Demography and Group Structure in Wedgecapped Capuchin Monkeys, Cebus Olivaceus
The extent to which population demography determines the age and sex composition of primate groups was examined using data from a population of wedge-capped capuchin monkeys Cebus olivaceus in central Venezuela. Demographic parameters were derived from censuses of individually recognized, aged, and sexed individuals living in nine groups over a ten year period. Animals were aged by extrapolation from census data. Animals of both sexes were classed as infants during their first year, and juveniles until they reached six years of age. Females reach sexual maturity at this time, while males were classed as subadults until they reached full adult size at age 12. Adulthood lasts at least 30 years in females, at least 24 years in males. Age-sex class specific mortality and fecundity rates generated a life table which indicated that the population was increasing (r = 0.087) between 1977 and 1986. The age and sex composition of the nine groups was described annually. On average, non-adults made up 60% of a group, with this percentage increasing with group size. There were more females than males in all groups in all years. The strong female-biased adult sex ratio (1:4.4) was a consequence of a biased birth sex ratio (1:1.9), higher female than male survivorship especially between the ages of 3 and 7 when males were dispersing, and a pronounced sexual bimaturism. The stable age distribution derived from the life table successfully predicted the observed average distribution of age-sex categories in groups. This suggests that the group structure of Cebus olivaceus groups is not a consequence of intragroup social interactions, but results from demographic parameters.