Can the activity budget hypothesis explain sexual segregation in western grey kangaroos?

Abstract

In many sexually dimorphic and polygynous species, individuals exhibit social segregation by grouping with others of their own sex. I tested one hypothesis proposed to explain this form of sexual segregation. The activity budget hypothesis proposes that segregation evolved because sex differences in body size induce asynchrony in activity between the sexes, leading to lower stability of mixed-sex than same-sex groups. Specifically, I examined two assumptions of this hypothesis in western grey kangaroos, Macropus fuliginosus, which show strong social segregation: (1) that differences in body size result in activity budget differences, and consequently activity asynchrony, and (2) that activity asynchrony causes mixed-body size groups to be less cohesive than same-body size groups. Definitive tests of the first assumption are lacking in ungulates, because body size is confounded with sex in most ungulates. However, kangaroo populations are composed of a spectrum of body sizes: young adult males are the same size as adult females, and older males may be more than double that size. By exploiting this heteromorphism, I found that differences in body size do not result in activity budget differences, and consequently activity asynchrony in kangaroos: individuals of different body size did not have different transition times between feeding and resting, sex-size classes were not least synchronized when in mixed-body size groups, and synchrony was not lower among mixed-body size than same-body size groups. Activity asynchrony did not cause mixed-body size groups to be less cohesive than same-body size groups in kangaroos. Although mixed-body size groups were fused for less time than same-body size groups, synchrony was not correlated with group stability. These results question the validity of the activity budget hypothesis as a universal cause for social segregation.