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In most animals, the sex that invests least in its offspring competes more intensely for access to the opposite sex and shows greater development of secondary sexual characters than the sex that invests most. However, in some mammals where females are the primary care-givers, females compete more frequently or intensely with each other than males. A possible explanation is that, in these species, the resources necessary for successful female reproduction are heavily concentrated and intrasexual competition for breeding opportunities is more intense among females than among males. Intrasexual competition between females is likely to be particularly intense in cooperative breeders where a single female monopolizes reproduction in each group. Here, we use data from a twelve-year study of wild meerkats (Suricata suricatta), where females show high levels of reproductive skew, to show that females gain greater benefits from acquiring dominant status than males and traits that increase competitive ability exert a stronger influence on their breeding success. Females that acquire dominant status also develop a suite of morphological, physiological and behavioural characteristics that help them to control other group members. Our results show that sex differences in parental investment are not the only mechanism capable of generating sex differences in reproductive competition and emphasize the extent to which competition for breeding opportunities between females can affect the evolution of sex differences and the operation of sexual selection.

In species of cooperative insects that live in large groups, selection for increased fecundity has led to the evolution of an increased body size among female reproductives, but whether this is also true of cooperative vertebrates is unknown. Among vertebrates, morphological modification of female breeders has only been documented in a single species; in naked mole rats (Heterocephalus glaber), acquisition of alpha status is associated with a significant increase in body size through an elongation of the lumbar vertebrae. Here we provide evidence of morphological modification among breeding females of a cooperative carnivore, the meerkat (Suricata suricatta), and demonstrate that this modification is likely to be adaptive. The same female meerkats were significantly larger when they were dominant than when they were subordinate. This increased body size was not explained by differences in age, foraging efficiency, or investment in offspring care, but may have arisen, in part, through increased levels of hormone that govern bone growth. Increases in body size are likely to result in fitness benefits, for large females delivered larger litters and had heavier offspring, both of which are known to correlate positively with measures of breeding success in meerkats. Our results suggest that the acquisition of alpha status in female meerkats is associated with an adaptive increase in body size and hence that morphological modification of female vertebrates may be more widespread than has been previously supposed.