Domesticating Chickens Shrunk Their Brains

The “domestication phenotype” of floppy ears, curly tails, and white spots on fur appear in a range of species, like horses, cows, foxes, and dogs. Another shared trait? Smaller brains. These commonalities may pop up from breeding based on the quality most important to ancient humans: friendliness. Increasing friendliness, or at least decreasing fear of humans, may influence a similar set of genes or brain structures across species.

Brain structure and behavior are wound together: changing one impacts the other. A team of scientists studied this process in red junglefowl (pictured here), the wild ancestor of the chicken. The team bred two different lines of junglefowl, based on their fear of humans. After ten generations, the least fearful birds had smaller brains, relative to body size, than the more fearful birds. Specifically, domestication reduced the brainstem, an area of the brain involved in unconscious fear and stress responses. This change may have helped chickens navigate life on a farm, surrounded by startling noises and busy humans.