Parental investment in the common marmoset (Callithrix jacchus) : demography and behavior

This research attempted to determine if the common marmoset (Callithrix jacchus, family Callitrichidae) is biasing its sex ratio and parental investment according to patterns predicted by Trivers and Willard (1973) and other current investment theory. Because the common marmoset has a polyandrous mating structure and high female reproductive variance, females in good condition were predicted to invest more in female infants and females in poor condition were predicted to invest more in male infants. Both demographic and behavioral data sets were examined. Demographic data sets from Oak Ridge Associated Universities Marmoset Research Center and the University of Tennessee Walters Life Science Colony were analyzed. Sex ratios were compared to frequencies of the cumulative binomial distribution. Interbirth intervals of females in poor and good condition were compared by the Kruskal-Wallis test. Data from Walters Life Science were compared to that of ORAU by means of Fisher's method for combining probabilities. Demographic data showed a bias towards females in all births and a bias towards females born to mothers in good condition. Behavioral data was collected at the Walters Life Science colony. Infant carrying was analyzed by condition of the mother, while nursing and food provisioning behavior was also analyzed by condition of the infant. Some relationships were observed qualitatively, although multivariate analysis was impossible due to the small sample size. It was concluded that a bias exists in the predicted direction in the birth sex ratio of the common marmoset. Some behavioral biases also exist according to the condition of the mother and the infants involved.