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The structure and evolution of primate societies are generally shaped by ecological and social forces of natural selection. The habitat and feeding ecology of primate populations, in particular, largely determine the size of the existing social groups and the pattern of interactions between individuals within and across such groups. The bonnet macaque (Macaca radiata), an Old World monkey endemic to peninsular India, usually lives in seasonal tropical deciduous forests and occurs in typically large multimale multifemale associations. This species, however, appears to have evolved, in re- cent years, a fairly high proportion of small, but rea- sonably stable, unimale troops within one particular population in the Bandipur–Mudumalai wildlife sanc- tuaries of southern India. Demographic analyses indi- cate that, as compared to multimale troops, unimale groups are relatively depleted in subadult and juvenile males, exhibit a unique female-biased birth sex ratio and display extensive female dispersal, all of which may have arisen in response to reproductive monopo- lization by the solitary resident male. Several ecological factors, including food provisioning, may have led to the evolution of this social organization, unique for a seasonally breeding cercopithecine primate. Provisioning of primate groups also leads to a significant increase in intra-troop competition among individuals for the newly available resources. Do such individuals, however, exhibit altered behavioural strategies to alleviate social tension? Changing patterns of social interactions between adult males were also analysed for one particular troop of bonnet macaques in the Mudumalai Wildlife Sanctuary under two ecological situations – as they foraged on their natural diet and when they gathered provisioned food from tourists visiting the sanctuary. Although feeding competition increased markedly as these individuals alternated between natural foraging and competing for provisioned food, individual macaques were able to adopt appropriate social strategies under such rapidly changing eco- logical regimes. These studies demonstrate the behavioural and social plasticity of a primate species and the value of demographic studies of multiple groups and populations in different ecological environments.