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The eastern grey kangaroo, Macropus giganteus, and the western grey kangaroo, M. fuliginosus, are sibling species. The behaviour of 48 eastern grey kangaroos and 15 western grey kangaroos was observed for 249 h at Melbourne Zoo. Up to 31 eastern grey kangaroos and 38 western grey kangaroos were observed for 305 h where they occurred syntopically in the Grampians National Park.

In total, 46 social acts were identified in four social contexts between conspecifics. All but one of the acts were performed by both species in captivity, whereas fewer were seen performed by either species in the field. Five general acts were performed by all age/sex classes, and 10 acts occurred in mother-young contexts. High-intensity agonistic acts and all sexual acts were performed exclusively by adult males. In captivity, western grey males performed a truncated range of sexual acts directed at eastern grey females, but no interspecific sexual acts were performed by males of either species in the field, supporting the distinctiveness of the two species. The close evolutionary relationship between the two species was confirmed by high degree of concordance between their behavioural repertories, which differed only in the structure of the isolation call given by large pouch young.