Mating System and Reproductive Success: a Comparison of Two African Forest Monkeys (Colobus Badius and Cercopithecus Ascanius)

Abstract

Reproductive success was compared between two species of rainforest monkeys. The folivorous red colobus lived in patrilineal, multi-male groups, whereas the omnivorous, redtail had matrilineal groups usually with only one adult male, but occasionally experiencing temporary multi-male influxes. Maternal investment by red colobus in offspring was demonstrated in two ways. Offspring survival was positively correlated with interbirth interval even when cases of neonatal mortality were excluded. Among male red colobus surviving to at least 24 months, the interbirth interval following their birth was positively correlated with the age at which they disappeared or became breeders. Female red colobus appear to invest more in sons than daughters. Among the primate species examined, females invested more in their sons and daughters depending upon variables such as intersexual differences in variance in reproductive success, the dispersing sex, and maternal dominance rank as it influenced the offspring's reproductive success. Lifetime reproductive success (LRS) of female red colobus was influenced most by offspring survivorship and less so by birth rate. In contrast, LRS of male red colobus was largely a function of reproductive rate and lifespan, with offspring survivorship playing a much smaller role. Variance in LRS was higher in males than females. This difference was much more pronounced in the one-male groups of redtails than the multi-male groups of red colobus. The coefficient of variation in male reproductive success in these two species was similar to that in other polygynous mammals, except elephant seals. Although offspring mortality and estimated reproductive lifespan were nearly identical for males of the two species, the harem breeding system and highly skewed adult sex ratio lead to greater, more variable LRS in redtail males. Male red colobus within groups produced about 20% more offspring per year than male redtails, but the large proportion of red colobus males with zero breeding success reduced mean LRS to 40% less than redtails. While annual fecundity among adult females of both species was similar, variance in annual fecundity among female redtails was over five times greater than in female red colobus. This, accompanied by a longer reproductive lifespan in red colobus females, lead to a higher LRS in female red colobus and a lower coefficient of variation than in female redtails. Although females are the primary dispersers in red colobus, they did not have a greater coefficient of variation in LRS than in species where females rarely disperse. This is because female red colobus were readilty accepted into new groups and their intergroup transfer was relatively rapid with little time, if any, spent as a solitary. In both red colobus and redtails adult males contributed, on average, more offspring to the succeeding generation than adult females, as in other polygynous mammals. This is because fewer males survived to adulthood, but once they reached adulthood, their estimated reproductive lifespans were similar to females.