Isolation from mammalian predators differentially affects two congeners

Evolutionary isolation from predators can profoundly influence the morphology, physiology, and behavior of prey, but little is known about how species respond to the loss of only some of their predators. We studied antipredator behavior of tammar wallabies (Macropus eugenii) and western gray kangaroos (Macropus fuliginosus) on Kangaroo Island (KI), South Australia, and at Tutanning Nature Reserve on the mainland of western Australia. Both species on KI have been isolated from native mammalian predators for several thousand years. On KI, wallabies (because of their size) are vulnerable to diurnal aerial predators. In contrast, on the mainland both species have been exposed continuously to native and introduced mammalian and avian predators. At both locations, wallabies modified the amount of time they allocated to vigilance and foraging in response to group size, whereas kangaroos did so only at the higher risk Tutanning site. Both species modified overall time budgets (they were warier at the higher risk site), and both species modified space-use patterns as a function of risk. At the higher risk site, tammars were closer to cover, whereas kangaroos were, on average, farther from cover. We hypothesize that the presence of a single predator, even if it is active at a different time of day, may profoundly affect the way a species responds to the loss of other predators by maintaining certain antipredator behaviors. Such an effect of ancestral predators may be expected as long as species encounter some predators.