Life on a Hostile Landscape: Success, Distributions, and Consequences of Ectoparasites on Their Hosts

In this dissertation, I explore the behavior and ecology of ectoparasites, particularly ticks, from the perspective of the parasite. Ectoparasites forage in environments with unique constraints. The decisions ectoparasites make to optimize their fitness determine their distribution and abundance on their hosts, and those decisions are greatly influenced by the threat of grooming from their host. To understand the impact of that threat, I designed two experiments to infest white-footed mice (Peromyscus leucopus) with blacklegged ticks (Ixodes scapularis). I manipulated grooming in two ways: first, to determine the impact of the initial period immediately after a larval tick contacts its host, and second, to determine the overall effect of grooming while a tick navigates, feeds, and ultimately escapes its host. Grooming-free time during the vulnerable period following a tick’s arrival directly and indirectly increased the proportion of ticks that engorged. Reduced grooming effort during the entire tick feeding cycle also had positive effects on the proportion of ticks that fed and escaped. I then develop a theoretical framework and agent-based model for how ticks might use information about their surroundings to make decisions. By modeling the attachment and feeding process under the threat of grooming, I demonstrate that most ticks actually feed in risky but highly rewarding areas. The ticks typically seen on hosts are those that are simply there longer and may not reflect the locations where the majority feed. This insight could inform our understanding of which hosts actually support and feed the majority of ticks in systems with vector-borne diseases. Finally, I address the intriguing paradox of parasites on large carnivore predators. Predators should be close to peak performance in order to subdue prey, particularly since encounters with prey have potential for injury. Parasites are a detrimental burden on their hosts, decreasing predator performance and increasing the predator’s risk of starvation. Yet predators often carry high parasite burdens, which should be detrimental to both predator and parasite. My model of this system indicates that this paradox may be resolved through anti-predator behaviors in the prey, and provides evidence for an optimal level of parasites for predators.