While scientists have long believed that male birds have brighter colored plumage in order to attract mates, new research from the University of Wisconsin-Milwaukee throws cold water on this theory by indicating that males and females aren’t so different after all.
A team of researchers led by UW-Milwaukee biological sciences professors Peter Dunn and Linda Whittingham examined nearly 1,000 different species of birds and found that while males still do typically have brighter feathers than females, the gap between the two sexes is closing as both evolve over time to blend into their surroundings and hide from predators.
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In a recent edition of the journal Science Advances, the authors wrote, “the bright colors of birds are often attributed to sexual selection on males, but in many species both sexes are colorful, and it has been long debated whether sexual selection can also explain this variation.”
“We show that most evolutionary transitions in color have been toward similar plumage in both sexes, and the color of both sexes (for example, bright or dull) was associated with indices of natural selection (for example, habitat type), whereas sexual differences in color were primarily associated with indices of sexual selection on males,” they added.
In short, they concluded that both natural selection and sexual selection were influential to bird coloration, but that each of those factors have been involved in different ways. Sexual selection led to reproductive differences, while natural selection caused color changes in both sexes, and there have been as many similarities as differences in male and female birds.
“Although most studies of bird plumage focus on dichromatism, evolutionary change has most often led to similar, rather than different, plumage in males and females,” Dunn, Whittingham and former UW-Milwaukee grad student Jessica Armenta wrote. “Our study shows that ecology and behavior are driving the color of both sexes, and it is not due to sexual selection.”
Birds of a feather
Armenta, who is now a teacher at Austin Community College in Texas, gathered data from 977 species of birds from museums in Australia and the US over a four-year period. She looked at six birds (three males and three females) from each species, and the data she gathered was examined by Dunn and Whittingham, who assigned each a color score based on brightness and hue.
They examined plumage color in relation to 10 measures of natural and sexual selection, finding that when the sexes became closer in color, they did so for reasons of natural selection, and when the color gap increased, it was due to reasons related to sexual selection. The study is said to be the first to examine the color of both sexes in terms of both natural and sexual selection in birds.
“A lot of research has focused on how plumage color is related to mating success, especially in males,” Dunn explained, “so this should hopefully get researchers to think more about how color affects survival, especially predation and foraging success, in both sexes.”