A new study has shown that a simple artificial enhancement in appearance boosted the amount of testosterone in male North American barn swallows, indicating the clothes—or in this case, the feathers—make the man. The study is the first to show that how a bird looks can significantly affect its physiology and may help scientists better understand the role of physical features in ecology and evolution.
"A simple change in appearance had striking physiological consequences for these barn swallow males, which was a big surprise," said University of Colorado-Boulder ecologist Rebecca Safran, who led the study. "The experimental manipulation didn't just improve their looks in the eyes of the females, it actually changed their body chemistry. The relationship between a male's physiology and the traits that win him mates is a lot more flexible than we had imagined."
The swallows, whose breast feathers were darkened to a deep red known to be most attractive to females, likely had more testosterone racing through their bodies because of amorous interactions with the opposite sex and more run-ins with jealous males, said Safran. The jump in testosterone was unexpected because it was observed at the time in the breeding cycle when levels of sex steroids like testosterone are typically declining, she said.
Safran likened the biochemical feedback mechanism to a man who has just walked out of a clothing store in a new suit. "When he says he feels like a million bucks, there probably is some biochemical feedback going on," she said. "The result is something we all might experience when given the chance to upgrade our status, like winning a prestigious award or being invited to an exclusive event."
"The traditional view is that internal processes of birds determine their external features—in other words, physiology forms the feathers," said Kevin McGraw of Arizona State University's School of Life Sciences who also worked on the study. "But our results indicate that a perceived change in the color of an animal can affect directly its internal physiological state. A barn swallow's hormonal profile is influenced by its outward appearance."
Male North American barn swallows use their breast colors to convey status, health and the ability to successfully raise young, said Safran. One of her earlier studies showed male barn swallows that were "made over" with darker breast colors bred earlier in the season and fathered more young, and the females that chose them cheated less often with other male suitors. "The results provide strong evidence that color is an important indicator of male quality," she said.
In the new study, the researchers captured 63 male barn swallows from six colonies in New Jersey at the start of the breeding season as the birds arrived at the breeding site and started forming pairs. The breasts of roughly half the birds were colored with a nontoxic marker to match the darkest, most-attractive feathers of males within the population.
The marked birds were released back into the wild, re-captured a week later and administered blood tests to measure androgen levels, including testosterone. In addition to showing increased levels of androgens, the marked birds also lost weight, perhaps because they were more active than their "duller" neighbors, or simply couldn't measure up to the expectations of other barn swallows because of their "counterfeit" sexual signals, she said.
Sexual signals by males in the animal world, from the stately antlers of elk to the gaudy tail feathers of peacocks, have evolved to convey "honest, accurate information," she said. Evolutionary biologists believe the top males in a population can afford the physiological costs of expressing the most exaggerated forms of sexual signals, like a conspicuous dark feather color that is either biochemically costly to produce or makes those individuals more susceptible to predators, she said.
"A male barn swallow can't look in a mirror and assess his social status," said Safran. "But if he flies into a group of other swallows, the birds will quickly assess it for him and give him a sense of where he fits in. Since status is relative to the company one keeps, a male could be a "stud in one group and a regular guy in another," she said.