1. Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK
2. Mammal Research Institute, University of Pretoria, 0002 Pretoria, South Africa
3. Department of Animal Ecology, Uppsala University, Norbyvägen 18D, SE-752 36 Uppsala, Sweden
4. Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield, S10 2TN, UK
5. Department of Zoology, University of Stellenbosch, Matieland 7602, South Africa
6. Zoologisches Institut, Universität Zürich, CH-8057 Zürich, Switzerland

Correspondence to: T. H. Clutton-Brock^1,2 Correspondence and requests for materials should be addressed to T.H.C.B. (Email: thcb@cam.ac.uk).

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^{1, 2}. However, in some mammals where females are the primary care-givers, females compete more frequently or intensely with each other than males^{3, 4, 5}. 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 group⁶. 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.

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