Progeny Sex Ratio in a Sexually Monomorphic Ungulate, the Collared Peccary (Tayassu tajacu)

Abstract

Polygynous ungulates are commonly used to test the Trivers–Willard hypothesis that high-quality females should produce a higher proportion of male than female offspring given certain assumptions. We studied relationships among age, maternal condition, and sex ratio of progeny in the collared peccary (Pecari tajacu), a sexually monomorphic ungulate, in southern Texas to examine this hypothesis. We collected data on litter size and sex ratio from 264 peccaries from 1989 to 2003. The overall fetal sex ratio was male-biased (P ¼ 0.04) at 54.8% male. Sex ratio varied by age, with individuals in age class 2 (of 4) being responsible for most of the skew in the ratio. Condition indices (body mass, carcass mass, and kidney fat index) did not vary in females categorized by sex ratio of litters. Logistic regression revealed that increasing condition of peccaries was associated with a more male-biased litter, but these relationships were weak (P ¼ 0.10–0.20). Our results were consistent with recent work showing sex ratio–age relationships and reviews documenting decreasing support for the Trivers–Willard hypothesis in ungulates with less sexual dimorphism.