Domestication Likely Made Smaller-Brained, Less Fearful Chickens

In 10 generations, junglefowl bred for tameness show changes in brain size.

Posted Sep 10, 2020

Per Jensen, used with permission.
Domestic chickens.
Source: Per Jensen, used with permission.

How did selective breeding by ancient humans change the timid, rainforest-dwelling junglefowl into today’s domesticated chicken? In a recent study, researchers from Linköping University in Sweden bred wild junglefowl with the least fear of humans for 10 generations, ending up with smaller-brained birds more tolerant of potentially frightening stimuli. The results, published in the journal Royal Society Open Science, show how the process of domestication may shape the brains and behavior of chickens and other animals that live among humans.

Every one of the 20 billion chickens alive today derives from the Red Junglefowl of southeast Asia. This species was domesticated by humans about 8,000 years ago.

Researchers Rebecca Katajamaa and Per Jensen set out to recreate the selection that occurred during the earliest phases of chicken domestication. Starting with a group of wild Red Junglefowl, they selected and bred together individuals that showed the least fear of humans. As a control group, they bred those birds showing the greatest fear of humans. After 10 generations, they measured the chicks’ brain size and composition as well as different types of learning.

Jensen, a professor of ethology at Linköping University, says that selecting for low fear of humans did more than just create tamer birds in a few generations.

“We also found a suite of morphological, physiological, and behavioral changes that occurred as correlated responses to the selection for increased tameness,” he says. “We think, therefore, that many of the typical characteristics of domestic chickens may originally have popped up as ‘side effects’ of the increased tameness that was a necessary first step in domestication.”

In chicks selected for low fear of humans, brains became smaller relative to body size, which mirrors changes seen in domesticated chickens compared to their wild ancestors. The difference was particularly pronounced in the brain stem, an ancient part of the brain that is involved in, among other things, certain stress reactions.

Jensen and Katajamaa also carried out two behavioral experiments to test the chicks’ learning ability.

In a fear habituation test, they investigated how quickly the birds became accustomed to something that could be experienced as frightening, but which was actually harmless (in this case a flashing light). The birds from the low fear line stopped reacting to the stimulus significantly faster than their counterparts from the high fear line. This ability to habituate rapidly could benefit birds that live among humans, where events that are unfamiliar and potentially frightening, but not dangerous, occur often.

The researchers also assessed the chicks in a test of associative learning (the ability to pair two things with each other, such as coupling a tone with food). They found no differences between the high fear and low fear groups, indicating no general effect on learning in the domesticated chicks.

Francesco Veronesi, via Wikimedia Commons. Distributed under a CC BY-SA 2.0 license.
Red Junglefowl.
Source: Francesco Veronesi, via Wikimedia Commons. Distributed under a CC BY-SA 2.0 license.

The changes Jensen and Katajamaa observed after 10 generations of selective breeding are in line with the domestication syndrome, a set of traits common among domestic animals. Compared to their wild ancestors, domesticated animals tend to have less fear of humans, smaller brains, changes in body shape and pigmentation, and increased reproductive capability. Early domestication may have been driven, either unconsciously or on purpose, by selection for tameness and this may have driven several of the other changes seen in the domestication syndrome. This is probably true for chickens as well as domesticated mammals such as dogs, mink, sheep, and more.

Jensen says it is not yet possible to say whether the differences in behavior shown in the study are functionally connected with the differences in brain size and composition. He is interested in the possible effects of selection for tameness on other cognitive processes and says that future research will examine the relationship between the size of specific brain regions and behavior in chickens.

Overall, this study is another step towards understanding the process by which chickens – and possibly other species – become domesticated, says Jensen.

“It means that if our ancestors selected the tamest chickens when they first started to domesticate them, they would automatically get larger and more productive birds, with a brain and behavior that is more adapted to a life among humans than that of a wild Red Junglefowl.”


Katajamaa, R. and Jensen, P. (2020). Selection for reduced fear in red junglefowl changes brain composition and affects fear memory. R. Soc. Open Sci. 7: 200628. Doi: 10.1098/rsos.200628.