Effect of group size on social organization in rats with restricted access to food.

In groups of rats subjected to a experimental design in which access to the feeder was made difficult, a behavioural differentiation developed. Some rats brought back food pellets to the cage (carrier rats) while the others stayed in the home cage (noncarrier rats). We compared the social organization underlying the adoption of these roles in groups of three and six rats. Reducing group size increased the incidence of carrier rats, leading to a smaller number of differentiated groups. However, differentiated groups seemed to adapt to the situation in the same way as the groups of six rats. In both cases, carriers could be distinguished from noncarriers by their way of obtaining food and by how food possession stopped. Moreover, behavioural variables generally exhibited better stability in small groups and noncarrier/carrier rat relations were more extensive, resulting in more typical organization. The potential use of such groups for pharmacological studies is discussed.