Abstract

Social foragers frequently show diminishing levels of per capita vigilance as the group size increases. This phenomenon, called the "group size effect", was studied in a natural population of the caviomorph rodent Octodon degus. Through field observations of groups of different size, I quantified the duration of bouts of vigilance and foraging. Results showed that degus spent significantly less time being vigilant as group size increased, which agrees with the group size effect. The reduction in vigilance was achieved through a decrease in the duration of vigilance bouts as well as in scanning rate. Further, foraging bouts of group members lasted longer with increasing group size. Total group vigilance also increased with group size. Degus adjusted their behavior in similar manner to that of other social feeding species. Time saved from vigilance was allocated to foraging. Group foraging may confer anti-predator as well as short-term feeding advantages to O. degus. Further studies in this area of research may help to understand the evolution of sociality in this species.