Based on previous work, we know Gyros can impose selection by affecting survival, growth rate, and reproductive success. We also know Gyros can have an effect on behaviour, body size, and colouration, so it is not unreasonable for one to think parasitism can impose selection on phenotypic traits. However, relatively few of these studies are field studies, so we seek to fill that lack of information by building on previous field-surveys.
So what did we find? Well, first, we found that when we sample around the same time in two subsequent years, the parasitism levels are pretty consistent, that parasitism levels among populations varied quite a bit, and that parasitism levels were generally higher in HP sites than in LP sites. This is consistent with previous surveys, and we speculate this could be because (1) flooding might sweep infected fish downstream from LP to HP populations, (2) HP guppies like to shoal more than LP guppies, possibly increasing transmission, (3) susceptibility might be higher in HP populations, and (4) HP sites might differ ecologically in ways conducive to parasite infection.
So we didn't find a significant effect of parasitism on guppy colour. But what if we throw predation back into the story? Does parasitism modify our conclusions about the effect of predation on colour? When considering predation, our results were congruent with what we expected a priori: males in LP sites had more colour than HP sites in general. But we had a lot of variation. It wasn't a clear cut story. Some HP sites had more colour than some LP sites. Previous work has also found similar differences and nuances, and from this, we can conclude that predation IS an important selective agent on guppies, but it's a lot more complicated than just predation alone.
We now ask "does the predation regime story benefit from a simultaneous consideration of parasitism?" Well, we didn't find an effect of parasitism on colour, and adding a parasitism term to our models almost never affected our conclusion about the effect of predation. We still must consider the potential drawbacks of field surveys, but based on our data, we find that parasitism does not modify our interpretation of the effects of predation.However, we are not saying parasitism has no effect, but rather that the signature of an effect of parasitism on male guppy colour might be swamped by other selective agents (e.g. predation) leaving us unable to detect a significant effect of parasitism on colour.
So, in a nutshell, we found that parasites vary between populations and were relatively consistent when assessed over two years, higher infection levels are often found at HP sites as compared to LP sites, parasitism doesn't appear to have an effect on male guppy colour, and considering parasites does not alter our current conclusions about predation.
That's all she wrote!