Roe deer (Capreolus capreolus) spacing behaviour, during the period between mother-fawn separation and home range establishment, was studied in southern Sweden during 1987-1992. Data were collected, using telemetry, in two non-hunted populations. Females dispersed either as yearlings, or as 2- or 3-year-olds after having migrated between non-overlapping summer and winter ranges for one and two seasons, respectively. Seasonal migration usually ended with permanent settlement in the new summer area. Males dispersed as one- or two-year-olds, or remained philopatric. Median distance moved was c. 2 km with no significant sex-bias. Both populations underwent one increase, and one stable phase. During the increase phase, the frequency of yearling dispersal was 70% in one of the populations, but dropped to 20% at high density. This drop paralleled a 14% decrease in yearling winter weights. In the other population, yearling dispersal frequency was 56% during the increase phase. This population stopped increasing at an intermediate density and dispersal frequency was kept at 75%. Weights remained high in this population. In neither of the populations, at intermediate and high densities, was a sex-bias in dispersal among yearlings evident. Dispersers were on average heavier than philopatrics. High juvenile winter weight did not predestinate yearlings to dispersal, whereas low weight seemed to prevent dispersal. On the basis of these relationships, we propose a hypothesis relating dispersal to body condition, in order to explain dispersal tendency at different population densities. Further, we suggest the reason for female migration is the advantage of residing in a group during winters when predation risk is high. Since it is difficult for females (but not for males) to join non-relatives, dispersers must go back to the natal area as long as they have too few offspring to form their own matriarchal group.
