Underlying mechanisms of sexual segregation amongst ungulates, and Trivers and Willard’s hypothesis that mothers can influence primary sex ratios, continue to be topical theoretical issues. Over two years, using monthly repeated road transects, we determined the habitat and social segregation of male versus female impala (Aepyceros melampus) and kudu (Tragelaphus strepsiceros) in a predator-free, versus a predator-laden, South African reserve. We also determined, by the same technique but over four years, the primary sex ratio of the impala population free from predation. Significant overlap in habitat usage (Schoener’s Index 0.63 to 0.8) was found between the two sexes when free from predation, but not (Schoener’s Index 0.46 to 0.47) when under predation. Whilst occupying the same habitats impala, kudu and wildebeest (Connochaetes taurinus) male and female groups maintained rigid social segregation throughout the year, even when at close quarters. Impala primary sex ratios were significantly biased towards females (male/female = 0.72; χ² = 4.3175, df = 1, p-value = 0.038) in the absence of predation. Our findings suggest that whilst risk of predation is a proximal cause of sexual segregation, thus lending support to the predator-risk hypothesis, the underlying, functional mechanism of sexual segregation is difference in the activity budgets of males versus females (the activity-budget hypothesis).Our findings also suggest that mothers may indeed be able to adjust primary sex ratios, with the postulated driver in this case being an abnormally high density of adult males.
