Even the plainest Perdue chicken traces its ancestry to the wild and flashy red junglefowl (Gallus gallus) of Southeast Asia. Now, animal behaviorists have shown that domestication was a two-for-one deal: Not only did the birds become fearless, they also got bigger and produced more and larger eggs. Changes in levels of a brain chemical called serotonin, which underlies depression in people, may have help coordinated these transformations, they report.

The new study offers “yet another piece of information” that selecting for changes in behavior can trigger a host of other changes in domesticated animals, says Greger Larson, an evolutionary biologist at the University of Oxford in the United Kingdom, who was not involved with the work.

Lead author Per Jensen, a behavioral geneticist at Linköping University in Sweden, has long been fascinated by domestication, which Charles Darwin called a “case study” in evolution. Jensen and his team were inspired by a decades-long domestication study in Russia that found foxes bred for friendliness developed floppy ears and coat colors similar to those of dogs. Tamer foxes, also like dogs, had more social smarts. The work suggested changing an animal's behavior also changes other traits.

To understand these changes in chickens, Jensen and his colleagues have been running a similar experiment. From an original pool of about 60 male-female pairs of red junglefowl, they have bred hundreds of birds. In each successive generation of chicks, they segregate 60 or so offspring according to how the birds react, for example, to being touched by a person. Eventually they ended up with one line of very fearful junglefowl and one line of fearless ones.

Even after just three generations, other traits had begun to change, Jensen's team reported last year. The tamer birds grew faster, laid larger eggs, and were bossier than their more fearful counterparts. "Low-fear [birds] seem to be developing the traits we want in domestication," Larson says.

Now, testing the fifth and sixth generation of the birds, Jensen and his colleagues have looked more deeply into these traits, focusing on metabolism, feeding efficiency, and body chemicals involved in boldness. The basal metabolic rate, an indication of how efficiently food is turned into energy, was higher in the tamer birds, who gained more weight per kilogram of food consumed, the researchers report online today in Biology Letters. "I am surprised that we see such strong effects in such a short time," Jensen says.

The tamed birds also had higher levels of serotonin, both a signaling molecule in the brain that may be involved with fear responses and a hormone associated with metabolism and feeding in chickens. Jensen suspects serotonin may be one of the lynchpins that leads to the suite of changes seen in domestication. He will be trying to find this out.

Larson applauds the work, as it is revealing the early steps of domestication. And by comparing the wild ancestor—the tamed junglefowl—with the fully domesticated chicken, it may be possible to get at the genetic basis of these trait changes, he says. "It's hard to start from scratch and do that for other animals."