Cheetah mothers bias parental investment in favour of cooperating sons

In species where members of one sex enjoy enhanced reproductive benefits through cooperation, parents are expected to invest disproportionately in offspring of that sex. Individuals in certain species have been shown to alter the sex ratio of their progeny in favour of the cooperating sex. But preferential postnatal investment is also expected if it increases the reproductive chances of cooperating offspring. In cheetahs (Acinonyx jubatus) living in the Serengeti, where male investment is restricted to the transfer of gametes, coalitions of males had a higher probability of securing and retaining tenure of territories, and residents encountered significantly more females than did non-territorial males. Coalitions were usually composed of littermates. Females, in contrast, lived alone and gained no reproductive advantages through their sisters. This provided an opportunity to test the prediction that cheetah mothers might favour pairs of sons over single sons. Results showed that mothers disproportionately provisioned litters containing two or more sons compared to those containing only one during the period that offspring depended upon them for solid food, whereas there was no such effect for increasing numbers of daughters. Mothers of litters containing two or more males had smaller belly sizes than those containing only one, and the proportion of time mothers fed from carcasses was negatively correlated with the proportion of time sons but not daughters fed. Although mothers provided slightly more flesh per unit time for litters with two or more sons than those containing one, males with brothers did not feed for appreciably longer than those with no brothers. Nevertheless, as adults, male coalition members were larger than singleton males, and it is known that juvenile nutrition is an important determinant of adult body size in mammals. The degree to which these results support the prediction that mothers favour the cooperating sex is critically discussed.