The function of vigilance in sympatric marsupial carnivores: the eastern quoll and the Tasmanian Devil

Sympatric marsupial carnivore (Marsupialia: Dasyuridae) species experience different levels of risk of predation and competition as a consequence of different body size and feeding adaptations. The small eastern quoll, Dasyurus viverrinus, is preyed upon by owls and is aggressively displaced from carcasses by its larger competitor and potential predator, the Tasmanian devil, Sarcophilus laniarius. Devils have no known predators but this specialized scavenger experiences intense intraspecific competition. I studied the vigilance behaviour of individuals of these species feeding on carcasses. (1) Eastern quolls were more vigilant in the presence than in the absence of devils, suggesting that their vigilance functions to detect predators/competitors. (2) Devils showed a decrease in vigilance with increasing body size and age, and a trend towards a decrease with increasing group size. These are the same patterns expected if vigilance was in response to predation risk, although devils have no predators. Vigilance behaviour did not change in the presence of eastern quolls, suggesting that vigilance is not directed towards interspecific competitors: (3) As expected from their greater risk of predation and injury from a larger competitor, eastern quolls appeared to invest more in vigilance than devils did: they used vigilance postures that gave better visibility but disrupted feeding more.