Differential investment in sons and daughters: Do white rhinoceros mothers favor sons?

In species in which one sex experiences greater variation in lifetime reproductive success than the other, the Trivers-Willard hypothesis predicts greater parental investment in offspring of the more variable sex. Support for this hypothesis has been inconsistent and few studies have determined whether differential allocation of resources can be attributed to the parent (as predicted by sex-biased parental investment) or to efforts by offspring to extract resources. We addressed this issue by characterizing maternal investment in wild white rhinoceros (Ceratotherium simum). In a 3-year study, we radiotracked 14 adult females and recorded behavioral activities related to both maternal investment and offspring extraction of maternal resources. Investment in sons appeared to be greater than in daughters: sons suckled significantly longer and more frequently and were weaned significantly later than daughters. Although mothers did not respond differentially to individual nursing solicitations from male versus female offspring, our results suggest that mothers invested more in sons that vocalized at higher rates, but did not alter their investment in daughters in response to call rate. This may indicate that mothers, in part, facilitate access to resources depending on the sex of their offspring. This study provides a useful model for detennining whether sex-biased investment is under matemal control or is a consequence of sex differences in the offspring's efforts to extract parental resources.