Social determinants of reproductive failure in male common marmosets housed with their natal family.

Postpubertal male common marmosets, Callithrix jacchus, engage in little or no sexual behaviour while living with their natal families. The social mechanisms underlying this phenomenon have not been identified but have been assumed to include reproductive suppression by dominant males and/or avoidance of mating with closely related females. We evaluated these two possible components of male reproductive failure. Seven postpubertal males and their fathers underwent a series of 45-min sex tests, in which the son, the father, or the son and father together were allowed to interact freely with the mother/mate or with an unfamiliar, unrelated adult female. We measured testosterone, luteinizing hormone and cortisol concentrations in blood samples collected from males immediately following each test, and in basal blood samples collected on three different occasions. Sons, but not fathers, engaged in very low rates of sexual behaviour when tested with the mother/mate. When tested with unrelated females, however, sons engaged in significantly more sexual behaviour than they had when tested with their mothers and showed no differences from their fathers. When sons and fathers were tested together, both males performed significantly less sexual behaviour than when tested alone with a female; however, they showed no overt competition for females and engaged in little agonism. Hormone levels did not differ significantly between fathers and sons. These results indicate that avoidance of mating with familiar females and possibly father-son competition, but not specific, rank-related suppression, contribute to reproductive failure among postpubertal male marmosets living with their natal family. Copyright 1999 The Association for the Study of Animal Behaviour.