Host and parasite interactions across biological scales

Parasites are ubiquitous across natural systems; however, they do not interact with their hosts equally. Parasites vary in their impact on hosts across different biological scales. We see variation in how host interact with their parasites (hereafter, ‘parasite variation’): parasite costs to individual hosts; transmission among individuals in a population; and distribution among hosts across populations. Individual-level differences in host traits can be a major driver in this parasite variation, but there is limited empirical evidence of how individual-level traits explain the variation observed across these biological scales. I used a Trinidadian guppy (Poecilia reticulata) and a monogenean worm (Gyrodactylus spp.) system to test how individual-level host traits impact parasite variation between hosts, within populations, and between population scales. In chapter 1 I briefly review what we know about host individual level traits and the interactions of hosts and parasites across scales. In chapter 2, I test the impact of individual-level host activity on parasite intensity. I found that hosts that invest in activity before infection did not have more intensive infections but those who increased activity during infection suffered higher parasite intensities. In chapter 3, I examined the impact of host sex and sociality on the spread of parasites at the beginning of epidemics. I found that females were much more likely to become infected than males at the beginning of an epidemic, even though males increased contact rate with more heavily infected fish. We did not find a significant impact of sociality on the likelihood of infection. In chapter 4, I tested how individual-level traits impact the distribution of parasites among their hosts at the between population level. I found that parasites are more aggregated on their hosts than expected, and this aggregation differed between the sexes and with body size of the host. I provide novel empirical evidence for how individual-level host trait variation impacts parasite intensity, spread of parasites, and the distribution of parasites between populations. These results highlight the need integrate individual-level host traits in both theoretical and empirical work that examine how hosts interact with their parasites.