The adaptive value of secondary males in the polygynous multi-level society of hamadryas baboons.

OBJECTIVES: One-male social systems are usually characterized by polygyny and reproductive exclusion by a single resident male. Sometimes, however, secondary males join these groups, and this may carry fitness costs and/or benefits to the resident male. In hamadryas baboons (Papio hamadryas hamadryas), which live in one-male units (OMUs) with female defense polygyny within a multi-level social system, secondary "follower" males often reside in OMUs. Our aim here is to examine possible benefits of these secondary males to hamadryas resident males. MATERIALS AND METHODS: Using 6 years of data from 65 OMUs in a band of wild hamadryas baboons in Ethiopia, we compared demographic and reproductive parameters of OMUs with and without secondary "follower" males to assess whether their presence conferred any reproductive benefits to resident "leader" males. RESULTS: Leaders with followers had tenure lengths almost twice as long, acquired more than twice as many females, retained females longer, and had three times as many infants during their tenure compared to leaders without followers. DISCUSSION: Hamadryas follower males enabled leaders to retain females for longer periods of time-likely through unit defense, social relationships with OMU members, and/or infant protection. Hamadryas leaders appear to be able to monopolize access to females despite the presence of followers, and as such any enhanced reproduction derived from the presence of followers likely increases the fitness of the leader rather than the follower. Thus the relationship between leaders and followers in hamadryas society appears to be a mutually beneficial one and tolerance of secondary males may be an adaptive reproductive strategy characterizing hamadryas leader males.

The rise and fall of leader males in a multi-level society: takeovers and tenures of male Hamadryas baboons.

As one means to maximize access to females, males of some species are intolerant of other males in social units, resulting in female defense polygyny, a mating system in which one male monopolizes mating access to as many females as he can for as long as possible. In such a system, the length of a male's tenure and the number of females he is able to acquire are important predictors of his reproductive success. Hamadryas baboons differ from many other taxa with female defense polygyny in that they acquire and lose females individually, thus patterns of acquisition and loss of females over time are additional factors contributing to a male's fitness. Here, we describe longitudinal patterns of female acquisition and loss over a 9-year period in a group of 250 wild hamadryas baboons. Complete tenures of leader males ranged from 310 to 2,160 days (N = 13) and results from a survival analysis yielded a median tenure length of 2,160 days, or 6 years (N = 49). The total number of females acquired, which increased with tenure length, ranged from 1 to 14 and averaged 3.5, and leader males acquired females both opportunistically and via challenging other males. The interval between acquisition of successive females ranged from 0 to 1,196 days with a median of 203, and males acquired all of their females less than halfway into their tenure. Females from outside of a leader male's social sphere (clan and band) were acquired relatively later in their tenure compared to females from within a male's social sphere. Leaders typically lost females gradually during the latter part of their tenure or all (or most) at once, suggesting an inverted U-shaped longitudinal arc of male competitive ability.

Female "dispersal" in hamadryas baboons: transfer among social units in a multilevel society.

Unlike most cercopithecines, hamadryas baboons (Papio hamadryas hamadryas) are characterized by female-biased dispersal. To clarify this pattern within the context of their hierarchical social system (comprising one-male units, clans, bands, and troops), we report here 7 years of data on female transfers among social units in wild hamadryas baboons in Ethiopia. Female tenure in one-male units (OMUs) ranged from 1 to 2,556 days (N = 208) and survival analysis revealed a median tenure length of 1,217 days (40 months). Changes in OMU membership consisted almost exclusively of takeovers by males, not voluntary transfer. Of 130 takeovers, 67% occurred within the band and 33% across bands, and, of the 22 takeovers for which we have clan membership data, 77% occurred within, not between, clans. These results reinforce the notion that hamadryas female dispersal is not analogous to sex-biased dispersal in other taxa, because (1) at least in Ethiopian populations, females do not disperse voluntarily but are transferred, often forcibly, by males; (2) only dispersal between bands will promote gene flow, whereas females are most often rearranged within bands; (3) hamadryas females undergo social dispersal but not usually locational dispersal; and (4) while male hamadryas are far more philopatric than females, they have been observed to disperse. It thus appears that the ancestral baboon pattern of female philopatry and male dispersal has evolved into a system in which neither sex is motivated to disperse, but females are forcibly transferred by males, leading to female-mediated gene flow, and males more rarely disperse to find females.

Alternative routes to the leader male role in a multi-level society: follower vs. solitary male strategies and outcomes in hamadryas baboons.

The nested one-male units (OMUs) of the hamadryas baboon are part of a complex social system in which "leader" males achieve near exclusive mating access by forcibly herding females into permanent consortships. Within this multi-level social system (troops, bands, clans and OMUs) are two types of prereproductive males--the follower and solitary male--whose different trajectories converge on the leader role. Here we compare OMU formation strategies of followers, who associate with a particular OMU and may have social access to females, with those of solitary males, who move freely within the band and do not associate regularly with OMUs. Data were derived from 42 OMU formations (16 by followers and 26 by solitary males) occurring over 8 years in a hamadryas baboon band at the Filoha site in Ethiopia. "Initial units" (IUs) with sexually immature females (IU strategy) were formed by 44% of followers and 46% of solitary males. The remaining followers took over mature females when their leader was deposed (challenge strategy) or disappeared (opportunistic strategy), or via a seemingly peaceful transfer (inheritance strategy). Solitary males took over mature females from other clans and bands, but mainly from old, injured or vanished leaders within their clan (via both the challenge and opportunistic strategies). Former followers of an OMU were more successful at taking over females from those OMUs than any other category of male. Despite this advantage enjoyed by ex-follower leaders, ex-solitary leaders were equally capable of increasing their OMU size at a comparable rate in their first 2 years as a leader. These results demonstrate the potential for males to employ both multiple roles (follower vs. solitary male) and multiple routes (IU, inheritance, challenge, opportunistic) to acquire females and become a leader male in a mating system characterized by female defense polygyny in a competitive arena.