Effects of group size and personality on social foraging: the distribution of sheep across patches

Group cohesion in social animals foraging in patchy environments depends on 2 opposite forces, intraspecific competition, and attraction. The decision to leave or stay in a group may vary according to the individual personality. We investigated the role of personality when feeding competition increases as a result of increasing group size. Individuals referred as "bold" and "shy" were identified from an indoor exploration test according to their propensity to leave the group to explore a novel environment, using 12 novel objects placed at increasing distances from the group. Groups of 2, 4, 6, or 8 shy or bold sheep were tested in 45 × 5--m grass arenas, with one 5 × 5--m patch of preferred vegetation at each end. Sheep grazed on or close to these patches, but the number grazing the patches seldom reached more than 4--5 individuals, suggesting that crowding might affect foraging at the highest densities. The smallest groups grazed together on the same patch, but there was an increasing likelihood of splitting into subgroups as group size increased, with equal-sized subgroups most commonly grazing the 2 patches simultaneously. This effect was greatest in bold sheep, which tended to split into subgroups at smaller group sizes than shy sheep. This study provides new insight into the mechanisms by which group-living herbivores distribute themselves across patchy resources in a way that minimizes interference competition and demonstrates the importance of individual variability for spatial organization at the level of the group. Copyright 2009, Oxford University Press.