An experimental test of hypotheses explaining social segregation in dimorphic ungulates PABLO MICHELENA*, PIERRE MARIE BOUQUET†, AGNES DISSAC*, VINCENT FOURCASSIE*, JACQUES LAUGA‡, JEAN-FRANCOIS GERARD§ & RICHARD BON* *Centre de Recherches sur la Cognition Animale, Universite ´ Paul Sabatier yDomaine du Merle, ENSA-M zLaboratoire Evolution et Diversite ´ Biologique, Universite ´ Paul Sabatier xComportement et Ecologie de la Faune Sauvage, INRA (Received 24 July 2003; initial acceptance 1 September 2003; final acceptance 8 April 2004; published online 30 November 2004; MS. number: 7798) 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 1-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 be 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. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. In many social and sexually dimorphic ungulate species, adult males and females spend most of their time in single-sex groups outside the mating period. Several not mutually exclusive hypotheses have been proposed to explain sexual segregation, in particular the reproductive strategy, the sexual dimorphism–body size and the scram- ble competition hypotheses (e.g. Main et al. 1996; Bleich et al. 1997; Ruckstuhl & Neuhaus 2000). Sexual differences in parental care and sensitivity to predation are at the origin of the reproductive strategy (predation risk) hypothesis (Ruckstuhl & Neuhaus 2000). This hypothesis states that females select safe habitat at the expense of the best forage, whereas males invest in body growth by using areas with high-quality food. The sexual dimorphism–body size (forage selection) hypothe- sis assumes that females, being smaller, need to feed on higher-quality diets than males do. For both hypotheses, social segregation is considered to be a consequence of habitat segregation in heterogeneous environments. Hab- itat segregation between the sexes in grazing species is also predicted by the indirect or scramble competition hy- pothesis (Clutton-Brock et al. 1987). In areas with short grass, females are thought to outcompete males, which will be forced to use areas with abundant but low-quality Correspondence: P. Michelena, Centre de Recherches sur la Cognition Animale, Universite ´ Paul Sabatier, 118 route de Narbonne, F-31062 Toulouse cedex 04, France (email: michelen@cict.fr). P. M. Bouquet is at the Domaine du Merle, route d’Arles, F-13300 Salon de Provence, France. J.-F. Gerard is at the Comportement et Ecologie de la Faune Sauvage, INRA, BP 27, F-31326 Castanet Tolosan cedex, France. 1371 0003–3472/04/$30.00/0 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2004, 68, 1371–1380 doi:10.1016/j.anbehav.2004.04.008