Influences of predation on African antelope

i) Sex ratios and resource-associated mortality

Smaller, enclosed reserves lacking large mammalian predators are an increasingly popular commercial model in southern Africa and elsewhere. The presence or absence of predation is likely to have major effects on the population dynamics of sexually dimorphic ungulates, with contradictory implications for multiple-use reserves, and to provide fundamental insights into predator–prey relationships. Over a two- and four-year period I determined the adult sex ratios and juvenile mortality of two substantial populations of impala Aepyceros melampus in South Africa – one in predator-free Ithala Game Reserve (IGR), the other in neighbouring predator-laden Hluhluwe-iMfolozi Park (HiP). Data were collected monthly, over a five-day period, by repeated road transects covering a representative sample of the reserves’ habitat types. I found that the impala adult male to adult female ratio was significantly lower in the presence of predation (HiP = 0.43,IGR = 0.69), whilst a generalized linear model revealed that the overall proportion of juveniles in breeding herds declined, over the breeding year, at a faster rate in the presence of predators. Impala juvenile mortality over the breeding year was not significantly affected by lower rainfall in the absence of predators, but under predation juvenile mortality declined at a faster rate over a drier year compared to years of near average rainfall – a novel finding amongst African antelope. Such fundamental insights into predator–prey relationships are especially relevant to predator-free reserves, where management and planners should be aware of these influences and, depending on the business model, consider replicating them artificially.

Project Publications

O’KANE, C.A.J., & MACDONALD, D.W. 2015. An experimental demonstration that predation influences antelope sex ratios and resource-associated mortality. Basic and Applied Ecology 17: 370-376.

ii) Sexual segregation and primary sex ratios

Underlying mechanisms of sexual segregation among ungulates, and Trivers and Willard’s hypothesis that mothers can influence primary sex ratios, continue to be topical theoretical issues. Over 2 years, using monthly repeated road transects, I determined the habitat and social segregation of male vs. female impala (Aepyceros melampus) and kudu (Tragelaphus strepsiceros) in a predator-free, vs. a predator-laden, South African reserve. I also determined, by the same technique but over 4 years, the primary sex ratio of the impala population free from predation. Significant overlap in habitat usage (Schoener’s Index 0.63–0.8) was found between the two sexes when free from predation, but not (Schoener’s Index 0.46–0.47) when under predation. While 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 = 4.32, d.f. = 1, P-value = 0.038) in the absence of predation. My findings suggest that while 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 the difference in the activity budgets of males vs. females (the activity-budget hypothesis). These 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.

Project Publications

O’KANE, C.A.J., & MACDONALD, D.W. 2017. An experimental demonstration of the impact of predation on sexual segregation and primary sex ratios amongst ungulates. Austral Ecology 42: 414-421.