Interactions between social and feeding motivations on the grazing behaviour of herbivores: sheep more easily split into subgroups with familiar peers

Social grouping tendencies are known to influence the grazing behaviour of domestic herbivores. Here, we investigate how the strength of social bonds in a flock influences the choice of grazing location in sheep. Eight experimental ewes of INRA 401 breed grazed in plots in which a preferred feeding site, i.e. a patch of taller grass, was located 35 m away from a socially attractive site, i.e. five sheep confined in a pen, at one end of the plot. These penned sheep were either all familiar or all unfamiliar to the experimental ewes. Each experimental ewe was tested with one or four companions, which were either familiar or unfamiliar to her. The two group sizes, two degrees of familiarity with the penned sheep and two degrees of familiarity with the companions were incorporated into a Latin square design. In a series of 20 min tests, we recorded the behaviour of the experimental ewes by focal sampling and the location of each animal in the plots by scan sampling. As in a previous experiment, ewes in a larger group were more likely to move away from the penned sheep and to graze the preferred feeding site, and were also less vigilant. There was an additive effect of the degree of familiarity: the ewes with familiar companions grazed the preferred feeding site for longer, vocalised less and were less vigilant than those with unfamiliar companions. Inter-individual distances were also reduced when ewes grazed with familiar companions. Conversely, the degree of familiarity with the penned sheep had no effect on the ewes' behaviour. Differences in the strength of social bonds within a flock are thus likely to affect the formation of subgroups and influence the way herbivores forage in patchy grasslands. (C) 2002 Elsevier Science B.V. All rights reserved.