Males collectively defend their one‐male units against bachelor males in a multi‐level primate society

Abstract

Group-level male-male co-operation, which has been documented in several primate and non-primate societies, may be mutualistically advantageous to the participants when confronted with threats such as takeovers and cuckoldry by external males. Co-operation among members of distinct social units-while universal among humans-is extremely rare in non-human primates. We present the first observations of collective action or co-operation among males of different one-male units (OMU) in a multi-level society of Rhinopithecus roxellana. A total of 59 instances of male co-operation were recorded. Male co-operation included coordinated chasing, joint vigilance, and patrolling behavior directed at lone adult males trying to enter an OMU. Male co-operation was significantly more frequent during the mating season when the risk of incursions and extra-group paternity was higher. Paternity of infants born in the subsequent birth season and kin relationships among resident males were identified using microsatellite genotype. All infants were sired by OMU males, which we interpret as possible evidence for their success at thwarting mating attempts by satellite males. OMU males were principally unrelated suggesting that male co-operation is best understood in terms of the mutual direct benefits individuals obtain through collective action. Our findings lend support to the bachelor threat hypothesis in which the cooperative behavior of several individuals is more effective than the lone action of a single individual in providing mate defense. Our research has implications for understanding male bonding, higher-level collective action, and the evolution of social co-operation in human societies.

Satellite telemetry and social modeling offer new insights into the origin of primate multilevel societies

“…Moreover, we observed leader males to form coalitions and aggressively chase members of the all-male band when individuals were in close proximity to the breeding band. This same behavioural pattern has been reported in a study of golden snub-nosed monkeys in Hubei Province, China58.…”

Primate model offers insights into male bonding in complex societies

“…Similar conclusions have been drawn for snubnosed monkeys in multilevel societies (15), and an absence of strong kin bias also typifies male social bonds in other social systems (10,11). This finding suggests that male cooperation is best understood in terms of the mutual direct benefits individuals obtain through acting together (20).…”

“…In hamadryas baboons males of different "harems" are linked through social bonds at the clan level and can also function as allies in a competitive interaction (13); moreover, they show "respect of possession" toward a leader with his females (14). Similarly, in snub-nosed monkeys, males belonging to different one-male units have recently been shown to engage in joint collective action, such as patrolling behavior (15). In Guinea baboons, an elaborate repertoire of bonding mechanisms, including coalitions and ritualized greetings, involve males residing in different units but being members of the same party (and occasionally different parties).…”

Deciphering the Social Organization and Structure of Wild Yunnan Snub-Nosed Monkeys (Rhinopithecus bieti)

“…We were unable to detect extraunit matings due to lack of individual recognition, but precisely such have been recorded in semiprovisioned groups of the closely related R . roxellana [Zhao et al, 2005;Xiang et al, 2014]. On the other hand, the low copulation rates documented in this study show that males have more or less a reproductive monopoly in their OMUs.…”