Social cohesion in groups of sheep: Effect of activity level, sex composition and group size

We investigated the effects of activity, group size and sex composition on the cohesion of merino sheep (Ovis aries) groups. Mixed-sex (50% of each sex) and single-sex groups of 2, 4, 6 and 8 sheep were placed within 491-m arenas located in natural pastures and were video recorded during 6 daily hours. The behaviour, orientation and location of each sheep were then extracted from the films at 1-s intervals. We analysed the polarisation of individual orientations, mean inter-individual and nearest neighbours’ distances, as well as the frequency of pairs of nearest neighbours according to their sex within mixedsex groups. Sheep were more aggregated than predicted under the null hypothesis of random spatial distribution for all group compositions and sizes. Sheep were more spread out and less aligned in halfactive than in all-active groups, showing that social cohesion was reduced by a lack of activity synchronisation. The highest proximity between individuals was found in resting groups, yet alignment was low. The polarisation peaked in all-active groups. Mean inter-individual distance did not vary and the nearest neighbour distance decreased as group size increased. When sheep were all-active or all-resting, mixed-sex groups were more spread out than single-sex ones, with a greater distance between opposite than between same-sex individuals. Nearest neighbours of the same sex were also more frequent than www.elsevier.com/locate/applanim Applied Animal Behaviour Science 112 (2008) 81–93 * Corresponding author at: Centre de Recherches sur la Cognition Animale, Université Paul Sabatier, 118 route de Narbonne, F-31062 Toulouse Cedex 04, France. Tel.: +33 561 556 441; fax: +33 561 556 154. E-mail address: [email protected] (P. Michelena). 0168-1591/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2007.06.020 random. Our results show that social cohesion can be modulated by activity synchrony but also by social affinity. # 2007 Elsevier B.V. All rights reserved.