Elsevier

Evolution and Human Behavior

Volume 36, Issue 5, September 2015, Pages 389-397
Evolution and Human Behavior

Original Article
Prosocial signaling and cooperation among Martu hunters

https://doi.org/10.1016/j.evolhumbehav.2015.02.003Get rights and content

Abstract

Among immediate-return societies, cooperative social relationships are maintained despite the lack of centralized authority, strong norms of ownership and the punishment of free-riders. The prosocial signaling theory of cooperation solves the puzzle of social cohesion in such societies by suggesting that costly forms of generosity can function as an honest signal of prosocial intent, and that the reputations for prosociality signalers build generate trust between individuals, supporting the formation of cooperative partnerships. However, not all forms of costly generosity are prosocial: we contrast two types of generosity, aggrandizing and prosocial, and suggest that only prosocial generosity provides benefits through cooperation. Prosocial generosity is accompanied by pecuniary distancing: the payment of a higher relative cost to share, and a manner of sharing that disengages the acquirer from ownership over the rights to benefit from his or her harvest. We test the prosocial sharing hypothesis among Martu hunters and find that there is a significant association between the propensity of an individual to share a higher proportion of her income and centrality in the cooperative hunting network. Those who consistently pay higher costs to share, not necessarily those who are better hunters, are preferred partners for cooperative hunting. While many have emphasized the direct, status enhancing, competitive aspects of generosity, we suggest here that prosocial generosity produces benefits indirectly, through the formation of trusting, cooperative partnerships.

Introduction

Under circumstances where there are no centralized institutions that ensure cooperation and promote social solidarity, how do societies remain cohesive? A substantial body of theoretical literature suggests one solution: that social cohesion is in part maintained by strong links between generosity and cooperation, generated through the honest signaling of one's intrinsic quality (Barclay, 2004, Barclay, 2013, Boone, 1998, Gintis et al., 2001, Lotem et al., 2003, Macfarlan et al., 2013, Panchanathan and Boyd, 2004). Much of this work has focused on how seemingly costly forms of generosity, those that are performed in ways that minimize the possibility of reciprocation (such as public goods provisioning), may be an evolutionarily stable strategy under a wide range of conditions. The proposed benefits for such signaling lie in the trust that a generous individual will play fair, facilitating the formation of cooperative partnerships in other ventures. Empirical support for the prosocial signaling theory of cooperation has primarily come from experimental games, where subjects often choose to cooperate more often with those who have established themselves as more generous (Barclay and Willer, 2007, Klapwijk and Van Lange, 2009, Sylwester and Roberts, 2013, Wedekind and Milinski, 2000, Willer, 2009). Here, generosity is a strategic cost that signalers pay to influence receivers to act cooperatively toward them; the guarantee for honesty is the potential cost to one's reputation for cheating (see Számadó, 2011).

However, the design of such games has tended to make it difficult to disentangle the pathways of benefit: do generous individuals gain rewards because they are trusted cooperation partners, or because others defer to them as a result of their display of intrinsic quality? While recent work supports the formation of trust for future cooperative partnerships as the main content of the signal (Fehrler & Przepiorka, 2013), there are also cases where generosity appears to operate more as a conspicuous display of quality, meant to enhance the prestige or social dominance of the donor (Hardy, 2006, Hawkes and Bird, 2002, Mauss, 1954, Milinski et al., 2002, Vugt and Hardy, 2010). As others have pointed out (Sugiyama & Sugiyama, 2003), and one of us has suggested for Meriam turtle hunting (Smith, Bird, & Bird, 2003), signaling one's prowess and skill may not result in cooperative outcomes because such signaling is designed primarily to establish competitive hierarchies between individuals, a form of personal aggrandizing.

As Smith et al. (2003) point out, Meriam turtle hunting carries two kinds of informative signals, sent by the ability of the hunter to pay different types of costs. Competitive (or aggrandizing) signals sent through the cost of hunting make visible the skill and ability of the hunter, particularly those who play the roles of jumper and hunt leader; more skillful individuals pay a lower cost to hunt than less skilled individuals. Prosocial signals sent through the cost of sharing the entire turtle makes visible one's political motives and prosocial motivations, mainly for hunt leaders. The material cost inherent in the way the turtle is distributed, in the hunter donating his labor to a feast-holder, in giving away a whole live turtle so that all can see he took nothing for himself, all provides information to others. Men who pay such high costs to supply another's feast are indexing their support of the feast-giver; the fact that they distance themselves from distribution and from expectations of return in kind is a major component of the costly signal that allows the feast-giver to trust that the hunter values the possibility of future social interaction or cooperation more than taking an immediate individual benefit. Importantly, what the hunter is doing is visibly distancing himself from the perks of ownership, the rights to control who receives how much of the turtle, including himself. This visible distancing, so common among successful hunters in immediate-return societies, is the opposite of what we might expect to see if the display of hunter skill was entirely a form of personal aggrandizement. As such, the term competitive altruism, often used in the literature on signaling and cooperation, which includes both competitive feasting and instances of what seem to be costly sharing (Barclay, 2004), collapses two very different modes of signaling: those that are aggrandizing, and those that involve pecuniary distancing.

Signaling theory suggests that key to the prosocial nature of generosity is that it is kept honest through a kind of pecuniary disinterest: a distancing of the signaler from the immediate material and immaterial profits of his or her own production (Bird and Bliege Bird, 2010, Gambetta, 2009). This distancing puts reciprocity at risk, and it is this risk that engenders trust, which provides a wide range of benefits in social interaction (Molm et al., 2007a, Molm et al., 2000). Opening yourself up to potential loss in the short term is one way to let others know that you are not out for the selfish, short-term gain in social interactions. One way to accomplish this is to produce large amounts of food but conspicuously avoid profiting from that production by giving much of it away. Key to the honesty of the cost paid is that one shares a higher proportion of one's income, not simply more in an absolute sense. While aggrandizing signals often involve giving away more than others, as in competitive feasting (Boone, 1998, Dietler and Hayden, 2010, Smith and Bird, 2000) paying absolutely more than everyone else does not necessarily guarantee any prosocial intent if that cost is more easily paid. Instead, the honesty of a prosocial signal is determined by the relative cost endured: those who pay personally greater costs are those who should be trusted in cooperative partnerships.

Another important component of pecuniary distancing lies in the way sharing is conducted. Building trust takes more than a single act of costly sharing: the truly prosocial sharer not only takes on the relatively higher cost of the act, she also expects no immediate payback contingent upon the provisioning of the collective good, either in deference (prestige) or in kind. Demonstrating this additional hidden quality requires cumulative signaling in ways that tell others that one's generosity does not come with hidden strings attached. Sharing in costly ways expecting to extract benefits from others contingent on the gift is a form of manipulation, not a demonstration of prosocial intent (Cronk, 1994). One of the most straightforward ways to distance one's self from the possibility of contingent sharing is to contribute to the provisioning of public goods (Smith & Bliege Bird, 2006). Other ways to subvert the manipulative power of the gift (Mauss, 1954) are to distance one's self from ownership over production by giving up all rights to distribute the goods in question, essentially making a potentially private good a collective good (Hawkes, 1993). Among many hunter gatherers, the sharing of especially large items is accomplished by someone other than the acquirer. Hunters of large animals are particularly likely to divest themselves of ownership by dropping the prey at the edge of camp, and quietly drawing no attention to their successful harvest (Hawkes, 2001). They do not participate in distribution, leaving others to determine how large a share they will receive of their own prey, and do not participate in deciding how large portions are to be that go to others (Bird & Bliege Bird, 2010). We suggest that there are at least two forms of pecuniary disinterest that ensure honesty of cooperative intent: disinterested distribution — a lack of obvious interest in contingent giving and immediate repayment (in material or immaterial benefits), and costly sharing — taking on relatively greater (but not necessarily absolutely greater) costs to benefit others.

Just as aggrandizing and prosocial signals are sent through a different set and scale of costs, so too do they result in a different set and scale of benefits. Aggrandizing signals of skill typically result in greater prestige and advancement in competitive hierarchies, which helps signalers garner attention and deference from others (Henrich and Gil-White, 2001, Macfarlan et al., 2012, Plourde, 2008). Prosocial signals may similarly result in heightened prestige for those who can cultivate a reputation for generosity, but the more crucial benefit may be heightened social support through the trust such signals engender. For the Meriam, it is not the competitive signals of hunter skill, but the distribution of the turtle that provides the real political benefits. As hunters themselves say, being a hunt leader that provides turtles for feasts, "ensures the old men will listen to you in meetings and support you in disputes over land, and impresses them enough to let you marry the best girl." Beyond these political benefits, empirical work suggests that while those who share more may receive benefits through bettering their position in competitive hierarchies, those who pay higher costs to share, not necessarily those who share the most, will be more likely to gain the benefits of cooperation. Gurven and colleagues (Gurven, Allen-Arave, Hill, & Hurtado, 2000) explored the relationship between generosity and the chances of receiving food when ill and found that it was those who shared a higher proportion of their production, regardless of how much of the absolute amount was shared, that were likely to be cared for when unable to produce themselves. Only those who had paid the higher relative cost in their sharing were identified as truly generous, and so received help from others confident in their cooperative partnerships. Similarly, Lyle and Smith (2014) found that contributions to public goods in an Andean village built reputations that led to greater access to cooperative partnerships, which, in turn, ultimately provided greater health-buffering benefits. Prosocial signals, then, not only build reputational standing (as aggrandizing signals do), but also appear to foster trusting, supportive relationships that have demonstrable impacts on people's livelihoods. In prosocial display, benefits do not come directly, via the generous act, but indirectly, through the benefits gained from cooperating with others. In contrast, an aggrandizing show-off would gain benefits mainly through producing more than others, sharing more but keeping more as well (Hawkes, 1991). Or, an aggrandizing producer might share only to extract obligatory repayments from others, leveraging the power of the gift to get more material benefits in return.

Here, we test predictions of the prosocial signaling theory of cooperation among Australian Western Desert Martu, asking whether those who share more reap the benefits of cooperation, and whether these social benefits are gained through aggrandizing or prosocial generosity. Are aggrandizing show-offs or pecuniary distancers more likely to be rewarded for production via cooperative partnership formation? Are those who share inducing obligatory repayment or generating trust?

Our analysis focuses on cooperation and sharing in the small-prey hunting context (sandplain foot hunting). To hunt small prey on foot, Martu form dinner camps, remote temporary hunting camps several kilometers distant from the main community where the products of the day's hunt are cooked and consumed as a shared meal (see (Bird, Bliege Bird, & Codding, 2009) for details). Such hunting is dominated by women, although some men, especially older men, actively hunt small prey, bringing in about 30% of the small game calories (Bliege Bird & Bird, 2008). Individual camp members arrive at the central place either on foot or by vehicle, radiate outward from the camp to hunt or collect plant foods, and then rendezvous back at the camp at the end of the day. Dinner camps range in size from 4 to 20 people, with almost all of those present, including children older than about 5 or 6, actively working to acquire food for the shared meal. Hunters tend to leave at the same time and time their arrival to return together just before sunset, whether they hunt cooperatively or alone. The main prey acquired on dinner camps is small game: sand monitors comprise the majority of food items, with feral cats, perentie monitors, snakes, fruit, and other small lizards making up a minority. When hunters return to camp, each hunter or team of hunters guts and cleans the sand monitors and then cooks them whole in the fire. Everyone shares the same cooking hearth, sitting around the fire drinking sweet, milky tea while the meat cooks. The sand monitors are taken out simultaneously, each hunter then receiving the cooked prey she acquired. After pulling the cooked lizards from the fire, an initial division between cooperating hunters may occur (if the hunt was cooperative). If not, each hunter simultaneously hands over individual prey to other consumers. Primary distributions between cooperating hunters are often difficult to see because hunters frequently divide the catch while still out hunting, so secondary distributions of cooked meat from hunter to consumer are the ones that are publically visible to all. Portions are nearly always handed directly to consumers (hunters and non-hunters alike), not to family groups, and if a single lizard is intended to be split between two people, say a wife and a husband, the hunter will instruct the recipient to split the prey: "This is for you and your nyupa, give him the tail," she'll say. Older children will receive portions directly, but very small children will need to be fed from their caretaker's share. During the rapid-fire tossing of almost identical lizards from one person to another in the secondary distribution, something which is incredibly difficult for a researcher to track, people may be giving with one hand while receiving with the other. Since giving and receiving is often simultaneous, a hunter has the opportunity to assess how much she is receiving and to give away more than she has actually acquired by giving away a lizard she has just received. Those who did not acquire anything at all can share by immediately redistributing all or a portion of what they have just received. The idealized outcome of secondary distribution is not for reciprocity to be balanced, but for all consumers to have the same amount to eat, and for hunters to eat very little, if any, of their own harvest, a goal which is often (but not always) met.

Martu women hunt cooperatively more often than men, and we have previously shown that cooperation provides different benefits to hunters of different skill levels (Bliege Bird, Scelza, Bird, & Smith, 2012). We have previously suggested that cooperation serves as a way for poorer hunters to increase their foraging returns and for better hunters to provide social support to poorer hunters and gain the title of mirtilya, a good hunter (Bliege Bird et al., 2012). Poorer hunters want to hunt cooperatively as a form of insurance against failure and to have more to share with others, while better hunters benefit by visibly supporting poorer hunters. To gain the benefits of cooperation, partners must split the cooperative harvest evenly in the primary distribution: a hunter who captures more lizards should share those with her less successful partner. Although most harvests are evenly split, 35% are not, so there is always the potential for cooperation to fail and for each hunter to take only those lizards she captured. A hunter who claims more will be able to either keep more for herself, or to share more with others (Bird and Bliege Bird, 2010, Bliege Bird et al., 2012). The temptation to divide harvests unevenly in the initial division of the prey, allowing better hunters to gain the benefits of their overproduction, leaving poorer hunters holding the (empty) bag, suggests that (as they are not assorting on the basis of skill) hunters may determine their hunting partners on their propensity to share equally. Since primary distributions are not always visible, hunters must be keying on the widely visible secondary distributions that always take place in camp when everyone is present.

We thus hypothesize that Martus use reputations for generosity built through secondary sharing with everyone at the dinner camp as a way to determine which partnerships to form for cooperative hunting: those who engage in more costly sharing (pecuniary distancers) gain the benefits of cooperation. We contrast this with an alternative hypothesis that Martu form cooperative hunting partnerships based on one's foraging production: those who produce and share the most (personal aggrandizers), gain the benefits of cooperation.

If hunters are practicing prosocial signaling, the way they share should involve distancing themselves from pecuniary reward. Honest signaling of prosocial intent requires some assurance that generosity is not intended as a form of manipulation, to extract an immediate benefit from others via deference or obligatory repayment, or that hunters are not benefitting materially from their own production by keeping more the more they acquire. Thus, we predict that those who produce more on any given day a) should not receive more from others in secondary distributions: that is, there is no immediate material in-kind exchange or intent to profit from producing more, and b) those who produce more should also share a higher percentage of the food they acquire. If, however, hunters are engaging in an aggrandizing show-off strategy, sharing may be a way to materially benefit by inducing obligatory repayment (balanced reciprocity) or to keep more for one's self. In that case, those who produce and share more should keep and/or receive more.

If hunters are signaling prosocially, sharing in more costly ways should generate trust for cooperative partnership formation. Then, a) one's average generosity (proportion of foraging income shared) should correlate with centrality in the cooperation network, independently of total foraging production. Alternatively, if foragers are behaving more like aggrandizing show-offs, b) those who consistently produce more but do not share relatively more may have higher centrality, and production rank should correlate with centrality independently of generosity.

Section snippets

Materials and methods

The analysis we report on here uses the same ethnographic data collection methodology as reported elsewhere (Bird and Bliege Bird, 2010, Bliege Bird and Bird, 2008, Bliege Bird et al., 2012). Here, we provide details of our specific predictions and the analysis of each derived dataset.

Analysis of prediction 1 uses a dataset of 167 observations on 52 individual dinner-camp members that recorded the proportion of each individual's daily production that was shared to others in secondary

Results

We find broad support for our predictions, which collectively suggest that Martu hunters share in ways intended to demonstrate prosocial intent, rather than to simply (or solely) demonstrate hunting prowess or to induce obligatory repayment in others. It is these prosocial signals of generosity, rather than aggrandizing signals of hunting proficiency, that leads to one being preferentially chosen by others as a hunting partner.

Discussion

Our results show that 1) those who acquire more small prey (sand monitor lizards) share a higher percentage of their daily income than those who acquire less — the amount (in grams) a successful hunter keeps diminishes with harvest size, while the total amount shared increases linearly with harvest size; 2) how much meat an individual receives from other hunters is not linked to how much meat he or she acquired that day; and 3) the best hunter on any given day keeps significantly less than

Conclusions

Better Martu hunters do not appear to directly claim the material benefits of their overproduction. Those who produce larger harvests do not receive more from others who share to them on that day. Better hunters not only share absolutely more than poorer hunters, they share a higher proportion of what they acquire and a higher proportion of their total daily food income. Those who are consistently more generous in sharing a higher proportion of all meat they could potentially consume that day

Acknowledgements

The authors are grateful for the support of the Parnngurr community in conducting this research, and for helpful comments and suggestions from Douglas Bird and Brian Codding, and two anonymous reviewers. Funding for this research provided by grants from the National Science Foundation BCS-0850664, BCS-0314406, SBR-0211265, BCS-0127681, BCS-0075289; The Leakey Foundation, The Christensen Fund, The University of Maine, Stanford University Office of Teaching and Learning, The Lang Fund for

References (60)

  • E.A. Smith et al.

    Turtle hunting and tombstone opening public generosity as costly signaling

    Evolution and Human Behavior

    (2000)
  • K. Sylwester et al.

    Reputation-based partner choice is an effective alternative to indirect reciprocity in solving social dilemmas

    Evolution and Human Behavior

    (2013)
  • S. Számadó

    The cost of honesty and the fallacy of the handicap principle

    Animal Behaviour

    (2011)
  • P. Wiessner

    Hunting, healing, and hxaro exchange: A long-term perspective on !Kung (Ju/’hoansi) large-game hunting

    Evolution and Human Behavior

    (2002)
  • C.L. Apicella et al.

    Social networks and cooperation in hunter-gatherers

    Nature

    (2012)
  • P. Barclay et al.

    Partner choice creates competitive altruism in humans

    Proceedings of the Royal Society B: Biological Sciences

    (2007)
  • D.W. Bird et al.

    Competing to be leaderless: Food sharing and magnanimity among Martu Aborigines

    The Evolution Of Leadership: Transitions In Decision Making From Small-Scale To Middle-Range Societies

    (2010)
  • D.W. Bird et al.

    In pursuit of mobile prey: Martu hunting strategies and archaeofaunal interpretation

    American Antiquity

    (2009)
  • R. Bliege Bird et al.

    Why women hunt

    Current Anthropology

    (2008)
  • R. Bliege Bird et al.

    The hierarchy of virtue: Mutualism, altruism and signaling in Martu women’s cooperative hunting

    Evolution and Human Behavior

    (2012)
  • J.L. Boone

    The evolution of magnanimity

    Human Nature

    (1998)
  • S.P. Borgatti et al.

    Ucinet for Windows: Software for social network analysis

    (2002)
  • L. Cronk

    The use of moralistic statements in social manipulation: A reply to Roy A. Rappaport

    Zygon

    (1994)
  • M. Dietler et al.

    Feasts: Archaeological and ethnographic perspectives on food, politics, and power

    (2010)
  • D. Gambetta

    Codes of the underworld: How criminals communicate

    (2009)
  • M. Gurven et al.

    Hunting, social status and biological fitness

    Social Biology

    (2006)
  • C.L. Hardy

    Nice guys finish first: The competitive altruism hypothesis

    Personality and Social Psychology Bulletin

    (2006)
  • K. Hawkes

    Why hunter-gatherers work: An ancient version of the problem of public goods [and comments and reply]

    Current Anthropology

    (1993)
  • K. Hawkes

    Is meat the hunter’s property? Big came, ownership, and explanations of hunting and sharing

    Meat Eating and Human Evolution

    (2001)
  • K. Hawkes et al.

    Showing off, handicap signaling, and the evolution of men’s work

    Evolutionary Anthropology

    (2002)
  • Cited by (100)

    • A cost for signaling: do Hadza hunter-gatherers forgo calories to show-off in an experimental context?

      2023, Evolution and Human Behavior
      Citation Excerpt :

      It is alternatively possible that foraging effort in certain contexts might signal not hunting skill, but some aspect of the sharer's magnanimity or pro-sociality (Gurven, Allen-Arave, et al., 2000; Smith, 2004; Smith & Bird, 2000) and this idea is considered explicitly in the earliest literature on the show-off hypothesis (Hawkes, 1991). Some work on hunting and costly signaling (Bliege Bird & Bird, 2008; Bliege Bird & Power, 2015) has brought these predictions to the fore, highlighting the value of hunting, food-sharing and ‘disengagement from property’ as a signal of generosity or magnanimity. Our experimental games provided opportunities to signal elements of foraging skill only.

    View all citing articles on Scopus
    View full text