Behavioural tactics of breeders in cooperative meerkats T. H. CLUTTON-BROCK *†, A. F. RUSSELL * & L. L. SHARPE ‡ *Department of Zoology, University of Cambridge yMammal Research Institute, University of Pretoria zDepartment of Zoology, University of Stellenbosch (Received 21 October 2002; initial acceptance 12 December 2002; final acceptance 24 October 2003; published online 15 September 2004; MS. number: 7505R) In eusocial invertebrates, queens commonly show morphological and behavioural modifications to their role as the principal breeders in their colonies. With the exception of naked mole-rats, Heterocephalus glaber , morphological modification of breeders has yet to be shown in cooperative vertebrates, but the behaviour of dominant individuals may be modified so as to maximize reproductive success. We studied the cooperative behaviour of dominant and subordinate adults in meerkats, Suricata suricatta, and found that the decision rules governing the contributions of dominant breeders differed from those of subordinate helpers. Dominant breeders contributed less than adult helpers to babysitting and pup feeding, but raised their individual contributions to pup care to a greater extent when helper:pup ratios were low. In contrast to subordinates, dominant breeders did not increase their contributions when they foraged successfully. Finally, while subordinates of both sexes assisted in rearing the young when dominants bred, dominant females contributed little when subordinates attempted to breed, and male helpers (but not females) reduced their contributions to the care of pups. Our results suggest that the division of labour between breeders and helpers in meerkats is intermediate between that of facultatively cooperative species, where parents are principally responsible for rearing young, and that of specialized eusocial species, which show a well-defined division of labour between breeders and workers. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. In many eusocial insect societies, breeding adults play little part in the care of young and show morphological adaptations to their role as the primary breeders in their group (Wilson 1971; Bourke & Franks 1995; Shellman- Reeve 1997). In contrast, the morphology of breeders closely resembles that of helpers in most cooperative vertebrates (but see O’ Riain et al. 2000; Russell et al. 2004). However, in vertebrates, breeders and helpers may vary in their contributions to offspring care, and there is some evidence that the factors affecting their levels of contribu- tions may also differ (Solomon & French 1997; Cockburn 1998; Hatchwell 1999). Contrasts in behaviour between breeders and helpers may be most pronounced in obligate cooperative vertebrates, where dominant individuals can breed several times a year and reproductive skew is high. Under these conditions, female breeders may conserve resources for future breeding attempts, by contributing less than helpers to rearing young, and by being less responsive to increases in demand. However, as yet, few studies of obligate cooperative breeders have been able to compare contributions of breeders and helpers quantitatively or to compare their responses to variation in group size, food availability or parentage (Clutton-Brock et al. 2002). A better knowledge of the contrasts in behaviour between breeders and helpers is important for attempts to predict optimal levels of investment in breeders and helpers (Hatchwell & Russell 1996; Hatchwell 1999), as well as for comparisons between cooperative and eusocial socie- ties (Wilson 1971; Crespi & Yanega 1995; Sherman et al. 1995; Costa & Fitzgerald 1996). We studied contributions to cooperative behaviour, and the factors that affect them, in dominant and adult subordinate meerkats, Suricata suricatta. Meerkats are one of the most cooperative vertebrates yet described: females rarely rear young without helpers and the survival and breeding success of group members increase with the number of helpers present (Clutton-Brock et al. 1999a, 2001a; Russell et al. 2002). Groups of meerkats consist of 2e40 individuals and include a dominant female, who produces up to four litters of three to eight pups per year (Clutton-Brock et al. 2001b; Russell et al. 2003a), and a dominant male who is the father of almost all of her Correspondence: T. H. Clutton-Brock, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, U.K. (email: thcb@cam.ac.uk) and Mammal Research Institute, University of Pretoria, Pretoria 0002, South Africa. A. F. Russel is at the Department of Animal & Plant Sciences, University of Sheffield, Western Bank, Sheffield S10 2TN, U.K. L. L. Sharpe is at the Department of Zoology, University of Stellenbosch, Matieland 7602, South Africa. 1029 0003e3472/03/$30.00/0 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2004, 68, 1029e1040 doi:10.1016/j.anbehav.2003.10.024