Washington, June 3 : A new study has revealed that the colour of barn swallows have a profound effect on their physiology.
During the study, the team, including one from Arizona State University found that artificially colouring the breast feathers of male barn swallows raised the testosterone levels of the birds in a short period of time.
The jump in testosterone, recorded after one week, was unexpected because it was observed at the time in the breeding cycle when levels of sex steroids like testosterone are typically declining.
"The traditional view is that internal processes of birds determine their external features -- in other words, physiology forms the feathers," said Kevin McGraw, an assistant professor at ASU's School of Life Sciences.
"But our results indicate that a perceived change in the colour of an animal can directly affect its internal physiological state. A barn swallow's hormonal profile is influenced by its outward appearance," he added.
"The experimental manipulation didn't just improve the males' looks in the eyes of the females, it actually changed their body chemistry," said lead author Rebecca Safran of the University of Colorado.
The new study has evolutionary implications for North American barn swallows since their breast colors are used to convey status, health and the ability to successfully raise young.
For the study, the researchers captured 63 male barn swallows from six colonies in New Jersey at the start of the breeding season and the breasts of nearly half the birds were coloured with a non-toxic 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.
The findings showed increased levels of androgens, the marked birds also lost weight, perhaps because they were more active than their "duller" neighbours, or simply couldn't measure up to the expectations of other barn swallows because of their "counterfeit" sexual signals, according to the researchers.
"The speed with which the internal qualities of the bird were affected by the plumage colour manipulation was surprising to me," added McGraw. This suggests a dynamic system," said McGraw
"Increased testosterone levels are often associated with increased rates of activity and competition in animals, which might be why darkened birds lost body mass.
"These studies cumulatively show that colours are revealing of the bird's individual qualities.
"We previously found that the darker guys were more fit, by fathering more offspring, but still didn't know what made a good, dark male. Here it suggests that testosterone and its associated behaviours are closely linked to a male's color and fitness," he added.
The findings are reported in the June 3 issue of Current Biology in the article "Sexual signal exaggeration affects the physiological state in a social vertebrate."