Abstract
Trivers and Willard predicted that when parental condition has differential effects on the fitness of male and female offspring, parents who are in good condition will bias investment toward the sex that benefits most from additional investment. Efforts to test predictions derived from Trivers and Willard's model have had mixed results, perhaps because most studies have relied on proxy measures of parental condition, such as dominance rank. Here, we examine the effects of female baboons condition on birth sex ratios and post-natal investment, based on visual assessments of maternal body condition. We find that local environmental conditions have significant effects on female condition, but maternal condition at conception has no consistent relationship with birth sex ratios. Mothers who are in poorer condition at the time of conception resume cycling significantly later than females who are in better condition, but the sex of their infants has no effect on the time to resumption of cycling. Thus, our findings provide strong evidence that maternal condition influences females' ability to reproduce, but females do not facultatively adjust the sex ratio of their offspring in relation to their dominance rank or current condition.
Highlights
When parental condition has differential effects on the fitness of male and female offspring at weaning, and variation in condition at weaning persists into adulthood, parents who are in good condition are expected to bias investment toward the sex that benefits more from additional investment [1]
There have been three studies of facultative adjustment of birth sex ratios in cercopithecine primate species that were based on measures of female condition rather than dominance rank [7,8, this study]
Current evidence indicates that cercopithecine primates represent an exception to Cameron’s [3] finding that analyses which are based on direct measures of maternal condition consistently support the Trivers and Willard model
Summary
When parental condition has differential effects on the fitness of male and female offspring at weaning, and variation in condition at weaning persists into adulthood, parents who are in good condition are expected to bias investment toward the sex that benefits more from additional investment [1]. In most sexually dimorphic species, large body size enhances male reproductive success, and males are likely to benefit more from additional investment than females [2]. In these species, parents in good condition are expected to bias investment in favor of sons. In a meta-analysis of more than 400 studies on mammalian species, Cameron [3] found that about one-third of the studies provided significant support for the Trivers and Willard model. Most of the remaining studies showed no consistent relationship between maternal condition and offspring sex, and a few found significant patterns that were in the opposite direction
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