Sex of Infants Produced by Male Rhesus Macaques

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Abstract
Deviations from a 50:50 sex ratio at birth are expected when differential parental investment improves the future reproductive success of a particular sex. The influence of the father's rank on infant sex was evaluated for 374 rhesus macaque infants (Macaca mulatta) of known paternity. Multiple linear regression was used to test the effects of the infant's year of birth, cage, mother's rank, father's rank, and all two-way interactions between these variables. Mother's rank and all interactions with mother's rank had no statistically significant influence on the infant's sex. The father's rank exerted a statistically significant effect on the infant's sex only when it interacted with the year of birth and the cage. The variation among groups and over time in the influence of paternal (and maternal) rank on the infant sex ratio found in this study suggests that sex-ratio biases previously reported in individual groups may be the result of sampling error.