Male birds possess some of the most striking and beautiful coloration in the animal world. Yet females from the same species can often be drab in comparison. There have been many hypotheses as to the evolutionary mechanisms that are linked to these differences. How well females can distinguish and perceive color has been one such hypothesis, but this has been difficult to study in the laboratory. After all, it’s tricky for scientists to ask a bird what it thinks of a mate’s color, much less prove it.
So, instead of looking at behavior, Natasha Bloch, PhD, a recent graduate student from the laboratory of Trevor Price, PhD, professor of ecology and evolution, looked at genes. She focused on opsins – specialized proteins in the retina that are responsible for detecting light. In a study published Nov. 26 in the Proceedings of the Royal Society B, Bloch discovered that the expression of a specific opsin gene in female birds is linked to the evolution of colorful plumage in males. Her findings confirm the essential role of female color perception in mate selection and sexual dimorphism.
“This is the first time an aspect of the visual system in birds has been directly associated with plumage evolution,” said Bloch, who is currently a post-doctoral fellow at University College London. “It tells us color perception plays an important role in the evolution of the spectacular diversity of colors we see in nature.”
In birds, four types of opsin contribute to color vision, each with different sensitivity ranges for certain wavelengths of light. Bloch measured gene expression levels of these opsins in males and females from 16 species of New World warblers, a family of songbirds common across the Americas (collected through Chicago Bird Collision Monitors). She found that opsin expression varied greatly between species. As this is a measure of opsin abundance and density, her results suggest the species varied in their sensitivity to and ability to perceive color.
A discriminating eye
When gene expression levels were measured against differences in plumage coloration between males and females, one trend stood out. In warbler species where females had high expression of the opsin Sws2, males were much more colorful. Species where Sws2 expression was low showed the reverse trend, with smaller coloration differences between sexes.
“The strong relationship seen between Sws2 expression and plumage coloration suggests the expression of this opsin changes female perception and thus female preference for color, which in turn drives the evolution of male plumage,” Bloch said.
Sws2 is sensitive to wavelengths around the blue region of light, which sits in the middle of the visible spectrum. Color discrimination, the ability to tell different colors apart, is known to require multiple opsins to work in tandem.
“Because sexual selection puts a premium on choosing the sexiest and best quality mate, which requires good color discrimination, it makes sense that it is an opsin in the middle of the spectrum that evolves in response to these pressures,” Bloch said.
Bloch also examined the role of the other three opsins. The expression of two of the other opsins varied with the light conditions of the habitats the birds occupied, suggesting they evolve in response to environmental pressures. The remaining opsin showed a weak correlation to sexual selection, but was not significant.
The relationship between vision and sexual selection is complex, Bloch cautions, but these results lay an important foundation for future studies. “Opsin expression is only one aspect of color vision. We still need to learn a lot about how the visual system varies across different species to understand how the beautiful colors of animals have evolved and why different species have evolved different colors,” she said.
The study “Evolution of opsin expression in birds driven by sexual selection and habitat,” was supported by the National Institutes of Health and National Science Foundation.