"It's another clear demonstration that altering social interaction for one individual can have quite profound effects on not only its behavior but its physiology too," Wingfield said.
The testosterone rise might be explained by an increase in courtship encounters with female birds—but male-male interactions may also play a role.
"There is a longstanding hypothesis called the 'challenge hypothesis,' [which suggests that] male-male interactions can increase testosterone—especially in socially monogamous species like barn swallows," Wingfield said.
It is even possible that naturally dominant males might "smell a rat" and somehow determine that their artificially enhanced competitors are cheating the system.
But in Safran's opinion, the data do not support this theory.
Previous studies have demonstrated a social-physiological link for testosterone levels, showing spikes in human males who watch a favorite sports team win or who witness a fight.
"Social feedback on a daily basis affects your physiology," Safran said.
"If you move into a crowd of males that are dark and you are not so dark, you get feedback that says you'd better not strut around in this crowd because it's pretty clear that you are not the top individual."
"[But] if you get feedback that says you're something special, that's sort of a sign for you to behave like a more dominant individual."
Safran's research, recently published in the journal Current Biology, was funded in part by a grant from the National Geographic Society's Committee for Research and Exploration.
(The National Geographic Society owns National Geographic News.)
Safran's team had previously found that naturally darker male birds had higher testosterone levels.
In a 2005 study the team revealed a relationship between coloration and male reproductive success. By evaluating males before and after color enhancement, they discovered that males who had their color enhanced sired more offspring.
The observation was telling about females as well.
"Females dynamically updated their sense of a male's quality," Safran said, noting that females tended to cheat on mates who could not keep their color up to snuff.
The barn swallows' dark plumage, like large antlers and other animal aphrodisiacs, has likely evolved as an indicator of an individual's overall health and ability to reproduce.
But this theory of sexual selection may be modified as research continues.
"The prevailing view has been that males who are not in great condition just don't look as good, and that males in top condition can afford to develop plumage traits that convey [their good health]," she said.
"Its [often considered] a fixed relationship, but this study suggests that there are a lot of interesting dynamics between the two."
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