One has all or all have one: Sex specific differences in aggregation within a guppy host parasitic worm system

David Clark Jr., Biology, University of Pittsburgh, Pittsburgh, PA and Jessica F. Stephenson, Biological Sciences, University of Pittsburgh, Pittsburgh, PA

Background/Question/Methods
The aggregation of parasites among hosts, such that most hosts are uninfected or have very few parasites and a minority are very heavily infected, is a near-ubiquitous pattern in wildlife disease systems. Aggregation can be caused by differences between hosts in characteristics like behavior, immune function, and size. Often, these factors can differ greatly by sex in natural systems. In trinidadian guppy (Poecilia reticulata) systems, females tend to be larger, more tolerant to parasites, more social, and live longer than males. Because of these sex differences in traits linked both to defence against parasites and parasite transmission, we hypothesize that parasites will be more aggregated among females than among males in guppy populations. We tested this hypothesis with data from 84 Trinidadian guppy populations encompassing 4,715 guppy individuals and 6,688 Gyrodactylus spp. parasites. We used constraint-based models to measure the difference between the observed and null distributions of parasites among their hosts to give us a measure of aggregation. We used this metric as the response variable linear mixed models to 1) test for differences in aggregation between the host sexes, and 2) determine which host variables were significant predictors of aggregation between populations.
Results/Conclusions
We found that Gyrodactylus spp. parasites were significantly more aggregated among female guppies than among males, and that the parasites were less aggregated in populations of larger fish. The sex-difference in aggregation is most likely driven by sex specific differences in how hosts interact with conspecifics in the population and with their parasites. Additionally, host size may be linked to tolerance in this system: our results suggest that populations of larger, more tolerant hosts have less aggregated parasites because there are a relatively large number of heavily infected hosts. Future work could usefully determine which sex specific characteristics can lead to variation in how host sexes interact with parasites, and thus how parasites are distributed among their hosts. Describing sex- and population-level variation in parasite distribution can therefore highlight potential drivers of meaningful variation in parasite load between individual wild hosts.