I had three overall objectives in this research. (1) I measured the relative importance of host characteristics (age, sex, weight) and abiotic variables (site, season, year) on ectoparasite prevalence (% hosts infested) and abundance and endoparasite species richness across 12 unmanipulated sites in mid-Missouri. In each case, I developed and tested a priori hypotheses using an information-theoretic framework. Ectoparasites had distinct patterns that were related to their host specificity, mobility, and ability to survive off the host. Tick (adult Dermacentor variabilis) abundance was dependent on the month of collection, as they are only active in summer and peak in July. However, the abundance of engorged ticks, which need to be present on hosts for 7-10 days prior to obtaining a full blood meal, was primarily related to host age and sex, with older raccoons and males infested by more ticks (Chapter 1). Lice (Trichodectes octomaculatus ) are highly dependent on raccoons and their prevalence and abundance were best predicted by host age and sex. In particular, male raccoons were infested by 2-3x the number of lice compared to females. Fleas (Orchopeas howardi), which can use multiple hosts and survive off hosts for weeks at a time, displayed only a weak seasonal pattern of infestation (Chapter 2). Among endoparasites, infection patterns diverged according to their life history; directly transmitted parasites declined over the life of the host while indirectly transmitted parasites increased (Chapter 4). Collectively, these results highlight the need to consider parasite characteristics and simultaneously assess the relative importance of multiple ecological variables between parasite species when describing general trends and constraints of host-parasite associations.
(2) I investigated how experimental increases in social aggregation and resource availability affected ectoparasite prevalence and intensity (number of parasites on infested hosts only) and endoparasite species richness of raccoons. Twelve independent raccoon populations were randomly subjected to differential resource provisions for two years; a clumped food distribution to aggregate hosts (n = 5 populations), a dispersed food distribution to control for the effects of food without aggregating hosts (n = 3), and a no food treatment (n = 4). The intensity of ticks was greater in aggregated populations, particularly among male raccoons. Conversely, the intensity of lice on male raccoons declined in aggregated populations due to greater overdispersion of lice and a larger number of male hosts harboring fewer parasites. The intensity of fleas did not differ among treatments and displayed no correlation with host characteristics (Chapter 3). Among endoparasites, there was strong evidence that food additions decreased the number of indirectly transmitted parasites, particularly among the oldest age classes at sites with clumped food. Conversely, food and social aggregation had little to no impact on the species richness of directly transmitted parasites (Chapter 4). These results suggest that the effects of increased resources and social aggregation of hosts are parasite-specific and can be dependent on parasite mobility and route of transmission, as well as sex-related differences in host behavior or physiology.
(3) I determined sampling constraints of measuring stress hormones (fecal glucocorticoid metabolites, FGM) of raccoons and conducted a parasite-reduction experiment to determine if nematodes and ectoparasites affect baseline levels of FGM in adult free-ranging raccoons. Parasite reduction treatments reduced the prevalence and abundance of the most widespread ectoparasites, the prevalence of most nematodes, and the number of parasite species per individual. No differences in FGM values were observed within individuals or between treatment and control groups following parasite reduction treatments, indicating that the reductions in nematodes and ectoparasites had no effect on stress hormone levels of raccoons during summer (Chapter 5). Because this study coincided with the most common and energetically expensive ectoparasite in the region (Dermacentor variabilis), I conclude that ectoparasites do not affect glucocorticoid levels of raccoons. However, given that helminth parasites are one of the most likely groups to influence the endocrine system, further experimental work should focus on methods that can more effectively reduce all endoparasite species and measure their influence on stress hormone levels across seasons.