Ideal free distribution and natal dispersal in female roe deer

We investigated whether adult (≧ 2 years) female roe deer Capreolus capreolus conform to an ideal free or an ideal despotic distribution, in order to understand whether natal dispersal is 'voluntary' or socially enforced. The study was undertaken in a high-density, free-ranging population close to Stockholm, Sweden, during 1989-1994. Data on population density, habitat quality, and five parameters on female reproduction and body condition, in addition to age distribution, were obtained in two nearby located areas, the 'field' and the 'forest', representing contrasting habitats. Population density was estimated by faecal pellet group counting in addition to total counts of culled animals after a major deer harvest. Density in the 'field area' was twice that in the 'forest area' (66 vs 33 deer/km²). Habitat quality was determined by analysing the amount of faecal nitrogen from samples of faeces collected in the two areas, and by comparing size of summer home ranges for adults of both sexes with the aid of telemetry. Both estimates indicated a higher nutritional quality and resource abundance, respectively, in the 'field area', with faecal nitrogen content being higher (2.96 vs 2.43%), and ranges being smaller (12.9 vs 20.9 ha). No significant differences were found in any of the parameters on reproduction or body condition obtained from culled females, i.e. number of corpora lutea (1.8 vs 1.8), proportion of females with offspring (71 vs 56%), body mass (16.9 vs 17.3 kg), kidney fat index (91.9 vs 98.5), and length of the lower mandible (152.8 vs 151.4 mm). Neither did the age distribution among females, as determined from tooth wear, differ between the areas. These results are consistent with the prediction of the ideal free hypothesis. This, in turn, suggests that female dispersal in this species is 'voluntary', the underlying proximate cause being maximization of resource gain.