COMPARATIVE MORTALITY-RATES OF MALE AND FEMALE KUDUS - THE COSTS OF SEXUAL SIZE DIMORPHISM

1. The aim of this study was to document the difference in age-specific mortality rates of male and female kudus, and to relate this difference to possible causes, in particular the influence of sexual size dimorphism. 2. The study was conducted over 10 years in the Kruger National Park, South Africa. Female survival was estimated by following cohorts of individually recognizable animals in closed social units. Male survival was estimated from year-class ratios, adjusted for population change. 3. Male mortality rate remained similar to that of females up to 3 years of age. Thereafter, male mortality accelerated sharply with age. A male reaching full weight at 6 years of age had only a 0.5 chance of surviving a further year. Female mortality started rising only after 6 years of age. Males lived up to 9 years, but females up to about 15 years. 4. The excess male mortality could not be explained by energetic expenditures in mate competition, increased hazards resulting from male dispersal, or the smaller group sizes typical of males. 5. Male kudus appeared more susceptible to malnutrition than females, possibly because of increased food demands resulting in accelerated tooth wear. However, male kudus showed higher mortality than females during periods of high rainfall when food shortages were less likely to be influential. 6. Male kudus are potentially less agile, and hence more susceptible to being caught by predators, than females. Loss of condition due to malnutrition may increase vulnerability to predation. Lions preferentially killed male kudus, especially large males. But the population sex ratio of kudus seemed to be equally female-biased in areas where lions were less abundant or absent. 7. Hence, the excess mortality incurred by male kudus could not be related primarily to any single cause. Males grow to a larger than optimal size for survival, relative to females, because of reproductive benefits, and thereby incur multiple costs. Nevertheless, predation is clearly an important factor amplifying the mortality predisposition due to large size. 8. Among African bovids there is no clear relation between sexual size dimorphism and the population sex ratio. Where a size difference between the sexes is less of an influence, other mortality factors arising from male competition come into play. In environments lacking large predators, mortality rates of both sexes may be atypically low, with implications for life-history theory.