VIGILANCE, GROUP-SIZE AND SOCIAL-STATUS IN CAPYBARAS

Capybaras, Hydrochaeris hydrochaeris, live in groups of varying size averaging 10 adults of both sexes. Vigilant behaviour was studied in capybaras under natural conditions, using focal-animal sampling on individual females, dominant males and subordinate males from groups of different sizes. There was a significant negative correlation between group size and individual alert rate, and a positive correlation with total group alert rate. Although the reduction in individual rates of vigilance levelled off at group sizes of 9-10, total alert rate continued to increase. The behaviour of the females accounted for most of the variation in individual alert rate and the behaviour of subordinate males accounted for most of the variation in total alert rate. These results suggest that females benefit the most from being in larger groups due to reduced vigilance required, while subordinate males 'pay' for their group membership by the vigilance they perform.