Structure and function of the eastern grey kangaroo (Macropus giganteus) foot thump

Most species of the family Macropodidae (kangaroos and wallabies) make a distinctive foot thump by striking the ground with their hind feet when they detect potential danger. I used the eastern grey kangaroo Macropus giganteus as a model to examine (1) the acoustic characteristics and structure of the thump, (2) the social context in which free-ranging kangaroos thumped when approached by a human observer on foot and (3) the intended recipient of the signal. Thumps were about two-thirds of a second in length, generally composed of two noisy pulses, and had the majority of the signal energy below 7 kHz. Only adult kangaroos, of both sexes, were observed to thump. A higher proportion of solitary kangaroos thumped than grouped kangaroos, and a higher proportion of kangaroos thumped when visibility was poor, either because of habitat type or low light levels. Given the context in which thumps were given, the foot thump appears to be a signal to a potential predator with three possible functions: to startle, signal detection or deter pursuit.