The Behavioral Ecology and Host-Parasite Dynamics of the Zebras of East Africa

Abstract

Species- or population-typical social organization is thought to result principally from the interplay between predation risk, resource distribution, and pathogen transmission risk. This thesis explores the ecological basis of social behavior in two socially-divergent zebra species – the Grevy’s zebra and the plains zebra – with a focus on host-parasite dynamics in each. The first chapter details a study of group size variation in each species conducted via photographic censuses of zebra groups at sites with different lion densities from 2015-2018. We found that Grevy’s zebras increased group size in the wet season, consistent with food-limited grouping behavior, whereas plains zebras increased group size in the dry season, in line with group sizes limited by water or by pathogen transmission risk. The second chapter investigates differential exposure risk as the basis for higher parasitism by gastrointestinal nematodes found in plains zebras. As arid-adapted equids that need to drink infre-quently, Grevy’s zebras can range much farther from water than plains zebras and graze in areas with lower dung density, and therefore lower exposure risk, as a result. The nematode community, or nemabiome, of each zebra was analyzed in Chapter 3, where nematode species co-occurrence analyses revealed evidence for a preponderance of positive species associations, potentially indicating facilitative relationships and a need for host resistance to regulate populations. The fourth chapter explores the genetic vs. environmental determinants of host immune strategies and parasitism in a zebra hybridization zone. The results suggest that Grevy’s zebras not only experience lower exposure risk, but are genetically more resistant to gastrointestinal parasites than plains zebras. Taken to-gether, our findings indicate that gastrointestinal nematodes impose more pressure on plains zebras than on Grevy’s zebras, congruent with the grouping behavior studied in Chapter 1. Pathogen transmission risk is not as well understood in relation to social behavior as are the other ecological determinants of social systems, and this thesis attempts to bridge this gap in our understanding of social evolution through studies on the zebras of East Africa.