Abstract: Close associations between adult males and lactating females and their dependent infants are not commonly described in non-monogamous mammals. However, such associations [sometimes called friendships (Smuts 1985)] are regularly observed in several primate species in which females mate with multiple males during the fertile period. The absence of mating exclusivity among friends suggests that males should invest little in infant care, raising questions about the adaptive significance of friendship bonds. Using data from genetic paternity analyses, patterns of behavior, and long-term demographic and reproductive records, we evaluated the extent to which friendships in four multi-male, multi-female yellow baboon (Papio cynocephalus) groups in Amboseli, Kenya represent joint parental care of offspring or male mating effort. We found evidence that mothers and infants benefited directly from friendships; friendships provided mother–infant dyads protection from harassment from other adult and immature females. In addition, nearly half of all male friends were the genetic fathers of offspring and had been observed mating with mothers during the days of most likely conception for those offspring. In contrast, nearly all friends who were not fathers were also not observed to consort with the mother during the days of most likely conception, suggesting that friendships between mothers and non-fathers did not result from paternity confusion. Finally, we found no evidence that prior friendship increased a male’s chances of mating with a female in future reproductive cycles. Our results suggest that, for many male–female pairs at Amboseli, friendships represented a form of biparental care of offspring. Males in the remaining friendship dyads may be trading protection of infants in exchange for some resources or services not yet identified. Our study is the first to find evidence that female primates gain social benefits from their early associations with adult males.

Abstract: When agonistic interventions are nepotistic, individuals are expected to side more often with kin but less often against kin in comparison with non-kin. As yet, however, few mammal studies have been in a position to test the validity of this assertion with respect to paternal relatedness. We therefore used molecular genetic kinship testing to assess whether adult female rhesus macaques (Macaca mulatta) from the free-ranging colony of Cayo Santiago (Puerto Rico) bias their interventions in ongoing dyadic aggressive interactions towards maternal and paternal half-sisters compared with unrelated females. It turned out that females supported maternal half-sisters significantly more often than paternal half-sisters or non-kin regardless of the costs associated with such interventions. Similarly, females targeted maternal half-sisters significantly less often than non-kin when this was associated with high costs. Unrelated females provided significantly higher mean rates of both high- and low-cost support to each other than did paternal half-sisters. However, females targeted paternal half-sisters significantly less often than non-kin when targeting was at low cost, suggesting that females refrain from intervening against paternal half-sisters. Our data confirm the general view that coalition formation in female mammals is a function of both the level of maternal relatedness and of the costs of intervention. The patterns of coalition formation among paternal kin were found to be more complex, and may also differ across species, but clear evidence for paternal kin discrimination was observed in female rhesus as predicted by kin selection theory.

Abstract: The formation of collaborating pairs by individuals belonging to two different classes occurs in the contexts of reproduction and intea-specific cooperation as well as of inter-specific mutualism. There is potential for partner choice and for competition for access to preferred partners in all three contexts. These selective forces have long been recognised as important in sexual selection, but their impact is not yet appreciated in cooperative and mutualistic systems. The formation of partnerships between members of different classes has much in common with the conclusion of trade agreements in human markets with two classes of traders, like producers and consumers, or employers and employees. Similar game-theoretical models can be used to predict the behaviour of rational traders in human markets and the evolutionarily stable strategies used in biological markets. We present a formal model in which the influence of the market mechanism on selection is made explicit. We restrict ourselves to biological markets in which: (1) Individuals do not compete over access to partners in an agonistic manner, but rather by outcompeting each other in those aspects that are preferred by the choosing party. (2) The commodity the partner has to offer cannot be obtained by the use of force, but requires the consent of the partner. These two restrictions ensure a dominant role for partner choice in the formation of partnerships. In a biological market model the decision to cooperate is based on the comparison between the offers of several potential partners, rather than on the behaviour of a single potential partner, as is implicitly assumed in currently accepted models of cooperation. In our example the members of one class A offer a commodity of fixed value in exchange for a commodity of variable value supplied by the other class, B. We show that when the B-class outnumbers the A-class sufficiently and the cost for the A-class to sample the offers of the B-class are low, the choosiness of the A-class will lead to selection for the supply of high value commodities by the B-class (Fig. 3a). Under the same market conditions, but with a high sampling cost this may still be the evolutionariy stable outcome, but another pair of strategies proves to be stable too: relaxed choosiness of class A coupled with low value commodities supplied by class B (Fig. 3b). We give a number of examples of mating, cooperative and mutualistic markets that resemble the low sampling cost situation depicted in Fig. 3a.

Abstract: Matrilineal dominance systems, which characterize several species of cercopithecines, are determined largely by the patterning of third-party aggressive interventions in conflicts. Although the role of interventions in structuring rank relations has received much attention, very few studies have dealt specifically with the development of intervention behavior. In other words,most studies have focused on the interventions received and their effect on the recipients rather than on the interventions performed and the goals of the interveners. We analyzed the intervention behavior of 10 juvenile females in a colony of 40 Japanese macaques (Macaca fuscata)housed at the University of Montreal Laboratory of Behavioral Primatology. The analysis of 749 interventions performed by the juveniles over their first 4 years and 2425 interventions received over the same period reveals that (1) juvenile females intervened selectively against females ranking below their mother, (2) they began to intervene at about the same time that they began to challenge the latter females in dyadic contests, (3) they sided with females as well as with males against these females, (4) juvenile interveners incurred little risks in terms of aggressive retaliation from their targets, (5) they derived immediate benefits in terms of conflicts won over stronger targets, (6) interventions often did not take place when the possible recipients needed support, and (7) interveners did not conform to a pattern of mutually preferential support. These results support the view that interventions by juveniles are selfish (vs altruistic) and constitute a low-cost and effective means to target and to outrank prospectively subordinate females.

Abstract: Coordination of primate group movements by individual group members is generally categorized as leadership behavior, which entails several steps: deciding where to move next, initiating travel, and leading a group between food, water sources, and rest sites. Presumably, leaders are able to influence their daily foraging efficiency and nutritional intake, which could influence an individual's feeding ecology and long-term reproductive success. Within anthropoid species, females lead group movements in most female-bonded groups, while males lead groups in most nonfemale-bonded groups. Group leadership has not been described for social prosimians, which are typically not female-bonded. We describe group movements in two nonfemale-bonded, lemurid species living in southeastern Madagascar, Propithecus diadema edwardsi and Eulemur fulvus rufus. Although several social lemurids exhibit female dominance Eulemur fulvus rufus does not, and evidence for female dominance is equivocal in Propithecus diadema edwardsi. Given the ecological stresses that females face during reproduction, we predict that females in these two species will implement alternative behavioral strategies such as group leadership in conjunction with, or in the absence of, dominance interactions to improve access to food. We found that females in both species initiated and led group movements significantly more often than males did. In groups with multiple females, one female was primarily responsible for initiating and leading group movements. We conclude that female nutritional needs may determine ranging behavior to a large extent in these prosimian species, at least during months of gestation and lactation.