In the bird world, flashy colors and boisterous songs are generally reserved for males. Females tend to be comparatively dull-colored and often stay quietly hidden in the background. Charles Darwin himself noted this difference between the sexes and proposed that it was mainly caused by past evolutionary changes in males, in a process he called sexual selection—a theory still widely upheld today.
Yet, these understated female birds have a hidden past that overturns something researchers have assumed for over 150 years.
In a recent study, biology researchers J. Jordan Price of St. Mary’s College of Maryland and Muir Eaton of Drake University provide evidence that, despite current appearances, most past evolutionary changes in color have occurred in females, not males. Many of today’s drab females had brightly colored ancestors, and current differences between the sexes are largely due to females losing bright colors rather than males gaining them.
“I think Darwin would have been very surprised by this idea,” said Price. “Darwin was a strong advocate for the idea that sexual dimorphism is due to sexual selection. Many male traits undoubtedly are the products of sexual selection, as he suggested, but our evidence suggests that differences between the sexes are largely due to selection on females.”
Price and Eaton focused on a group known as the New World blackbirds, which includes a variety of songbird species ranging from Canada to Argentina. In New World blackbirds some females are dull in color compared to their male counterparts and some are just as brightly colored as males. Using relationships based on DNA sequence data, they were able to reconstruct the likely evolutionary histories of each sex, which revealed the surprising differences in the speed with which males and females have evolved.
“We mapped out differences using a phylogenetic tree to figure out what happened in the past and to look at color differences over time, and we found that although evolution occurs along the same period of time for both sexes, females have covered a lot more evolutionary ground,” said Price.
To paint the picture of the female birds’ rapid, teeter-totter evolution—away from and toward male colors—Price said to think of something many do every day. “It’s like when you are out walking your dog and you cover a distance and the dog covers three times the distance by running back and forth,” he explained.
“What’s often assumed is that sexual selection operates mainly on male appearance, and the result is that males then look different from females,” said Eaton. “Our results strongly suggest the opposite. Females, with their dull colors, are under strong natural selection to not stand out, thus they look very different from males. Perhaps this is because they spend more time on or near the nest and must be inconspicuous.”
Eaton also noted that the color differences in male and female birds go beyond what the human eye can see. “Our use of objective measurements of feather coloration, and quantification of color differences from the perspective of how birds see color differences, allowed us to uncover these complex evolutionary patterns that might otherwise go unnoticed,” he said.
The study, entitled “Reconstructing the Evolution of Sexual Dichromatism: Current Color Diversity Does Not Reflect Past Rates of Male and Female Change,” is available online today in the journal Evolution.