Group size, methods of surveillance and foraging rates of free-ranging eastern grey kangaroos, Macropus giganteus, were compared between night and day, from data on 24-h followings of 24 individuals. Kangaroos occurred in smaller groups in the dark but the reduction in group size took place before or around dusk, and could not be attributed to animals drifting apart after dark. Instead, changes in group size coincided with much movement by animals around dusk. Both males and females tended to spend less time surveying after dark, but females surveyed in fewer bouts at night than in the day. Females used the upright posture more at night than in the day; this is a conspicuous posture but one that probably enhanced their view of the surroundings. Foraging rates of kangaroos were lower after dark than before dusk because of an increased search time per bite. There was no evidence that other foraging parameters, such as rare of movement whilst feeding, were affected by darkness. Eastern grey kangaroos are one of the most social and diurnally active species of macropod and their highly flexible social system of open-membership groups appears to allow individuals to join and leave groups according to their foraging and safety requirements.
