Testicular Volume and Reproductive Status of Wild <Emphasis Type="Italic">Callithrix jacchus</Emphasis>

The social relationship dynamic among callitrichid females is well known. Breeding exclusivity by dominant females involves female-female competition, usually resulting in the inhibition of subordinate reproduction. However, the strategies to maintain the male breeding position are still unclear. Researchers have observed no overt aggression between males, and differences in testosterone levels between dominant and subordinate individuals do not correlate with differences in reproductive success. In Callithrix monogamy is the predominant mating system, and testicular size is compatible with the absence of sperm competition. We analyzed testicular volume during development in 95 individuals at different ages (infant n = 12, juvenile n = 9, subadult n = 15, and adult n = 59). We also investigated if the ratio between testicular volume and body mass correlates with breeding position in the social group. The ratio was significantly higher in breeding males and a positive correlation between body mass and testicular volume is significant only for nonbreeding males. The findings suggest that testicular size varies with male reproductive status in the social group and that the enlargement of testicular volume in breeding common marmoset males seems to be a result of proximate causes and to depend on social and reproductive contexts acting together or separately.