Sex-biased investment in nonhuman primates: can Trivers and Willard's theory be tested?

The study of how parents divide investment between sons and daughters has become one of the most popular areas of discussion in evolutionary biology. Trivers & Willard (1973, Science, 179, 90-92) suggested that the physical condition of the mother may influence whether she maximizes her lifetime reproductive success by biasing her investment towards sons or towards daughters. in this review I discuss the evidence for sex-biased maternal investment in primates. No consistent pattern emerges from data sets on sex differences in infant birth weights and growth rates, measures of nipple contact, and lengths of interbirth intervals with respect to maternal condition. Studies on birth sex ratios have also proven highly inconsistent. To test the Trivers & Willard hypothesis, information on birth sex ratios, sex-biased mortality before and after birth, and patterns of maternal care during prenatal and postnatal life are required in a single population. Currently, no such information is available for any one population of primates, and will be difficult to obtain, owing to the long life span and complexity of social interactions of primates. An additional difficulty of testing the Trivers & Willard hypothesis in primates is that it is hard to conceive of any empirical evidence that would disprove it as long as alternative adaptive explanations continue to be suggested for particular species and populations. A lack of correlation between maternal condition, birth sex ratio and maternal care could be interpreted as suggesting: that additional factors have been overlooked. Direct predictions from the theory are required. (C) 2001 The Association for the Study of Animal Behaviour.