Jun 21, 2018 | Updated: 09:54 AM EDT

The Curious Case of Birds—How Evolution Has Led to Less Differences Than Charles Darwin Thought

Mar 28, 2015 02:09 PM EDT

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It's a well-known fact that in nature it's often the boys that have the better looks. Without the task of investing their energy and resources into the next generation of children males are able to reallocate their resources into preening and looking pretty. But it's a curious case that is far from what the original evolutionary biologists once thought.

When it comes to bird species, the father of evolution Charles Darwin once documented that male finches were the brighter of the bunch, displaying much more vibrancy in their colors and much more ornate plumage than their female counterparts. But it's an evolutionary tactic that has left many researchers since then questioning the efficacy of being bright and beautiful, when most of survival relies on blending in versus standing out in the crowd.

Looking at 977 different species of birds researchers with the University of Wisconsin, Milwaukee have discovered, however, that not everything is as it seems when it comes to the plumage and sexual selection in evolution. Publishing their results this week in the journal Science Advances the team of researchers led by Peter Dunn and Linda Whittingham sought the answer to an alternative hypothesis that posited evolution actually favored similarities as much as it did the vibrant differences we see in nature. 

"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" the authors say. "Our study shows that ecology and behavior are driving the color of both sexes, and it is not due to sexual selection."

While males maintained their brighter feathers as signs of health, fertility and the ability to find a desirable mate, over time the sexes came far closer together in terms of appearances than researchers ever noticed. Analyzing data of six birds of each species, gathered from six museums across Australia and the US, the researchers were able to base their case examining plumage color in relation to 10 measures of natural and sexual selection. And what they found was that when it was a case of survival, the sexes converged, and when it was a case of procreation the sexes diverged far more than in times of stability. 

The unique study revealed that when the sexes became more similar in color it could be attributed to natural selection, whereas when the color gap increased it was attributed to do with sexual selection instead. And it's a revelation that Dunn hopes will set a precedence in sending future studies in new directions, breaking free from tradition.

"Researchers have called for separate analyses of each sex for over a decade, but this is the first large-scale study to examine the color of each sex in relation to indices of both natural and sexual selection" Dunn says. "A lot of research has focused on how plumage color is related to mating success, especially in males, so this should hopefully get researchers to think more about how color affects survival, especially predation and foraging success, in both sexes."

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