Evolutionary parasitology

Parasites and sexual selection

Parasites have long been thought to play a role in sexual selection, but exactly how important a role is a bit debated. Hamilton and Zuk really started the ball rolling on this idea by suggesting that all the fancy bright colours (usually) males use to attract females would get duller when they’re infected with parasites, so females choosing the brightest males would be choosing the least infected males and potentially passing on resistance genes to their offspring. They developed this idea to specifically tackle the issue of why variation in attractiveness exists – if bright red males get all the ladies, why aren’t all males bright red? HZ wrote that as there are coadaptational cycles between hosts and parasites (nice idea, still not much data on these), there will never be one single most resistant genotype, so there will never be a single genotype that’s consistently brighter red than all the others. It’s such a lovely idea, isn’t it?

Here’s the bad news. One issue with the HZ hypothesis is, as we know, parasites are aggregated among hosts, such that very few individuals in a population will be infected, and among those even fewer will have parasite loads high enough to affect their colouration… SO for females, a bright male *might* be resistant, or he might just not have been infected yet. I thought that one way to sidestep this issue with the HZ hypothesis is if the brightest males are the most resistant, even in the absence of infection!

So, I photo’d a lot of male guppies before infecting them and monitoring how they dealt with the infection… and the results suggest that actually some bits of male ornaments do signal parasite resistance! and others signal parasite susceptibility!

Fig 1. Male colour elements predict their response to parasites.

I’m still working on getting this published, but look out for:

J. F. Stephenson, M. Stevens, J. Troscianko, J. Jokela. The size, symmetry and colour saturation of male guppy ornaments forecast their response to parasites.

so what does this mean for natural populations?

Well… one of the best things about working on guppies is that we know a LOT about them in their natural habitat. People have been out there in the rivers of Trinidad for decades, measuring all sorts of things. Including female preference for orange on their males, and the proportion of fish that are infected at lots of different sites. We might think these two things should be correlated: if females at a site prefer orange males, the data above suggest that they prefer the males with the fewest parasites/no parasites, so transmission might be lower and we’d predict prevalence of the parasite to be lower at the site too. We went through the published data to test if this is the case.. and it looks like the pattern is consistent with this hypothesis (but obviously experiments will help disentangle all the many things that could contribute to this pattern):

Fig 2. Guppy populations in which females have been shown to significantly prefer males with larger areas of orange have lower prevalence of gyrodactylid parasites. We argue that this could be because oranger males are more resistant, so transmission at these sites should be lower, resulting in lower prevalence.

This analysis is ongoing (if you’ve got data you think might be useful, please please email me!), but hopefully one day will be published as Hansen & Stephenson.