Abstract
In many sexually dimorphic mammal species, the sexes live outside the mating season in separate social groups ('social segregation'). Social segregation occurs in a wide range of environmental conditions, but its cause in unknown. I suggest that social segregation is caused by a lower level of activity synchrony between individuals in mixed-sex groups than in single-sex groups, owing to sex differences in activity rhythm. As a consequence, mixed-sex groups are more likely to break up than single-sex groups, resulting in a predominance of single-sex groups at equilibrium. To test this hypothesis in red deer (Cervus elaphus L.), I developed an index of activity synchronization and showed that deer in mixed-sex groups were significantly less synchronized in their activity than deer in single-sex groups. Thus, low intersexual synchrony in activity can lead to social segregation. However, a lower level of intrasexual (female-female and male-male) activity synchrony within mixed-sex than within single-sex groups implies that additional factors (other than sex differences in foraging rhythm) contribute to the higher degree of instability of mixed-sex groups.
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