Survival of the Sexiest – is Climate Change Altering Sexual Selection in Birds?

By Calvin J.

Is climate change altering sexual selection in birds? A new paper published by Evans and Gustaffson in the journal Nature – Ecology and Evolution seems to suggest so. The researchers at the Upssala University in Sweden, found that with increasing annual spring temperatures, a large white forehead patch, which used to be a heavily desired sex characteristic for the male collared flycatchers (Ficedula albicollis), seems to have become a major disadvantage.

Secondary-sex characteristics are physical characteristics and traits that can distinguish the sexes of each organism. But while they have no direct function in sexual reproduction, they are useful in conveying the organism’s fitness. Males and females with certain secondary-sex characteristics are more likely to successfully mate and reproduce. And in the collared flycatchers, females have been shown to prefer males with larger white forehead patches (shown in the photo above by Johan Träff), as they signify increased genetic fitness and likelihood of establishing breeding territories. Or at least that used to be the case. The researchers found that while white forehead patches were gradually increasing during the first 8 years of the study due to sexual selection, they discovered a clear significant decrease in the more recent decade. Why might this be the case? Well, the researchers found that while the large white forehead patches were advantageous in attracting mates during breeding seasons following cold, harsh winters; as annual spring temperatures have been steadily rising in the past several decades (with breeding sites rising ~1.5°C between 1980 and 2012), this characteristic has become slowly selected against. Male collared flycatchers with larger forehead patches now became less likely to survive the following winter season.

But why might forehead patches be affecting the bird’s survival? Well, a lot more research still needs to be done. But the researchers believe that large white forehead patches may incur some kind of biological cost to the birds themselves. Meaning there is likely a tradeoff between the energy being utilized for growing or maintaining a white patch versus investing that energy into survival. And while that tradeoff may have been worth it to help attract mates in the past, increasing breeding ground temperatures are tilting the scale the other way, making it much more of a costly investment. It’s clear that a lot more work still needs to be done to clarify this relationship between breeding ground temperature and forehead patch size, but the researcher’s observation illustrates a clear dynamic nature in sexual selection and the growing importance of ecological research. With climate change leading to dramatic and varied effects across the globe, how might this be affecting  reproductive success in other organisms? And what are the even broader impacts of climate change on their evolutionary dynamics?

References:

Gustaffson, L. and Evans, S.R. (2017). Climate change upends selection on ornamentation in a wild bird. Nature. http://www.nature.com/articles/s41559-016-0039

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