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

Food sharing is ubiquitous among many foraging populations (Damas, 1972; Dowling, 1968; Kent, 1993; Lee, 1979; Sahlins, Graeber, Sahlins, & Graeber, 1972; Smith, 1985). Within these groups, men and women typically pursue different sets of resources, sometimes with little overlap (Bird, 1999; Bliege Bird & Power, 2015; Brown, 1970; Jochim, 1988; Marlowe, 2007). Often, men target high-risk, high-variance resources (e.g., large game) while women prioritize lower-risk, lower-variance resources (e.g., berries; tubers) (Blurton Jones, 2016; Codding, Bird, & Bird, 2011; Hawkes, O'Connell, & Blurton Jones, 1991; Hurtado, Hill, Hurtado, & Kaplan, 1992; Marlowe, 2007). Importantly, certain foods, especially those procured by men, are habitually redistributed widely beyond the immediate family of the procurer (Codding et al., 2011; Gurven, 2004; Gurven & Hill, 2009). These observations prompted anthropologists to consider why (Brown, 1970; Kaplan, Hill, & Cadeliña, 1985) foragers may be targeting high-risk foods and giving away caloric resources at a ‘significant opportunity cost to their own families’ (Hawkes, O'Connell, & Blurton Jones, 2014, p.596).

Many explanations for caloric redistribution among foragers hinge on considerations of long-term self-interest (Gurven, 2004; Hill, Kaplan, & Hawkes, 1993; Kaplan et al., 1985; Winterhalder, 1996b). For instance, some have proposed that food-sharing might be motivated by future reciprocal exchange (Cashdan, 1985; Gurven, 2004; Gurven, Allen-Arave, Hill, & Hurtado, 2000,Gurven, Hill, Kaplan, Hurtado, & Lyles, 2000; Sahlins et al., 1972) which could smooth variance in the availability of otherwise unreliably attained resources (Winterhalder, 2001). Others argue that food-sharing is motivated by persistent requests and demands from camp-mates (Blurton Jones, 1991; Peterson, 1993 but see Kaplan & Gurven, 2005; Stibbard-Hawkes, Smith, & Apicella, 2022), especially where individuals procure surplus perishable food (Winterhalder, 1986, Winterhalder, 1996a). Further, some propose that food redistribution might be motivated by inclusive fitness, such that resources flow to consanguineal kin (Wood & Marlowe, 2013, but see Allen-Arave, Gurven, & Hill, 2008; Hill et al., 2011) a pattern observed in certain studies of Hadza food redistribution (Wood & Marlowe, 2013 but see Hawkes et al., 2014), and in gift-giving games (Apicella, Marlowe, Fowler, & Christakis, 2012). Finally, many researchers see hunting as foremost a nuclear family provisioning strategy (Lovejoy, 1981; Washburn & Lancaster, 1968; Wood & Marlowe, 2013; Wood, Pontzer, Raichlen, & Marlowe, 2014).

Among the most enduring explanations for forager food acquisition (Hawkes, O'Connell, & Blurton Jones, 2018, 2014), redistribution, and the sexual division of labor (Bird, 1999; Jochim, 1988; Marlowe, 2007) is the notion that men might gain reproductive benefits from their food sharing (Gurven & von Rueden, 2006; Kaplan et al., 1985). Two key formulations of this idea are the show-off and costly signaling hypotheses (Hawkes et al., 2018; Hawkes & Bird, 2002). These predict that large food packages, such as large animal carcasses, ‘are more like public than like private goods’ (Hawkes et al., 2018, p.78) but men continue to procure them to ‘establish and maintain their relative social standing by showing off their hunting prowess’ (p.59). By targeting such resources, men forsake caloric optimality to reveal ‘information about an otherwise hidden quality’ (Hawkes & Bird, 2002, p.59) to third parties, and gain fitness benefits by doing so. Public goods, such as food items, attract wide and general interest in a way that other forms of signaling with no direct benefit to the recipient do not (Hawkes et al., 2018). In this way, people forsake calories to publicly show-off.

Sex differences in resource acquisition were traditionally framed as cooperative labour specialization, where women prioritize resources which are and aren't compatible with nursing and/or childcare (Brown, 1970; Hurtado, Hawkes, Hill, & Kaplan, 1985; Lancaster & Lancaster, 1987). Costly signaling models, by contrast, often highlight inter-sexual conflict, proposing that women gain fitness benefits by prioritising reliable food, while men gain fitness benefits through pursuing sometimes calorically suboptimal resources that are more widely shared and, often, less reliably attained (see Hawkes et al., 2018). Indeed, male hunting success is associated with measures of reproductive success in many forager populations (e.g., Gurven & von Rueden, 2006; but see Kraft, Venkataraman, Tacey, Dominy, & Endicott, 2019). While data on the relationship between Hadza men's long term hunting income and reproductive success [RS] are lacking, there is indirect evidence that Hadza men with better hunting reputations have higher RS (Apicella, 2014; Blurton Jones, 2016), more sequential marriages (Blurton Jones, 2016) younger and harder-working wives (Blurton Jones, 2016; Hawkes, O'Connell, & Blurton Jones, 2001) and are preferred as husbands (Marlowe, 2010). Moreover, there is evidence from multiple populations that women tend to target foods which are more reliably attained and shared less widely (Codding et al., 2011 though see Starkweather, Shenk, & McElreath, 2020), a pattern observed among Hadza children (Crittenden, Conklin-Brittain, Zes, Schoeninger, & Marlowe, 2013). For instance, variance in Hadza boys' caloric returns steadily increases throughout childhood and adolescence, but for girls, it remains stable (Apicella, Crittenden, & Tobolsky, 2017), and among both adults and children there are clear sex differences in competitiveness (Apicella & Crittenden, 2015; Apicella & Dreber, 2015).

Barker, Power, Heap, Puurtinen, and Sosis (2019), provide a counterpoint for the view that signaling is exclusively the purview of men, and highlight links between female foraging effort and likelihood of being named as friends or best friends (Marlowe, 2010) as well as links between female generosity and network centrality (Apicella et al., 2012). For this reason, they propose that Hadza women's foraging, ‘may additionally hold signal content of the skill and dedication of the forager, as well as her potential value as a foraging partner’ (Barker et al., 2019, p.89). In support of this, in a picture ranking task, Hadza women themselves rated skill-signaling highly as a motivation for foraging (Stibbard-Hawkes et al., 2022).

The show-off and costly signaling hypotheses have been divisive (Gurven, 2004; Gurven & Hill, 2010; Hawkes et al., 2014; Hawkes, O'Connell, & Coxworth, 2010; Stibbard-Hawkes, 2019; Stibbard-Hawkes, Attenborough, & Marlowe, 2018; Wood, 2006; Wood & Marlowe, 2013) and there is no universal consensus on the extent to which foraging, and sharing are motivated by showing-off. Gurven (2004) and Gurven and Hill (2009) have argued that food redistribution is multi-causal and varies based on circumstance. Moreover, as meat provides important nutrients not easily obtained from other sources (see, Milton, 2003; Tennie, Gilby, & Mundry, 2009; Watts, 2020), small quantities of meat may provide benefits to the procurer even when widely shared. The notion that large game are public goods, and that producers have minimal control over food distribution has been divisive (Hawkes et al., 2014; Wood & Marlowe, 2013; Woodburn, 1998) and varies across cultures (reviewed by Kaplan & Gurven, 2005; Testart, 1987). Wood and Hill (2000), Wood (2006) have found that men among the Aché and Hadza often state a preference for living with better hunters so that they get more food, suggesting intra-sexual cooperation trumps competition. Moreover, while assessments of hunting ability well predict success at tasks designed to test foraging skills (Apicella, 2014; Stibbard-Hawkes, 2019), and hunting reputations track individual return rates among Aché hunters (p.333 Hill & Hurtado, 1996), the error introduced by high stochasticity in acquisition (Hill & Hurtado, 2009) renders hunting a potentially noisy signal (Stibbard-Hawkes, 2019).

To date, most empirical support for the show-off and costly signaling hypotheses comes from cross-sectional or correlational research. This includes evidence that better hunters have higher status (von Rueden, Gurven, & Kaplan, 2008) and greater reproductive success (Gurven & von Rueden, 2006), or evidence that sharing is calorically costly or suboptimal (e.g., Hawkes et al., 2018, but see discussion in Stibbard-Hawkes, 2019). Apicella (2014) and Stibbard-Hawkes et al. (2018) both found statistically real but noisy associations between hunting reputation and tests of hunting skill, suggesting that hunters' skills are not entirely opaque. Moreover, Bishop, In press, Bishop, 2019 measured the signaling value of prey harvest composition across two foraging societies and found that different prey types altered perceptions of the hunter.

Several further studies have used experimental methodologies to assess the motives underlying food sharing behaviour. In two computer-simulated resource optimisation experiments (Kaplan, Schniter, Smith and Wilson, 2012, Kaplan, Schniter, Smith and Wilson, 2018), high variance resources leads to emergent reciprocal sharing relationships between participants, rather than relationships of tolerated theft. Moreover, economic games (dictator, ultimatum & public goods games) have been extensively used to assess giving behaviour in small-scale societies (e.g., Gurven, 2004; Henrich et al., 2005; Marlowe, 2005; Smith et al., 2019; Stagnaro, Stibbard-Hawkes, & Apicella, 2022). Notably, Gurven, Zanolini, and Schniter (2008) showed that public games giving was slightly increased relative to private games, indicating the importance of generosity. Wood and Hill (2000); Wood (2006) showed, in the context of interviews, that both Hadza and Ache men prefer residential camps/villages which maximize food availability rather than signaling opportunities. However, we know of no other studies that have given participants the opportunity to choose directly between caloric optimality and signaling opportunities in the context of a controlled, incentivised behavioural experiment.

Here, we explore how adults behave in a paradigm that pits caloric maximization against showing-off aspects of foraging skill (but not generosity). We introduced n = 196 Hadza adults (93 men; 93 women) to two games: the aim game, a test of target accuracy, and the search game, a test of search efficacy. Both games were designed to be challenging, but winnable. Although both tasks had elements of luck, both require skills relevant to hunting or gathering (hand-eye coordination in the first instance; search efficiency in the second). Success in each game was rewarded with a colored bracelet specific to that game. Campmates were aware that the bracelet denoted success at the corresponding task. Participants were given an initial allotment of honey sticks - a valued food resource. After playing the games once without cost, participants were able to pay for subsequent attempts at the game by trading in their honey sticks. Paying one honey stick meant one extra attempt at the game of their choice - a direct caloric cost in exchange for a signaling opportunity.

We predicted that if individuals were motivated specifically by skill signaling, they would be willing to forsake calories in return for an opportunity to visibly advertise foraging-relevant skill to campmates.

The second aim of our study is to assess sex differences in both willingness to participate and willingness to pay a cost to do so. Some propose men reap greater fitness benefits from allocating effort into status-seeking than into self- and nuclear-family-provisioning strategies (e.g., Hawkes et al., 2018; Hawkes, Rogers, & Charnov, 1995). Recent work has challenged or otherwise revised the notion that men and women have entirely conflicting motivations (Barker et al., 2019; Mulder & Ross, 2019; Starkweather et al., 2020; Stibbard-Hawkes et al., 2022). It would be useful to assess cost and risk tolerance in an experimental setting and to test whether, ceteris paribus, men are more willing to exchange calories to participate in a visible display of aim and dexterity than women.

Finally, the present study also affords the opportunity to explore age-related differences in willingness to pay caloric costs for signaling opportunities. If hunting is a means of advertising to prospective partners, we should expect younger people will be more likely to exchange calories to show off.

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