Breeding for tameness tends to produce animals with floppy ears, patches of white fur, juvenile faces, small jaws and other features, a new study reports.
Authors of the paper, published in the journal Genetics, believe the suite of features are tied to what they call "domestication syndrome," which can apply not only to mammals like dogs, foxes, pigs, horses, sheep and rabbits, but also to domesticated birds and fish, even if the latter two groups don't display all of the anatomical changes associated with tame mammals.
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The researchers theorize that domestication with tameness as a goal leads to genetic alterations that can affect a group of embryonic stem cells called the neural crest. Scientists, including Charles Darwin, have been wondering why domesticated animals seem to have so many features and behaviors in common.
"Because Darwin made his observations just as the science of genetics was beginning, the domestication syndrome is one of the oldest problems in the field," co-author Adam Wilkins from the Humboldt University of Berlin said in a press release. "So it was tremendously exciting when we realized that the neural crest hypothesis neatly ties together this hodge-podge of traits."
He explained that neural crest cells are formed near the developing spinal cord of early vertebrate embryos. As the embryo matures, the cells migrate to different parts of the body and give rise to many tissue types. These tissues include pigment cells and parts of the skull, jaws, teeth, and ears–as well as the adrenal glands, which are the center of the "fight-or-flight" response. Neural crest cells also indirectly affect brain development.
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"When humans bred these animals for tameness, they may have inadvertently selected those with mild neural crest deficits, resulting in smaller or slow-maturing adrenal glands," Wilkins says. "So, these animals were less fearful."
Because the neural crest influences more than the adrenal glands, the domestication process could also lead to all of the aforementioned physical signs of tameness. They aren't always beneficial. Floppy ears, for example, may look adorable on dogs and rabbits, but they actually are a result of malformed ear cartilage. An animal hoping to hear well isn't going to benefit from having the ear flopped down alongside its face.
Domesticated animals even have smaller brains than their wild counterparts. The authors suggest that the reduced forebrain size of most domestic mammals could be an indirect effect of neural crest changes, because a chemical signal sent by these cells is critical for proper brain development.
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"This interesting idea (the new theory) based in developmental biology brings us closer to solving a riddle that's been with us a long time. It provides a unifying hypothesis to test and brings valuable insight into the biology of domestication," Mark Johnston, Editor-in-Chief of Genetics, said.
Wilkins added, "Animal domestication was a crucial step in the development of human civilizations. Without these animals, it's hard to imagine that human societies would have thrived in the way they have."