Social influences on intergroup transfer by pine voles (Microtus pinetorum) Nancy G. Solomon, John G. Vandenbergh, and William T. Sullivan, Jr. Abstract: Selection to obtain mating opportunities is hypothesized to be a major factor in dispersal. Thus, if individuals move to another group or switch nests to maximize their reproductive opportunities, they should move to groups or sites where they minimize competition for resources or mates and where they can find potential mates. We examined movement of pine voles (Microtus pinetorum) in an orchard habitat. Males dispersed later than females, but the two sexes showed similar patterns of movement with respect to conspecifics. Males and females immigrated to territories that contained some opposite-sex conspecifics; they also immigrated to territories without same-sex conspecifics. The latter suggests that they may avoid competitors and move to territories where an opening exists. When female breeders were removed from a social group, a replacement female arrived and began to breed relatively rapidly. In control groups where breeding females were not removed, no replacement females appeared. These results suggest that pine voles, at least females, may monitor vacancies and immigrate if a vacancy appears. Résumé : La sélection favorisant les occasions d’accouplement est probablement un facteur important de la dispersion. Selon cette hypothèse, si les individus s’intègrent à un autre groupe ou font des échanges de nids pour maximiser leurs chances de s’accoupler, ils devraient se diriger vers des groupes ou des sites où ils peuvent minimiser la compétition pour les ressources ou les partenaires et où ils peuvent trouver des partenaires. Nous avons examiné les déplacements des Campagnols sylvestres (Microtus pinetorum) dans un verger. Les mâles se sont dispersés plus tard que les femelles, mais mâles et femelles avaient les mêmes patterns de déplacements par rapport aux autres individus de la même espèce. Les animaux des deux sexes ont immigré vers des territoires contenant des individus conspécifiques du sexe opposé; ils ont également gagné des territoires ne contenant pas d’individus conspécifiques du même sexe. Cette observation indique que les campagnols tentent d’éviter les compétiteurs et migrent vers des territoires où il y a des ouvertures. À la suite du retrait des femelles reproductrices d’un groupe social, une femelle de remplacement s’est présentée et a entrepris de se reproduire relativement rapidement. Chez des groupes témoins d’où les femelles reproductrices n’ont pas été retirées aucune femelle de remplacement ne s’est présentée. Ces résultats indiquent que les Campagnols sylvestres, du moins les femelles, repèrent les disponibilités et entreprennent d’immigrer lorsqu’une ouverture se présente. [Traduit par la Rédaction] Solomon et al. 2136 Introduction The strongest pressures shaping the evolution of dispersal are associated with reproduction, specifically with obtaining mating opportunities (Harcourt 1978; Brant 1992). Two ma- jor hypotheses (inbreeding avoidance and intrasexual com- petition) have been proposed to explain dispersal, but these are not necessarily mutually exclusive (Dobson and Jones 1985). Dispersal to increase access to potential mates may involve emigrating to avoid inbreeding (Greenwood 1980; Shields 1982) and gain access to unrelated opposite-sex conspecifics or to avoid intrasexual competition with close relatives for mates or resources needed to rear offspring (Dobson 1982; Moore and Ali 1984; Waser 1985). Some, but not all, studies suggest that individuals move to avoid competition with same-sex conspecifics. Male Califor- nia mice (Peromyscus californicus) from large litters and fe- males with many sisters dispersed, which suggests that competition with littermates might influence dispersal (Ribble 1992). Contrary to the prediction of the intrasexual competition hypothesis, male and female prairie voles (Micro- tus ochrogaster) remained philopatric more frequently if same- sex adults were present (McGuire et al. 1993). Data from other studies are consistent with predictions of the inbreeding avoidance hypothesis. Presence of the opposite-sex parent affected dispersal and settlement in Richardson’s ground squirrels (Spermophilus richardsonii) (Michener and Michener 1973), Peromyscus maniculatus and P. leucopus (Wolff 1992), and male Townsend’s voles (Microtus townsendii) (Lambin 1994), respectively. Further- more, female grey-sided voles (Clethrionomys rufocanus) from male-biased litters tended to emigrate more frequently than females from litters with female-biased or unbiased sex ratios, but litter sex ratio did not affect male dispersal (Ims 1990). Can. J. Zool. 76: 2131–2136 (1998) © 1998 NRC Canada 2131 Received September 5, 1997. Accepted June 16, 1998. N.G. Solomon. 1 Department of Zoology, Miami University, Oxford, OH 45056, U.S.A. J.G. Vandenbergh. Department of Zoology, North Carolina State University, Raleigh, NC 27695-7617, U.S.A. W.T. Sullivan, Jr. Department of Zoology, Mountain Horticultural Crops Research and Extension Center, North Carolina State University, Fletcher, NC 28732, U.S.A. 1 Author to whom all correspondence should be addressed (e-mail: solomon@msmail.muohio.edu).