Striking variation in sexual traits between closely related populations is a widespread pattern across a diversity of taxa, and is thought to play an important role in the process of speciation. Despite this, we currently do not have a good understanding of the evolutionary mechanisms that generate this common pattern. It is increasingly recognized that variation in the ecological context can play an important role trait divergence and speciation between closely related groups. However, it is still unclear if variation in ecology is also driving sexual trait divergence. One way to test this hypothesis is to determine if the information females gain by evaluating sexual traits during mate choice is tied to the ecological context, and if this information changes across populations with divergent sexual traits.
One of the best-studied interactions between sexual selection and ecology is the relationship between parasites and the sexual traits of their hosts. Within populations, sexual trait expression is often correlated with parasite abundance, yet it remains unclear if adaptation to variable local parasite communities could be driving sexual trait divergence. To test this hypothesis, I conducted comparative field research on three barn swallow subspecies that differ in the relative elaboration of two sexual traits, tail streamer length and plumage color. Females prefer the most elaborated traits within each subspecies. For each subspecies, I first characterized the local parasite community infecting adults and nestlings. I assessed the costs of each parasite to male condition, survival, and reproductive success. Finally, I tested for associations between sexual trait expression and parasites to ask if elaborate sexual signals are honest indicators of locally costly parasites.
Results/Conclusions
I found that the composition of parasite communities varied significantly between subspecies. The same parasites also had different effects on fitness in different subspecies. Within each population, the sexual traits used by females always provided information about the costliest parasites from the local community, particularly parasites that reduced reproductive success. Thus, divergent sexual signals in each subspecies advertise adaptive and locally relevant information about parasites. Across populations, the same sexual traits were associated with different parasites, suggesting that parasite-trait links are flexible and can evolve relatively quickly between closely related subspecies. This research provides evidence that sexual traits are sensitive to the local ecological context, and that parasites may play an important role in shaping divergence in sexual signals among closely related populations.