Anim. Behav., 1991, 42, 913-919 Agonistic and spacing behaviour in white-footed mice, Peromyscusleucopus ANDREW I. KORYTKO* & STEPHEN H. VESSEY? Department of Biological Sciences, Bowling Green State University, Bowling Green, Ohio 43403-0212, U.S.A. (Received 8 February 1991; initial acceptance 10 February 1991; final acceptance 25 March 1991; MS. number: A5730) Abstract. The agonistic and spacing behaviour of white-footed mice, Peromyscus leucopus, was quantified in the field throughout an entire year to test the hypothesis that mice engage in interference competition for resources such as food, shelter and mates. Rates of aggression and displacement in paired arena tests increased markedly from winter to spring and summer, and declined in autumn, corresponding to the breeding season but not to population density, which remained low. Mice caught for the first time lost significantly more encounters than previously captured mice and the majority of the mice that lost encounters were transients. In spring, residents displaced transients, and mice paired within their home range usually won. Adults usually displaced juveniles, and aggression occurred more often between males than between females. The mean distance from non-pregnant females to the nearest male was negatively correlated with pregnancy rate, suggesting that non-pregnant females were receptive to males. When few receptive females were available, the distances between neighbouring males were low and displacement rates were high, suggesting that males competed for mates. These findings generally support the idea that interference competition is an important component of the lives of these small mammals. Agonistic behaviour is an important component of most models of population dynamics in small mammals (reviewed in Heske et al. 1988), mainly because of its presumed role in controlling the numbers and types of recruits into the population (e.g. Sadleir 1965; Healey 1967; Halpin 1981). Although the dynamics of P. leucopus populations are well described (e.g. Harland et al. 1979; Wolff 1985a), the roles of agonistic and spacing behav- iour in influencing those dynamics are not well understood. Aggression is thought to have different functions in the two sexes (reviewed in Wolff 1989). While females may defend food, nest sites or offspring against conspecifics, males are thought to defend access to females, as well as food and nest sites, especially at high densities. There is disagreement about the extent of adult aggression towards juveniles in Peromyscus spp. For instance, adult male P. maniculatus displace juvenile males, causing the latter to disperse or grow poorly (Sadleir 1965; Healey 1967). The importance of male aggression in limiting recruit- ment in P. leucopus was questioned, however, by *Present address: Department of Biological Sciences, Kent State University, Kent, Ohio 44242-0001, U.S.A. tTo whom correspondence should be addressed. Harland et al. (1979), and Halpin (1981) argued that adult female P. maniculatus could be just as important as males in limiting recruitment into the population. In the laboratory, lactating female P. maniculatus behave aggressively towards juveniles (Savidge 1974). Wolff et al. (1988) found little evi- dence of adult aggression towards juveniles for P. leucopus; young mice, especially males, dispersed even in the absence of competing adults. Wolff (1985b) has argued that aggression among adult females functions to reduce infanticide. The lack of agreement on the nature and importance of adult aggression may be due, in part, to differences among species, habitats andmethods of measuring aggression. Season is thought to influence spacing patterns and rates of aggression. In winter male P. leucopus frequently nest in mixed sex and age groups not observed during the breeding season (Madison et al. 1984). At the onset of breeding in the early spring, male P. maniculatus increase inter-individual dis- tance while females do not (Fairbairn 1977). If aggression is linked to increased competition for mates, then levels of aggression should increase at the onset of the breeding season (usually spring) and decline at the termination of breeding (usually autumn). If males are competing for mates by 0003-3472/91/120913+07 $03.00/0 9 1991 The Association for the Study of Animal Behaviour 913