The mating system of callitrichid primates: II. The impacy of helpers

Abstract

The implications of ‘helpers at the nest’ for a male's willingness to remain loyal to his female in a monogamous mating system where successful reproduction depends on biparental care are considered. Three possible types of helpers are distinguished: (1) immatures who remain in their natal groups to act as helpers until they can acquire breeding territories of their own, (2) adult males who act as non-breeding satellites on other males' territories in order to inherit a female and (3) coalitions of males who defend access to a female and mate polyandrously. Observed migration rates for males are too low to make the first hypothesis a viable strategy if males are seeking to maximize their fitness, and the third hypothesis is weakened by the fact that males neither transfer as alliances nor do so into groups where they are likely to have a close male relative. The satellite hypothesis seems to predict the observed behaviour of callitrichid males best. Variation in callitrichid mating systems may be related to environmentally determined differences in fecundity and mortality rates in different habitats. The high costs of reproduction in thermally stressed environments seem to bias adult sex ratios in such a way as to predispose younger males to pursue a satellite strategy. The presence of satellites willing to help rear the female's offspring in exchange for future breeding opportunities makes roving-male polygyny both a viable and a profitable strategy for older breeding males. This produces an unstable mating system in which groups are likely to oscillate between monogamy, polygamy and polyandry in just the way observed among callitrichids.