An experimental test of hypotheses explaining social segregation in dimorphic ungulates

in many sexually dimorphic ungulates, adults spend most of their time in single-sex groups outside the mating season. We tested two hypotheses proposed to explain this social segregation. The activity budget hypothesis predicts that females spend more time grazing than males, and that activity synchrony is higher within single-sex than mixed-sex groups. The social affinity hypothesis states that higher spatial proximity and aggregation within the sexes results from sexual differences in motivation to interact socially. fifteen merino sheep, Ovis aries, 2 or more years old, of each sex were penned together in three I-ha paddocks in succession during 7 winter weeks. Despite the high sexual dimorphism in body weight, both sexes were found together, and did not differ substantially in activity budget. Males grazed less than females but the difference was small. The degree of activity synchrony was similar within and between the sexes. When grazing, males walked more rapidly and tended to tie at the front of the group more often than females. Ewes were rarely seen interacting with other sheep, in contrast to males, which interacted twice as often with females as with males. However, same-sex pairs of nearest neighbours were found more frequently than expected, whether or not the males were at the front of the group. These results indicate a social segregation at a small spatial scale, which can be explained by differences in speed of movement between males and females and/or higher social affinity between same-sex than opposite-sex peers. The lack of single-sex groups can be explained by the strong gregariousness of merino sheep, and possibly the young age of the adult males. (C) 2004 The Association for the Study of Aninial Behaviour Published by Elsevier Ltd. All rights reserved.