists for studying adaptive patterns in contemporary nonforaging contexts, and for measuring degree of adaptation via fitness outcomes or correlates. Dual inheritance theory (DIT) views culture and genes as providing separate (but linked) systems of inheritance, variation, and hence evolutionary change.58–63 Most DIT practitioners posit that the spread of cultural information is affected by multiple forces, including natural selection (differential fitness of culturally inherited variation), decisionmaking (based on genetically or culturally evolved preferences and constraints), and transmitter influence or promi- nence. They differ in the relative strengths or attention they give to these various forces. In contrast to classical forms of cultural evolutionism64,65 and to evolutionary archaeology,66,67 DIT practitioners posit that cultural evolution is embedded in and constrained by genetically evolved psychological propensities. They often build these assumptions into explicit mathematical models. Since cultural inheritance differs from genetic inheritance in key ways (e.g., nonparental transmission, multiple transmission events over a lifetime), the evolutionary dynamics of culture will also differ in important but analytically understandable ways. Hence, genetically nonadaptive cultural evolution is possible, and may be likely in hierarchical and modern bureaucratic societies. However, this divergence of cultural and genetic evolution can cut both ways: several DIT theorists60,68,69 posit that mechanisms of cultural evolution often find ways of improving on adaptive outcomes compared to what can be achieved by ordinary genetic evolution plus phenotypic adaptation. Although EP, DIT, and HBE are often portrayed as competing alternatives, a case can be made for viewing them as complementary.46,48,49 Competition is fueled by different assumptions, analytical methods, and alliances with other disciplines, as well as by ordinary academic politics. Complementarity between the three styles is increasingly recognized, however, and can be fostered by taking advantage of differences in their methods of investigation (e.g., observation vs. experiment), empirical foci (e.g., psychological vs. behavioral vs. cultural), selection of constraints (cognitive, ecological, informational), time scales of adaptation, and behavioral domains (see table, above). There are limits to this complementarity, however. A behavior cannot simultaneously be the product of a genetically programmed cognitive algorithm that no longer produces adaptive results, a product of a culturally inherited meme that persists because it has a high replication rate, and a product of phenotypic adaptation that is optimally geared to local environmental conditions. Nevertheless, these hypotheses could be simultaneously true for different behavioral domains or instances.46 54 Evolutionary Anthropology ARTICLES benefits associated with potential al- Prey Choice: An Example of the independent variable might be the ternatives in this behavioral catego- the HBE Approach encounter rate with various resource ry?” Models of mechanism require that types. Other constraints (e.g., foraging we specify how natural selection, or a As with all HBE models, the prey- technology, information processing) variant such as sexual or kin selection, choice model incorporates a goal (op- could take this role, depending on will act on these costs and benefits. By timize net acquisition rate), a currency their rate and degree of change, and combining these two elements, the (for measuring the relevant costs and the researcher’s interest. HBE approach avoids some of the benefits), a set of constraints (charac- The alternative set in the prey- problems associated with sociological terizing the social and environmental choice model is the diet combinations functionalism.3 In particular, neo- context), and a decision or alternative (or breadth) achieved by stepwise ad- Darwinism restricts the range of set (the range of behavioral options to dition of resources which have been units, costs, and benefits that we ex- be examined). ranked by their pursuit and handling pect to have causal salience in evolu- Different evolutionary goals may re- profitability (or 1, 1 ϩ 2, 1 ϩ 2 ϩ 3, tionary processes. quire different optimization methods, etc.). Diet breadth is said to expand as HBE usually frames the study of and HBE analyses may draw on either more resources are added to the opti- adaptive design in terms of decision deterministic, stochastic, or dynamic mal set, i.e., those pursued on encoun- rules72 or conditional strategies: “In optimization, as well as on game-the- ter. 79 context X, do ␣; in context Y, switch to oretic analysis. The prey-choice Given the universal and recurrent ␤.” For example, the polygyny thresh- model typically uses a deterministic short-term need for metabolic en- old model30,73 assumes that female optimization approach, and it speci- ergy, it is reasonable to assume that mate choice has evolved to follow the fies which resource types can be har- foraging strategies that maximize decision rule, “If the bachelor suitor vested most efficiently from among the net acquisition rate of energy has at least half the resources of an those available in a given locale. Prey- while foraging have higher fitness, at already-married suitor, accept his of- choice model predictions test our as- least within broad limits. We should fer; otherwise, become the second sumption that foragers have the goal expect selection to favor cognitive wife of the married suitor.” Behav- of responding to changing environ- mechanisms (and culturally inher- ioral variation arises as individuals mental constraints with choices that ited rules of thumb) to produce be- match their conditional strategies to optimize net yield. haviors keyed to this goal. However, their diverse socioenvironmental set- Because it can be readily measured most optimal foraging models are tings. More generally, HBE assumes and is quite general, the currency used general enough that the currency that the details of genetic, phyloge- in the prey-choice model is usually the could be any rate measure of re- netic, and cognitive mechanisms do net acquisition rate of energy. Net ac- source value—protein capture, ma- not, to a first approximation, seriously quisition rate can be used when for- terial need, monetary return, or constrain human adaptive responses agers are time-limited (i.e., gain more prestige. Recent applications of the to ecological variation.74 from freeing time for other activities prey-choice model have examined than from harvesting additional re- the circumstances under which sex- sources), energy-limited (i.e., gain ual selection might favor different more from additional units of harvest currencies for males and females PRODUCTION: FORAGING than from reduced foraging time), or (see Sexual Division of Labor, be- BEHAVIOR AND RESOURCE face foraging conditions that expose low). SELECTION them to hazard levels greater than Although we abbreviate their pre- those they experience when not forag- sentation, it is important to keep in Analyses of resource selection and ing (e.g., predation, higher risk of in- mind that all hypotheses or interpre- harvesting behavior—“production” in jury, or climate stress).80–85 Thus, net tations discussed in the balance of this economic terms—formed the greater acquisition rate may be important paper are derived from models con- part of the HBE literature through the even if food energy is not strictly lim- structed of this same set of elements: 1980s (see Fig. 1). This research draws iting (see Smith,81 contra Vayda and an evolutionary goal, a currency, con- from “optimal foraging theory” (or McCay86). straints, and an alternative set. OFT).75,76 Although OFT was initially The constraints of the prey-choice developed by biologists, anthropolo- model include those endogenous to gists have made some original theo- the forager, such as the information Ethnographic Applications retical contributions in recent years.77 available, cognitive processing capac- and Extensions of OFT OFT consists of a family of models ities, and technology, as well as exog- addressing resource selection, time al- enous factors such as the distribution, A small set of ethnographic analyses location, and habitat movement or density, and nutritional content of the have tested basic OFT models among “patch choice.” By far the most popu- available resources. In applying the hunter-gatherers.87,88 These range lar of these foraging models has been model, all constraints but one are con- from primarily qualitative and heuris- the diet breadth or prey-choice mod- sidered to be relatively fixed. The re- tic applications to detailed, quantita- el.78 We focus on this particular maining constraint becomes the inde- tive analyses of foraging deci- model to illustrate the logic of the pendent variable that predicts choices sions.89–92 Specific applications HBE approach in general. among the decision set. For instance, explain shifts in subsistence patterns ARTICLES Evolutionary Anthropology 55

novel research questions illustrates Box 2. Leapfrogging From Prehistoric to Postmodern: how HBE has stimulated important Foraging for Information on the World Wide Web new research (Box 2).

Foraging theory has established a secure place in archaeological analyses Children’s foraging of economic decisions in prehistoric societies. Much of the ethnographic use of these models has focused on the subsistence behavior of extant hunter- Children exhibit different levels of gatherers, pastoralists, or, less commonly, agriculturists (see Production: foraging effort and self-provisioning Foraging Behavior and Resource Selection). One can imagine applications in across hunter-gatherer groups (Fig. urbanized, industrial economies, although we know of few specific studies. 2). The !Kung are low-effort foragers For instance, taxi drivers act as predators seeking to optimize the benefit-to- whose children do almost no produc- cost ratio of encountering and “harvesting” their prey. Routinely, they must tive work. Among the Hadza, even decide whether to actively search or to use a sit-and-wait, ambush tactic, young children provide substantial when to leave a patch that is declining in its yield, etc. In an actual study, portions of their food needs in some political scientists Gray and Lowery98 make a preliminary case that behavioral seasons of the year. Using cost-benefit ecology models of group formation can be used to examine competition and measures derived from OFT, Blurton 101 102 alliance formation among political groups lobbying state legislatures. Jones, Blurton Jones et al., and 103 One of the more interesting recent applications leaps directly to the hunter- Hawkes et al. argued that these dif- gatherers of the postmodern world: software engineers are using HBE theory ferences occur because Hadza coun- to analyze and optimize patterns of “information foraging” on the internet. try periodically offers children higher Computer scientists Pirolli and Card99 argue that foraging theory models are returns than their !Kung counter- applicable to the analysis and design of effective information-gathering mech- parts, and Hadza children are less anisms from the world wide web and other large-scale, patchy, informational likely to get lost or be attacked by databases. In a similar analysis, library scientist Pamela Sandstrom100 sug- predators. gests that foraging theory will be important for her field, as librarians seek to The question of why children differ optimize the search capacities of their patrons. from adults in their foraging choices This presents us with one of those marvelous, time-transcending ironies and net acquisition rate is currently a that appeal to postmodernist sensibilities: information-seeking readers may matter of debate. Working with 104 have found this paper because their internet software or library reference Hadza, Blurton Jones et al. suggest services use HBE principles designed originally to understand the subsis- that children select different re- tence-seeking behavior of prehistoric hunter-gatherers. sources because of their lesser upper- body strength, not because of limita- tions of cognition or experience. In contrast, Kaplan et al.12 propose that humans have evolved to specialize in over time, in response to such factors (grass seed harvesting among Austra- nutrient-dense resources that are dif- as changes in technology,93,94 climatic lian Aborigines),96 or hunting com- ficult for children to procure and pro- fluctuations,95 the availability of im- panions.97 cess. Effective use of these resources, ported substitutes (wheat flour) for re- Fieldwork application of OFT mod- they argue, requires maturity, experi- sources with high handling costs els to several long-standing and some ence, and lengthy skill acquisition.

Figure 2. Among Mikea forager-farmers of southwestern Madagascar, children provide for much of their own subsistence needs through tuber foraging. Wild tubers are the dietary basis for many Mikea house- holds, especially during years of agricultural shortfall. The potential impact of self-provisioning by children on socioecological adaptations of hominids and contemporary hunter-gatherers is an active area of HBE inquiry. Photograph courtesy of Bram Tucker. 56 Evolutionary Anthropology ARTICLES

Bird and Bliege Bird105 show that Me- horticulturalist-foragers of lowland riam children foraging for shellfish on Peru select prey species matching the tropical reef-tops quickly learn to pur- prediction of shorter-term optimiza- sue an optimal set of prey which, tion models, irrespective of their vul- given their slower travel speeds and nerability to overexploitation and de- lesser strength, differs from the opti- pletion.121 When moving through mal set taken by adults in the same depleted zones near to villages in or- environment. In this case, body size der to hunt in more favorable distant rather than cognitive or experiential areas, Piro (Peru)122 and Yanomamo¨ constraints explain the differences be- and Ye’kwana (Venezuela)120 foragers tween adult and child foraging strate- do not forego opportunistic chances gies. In some environments, children to harvest individuals of valuable but are able to forage quite effectively and overexploited species. Piro do not se- match OFT predictions, even if their lectively avoid the more productive different capabilities require that they sex-age classes in favor of those min- diverge from the resources selection imizing impact on population sustain- of their parents. ability.123 Further, as prey become depleted, tropical foragers increase the Nonforager applications time they spend hunting, opposite Behavioral ecologists have now suc- what would be predicted under the cessfully applied optimality and selec- conservationist view.120,124 tionist logic to horticultural106,107 and Some important modeling work has 108 pastoral production systems. Dy- Figure 3. The Huaorani Indians of Ecuador’s examined how population dynamics namic optimization analysis has been Amazon forest use blowguns and darts used to examine the cost-benefit tipped in curare poison to hunt monkeys trade-offs faced by pastoralist families and birds from the canopy. Access to fire- Behavioral ecologists deciding between inexpensive, high- arms and markets has changed the dynamic between indigenous hunters and fertility but environmentally sensitive have now successfully their faunal prey in ways being investigated small stock, and expensive, low-fertil- by behavioral ecologists. At its best, this applied optimality and ity but reliable large stock.109 Data work integrates anthropology, evolutionary selectionist logic to from several East African societies ecology, population ecology, and conser- (Somali, Twareg, Meidob, Turkana) vation biology. Photograph courtesy of horticultural and fit the model predictions well.110,111 Flora Lu. pastoral production Because they manage the movement and feeding of their herds, pastoralists systems. such as the Fulani can be analyzed as ships between observed behaviors and making foraging decisions on behalf inferences. Sustainable harvests have of their domestic stock.112 Models of been cited in favor of conservation patch choice and optimal informa- when they may be incidental to good and production strategies inter- tion-sharing have also been employed hunting tactics or a consequence of act.125,126 For instance, simulation has in an industrialized context, to help other constraints. For instance, rota- been used to show how the different explain the behavior of Alaskan com- tion of hunting ranges has been cited mercial fishermen.113 both as conservation behavior (tracts parameter values for population den- recently hunted are allowed to “rest,” sity, foraging effort, and resource se- Conservation biology while more productive patches are be- lection strategies of hunter-gatherers interact with the sustainability and Some writings portray indigenous ing exploited),116,117 and as a tactic to population dynamics of the resource peoples as living in ecological har- maximize yields through optimal 127,128 82,93,118,119 on which they depend. For ex- mony with their environments and movement among patches. 20 120 ample, if a predator exploits two prof- carefully conserving resources against Alvard and Hames use foraging overexploitation. Others cite archaeo- theory to address these issues (Fig. 3). itable species that differ in their in- logical examples of apparent anthro- Alvard20 defines as conservationist trinsic rate of increase (“r”), the low pogenic extinction, and argue that any resource use restraint that sacri- “r” species of the pair is vulnerable to sustainability may be due more to low fices short-term yield and fitness gains depletion and local extinction. population densities, simple technolo- in order to realize long-term benefits Three characteristics of resources— gies, or absence of markets than to from heightened sustainability or open access, high abundance relative to ecological restraint.114,115 This inter- yield. Because OFT models identify need, and a reproductive potential pretive disagreement is partly an em- precisely the behaviors expected by lower than evolved human discount pirical problem—evidence is scant— short-term optimization, they provide rates129—may help to explain why gen- and it is partly due to unclear the null hypothesis against which con- uine conservation of game appears to definitions and ambiguous relation- servation can be assessed. The Piro be rare in indigenous societies.20 OFT ARTICLES Evolutionary Anthropology 57

Figure 4. Huaorani females in the lowland tropical forests of northeastern Ecuador return from the garden burdened with manioc tubers and cuttings. Although the men assist in garden clearing and weed- ing, much of the horticultural work is done by women, a typical division of labor. Quantitative, ethnographic studies of time allocation have illuminated the work patterns and subsistence contributions of different sex-age groups, e.g., the young females pictured here, but a satisfactory explanation for male-female division of labor seen in foraging and forager-horticulturalist societies has remained elusive. Pho- tograph courtesy of Flora Lu.

allows us to isolate the costs and bene- in household provisioning and macro- model that predicts when hunter- fits that must be assessed in order to nutrient balance,137–139 or it may re- gatherers who travel from their resi- separate incidental from genuine con- flect male status competition and mat- dence (A) to a distant site (B) to gather servation, in ethnographic130 and in ar- ing investment.21,140,141 The debate resources logistically, should reverse chaeological131,132 contexts. Combined among these alternatives (and inter- the pattern and relocate to that sec- with population ecology models, OFT mediate possibilities) has been pro- ond site, making logistic foraging provides insights into the dynamic pro- tracted, and no clear resolution is in trips back to (A). cesses linking foraging decisions to sight (see below, Parenting-Mating their impacts on resource popula- Strategy Interactions). Field processing tions.128 Various social constraints (including inequalities in social power) Archaeologists also have adapted may fundamentally alter the trade-off Archaeological Applications CPF models to predict when resources between efficiency and conservation.133 and Extensions will undergo field processing to re- move low utility portions prior to More generally, success in solving con- Archaeological uses of OFT have their transport back to a camp.77,91,147 temporary environmental problems been hampered by the fact that direct Transport of an unprocessed resource may require appreciation of our tests of the models require data on takes less handling time in the field evolved dispositions with respect to pat- individual decisions, taken in behav- and thus allows for more round trips, terns of self-interest and temporal dis- ioral time. In contrast, the recoverable 134–136 but some part of the carried load has counting in the use of resources. archaeological record consists of ma- little or no value. Field processing be- terial remains that aggregate over fore transport lessens the number of Sexual division of labor multiple foragers and foraging epi- round trips per unit time, but the load sodes. The data only indirectly reflect Overall, ethnographic tests and in- is composed of high-value materials behavior and are further obscured by terpretive applications of OFT models only. Given estimates of load capaci- various taphonomic factors. However, have been reasonably successful over ties, processing costs, and the utilities there has been progress in revising a broad range of systems. But failures of high and low value portions,148 the OFT predictions to fit the archaeolog- of model predictions can be a stimu- model predicts the travel distance at ical context.142 In addition, ethnoar- lus to research as well. An example is which field processing becomes the chaeological applications have been the ongoing debate over the male-fe- best option. Separating nut meats or devised to serve as a bridge between male division of labor in many forag- shellfish from their shells, edible ani- the short time-frame of foraging mod- ing societies (Figs. 3, 4). In some mal tissues from low utility skin, car- els and the extended time-frame of groups, men pursue resources (e.g., apace, and hooves, or high-grade prehistorians.143–145 large game) that yield a high variance, stone tool material from low-quality entail greater between-household matrix at a quarry are potential exam- Residential movement sharing, and have greater protein and ples. Similar analyses shed light on fat content than the resources prefer- Drawing on archaeological observa- how the distribution and composition entially harvested by women. This di- tions from the Owens Valley, of prehistoric faunal remains will be morphism in foraging strategy may be Zeanah146 (see also Kelly88) has devel- shaped through selective processing due to male-female complementarity oped a central place foraging (CPF) and transport.149,150 58 Evolutionary Anthropology ARTICLES

Intensification diets of arid-zone Australian Aborigi- terpreted from the diminishing size nes. and apparent depletion of easily har- According to the prey-choice model, Stiner et al.154,155 use a similar OFT vested “slow” species and their re- as the forager’s encounter rate with approach in their study of human placement in the foraging schedule high profitability resources declines, population growth and resource in- with low profitability, “fast” small overall foraging efficiency falls and tensification in early Middle- to Up- game. Biotic communities were rela- the best-choice diet expands to en- per- and Epi-Paleolithic sites in Italy tively stable through the examined compass previously neglected types of and Israel. Their analysis turns on the changes in harvests, eliminating one lower and lower profitability. Encoun- implications of two categories of alternative explanation for the pat- ter rates with the most desirable re- small game. In one category are tern. sources might decline due to a num- “slow,” easily caught and processed, Given their life history variability ber of reasons, including forager high profitability species with low fer- and potential disparities in their rank- population growth and resource over- tility and thus, low resistance to ex- ing as resources, small game may exploitation. Empirical studies of ploitation. Tortoises and marine shell- prove to be more sensitive indicators hunter-gatherers stimulated by forag- fish are “slow.” The other category of basic changes in the prehistoric for- ing theory have shown that low- consists of “fast,” more difficult to aging economy than are the large ranked resources typically are diffi- catch, low profitability species with game species that typically command cult-to-catch or difficult-to-process high fertility and thus high resistance the most archaeological attention. items such as small game, nuts, and Stiner et al.154,155 paleoanthropoloical seeds. offer a too rare example of the poten- Prehistorians have combined these Given their life history tial contribution of behavioral ecology observations on foraging logic, costs, models to interpretation. and benefits, to give a new and more variability and potential specific purchase to an old idea: re- disparities in their Domestication and agricultural source intensification or, more specif- origins ically, the broad spectrum revolution. ranking as resources, Recent work shows that OFT may For instance, Broughton131,132 and small game may prove be useful in understanding the condi- Broughton and Grayson151 have tions under which domestication and shown a remarkably consistent pat- to be more sensitive agricultural production might emerge tern through late Holocene archaeo- indicators of basic and spread.156–159 The cost-benefit logical sites in northern California. Al- changes in the trade-offs recognized in OFT models though the precise timing and the prehistoric foraging underwrite a set of hypotheses con- species set differ by site and environ- cerning the circumstances in which mental zone, in each case harvests of economy than are the prehistoric agriculturalists would highly ranked species (e.g., sturgeon, large game species that adopt or domesticate new crops, and elk, white-tailed deer) drop steadily their impact on existing agro-ecologi- relative to low-ranked species (e.g., typically command the cal relationships in the prehistoric shellfish, acorns, small mammals). most archaeological southeastern United States. For in- Other evidence (e.g., declining size of stance, maize remained a minor com- the species presumed to be subject to attention. ponent of garden production for some heavy exploitation; indicators of in- 600–1,000 years after its introduction creasing human density) is consistent into the Midwest and Midsouth, but it with a hypothesis of resource deple- quickly came to dominate production tion and diet breadth expansion. Al- after its later arrival in the interior to exploitation. Partridges, hare, and ternative explanations for these Southeast.160 The use of acorns de- rabbits are “fast.” In the sites exam- changes—new technology, of climate clined relative to hickory nuts coinci- ined, “slow” species appear steadily in or habitat change affecting resource dent with the emergence of plant hus- the archaeofaunal record throughout population densities—can be re- bandry in upland regions of the jected. Whereas acorns once sug- the Middle Paleolithic but are increas- Cumberland Plateau, eastern Ken- gested the easy bounty of the Califor- ingly supplanted in importance by tucky.161 nia environment, experimental “fast” species in the Upper and Epi- archaeology studies coupled to OFT Paleolithic. The Middle-Paleolithic GROUPS AND RESOURCE models suggest that acorn use marks pattern is consistent with a mobile, the latter stages of a long period of low-density population of human for- TRANSFERS declining production efficiency.152 In agers, opportunistically harvesting Foraging models concern them- a like study, Edwards and high-value “slow” species without the selves with the short-term production O’Connell153 use foraging theory to exploitation intensity that would decisions of individuals. The input evaluate the plausibility of several hy- cause their extirpation. Increasing hu- variables to these models are environ- potheses for the relatively late appear- man density and greater sedentism in mental factors such as the profitabil- ance of “broad spectrum” seeds in the the Upper and Epi-Paleolithic are in- ity, density, distribution, and re- ARTICLES Evolutionary Anthropology 59 sponse to exploitation of potential in an ongoing social relationship? or more of the proposed resource trans- resources. Foraging models are the What cost-benefit constraints are fer hypotheses. Those that exist diverge only models a Robinson Crusoe would present? What is the nature of the re- on which mechanism appears to be need. However, for humans in gen- source?), and in the evolutionary causally dominant: scrounging and the eral, their hominid antecedents, and mechanism they apply (e.g., individ- show-off hypothesis,179 trade,175,180 kin- many other species, the harvest ual, inclusive, sexual, or group selec- provisioning,181 reciprocity-based risk- and/or consumption of resources tion). Simple individual-level selec- minimization,182,183 and costly signal- likely occurs in a group. Attention to tion will generate transfer by ing.141 Collectively, these studies leave the resulting social interactions adds scrounging167 (also known as tolerated the impression that transfer behaviors the evolutionary dynamics of resource theft168) when those not possessing a are much more diverse and situation- distribution to those of production. resource packet benefit more by tak- ally specific than has been appreciated ing portions than the holder can ben- in the standard ethnographic literature Models efit by defending them. Resource shar- on “sharing.” These studies also suggest ing eliminates the involuntary that transfer behaviors are probably Group-based subsistence efforts element of coercion, replacing it with multicausal in their origin, the result of may offer several advantages: 1) in- the delayed, cost-benefit calculus of several selective pressures acting con- creased per capita encounter rate with reciprocal altruism.169 In by-product currently, their relative causal impor- resources, 2) reduced variation in en- mutualism170 the individual discover- tance depending on the situation. Labor counter rates, 3) reduced losses to ing or possessing a resource obtains a exchange has been less studied, but competitors, and 4) increased vigi- net gain as a result of others partici- may show comparable diversity of lance and predator detection.162 pating in its capture, defense, or con- forms and the same range of causal in- Nonetheless, modeling has shown sumption. In this case, short-term co- fluences.184–186 that optimal group size itself may be operation is mutually beneficial, and unstable due to conflicts of interest defection or cheating, so important to between existing members and poten- reciprocal altruism, is not an issue. Archaeological/Prehistoric 163,164 tial joiners. Once groups exist, Costly signaling,171–173 a form of by- Studies they provide the context for complex product mutualism, is another poten- Provisioning by grandmothers social dynamics, including competi- tial mechanism for resource trans- 165 tion and conflict. Here we focus on fers.174 By successfully harvesting and In conventional wisdom, the fea- resource transfers, which are funda- then distributing difficult or danger- tures by which Homo erectus is be- mental to these social dynamics and ous-to-capture resources, individuals lieved to anticipate later hominids— are well-studied in HBE. reliably signal their prowess, benefit- male-female division of labor, lengthy Hand-to-mouth feeding provides no ing themselves as well as participants juvenile dependence, nuclear families, incentives to food transfers beyond who gain both food and useful infor- and central place foraging—devel- provisioning of related infants and ju- mation.141 In trade or exchange,175 in- oped from a pattern in which males veniles until they are competent as individuals swap unlike resources or provisioned their offspring through dependent foragers. However, the sit- services because both, by marginalist large-game hunting (viz., the hunting uation is different if group members logic, stand to gain by doing so. The hypothesis). O’Connell et al.187 ques- harvest one or more of their resources show-off hypothesis176 is a transfer tion this scenario and present an al- as a divisible “packet,” a unit large model built on the mechanism of sex- ternative, the grandmother hypothesis. enough that it cannot be consumed ual selection. Male hunters target high In their model, the development of immediately by the individual holding value, high-variance resources, de- more arid and seasonal environments it. The technical equivalent of this spite unfavorable average returns and following 1.8–1.7 million years ago re- condition is that the value of the losses to group-wide sharing, because duced the capacity of juveniles to pro- packet is subject to short-term, dimin- they are able to translate the resulting vision themselves with easily gathered ishing marginal returns to the individ- “social attention” into other benefits fruits, as occurs in most primate spe- ual possessing it.166 This virtually such as enhanced mating opportuni- cies and some hunter-gatherers. This guarantees that portions of the packet ties. Finally, inclusive fitness selection put pressure on their caretaking will be valued differently by different should lead to transfers through kin- mothers, who turned to a low-ranked group members. Given this model of provisioning that balances costs and food source which was plentiful and circumstance, a variety of evolution- benefits against degrees of related- geographically widespread, but which ary mechanisms may come into ness. If individuals produce signifi- demands adult skill and strength to play.71 cant parts of their diet unpredictably, harvest and process: tubers. The All resource transfer models ad- risk minimization is among the impor- growing demands of provisioning dress a common causal circumstance, tant benefits of resource trans- their developing offspring with tubers the unsynchronized acquisition of fers.177,178 sharply constrained adult female valuable resource packets by individ- birth intervals and consequently their uals within a group. The models differ fertility. According to the grand- Ethnographic Studies primarily in their structural refine- mother hypothesis, this constraint ments (e.g., to what extent are the There are several empirical studies provided an opportunity for selection: group members genetically related, or assessing the relative importance of one aging female relatives of these moth- 60 Evolutionary Anthropology ARTICLES ers could offset declines in their own theft. However, once effective systems analyses focus on variation in repro- fitness by stepping in to provision of reciprocity or exchange augment ductive behavior as a function of lo- their grandchildren, especially the off- the effective value of very large pack- cal ecological context. And in con- spring of their daughters. Compara- ets to the hunter, such prey items trast to evolutionary psychology,200 tive life history studies of existing ver- would be more likely to enter the op- HBE posits that this variation in- tebrates188 suggest that a variety of timal diet. volves phenotypic tracking of cur- features might have followed: reduced Given social foraging and harvest of rent circumstances, rather than the juvenile mortality, delay to age of ma- large-packet resources, both of which playback of species-, sex-, or age- turity, larger size, shortened birth in- are archaeologically visible, the vari- specific behavioral routines that tervals (higher fertility), and larger ety of evolutionary mechanisms that were adaptive in our remote evolu- 201,202 group size. This also makes grand- can generate intragroup transfers sug- tionary history (see Box 1). mothering a leading evolutionary hy- gests that they will be both common- Nevertheless, HBE approaches over- pothesis for the extended, postmeno- place, and diverse in form and func- lap considerably with these other pausal female life span of two traditions and with certain ver- tion, among our hominid ancestors. hominids,189–192 which would evolve sions of cultural evolution, as well as Comparative ethological and ethno- and be maintained by natural selec- with less explicitly neo-Darwinian graphic evidence is consistent with tion if women near the end of their life fields such as demography203 and re- this prediction.196 Resource transfer span gain inclusive fitness by invest- productive ecology.204 models also expand our sense of aping in their grandchildren (or other HBE analyses of reproductive be- propriate currencies beyond kilocalo- close relatives still in or before their havior can be divided into three broad reproductive prime). ries, perhaps modifying optimal re- areas: mating, parenting, and life his- If correct, the grandmother hypoth- source selection. Some prey types tory.205–207 esis shows that neither big-game may be valued precisely because they hunting nor male feeding of a female Mating Strategies and her children is required to explain key features of hominid emergence. If correct, the The distribution of key resources The hypothesis combines foraging grandmother hypothesis strongly shapes the distribution of theory observations on the gathering males and females, generally through tactics of Hadza mothers and their shows that neither big- different routes.208,209 If some males children193,194 and life history pat- game hunting nor male can monopolize resources necessary terns suggested by comparative stud- for female survival and reproduction, ies and the models of Charnov and feeding of a female and they can use this control to attract Berrigan.14 The precise form and her children is required mates, or to compete with other males for social dominance. Polygyny and plausibility of the hypothesis depend to explain key features substantially on research stimulated increased variance in male mating by HBE. of hominid emergence. success will result. Male resource control coupled with female-initiated Transfers and the exploitation mate choice is the basis for the “polygyny threshold” model30 referred to of large game earlier. The outcome predicted by the Paleoanthropologists195 sometimes are dangerous, rare, and costly to ob- simplest versions of this model is an argue that it was the active hunting of tain,141 e.g., the albino deerskins, red ideal free distribution,210 in which the big game that provided the opportu- woodpecker scalps, and giant obsid- number of mates per male will match nity and incentive for food sharing. In ian blades displayed in certain Native the resources each can offer, and fe- this view, big game were a resource Californian ceremonies.197 Intragroup male fitness will be equal across mate- made available to hominids by tech- resource transfer models give HBE ships. nological or cognitive advances late in analytical linkages between the origi- Despite its simplifying assump- our evolutionary history. However, a nal production focus of OFT and the tions, tests of the polygyny threshold good HBE case can be made for the dynamic social realm of resource dis- model have generally supported its reverse: only with the evolution of rec- tribution. applicability to human mating sys- iprocity or exchange-based food tems.73,211–218 The male-controlled re- transfers did it become economical sources may be political rather than for individual hunters to target large REPRODUCTION: MATING, economic.219 Male coercion (especially game. The effective value of a large PARENTING, AND LIFE HISTORY by agnatic kin groups) may severely mammal to a lone forager—what constrain female choice.220 Females STRATEGIES could be eaten and converted directly mated polygynously may face reduced to inclusive fitness on the short term— While classical sociobiology198,199 reproductive success due to competi- probably was not great enough to jus- analyzed reproductive behavior in tion with co-wives.221,222 The effects of tify the cost of attempting to pursue terms of factors inherent in sexual co-wife competition may be compen- and capture it, even if unrelated group reproduction, such as genetic relat- sated, however, if the sons of polygy- members benefited through tolerated edness and gamete asymmetry, HBE nously married women have in- ARTICLES Evolutionary Anthropology 61 creased chances of inheriting wealth tive success. Blurton Jones and available to us obscure this in- and mating polygynously them- Sibly236 use this approach and data on trasample variability. There is abun- selves.223 !Kung San work effort and demogra- dant evidence for such adaptive vari- Polygyny has been the preferred phy235 to show that interbirth inter- ability in human reproductive marriage form for the great majority vals much shorter than the actual ecology.203,204 One possible solution, of societies in the ethnographic mode of 4 years resulted in increased multivariate analysis, requires larger record.208 Even in putatively monoga- offspring mortality, sufficient to cause samples than anthropologists are usu- mous societies, extramarital mating a net loss in expected reproductive ally able to generate. Another is to use and remarriage biased towards success.236 This effect did not occur historical data on individual families wealthier or more powerful males cre- for a mother’s first interval, nor when within the sample to track functional ates a situation of effective polygy- birth followed the death of the preced- links between birth spacing and re- ny.218 Human behavioral ecologists ing offspring; in these cases, inter- sources, but this is obviously not an have also analyzed the rare but in- birth intervals were significantly option if the requisite time series is triguing polyandrous case,60,202,224–226 shorter (as the model predicted).237,238 lacking. as well as monogamous sys- At least one careful attempt to rep- tems,138,227,228 especially those involv- licate the !Kung results among Ache 229–232 ing stratification and dowry. foragers239 failed, possibly because Differential investment Recent work is particularly concerned offspring mortality is more sensitive with analyzing the trade-offs between to variation in interbirth interval in Parental investment throughout a parental investment (see below) and the !Kung setting. It has also been ar- child’s development affects that mating effort, as noted above (Sexual gued that the !Kung results are spuri- child’s health, survival, and future Division of Labor). ous, with the long interbirth intervals mating success, and thus the parents’ Ethnoarchaeologists are just be- a result of female infertility due to inclusive fitness. Parental fitness pay- ginning to make use of HBE ap- sexually transmitted disease,240,241 an offs depend on three sets of variables: proaches to reproduction. For in- argument rebutted by Blurton 1) the genealogical relatedness be- stance, the mating tactics of hunter- Jones.242 Finally, it may be that in the tween parent (or other caregiver) and gatherer males may lead to mobility Ache case (and likely many others), offspring, 2) the effect of investment and ranging patterns different from the relationship between fertility and on the expected reproductive value of those optimal for reliably securing offspring survival is confounded by the offspring (as well as present and 233 resources. phenotypic correlation, which gener- future siblings), and 3) the effect of ally will mask the predicted functional investment on the caregiver’s own re- Parenting relationship of the Lack mod- productive value. Set (1) is indirectly el,206,239,243 as illustrated in Figure 5b. subject to ecological variation (e.g., Whatever the mating system, hu- Phenotypic correlation occurs when patterns of coresidence will affect man offspring require extensive and hidden heterogeneity in uncontrolled the likelihood that a social parent or extended parental care. This parental variables confounds the effect of the other caregiver is a close genetic rela- 234 investment begins with the moth- causal variable under investigation. tive), while sets (2) and (3) are more er’s resource allocations during gesta- For example, wealthy individuals directly affected by ecological vari- tion. HBE analyses ask how the tim- might tend to have more expensive ables, and hence at the center of HBE ing and amount of such investment houses and more expensive cars (a analyses. might vary according to social and en- phenotypic correlation), even though Postpartum parental investment de- vironmental constraints. Most re- we have good reason to expect a neg- cisions range chronologically from search falls into three categories: ative correlation between investment whether or not to keep the child—the birth-spacing, differential investment in houses and investment in cars due alternatives being infanticide, aban- in offspring (by sex or expected repro- to the fact that the same dollars can- donment, and adopting out—to dele- ductive value), and interactions be- not be spent on both.244 While the gation of care to others, generally ei- tween mating and parenting. logic of phenotypic correlation is ther relatives or hired laborers such as straightforward, uncovering hidden wet nurses, to the legacy the child may Birth spacing heterogeneity and controlling for its receive prior to or upon death of the If parental time and resources are effects in nonexperimental studies parents. Fitness payoffs for nearly all finite, a shorter interbirth interval like those undertaken by HBE (and of these decisions may differ accord- should result in less parental invest- other fields studying human behavior ing to sex of offspring. The Trivers- ment per offspring and may eventu- in naturalistic settings) are difficult. If Willard245 prediction that parents in ally reduce each offspring’s fitness. selection has designed the reproduc- poor condition or with limited re- This is the basis of the optimal clutch- tive system to adjust facultatively to sources will invest more heavily in off- size model (Fig. 5a) of Lack.26 It pre- situational constraints, then inter- spring with the lowest expected vari- dicts that beyond a certain point, in- birth intervals will be short when the ance in reproductive success (usually creased fertility (larger clutches, or parent’s resources are relatively abun- daughters) has received HBE atten- shorter interbirth intervals) will result dant and long when their condition is tion, as have a variety of other hypoth- in lowered overall parental reproduc- poor. The modal measures typically eses (Box 3). 62 Evolutionary Anthropology ARTICLES

Figure 5. A graphical model of optimal fertility rate, based on Lack.26 Solid diagonal lines represent births per unit time, while dashed curves represent mortality as an increasing function of fertility rate. The net difference between fertility and mortality is the number of surviving offspring, with ␣ representing the maximum value this can take for a given pair of fertility/mortality curves. The graph assumes that selection favors maximum values of ␣, given the constraints a parent faces, and hence an op-

timal fertility rate Fo. a: A single parent is considered. b: Parents with different resource or phenotypic endowments have distinct offspring mortality curves as a function of fertility rate. Thus a “poor” parent faces higher offspring mortality at a given level of fertility, and hence a lower optimal

fertility rate Fp than a higher-quality parent

with optimal fertility rate Fh. If analyses of fertility and mortality do not control for this heterogeneity, the lumped data may make it seem that fertility rate and offspring mortality are negatively correlated. The result is that the pattern of fertility optimization may be masked by phenotypic correlation (see text), and the prediction of positive correlation will be rejected even though correct.

Parenting-mating strategy paternal care and resource provision- Xhosa men in South Africa invest interactions ing, rather than simply being forms of more time and resources in stepchil- parental investment, may be designed dren who are offspring of their cur- Following the lead of primate and to attract or maintain a relationship rent mates than they do in stepchil- avian behavioral ecology, HBE re- with a mate (Fig. 6).21,138,276–278 For dren from former relationships searchers have begun to consider if example, Albuquerque men as well as (though less than they invest in ge- ARTICLES Evolutionary Anthropology 63

Box 3. Selected HBE Tests of Parental Investment Theory

The evolutionary analysis of parental investment need prospects for offspring that may vary by their sex or phe- not include a direct focus on ecological factors. However, notypic condition, as well as by the social status and HBE studies generally examine the ways in which ecolog- relationships of the parents. Some of the primary HBE ical constraints shape the ﬁtness-related costs and bene- predictions about parental investment are listed below, ﬁts of various alternative patterns of parental care, which in along with a representative sample of empirical studies turn leads to predictions concerning variation in parental that attempt to test these predictions. Not all studies listed investment decisions. These constraints can include a va- below necessarily support the prediction under test. riety of economic, demographic, political, and mating

Predictiona Representative studies Reduced genetic relatedness or reproductive conflicts of interest Daly and Wilson;246 Anderson et al.;247,248 lead to lower PI Strassman249 Offspring with reduced prospects of survival receive lower PI Daly and Wilson250 Parents reduce PI per offspring as number of offspring increases Borgerhoff Mulder;251 Hill and Hurtado239 PI per child is increased when marginal benefit of PI is higher Bereczkei;252 Kaplan et al.253 Adoption and fostering are adaptively modulated according to Silk;254 Pennington255 parental circumstances Mothers able to delegate nursing and infant care gain Bereczkei;256 Blaffer Hrdy;257 Turke258 increased reproductive success Postmenopausal women allocate resources and care to Hill and Hurtado;191,239 Hawkes et al.194,259 grandchildren or other close relatives Parents with fewer resources preferentially invest in offspring Boone;260 Borgerhoff Mulder;251,261 Gaulin and (usually daughters) with lower variance in expected RS Robbins;262 Judge and Blaffer Hrdy;263 Mealey and Mackey264 Offspring of the sex that faces better adult economic Hewlett;265 Low;266 Low and Clarke;267 Voland et opportunities receive higher PI al.268 Offspring of the sex that has greater probability of contributing Borgerhoff Mulder;251 Hewlett;265 Hill and to future support of siblings receive higher PI Hurtado;239 Smith and Smith269 Offspring of the sex that has greater probability of competing Borgerhoff Mulder;251 Mace;270 Voland et al.271 with siblings for resources or mates receive lower PI If daughters (or sons) have greater future mating opportunities, Dickemann;217 Boone;260 Bereczkei and Dunbar; they receive higher PI 272 Cronk273,274 If daughters (or sons) are better able to claim and hold political Blaffer Hrdy and Judge;275 Boone;260 Hewlett265 power, they receive higher PI (especially inheritances) Parental resources with increasing marginal benefits to offspring Boone;260 Voland et al.268 are characterized by unigeniture (single heirs) Increasing marginal benefit of biparental care leads to Hurtado and Hill;138 Blurton Jones et al.104 increased pair-bond stability a PI, parental investment; RS, reproductive success.

netic offspring under comparable cir- dispersal patterns, mortality patterns, based on Charnov279 that successfully cumstances).247,248 This pattern is not and senescence. Most HBE work on predicts variation in age of reproduc- adaptive as parental investment per life history thus far has focused on tive maturation of women in three dif- se, but does make adaptive sense if three topics: 1) links between produc- ferent populations: Ache, !Kung, and one views it as investment in main- tion and reproduction, 2) reproduc- North America. Analyzing data from taining a current mating relationship tive effort and maturation, and 3) evo- contemporary Chicago broken down (i.e., as mating effort). Unraveling in- lutionary explanation of the so-called by residential neighborhoods, Wilson teractions such as these is a promising demographic transition. and Daly280 show how life history area for future HBE research, but will The grandmother hypothesis (de- variables such as life expectancy and require a great deal more theoretical scribed above) and related proposals reproductive timing respond to varia- and empirical effort. take up the issue of reproductive ef- tion in social and ecological factors fort and maturation, particularly the such as economic opportunity and ho- possible evolutionary causes and con- micide rates (Box 4). Life History Strategies sequences of menopause. Perhaps the Demographic transition Life history theory is concerned most impressive work to date is that with the evolution of maturation by Hill and Hurtado,239 who devel- “Demographic transition” can refer rates, reproductive rates and timing, oped a model of reproductive timing to any dramatic and sustained change 64 Evolutionary Anthropology ARTICLES in patterns of fertility and mortality, family sizes still yield larger num- but conventionally it is used to label bers of grandchildren. the rapid decline in completed family The dynamic optimization model size observed throughout Western Eu- developed by Mace285 is meant to ap- rope in the late 18th and early 19th ply to subsistence-based societies centuries.281 This decline was due to a rather than economically modern combination (not generally simulta- ones, but nevertheless has implica- neous) of reduced mortality and an tions for understanding demographic even greater reduction in fertility; transition. The model predicts that in- elsewhere in the world, populations creased productivity and reliability of have shown various degrees and com- subsistence lead to increased fertility binations of these two trends, with re- (as observed in traditional societies): sultant levels of population growth or “when living conditions are easier, equilibrium. While demographic tran- parents can afford more children, and sition is primarily viewed as a matter do not need to be so generous to each of demographic rates, it is also char- child” in order to maximize the num- acterized by greatly increased levels of ber of surviving grandchildren (p. material investment (including inher- 395). It also predicts that an increase ited wealth) in each child; to the ex- in the cost of raising children leads to tent that this material investment re- duces the number of additional a marked reduction in the optimal fer- children that can be reared, demo- tility rate, as well as an increase in graphic transition also involves an in- Figure 6. A Tanzanian Datoga pastoralist crease in parental investment as de- and his child. The extent of male parental fined in evolutionary theory.234 effort and its relationship to mating tactics in ...rather than Historical and economic demogra- subsistence societies are a complex and hotly debated issue in human behavioral maximizing the number phy has revealed several socioeco- ecology. In pastoralist societies like this, of offspring raised, under nomic factors correlated with modern even though men engage in mating effort low-fertility regimes, but has failed to throughout their lives by accumulating po- at least some develop a robust explanation of demo- lygynously married wives, deep affection is circumstances humans graphic transition that can account observed between fathers and their children. Photograph courtesy of Monique for its varied occurrence in time and might adjust fertility Borgerhoff Mulder. space. The conjunction of increased behavior in a way that wealth and decreased fertility charac- teristic of post-transition populations maximizes longer-term presents an apparent paradox for evo- it might be a weighted product of 286 fitness. lutionary models of fertility and pa- (heritable) wealth times children, a 287 rental investment. Indeed, it stands in fertility function weighted by risk, stark contrast to a growing body of or asymptotic (long-term expected) 288 data showing that in pretransition fitness. (preindustrial) societies, wealth and Kaplan et al.253,289 and Kaplan and average inheritance and average fertility (as well as survivorship, and Lancaster290 have developed an ex- wealth, all features associated with hence reproductive success) are posi- planation of fertility reduction that post-transition societies. Interest- tively correlated.282,283 focuses on parental investment to ingly, reduced mortality also lowers A number of HBE researchers have prepare offspring for successfully fertility, but not by a large amount. recently tackled this issue with a com- competing in labor markets where In another original approach to the bination of analytical models, simula- “human capital” (education and ac- issue, Winterhalder and Leslie287 use tions based on dynamic modeling quired skills) is a prime determinant a risk-sensitive model of fertility opti- techniques, and empirical studies, of success. In their formulation, re- mization to predict that reduced fer- with some novel and promising reduced fertility and demographic tility will result from any socioeco- sults.282,284 The key theoretical idea transition result from high levels of nomic or environmental factors that explored in this work is that rather parental investment per child cou- reduce the degree of unpredictable than maximizing the number of off- pled with maladaptive levels of fer- variance in completed family size, de- spring raised, under at least some cir- tility reduction aimed at reducing crease the costs of unusually low fer- cumstances humans might adjust fer- these costs across a parent’s set of tility (such as reduced reliance on tility behavior in a way that offspring. Support for the model in- household labor by offspring), or in- maximizes longer-term fitness. In the cludes the finding that parents re- crease the costs of unusually high fer- simplest formulation, this might be spond to increased skills-based labor tility. From the perspective of this measured as number of grandchil- market opportunities with reduced model, the lowering of mortality due dren.253,285 In more complex versions, fertility, despite the fact that larger to stochastic events like epidemics or ARTICLES Evolutionary Anthropology 65

subsistence failures seen in the transition to modern economies would favor reduced fertility simply by damp- ening the stochastic variation in realized family size. Whether these effects of risk-sensitive fertility optimization are large enough to account for a substantial portion of the fertility reduction observed in historic transitions is as yet unknown.

HBE AS A PROGRESSIVE Box 4. The Wilson-Daly Study of Mortality and Reproductive RESEARCH TRADITION Timing in Chicago Although it may seem a brash claim In an innovative application of life-history analysis, Wilson and Daly280 given the current anthropological examine the relationship between risk-taking (measured by homicide rates as fashion for relativism, the 25-year his- well as other risk-based causes of mortality), life expectancy (with homicide tory of HBE is one of clear scientific effects statistically removed), reproductive timing, and economic inequality. progress. To make this point, we sum- Wilson and Daly propose that poor economic opportunities and lowered life marize the accomplishments of HBE 291,292 expectancy from extrinsic factors lead to elevated risk-taking behavior, in- in terms of epistemic values. cluding status contests that sometimes escalate into homicides. This steep These are six characteristics that discounting of future prospects should in turn favor early onset of reproduc- guide pragmatic assessment of scien- tion. tific progress. In the contemporary Analyzing data by neighborhood units (n ϭ 77), Wilson and Daly show that ambience of anthropology, even cau- these measures do covary across neighborhoods in precisely the manner tious partisans of scientific knowledge predicted: 1) Homicide rates vary 100-fold, and are highly correlated with both aid themselves and the discipline by male and female life expectancy, even after homicide mortality is removed an occasional and reflective defense of (see table, below). 2) A stepwise regression shows that the “Robin Hood its possibilities. index” (a measure of income variance within neighborhoods) is an independent predictor of homicide rates as well—local income inequality encourages Predictive Accuracy lethal competition. 3) Women respond to low life expectancy with earlier onset of reproduction, and high fertility rates in the early childbearing years (see Theory development often begins figure). The detailed results of this study provide compelling evidence that without the data or even the opera- humans respond to perceived variation in life chances with major shifts in life tional methods needed to make test- history variables, even in environments far different than those experienced ing feasible and convincing. Nonethe- during most of human evolution. less, given the premium on predictive accuracy in science, empirical confir- VARIATION IN LIFE HISTORY ACROSS CHICAGO NEIGHBORHOODSa mations must be forthcoming. Taken Variable Lowest Lx Highest Lx individually, HBE research projects Male life 57.1 74.5 generally use more rigorous, quantita- expectancy tive methods than their ethnographic

(Lx) counterparts, with good predictive Homicide 85.2 7.0 success. However, the number of rate compelling, data-rich HBE studies is Birth rate 414.0 135.0 still quite small. The reasons are sev- (women eral. Relatively few anthropologists aged 15–24) work with behavioral ecology theory, Age of 22.6 27.3 and its data demands are extensive women and exacting. Concepts and methods giving are advancing rapidly, and hence birth early studies look primitive by com-

a 280 parison to current standards. The use Notes: Data from Wilson and Daly. Lx, mean life expectancy at birth. The of confidence-inspiring tools such as “lowest” column contains data on the 10 neighborhoods with the lowest Lx values, and “highest” column is for the 10 with the highest Lx. Homicide rates are independent estimate of parameters, neighborhood averages for males and females combined, per 100,000 per year; null hypotheses, and significance birth rates are neighborhood averages per 1,000 women aged 15–24 per year; measures have lagged behind the de- and the last row provides the median age of women giving birth in each set of velopment and interpretive use of neighborhoods. HBE models. On this most important of desiderata—superior agreement 66 Evolutionary Anthropology ARTICLES with observation—the HBE record is history theory, in a theoretically co- ory, present advocates can more se- positive but altogether too thin. herent manner. curely claim that its topical expansion This qualified assessment does not represents genuine and unexpected detract from solid and important em- explanatory fertility. pirical gains. For instance, optimal External Consistency foraging theory has stimulated the A theory should be consistent with Unifying Power gathering of extensive data on the related theories that have a general, profitability of various classes of re- Unifying power is the cumulative background claim of applicability to sources. These data now are at the result of a coherent and fertile theory. the subject matter. HBE’s congruence heart of recent analyses of prehistoric Does the theory succeed in establish- with neo-Darwinism and micro- resource intensification and plant doing relationships among previously economic theory is primary here. mestication. Likewise, quantitative disparate subjects? Does it achieve HBE benefits by distancing itself HBE data on time allocation to food broad scope by uniting these subjects from claims of human exceptionalism production and net yield of foraging, within a common explanatory frame- and the historical attempt to isolate both as a function of sex and age, are work? For HBE, the initial signs look anthropological from collateral the- central to debates about the show-off good. Central place foraging models ory in the natural and some social and grandmother hypotheses,12,193 have been adapted to generate in- sciences. Red-winged blackbirds, sights about field processing and with their extensive implications for hunter-gatherers, and search engines hominid transport. Diet selection hominid life history and socioecologi- on the world wide web face some models have generated new insights cal evolution. of the same resource selection into population ecology and have trade-offs. General models of given new purchase to long-standing Internal Coherence scrounging apply to house wrens and archaeological concepts such as the hunter-gatherers; those of risk-sensi- The parts of a theory should cohere broad spectrum revolution. Cost-ben- in a logically consistent manner, with- efit methods that were developed to out gaps or ad hoc elements. Relative study patch choice are now being to sociocultural anthropology gener- Simplicity is valued for used to examine life-history trade-offs ally, HBE is strong on coherence, ben- analytical and aesthetic and possible causes of demographic efiting from its close association with reasons but may be the transitions. How far this trend will go, two relatively mature fields: econom- and how well it withstands competing ics and evolutionary theory. Although most troublesome theoretical developments, are issues HBE is in the early stages of devising for the future. “families”293 of subject-specific mod- quality of a research els, there is every reason to project tradition to define and Simplicity cumulative and coordinated develop- assess. ment. Because of the ambitious reach Simplicity is valued for analytical of the topics it addresses, mainte- and aesthetic reasons but may be the nance of internal coherence is an es- most troublesome quality of a re- pecially important accomplishment of search tradition to define and assess. HBE. tive adaptive tactics are equally at If we focus on its fundamental theo- Changing ways of valuing resources home in biology, anthropology, and retical premises, and view them rela- provide an example. Originally, re- economics.294 Life history compro- tive to potential explanatory reach, source value was appraised in terms mises affect all primates, be they then HBE is extraordinarily simple. If of the nutritional yield (e.g., calories) chimpanzees, early hominids in Af- we focus on the dozens of models and of the prey, an absolute and resource- rica, or the contemporary residents of myriad of hypotheses that make these specific measure. Scrounging models Albuquerque. basic premises operational, then the highlighted the importance of assess- situation can appear quite complex. ing marginal value, with its emphasis Fertility An evaluation of simplicity depends on the state and needs of the forager where, along this axis of premises-to- relative to others in the group. Hy- Does the theory, over the long term, empirical research, one wishes to potheses concerned with trade-offs continue to generate novel predic- place the assessment. between parental investment and tions, extensions, and modifications mating effort suggest that value (and not anticipated in its original formu- AUGURY currency) may be sex- and age-spe- lation? Those of us who set out in the cific. Each of these refinements gives 1970s to test diet breadth and related Early work in behavioral ecology by us new, coordinated insights into the OFT models had no inkling of the ex- biologists, including papers now diversity possible in optimal patterns tent and diversity of the HBE applica- among the most cited in the field, of resource selection. Another exam- tions that would be forthcoming. Be- were routinely rejected by the major ple is the grandmother hypothesis, cause its early critics so tightly and ecological journals.76 By contrast, the which links together subsistence, mat- confidently circumscribed the “nar- anthropologists who took up HBE ing and parenting behavior, and life rowly defined domain”295 of HBE the- typically fared well at the hands of ARTICLES Evolutionary Anthropology 67 referees and editors. In subsequent in- integrated with more traditional tation and human behavior: an anthropological fluence on their respective disciplines, forms of social science theory and re- perspective. Hawthorne, NY: Aldine de Gruyter. 297 8 Smith EA. 1992. Human behavioral ecology: these roles have reversed. Behavioral search. What HBE can offer in this I. Evol Anthropol 1:20–25. ecology in biology has become a thriv- partnership is such tools as evolution- 9 Smith EA. 1992. Human behavioral ecology: ing subfield. Although readily pub- ary game theory133 and costly signal- II. Evol Anthropol 1:50–55. lished, HBE work has not yet achieved ing theory.174 Finally, there are out- 10 Hill K. 1993. Life history theory and evolutionary anthropology. Evol Anthropol 2:78–88. comparable scale or impact, and HBE standing questions of synthesis. 11 Low BS. 1993. Ecological demography: a remains a modest and marginalized Explicitly reductionist in its methods, synthetic focus in evolutionary anthropology. enterprise in contemporary anthro- as HBE matures it will have to dem- Evol Anthropol 1:177–187. pology. Some archaeologists and the onstrate that it can successfully rein- 12 Kaplan H, Hill K, Lancaster J, Hurtado AM. 2000. A theory of human life history evolution: occasional cultural anthropologist fol- tegrate topically isolated models into diet, intelligence, and longevity. Evol Anthropol. low the field, but it has had little im- a compelling and more holistic under- 13 Smith BH. 1992. Life history and the evolu- pact, for instance, on general text- standing of human adaptive behavior. tion of human maturation. Evol Anthropol 1:134–142. books or broader theoretical 14 Charnov EL, Berrigan D. 1993. Why do fe- discussions. Given the expansion of ACKNOWLEDGMENTS male primates have such long lifespans and so HBE from foraging to take up tradi- few babies? or life in the slow lane. Evol An- thropol 1:191–194. tional problems in archaeology and If we had known at the beginning 15 Cashdan E. 1996. Women’s mating strate- anthropology, this seems likely to all that it would entail, we probably gies. Evol Anthropol 5:134–143. change. By generating sound empiri- would not have undertaken this re- 16 Jones D. 1996. An evolutionary perspective cal results on topics of broad interest, on physical attractiveness. Evol Anthropol 5:97– view. Nevertheless, we thank John 109. HBE surely will pique the curiosity of Fleagle for inviting us to do so. We are 17 Bentley GR. 1999. Aping our ancestors: com- a broader array of anthropologists. grateful to colleagues and students parative aspects of reproductive ecology. Evol We see several fruitful directions for who made numerous helpful sugges- Anthropol 7:175–185. HBE. First, old models are being re- 18 Strassmann BI. 1996. Energy economy in tions on earlier drafts: Michael Al- the evolution of menstruation. Evol Anthropol visited in light of new discoveries and vard, Rebecca Bliege Bird, Monique 5:152–156. applications. The original diet Borgerhoff Mulder, Sheryl Gerety, 19 Winterhalder B. 1996. Social foraging and breadth model is being applied with Don Grayson, Craig Hadley, Ray the behavioral ecology of intragroup resource transfers. Evol Anthropol 5:46–57. new appreciation that resource valua- Hames, Richard Klein, Geoff Kush- 20 Alvard MS. 1998. Evolutionary ecology and tion may be differentiated by sex, and nick, Jim O’Connell, Joanna Scheib, resource conservation. Evol Anthropol 7:62–74. it is being used on new problems such and Bram Tucker. We hope we have 21 Bliege Bird RL. 1999. Cooperation and con- as the broad spectrum revolution. done justice to their generous and in- flict: the behavioral ecology of the sexual division of labor. Evol Anthropol 8:65–75. Second, old models also are being re- sightful efforts. Our families took note 22 Winterhalder B, Smith EA. 1992. Evolution- vised in light of new methods such as of reduced parental investment and ary ecology and the social sciences. In: Smith EA, evolutionary game theory and risk- mate attention, but tolerated them Winterhalder B, editors. Evolutionary ecology and human behavior. New York: Aldine de sensitive or dynamic programming with humor and patience. Finally, we Gruyter. p 3–23. approaches. Third, the theory is being wish to acknowledge the senior men- 23 Charnov EL, Orians GH. 1973. Optimal for- extended from its base in ethno- tors who helped us become behavioral aging: some theoretical explorations. Seattle, graphic, hunter-gatherer studies to ecologists: Ric Charnov, Steve Emlen, WA: privately published manuscript. 24 Hutchinson GE. 1959. 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