Introduction The European wild rabbit (Oryctolagus cuniculus) lives in small, stable breeding groups consisting of one to three adult males and one to seven adult females (Myers & Mykytowycz 1958). Within these groups, stable linear dominance hierarchies are observed in both males and females, with males competing for access to females and females competing for breeding sites. Dominant individ- uals generally enjoy greater reproductive success (Bell 1983; Webb 1988). Rabbits exhibit gender-biased natal dis- persal with males leaving their social group prior to their first breeding season and females remaining within the group (Dunsmore 1974; Webb 1988; Webb et al. 1995). The social system of the European wild rabbit proba- bly evolved in response to a number of adaptive pres- sures, including predation and competition for patchily distributed resources such as food and nesting sites (Bell 1983). Group living can result in a reduction in predation risk (Russ et al. 1998), better predator detection and allow more time for foraging (Sadedin & Elgar 1998). Furthermore, being part of a group may be a necessary prerequisite for the construction of burrows (Faulks et al. 1997). Species that live in groups because of a require- ment for a limited resource, which is patchily dis- tributed, may evolve social behaviour as this reduces detrimental effects of living together (Alexander 1974). Such detrimental effects could include aggressive inter- actions between individuals. Under these circumstances, such agonistic interactions can be reduced by co-opera- tive behaviours. By remaining within the natal group, female individual rabbits are expected to be highly related to one another. With all the females within a group being kin, this is likely to lead to a greater co-oper- ation between group members and hence may reduce aggressive conflicts between individuals. Although philopatry may increase co-operation within group members, it also increases the chance of inbreed- ing. This does not necessarily reduce the total genetic variation, it reapportions it among the various hierarchi- cal levels (Sugg et al. 1996). Hence, female philopatry is expected to lead to lower relatedness between female lin- eages from different social groups relative to random individuals within the population (Chesser 1991). As the spatial (or hierarchical) scale is increased from within breeding group to population level, the rate of Molecular Ecology (1999) 8, 299—307 ' 1999 Blackwell Science Ltd Fine-scale genetic structuring in a natural population of European wild rabbits (Oryctolagus cuniculus) A. K. SURRIDGE, K. M. IBRAHIM, D. J. BELL, N. J. WEBB, C. RICO and G. M. HEWITT Population & Conservation Biology Sector, School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK Abstract The genetic structure of a free-living tagged population of European wild rabbits (Oryctolagus cuniculus) was investigated for two consecutive years (1990 and 1991) using 10 polymorphic microsatellite loci. A specific social behaviour, the formation of stable breeding groups, influenced the genetic structure of the population. These breeding groups were shown to constitute genetically differentiated units with low levels of gene flow between them. The average relatedness among members of a social group was higher than within the population as a whole. As a result of female philopatry coupled with male-biased natal dispersal, the relatedness of females was higher than that of males, both within social groups and in the whole population. Furthermore, the average relatedness of females within groups was twice the relatedness of females between groups. This study reveals marked fine-scale, intrapopulation genetic structure, which is attributable to the social behaviour of the European wild rabbit. Keywords: European rabbit, female philopatry, microsatellites, natal dispersal, Oryctolagus cuniculus, social structure Received 6 May 1998; revision received 20 August 1998; accepted 9 October 1998 Correspondence: A. K. Surridge. Fax: +(44) 01603-592250; E-mail: a.surridge@uea.ac.uk