Selfish or altruistic? An analysis of alarm call function in wild capuchin monkeys, Cebus apella nigritus

Alarm calls facilitate some antipredatory benefits of group living but may endanger the caller by attracting the predator's attention. A number of hypotheses invoking kin selection and individual selection have been proposed to explain how such behaviour could evolve. This study tests eight hypotheses for alarm call evolution by examining the responses of tufted capuchin monkeys to models of felids, perched raptors and vipers. Specifically, this study examines: (1) differences between individuals in their propensity to call in response to different threat types, (2) whether there is an audience effect for alarm calling and (3) the response of conspecifics to alarms. Results indicate that the benefits likely to be afforded to the caller vary with stimulus type. Alarm calling in response to felids is most likely selfish, with calls apparently directed towards both the predator and potential conspecific mobbers. Alarm calling in response to vipers attracts additional mobbers as well, but also appears to be driven by kin selection in the case of males and parental care benefits in the case of females. Alarm responses to perched raptors are rare, but seem to be selfish, with callers benefiting by recruiting additional mobbers

Demography of agile gibbons ( Hylobates agilis )

Demographic processes and the structure of a population of agile gibbons ( Hylobates agilis ) were investigated over 6 years in the Gunung Palung Reserve, Indonesia. Estimates of population size, density, and biomass revealed a population whose groups were stable in size and composition. Demographic processes place gibbons at risk, however, to short-term changes in their environment. Patterns of survival, fecundity, mortality, and dispersal combined to produce negative rates of growth. In addition, a top-heavy age-class distribution, with adults forming a large fraction of animals, makes it unlikely that this population could recover rapidly from a decline in numbers. Two behavioral factors, territoriality and monogamy, account for the size and stability of gibbon groups. Monogamy imposes limits on group size, while mating patterns and territoriality decrease the impact of sources of high mortality common in other primate species. These relationships underscore the fundamental importance of behavioral influence on demographic processes and social structure.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44559/1/10764_2005_Article_BF02196129.pd

Experimental field studies of Asian ape social systems

The Asian apes, orangutans and gibbons, possess unusual social systems among anthropoid primates. Social groups of gibbons consist of mated adult pairs and their offspring; mature orangutans are primarily solitary. Recent experimental field research has begun to yield insights into the behavioral mechanisms employed by these animals to maintain their characteristic patterns of social dispersion. While spatial separation between female orangutans appears to be maintained passively, aggression, which is manifest during direct encounters and long-distance vocal interactions, mediates male asociality. Male-male aggression is the result of intense intrasexual competition occurring between animals for mating access to females. To reduce intrasexual competition, male orangutans have adopted alternative mating tactics. In contrast to female orangutans, female gibbons show marked agonistic tendencies toward conspecifics. Female territoriality contributes to preventing males from becoming polygynous. Male gibbons, restricted to monogamous relationships, attempt to ensure their paternity through intrasexual aggression. These observations suggest that the spatial dispersion of females constrains male mating options in both species. However, variations between orangutan and gibbon social systems can be understood as consequences of the temporal dispersion of sexually receptive females. The temporal clumping of females, due to relatively high operational sex ratios, limits the ability of male gibbons to acquire multiple mates. Conversely, an extremely low operational sex ratio in orangutans creates a strong selection pressure for intrasexual competition and polygamous mating. These considerations provide a novel framework for interpreting the social systems of the African apes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44557/1/10764_2005_Article_BF02192784.pd