Why Do Polygynous Ungulates Segregate in Space? Testing the Activity-budget Hypothesis in Soay Sheep

Sexual segregation is the behavior in which animals of different sex in a species live in separate groups outside the mating season. Recently a new concept, namely, the ‘‘activity-budget hypothesis,’’ has claimed to be the ultimate explanation of this behavior. The new hypothesis explains not only sexual segregation, but also segregation between animals of different size within sex (i.e., social segregation). The hypothesis states that the activity patterns of animals will differ when big differences in body mass exist between them, because of the associated difficulties of the synchronization in behavior making it costly to form groups, leading to segregation by size. Here we tested the assumptions and predictions of the activity-budget hypothesis using 40 Soay sheep (Ovis aries) as the model species in a 2.3-ha experimental arena. Sheep were divided into treatment groups to test the effect of sex, body mass, and food supplementation in their activity budgets, behavioral synchronization, diet composition, intake, food digestibility, and spatial segregation. Our animals segregated by sex but not by size, and food supplementation did not affect the spatial distribution of any sex, which is all against the predictions of the hypothesis. We also found sexual differences in drymatter digestibility independent of body mass, which questions the Jarman-Bell principle at the intra-specific level, which is the foundation for some other hypotheses of sexual segregation. Increasing behavioral synchronization led to segregation, but at the same time forming groups facilitates synchronization, so it is unclear which (i.e., synchronization or segregation) is the cause and which the consequence. Our results do not support the activitybudget hypothesis and clearly indicate that there is no strong association between behavioral synchronization and segregation.