Reconsidering the null hypothesis: Is maternal rank associated with birth sex ratios in primate groups?
- *Sub-Department of Animal Behaviour, Department of Zoology, University of Cambridge, Madingley, Cambridge CB3 8AA, United Kingdom; and ‡Department of Anthropology, University of California, Los Angeles, CA 90095
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Communicated by Robert A. Hinde, University of Cambridge, Cambridge, United Kingdom (received for review March 25, 2002)
Abstract
Trivers and Willard hypothesized that vertebrates adaptively vary the sex ratio of their offspring in response to the mother's physical condition [Trivers, R. L. & Willard, D. (1973) Science 179, 90–92]. This hypothesis has produced considerable debate within evolutionary biology. Here we use meta-analysis techniques to evaluate claims that nonhuman primate females facultatively adjust the sex ratio of their progeny in relation to their own dominance rank in a uniform way. The magnitude of the difference in birth sex ratios of high- and low-ranking females declines as sample sizes increase, and the mean difference in birth sex ratios of high- and low-ranking females is zero. These results suggest that the observed effects could be the product of stochastic variation in small samples. These findings indicate that presently we cannot reject the null hypothesis that maternal dominance rank is unrelated to birth sex ratios.
Footnotes
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↵ † To whom reprint requests should be addressed. E-mail: grb1000{at}hermes.cam.ac.uk.
- Copyright © 2002, The National Academy of Sciences
