The link between sexual dimorphism, activity budgets, and group cohesion:: the case of the plains zebra (Equus burchelli)

Animals that differ in reproductive status and body size have different needs in terms of foraging and resting. In most social ungulates this leads to sexual segregation, probably because of incompatibilities between the activity budgets of males and females. Since most studies on behavioural differences between the sexes have been done on sexually dimorphic species, we decided to look at a system in which males and females are similar in body size. We studied time budgets, synchrony of behaviour, and bite rates of plains zebras (Equus burchelli) to evaluate the factors that enable these extremely social ungulates to stay in stable mixed-sex family groups throughout the year. As we predicted, time budgets were similar among males, lactating females, pregnant females, and non-reproductive females. Furthermore, we observed close synchronization of behaviours between females that differed in reproductive status and males. Lactating females, however, did take more bites per minute when foraging than either pregnant or nonreproducing females or males. We assume that the higher bite rates of lactating females were due to the extra costs of producing milk for their foal. We concluded that the special harem mating system, and for females the year-round possibility of conceiving, force the animals to synchronize their time budgets, which might be a major reason for the lack of difference in body size between males and females.