Sex Ratio at Birth and Age-Reversed Dominance among Female Varecia

Abstract

Analysis of 283 offspring born at the Duke University Primate Center show that young female ruffed lemurs, Varecia variegata, produce significantly more daughters, whereas old mothers produce more sons than expected. Data are compared to 3 hypotheses for sex ratio bias: the Trivers-Willard hypothesis which predicts that dominant females produce more males, the local resource competition (LRC) hypothesis which predicts that subordinate females produce more males, and the local resource enhancement (LRE) extension of the LRC hypothesis that females produce the sex that provides later cooperative benefits. In the case of Varecia, this may include alloparenting or cooperative breeding. The results are more consistent with the LRC model. However, observations of 8 mother-daughter relationships show that female dominance rank in free-ranging Varecia groups is age reversed, with daughters aggressively outranking their mothers. Daughters born into the group were not beneficial as future supporters in within-group intermatriline competition as assumed by the LRE model, or as subordinate alloparents, cooperative breeders or aids in territorial defense. Both sex ratio and ranking observations are consistent, however, with the hypothesis that mothers produce daughters when they are young and able to invade elsewhere and sons as they age and are less able to do so. This is supported by a single observation of a mother dispersing which resulted in her daughter inheriting the natal territory. These data are supportive of the LRE model with female alliances and cooperative breeding among dispersed females.