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Abstract
The role of infectious diseases in the population ecology and conservation of wildlife has moved into the scientific limelight in recent years, and is likely to remain there as biologic homogenization continues to expose wildlife populations to a plethora of introduced pathogens.
I studied the effects of a chronic bacterial disease, bovine tuberculosis, on African buffalo at Hluhluwe-Umfolozi Park, South Africa. I used longterm population records and demographic data from over 800 buffalo to characterize buffalo population dynamics. I collected information on infection status, age, sex, and fecundity of nearly 1200 buffalo to estimate the effects of TB on vital rates. I then examined possible effects of TB on buffalo population size, given climatic variability, and different scenarios for interactions between disease and density dependence. Finally, I studied stress and behavior in the buffalo to understand the sex difference in TB prevalence I observed in adult buffalo. Throughout the study, I used radio telemetry to track 30 buffalo and their social groups, to gain an understanding of the animals' behavior and ecology.
In the absence of TB, the buffalo population attains a maximum annual growth rate of 10–12%, but in recent years growth has averaged 4% due to density dependent effects. TB reduces adult survival from approximately 97% to 86%, and fecundity from 62% to 45% in most age classes. Modeling results suggest that TB can reduce population growth rate and therefore, resilience to disturbance. The response of population size to TB depends on interactions between density dependence, weather variability and disease.
TB prevalence in adult buffalo bulls was higher than in adult females, while there was no sex difference in prevalence for younger buffalo. Adult herd bulls had higher levels of cortisol, the primary hormone mediating stress physiology, than did females, or males in bachelor groups. High cortisol levels in the bulls appear to be driven by seasonally available mating opportunities, and correlate with an increase in mating and competition—related behaviors. Chronically elevated cortisol levels can have immune suppressive effects, and may in part be responsible for the difference in TB prevalence between male and female adult buffalo.
