Kin bias and male pair-bond status shape male-male relationships in a multilevel primate society

Abstract

Male-male social relationships in group-living mammals vary from fierce competition to the formation of opportunistic coalitions or the development of long-lasting bonds. We investigated male-male relationships in Guinea baboons (Papio papio), a species characterized by male-male tolerance and affiliation. Guinea baboons live in a multi-level society, with units of one reproductively active “primary” male, 1–6 females, and offspring at the core level. Together with “bachelor” males, several units form a party, and 2–3 parties constitute a gang. We aimed to clarify to which degree male relationship patterns varied with relatedness and pair-bond status, i.e., whether males had primary or bachelor status. Data were collected from 24 males in two parties of Guinea baboons near Simenti in the Niokolo-Koba National Park in Senegal. Males maintained differentiated and equitable affiliative relationships (“strong bonds”) with other males that were stable over a 4-year period, irrespective of their pair-bond status. Remarkably, most bachelor males maintained strong bonds with multiple primary males, indicating that bachelor males play an important role in the cohesion of the parties. A clear male dominance hierarchy could not be established due to the high degree of uncertainty in individual rank scores, yet bachelor males were more likely to be found at the low end of the dominance hierarchy. Average relatedness was significantly higher between strongly bonded males, suggesting that kin biases contribute to the social preferences of males. Long-term data will be needed to test how male bonds affect male tenure and ultimately reproductive success. Significance statement Males living in social groups may employ different strategies to increase their reproductive success, from fierce fighting to opportunistic alliance formation or the development of long-term bonds. To shed light on the factors that shape male strategies, we investigated male-male social relationships in the multilevel society of Guinea baboons (Papio papio) where “primary” males are associated with a small number of females and their offspring in “units” while other males are “bachelors.” Strong bonds occurred among and between primary and bachelor males and strongly bonded males were, on average, more closely related. Bachelor males typically had multiple bond partners and thus play an important role in the fabric of Guinea baboon societies. Across primate species, neither dispersal patterns nor social organization clearly map onto the presence of strong bonds in males, suggesting multiple routes to the evolution of male bonds.

Male-Male Social Bonding, Coalitionary Support and Reproductive Success in Wild Guinea Baboons

“…Regarding coalition formation, Guinea baboon males with stronger bonds supported each other more often during agonistic events, corroborating previous analyses in the same [23] and several other species [17,18,57,62]. Compared to macaques, however, rates of coalitionary support in Guinea baboons are low (0.001/hr; Assamese macaques, Macaca assamensis: 0.11/hr [17]; Barbary macaques: 0.01-0.21/hr [57]), mirroring the low rate of aggression [24]. Given the lack of a clear dominance hierarchy between males [24] and the presence of frequent instances of coalitions targeting other coalitions [29], it is unlikely that coalitions serve in rank ascension.…”

“…Male-male sociality and reproductive success Twenty-one of the 30 study males had at least one associated female during part or the entire study period, while the remaining males were not associated with a female during the study period (see figure 2). Of the nine males that never had primary status, seven were subadult males and two were old adult males for most of the study time during which they were present in the study party, corroborating the observation that bachelor males are often subadult or lateprime/old males [24]. Of the 21 males that had primary status at least once, twelve were adult males for their entire presence time, eight transitioned from subadult to adult during the study period, and one was a large subadult male during his presence time (see figure S1 for male age category changes).…”

“…Males were included as focal subjects when they were first classified as small subadult (∼ 6 years old). At this age, they already establish close affiliations and display strong bonds and coalitionary support [24,29] with adult males. All details about male presence and age category changes, age category assessment, and criteria for subject selection/exclusion can be found in supplementary appendix 1, tables S1 and S2, and figure S1.…”

“…Guinea baboons live in a nested multimale, one to six associated females, and immatures at the core of the society [22] ng home ranges [23]. Most primary males (76.5%) have one or more associated bachelors and bachelor males are often (66.7%) associated to multiple units [24]. Primary males maintain largely exclusive affiliative and sexual relationships with the females in their unit, while bachelors exchange a smaller proportion of social interactions with females and are usually not reproductively active [25].…”

“…We predicted that strong bonds between males - enhanced by coalitionary support - would result in higher male reproductive success via the attraction of more females, resulting in a higher number of offspring. To test this prediction, we determined bond strength following Dal Pesco et al [24] and assessed the link between bond strength and coalitionary support. We predicted that dyads with stronger bonds would be more likely to cooperate during conflicts.…”