Walk into any clothing store and you’ll see that the women’s section is more colourful (and bigger!) than the men’s section. Not so in the bird world. In most bird species, males are more colourfully and elaborately dressed than females. This type of sexual dimorphism in which males and females look dramatically different is frequently driven by sexual selection. That is, female preferences for bright colours and ornate decorations have pushed each successive generation of males to evolve more and more flamboyant plumage. But why do female birds prefer brightly coloured males in the first place?

One idea is that ornamental traits like coat colour are indicators that can provide useful information about important survival traits. For example, the theory of parasite-mediated sexual selection proposes that the quality of a male’s ornamental display signals how well he can resist infection by parasites. To test this theory, many researchers are turning to the house finch, a common bird found across North America. Males have variable red-to-yellow colouration on their head, breast and rump whereas female finches are a rather boring greyish brown in colour. The bright red males enjoy more popularity among the females than their more drab yellow rivals. Many factors contribute to male colouration, including nutrition and parasite exposure during feather growth.

Speaking of parasites, let’s talk about the complex relationship between house finch feather colour and parasite infection. Several studies, including this one in 2004, showed that male house finches infected with the bacteria Mycoplasma gallisepticum develop more yellow and less bright feathers than uninfected males fed the same diet. Yellow males also seem to be at a disadvantage when it comes to surviving an infection. In the mid-to late-1990s, an M. gallisepticum epidemic hit the house finch population in the eastern United States. Surveys conducted before and after the epidemic found that red males survived better than yellow males, which drove the eastern house finch populations to become more homogenously red than populations in the rest of the country. In laboratory experiments, red males from unexposed populations resolved symptoms of M. gallisepticum infection faster than yellow males. So what exactly is going on? How does feather colour affect parasite infection and vice versa?

In an effort to answer these questions, a team of scientists led by Dr. Geoffrey Hill at Auburn University looked for genes that were expressed differently between infected red and yellow male house finches and uninfected finches. The scientists captured red and yellow males form the southwestern U.S. where the population had not been exposed to M. gallisepticum before. They predicted that red males would respond better to M. gallisepticum infection than yellow males and that this difference was due to changes in gene expression.

The researchers identified seven genes that were expressed differently between infected red and yellow male house finches. These genes are involved in immunity, stress response, metabolism and other cellular functions. They are also excellent candidates that will help inform future studies aiming to understand the molecular mechanism underlying the link between feather colour and disease response.

One of these is a gene called hsp90 whose expression is normally turned on in response to stress. In red males, hsp90 expression decreased in response to infection whereas expression of hsp90 increased in infected yellow males. What’s more, hsp90 expression was the same in both yellow infected birds and red uninfected birds. One possible explanation for these perplexing findings is that red male finches have a higher baseline level of stress than yellow male finches. This elevated level basal stress could translate into increased stress resistance and an improved ability to survive an infection.

By choosing redheads over blonds, female house finches are selecting mates that can better resist and respond to a parasite infection and making sure that those beneficial disease-fighting traits are passed on to her offspring. Whoever said redheads are unlucky clearly didn’t know about male house finches!

Reference:
Balenger, S., Bonneaud, C., Sefick, S., Edwards, S., & Hill, G. (2015). Plumage color and pathogen-induced gene expression in a wild bird Behavioral Ecology DOI: 10.1093/beheco/arv055