Humans and chimpanzees are unusual among primates in that they frequently perform group hunts of mammalian prey and share meat with conspecifics. Especially interesting are cases in which males give meat to unrelated females. The meat-for-sex hypothesis aims at explaining these cases by proposing that males and females exchange meat for sex, which would result in males increasing their mating success and females increasing their caloric intake without suffering the energetic costs and potential risk of injury related to hunting. Although chimpanzees have been shown to share meat extensively with females, there has not been much direct evidence in this species to support the meat-for-sex hypothesis. Here we show that female wild chimpanzees copulate more frequently with those males who, over a period of 22 months, share meat with them. We excluded other alternative hypotheses to exchanging meat for sex, by statistically controlling for rank of the male, age, rank and gregariousness of the female, association patterns of each male-female dyad and meat begging frequency of each female. Although males were more likely to share meat with estrous than anestrous females given their proportional representation in hunting parties, the relationship between mating success and sharing meat remained significant after excluding from the analysis sharing episodes with estrous females. These results strongly suggest that wild chimpanzees exchange meat for sex, and do so on a long-term basis. Similar studies on humans will determine if the direct nutritional benefits that women receive from hunters in foraging societies could also be driving the relationship between reproductive success and good hunting skills.

Humans are well known for their ability to keep track of social debts over extended periods of time, and for their tendency to preferentially cooperate with closely bonded partners. Non-human primates have been shown to cooperate with kin and non-kin, and reciprocate helpful acts. However, there is ongoing debate over whether they keep track of previous interactions and, if so, whether they can do it over extended periods of time, or are constrained to finalize exchanges within a single encounter. In this study, we used 3000 hours of all-day focal follows of wild chimpanzees (Pan troglodytes verus) to investigate whether both females and males reciprocate grooming within a single interaction, throughout the day, or over longer periods of time. We found that grooming was reciprocated more symmetrically when measured on a long-term, rather than on an immediate or short-term basis. Random giving, general allocation of grooming efforts, similarities among individuals and kinship do not appear to explain these highly reciprocal exchanges. Previously collected consecutive focal follows of single individuals revealed that dyads groomed an average of once every 7 days. Our findings strongly suggest that chimpanzees, similar to humans, are able to keep track of past social interactions, at least for a one-week period, and balance services over repeated encounters.

Shariff and Norenzayan (2007) discovered that people allocate more money to anonymous strangers in a dictator game following a scrambled sentence task that involved words with religious meanings. We conducted a direct replication of key elements of Shariff and Norenzayan's (2007) Experiment 2, with some additional changes. Specifically, we (a) collected data from a much larger sample of participants (N = 650); (b) added a second religious priming condition that attempted to prime thoughts of religion less conspicuously; (c) modified the wording of some of their task explanations to avoid deceiving our participants; (d) added a more explicit awareness probe; (e) reduced prime-probe time; and (f) performed statistical analyses that are more appropriate for non-normal data. We did not find a statistically significant effect for religious priming. Additional tests for possible between-subjects moderators of the religious priming effect also yielded nonsignificant results. A small-scale meta-analysis, which included all known studies investigating the effect of religious priming on dictator game offers, suggested that the mean effect size is not different from zero, although the wide confidence intervals indicate that conclusions regarding this effect should be drawn with caution. Finally, we found some evidence of small-study effects: Studies with larger samples tended to produce smaller effects (a pattern consistent with publication bias). Overall, these results suggest that the effects of religious priming on dictator game allocations might be either not reliable or else quite sensitive to differences in methods or in the populations in which the effect has been examined.

Humans are the longest living and slowest growing of all primates. Although most primates are social, humans are highly cooperative and social in ways that likely co-evolved with the slow human life history. In this paper we highlight the role of resource transfers and non-material assistance within and across generations in shaping low human mortality rates. The use of complex cooperative strategies to minimize risk is a necessary precursor for selecting further reductions in mortality rate in late adulthood. In conjunction with changes in the age-profile of production, the impacts of resource transfers and other forms of cooperation on reducing mortality likely played an important role in selection on post-reproductive lifespan throughout human evolution. Using medical data and ethnographic interviews, we explore several types of common risks experienced by Tsimane forager-horticulturalists, and quantify the types and targets of aid. Our results illustrate the importance of transfers in several key domains and suggest that the absence of transfers would greatly increase human mortality rates throughout the life course.