Predation-sensitive grouping and habitat use by eastern grey kangaroos: a field experiment

Eastern grey kangaroos, Macropus giganteus, are thought to respond to high risks of predation by foraging closer to refuge and by forming larger groups, but females with newly emergent offspring mostly forage alone, a response thought to reduce predation risk on the latter. I tested these hypotheses for wild kangaroo populations in southeastern Australia, using a field-based experimental reduction of a potential predator, the red fox, Vulpes vulpes. I monitored kangaroo foraging behaviour in two fox removal and two control sites by counting scats at 30 and 150m from refuge during September 1994 and 1995 (after 5 and 16 months of fox control). I also monitored kangaroo group sizes in all sites, using monthly or bimonthly direct observations over 22 months of fox reduction. In both years, fox removal caused kangaroos to deposit relatively more scats further from refuge than in control sites where fox numbers were more than 85% higher. This suggests strongly that kangaroos spend more time foraging close to refuge when predation risk is high. Group sizes were also smaller in fox removal sites although only during June–October when the majority of females had large pouch young. In November–March, group size decreased in all sites owing to the solitary behaviour of many females with young at foot. However, where foxes were removed, the mean group size of such females was smaller than in control sites and females were also more often alone where foxes were removed. These results suggest that solitary behaviour increases rather than decreases predation risk. This experiment highlights that grouping behaviour is risk sensitive and that some individuals accept higher risks of predation to mitigate other costs of group foraging.