Domesticated fowl who are less fearful of humans also grow more and lay bigger eggs. According to new work published in Biology Letters, tameness may have been the driving force behind various other desirable traits, and they appear to be all linked by changes in the brain chemical serotonin.
The red junglefowl (Gallus gallus) is the ancestor of all domesticated chickens, and they can be found living in Southeast Asia today. To study the biological mechanisms underlying the change from wild to domesticated, Beatrix Agnvall and colleagues from Linköping University artificially selected red junglefowl for high or low fear of humans over the course of six generations.
To test for fearlessness, the team cracked an egg in the feed trough, and when the 21-week-old birds started eating from it, a novel object (in this case, a Coca Cola can) was placed 10 centimeters (4 inches) from the egg. The amount of time it took for the bird to start feeding again was a measure of boldness. The researchers picked out the birds with the least fear of humans and bred their offspring. They did the same thing for the most fearful fowl. “This method resembled the conditions during the very first stage of fowl husbandry 8,000 years ago," Agnvall said in a statement.
After a few generations, fearless birds weren’t just bolder around people, but they also had a higher metabolic rate and higher feed efficiency. That is, they grew more even though they ate less, compared to the more fearful fowl. They also laid bigger eggs.
Furthermore, levels of the hormone serotonin were higher in the tame roosters, and the team thinks that this might be one of the mechanisms driving these results. The findings suggest that traits commonly associated with domestication may have evolved as secondary effects to tameness. "It can automatically have led to many of the characteristics that we and our ancestors liked about domesticated animals,” added Linköping’s Per Jensen. “We can suppose that our ancestors didn't necessarily select animals because they were good at producing food, but mainly because they were easy to manage.”