Parental investment theory predicts that natural selection will favour a parental ability to bias the offspring sex ratio in favour of that which will yield the highest benefit–cost payoff. The local resource enhancement hypothesis posits that in cooperative breeders, parents may favour offspring that will mitigate parental effort by helping to raise younger siblings. Conversely, the fragile male hypothesis suggests that parents will adjust the sex ratio in favour of the sex that experiences higher mortality. We examined international studbook data of two cooperatively breeding primate species with differing reproductive strategies: golden lion tamarins, Leontopithecus rosalia, which typically give birth to twins, and callimicos, Callimico goeldii, which almost always give birth to singletons. The tamarins showed a stronger secondary sex-ratio bias in favour of males than did the callimicos, as predicted by the local resource enhancement hypothesis, because tamarins sustain higher per-parturition costs and because sons may invest more effort in caring for their younger siblings than do daughters. However, a generalized linear regression failed to reveal factors that may influence this variation. There was no significant predictor of male bias based on mother's age, mother's number of previous parturitions or group size. Contrary to predictions of the fragile male hypothesis, sons were no less likely to survive the postnatal period of parental investment than were daughters. A greater understanding of this phenomenon is needed because the adult sex-ratio bias in favour of males has become so pronounced, especially in golden lion tamarins, as to cause management difficulties for the conservation of these endangered species.
