Differential effects of experimental increases in sociality on ectoparasites of free-ranging raccoons.

1. Parasite transmission depends on the rate at which hosts come into contact with one another or the infectious stages of parasites. However, host contact rates and their influence on parasite transmission are difficult to quantify in natural settings and can fluctuate with host behaviour and the ecological constraints of parasites. 2. We investigated how experimental increases in rates of contact and social aggregation affected ectoparasite prevalence and intensity of free-ranging raccoons (Procyon lotor). Twelve independent raccoon populations were subjected to differential resource provisions for 2 years: a clumped food distribution to aggregate hosts (n = 5 aggregated populations), a dispersed food distribution to control for the effects of food without aggregating hosts (n = 3) and a no food treatment (n = 4). 3. Remote cameras indicated that aggregation sizes and rates of contact were three to four times greater in aggregated compared with that in non-aggregated populations. The number of ticks (adult Dermacentor variabilis) on raccoons in aggregated populations was 1.5-2.5 times greater from May to July, the primary time of tick seasonal occurrence. Conversely, louse (Trichodectes octomaculatus) populations were c. 40% sparser on male raccoons in aggregated (compared with that in non-aggregated) populations because of greater overdispersion of lice and a larger number of male hosts harbouring fewer parasites. No treatment-related differences were found among fleas (Orchopeas howardi). 4. These results were not consistent with our current understanding of parasite transmission; greater rates of host sociality led to increases in a parasite that does not rely on host contact for transmission (ticks) and declines in a parasite that depends on host contact for transmission (lice). We concluded that D. variabilis increased in aggregated sites because they can detect and seek out hosts and were more likely to drop off after obtaining a blood meal and re-attach to raccoons in these locations. Several factors may have contributed to sparser louse populations on male hosts, including a dilution effect that lowered per capita infestation levels. 5. These results indicate that ectoparasites can interact in unique ways with their hosts that are not consistent with other types of parasite species or models of their transmission.