A new study reveals that animals that are bred to be tame to humans feature floppy ears, white patches of fur, small jaws and juvenile faces, among other features.
The study, which was released in the Genetics journal, refers to the features as part of the "domestication syndrome" that encompasses not just mammals such as dogs, foxes and rabbits, but also domesticated fish and birds, despite the latter not portraying most of the changes that can be seen in domesticated mammals.
The researchers of the study believe that domesticated animals undergo genetic changes in a group of stem cells found in embryos known as the neural crest, leading to shared features and behaviors among all species of domesticated animals.
Charles Darwin has been known to wonder why animals that have been domesticated have many behaviors and features in common. The phenomenon has not been explained until now.
"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," said Adam Wilkins, from Humboldt University of Berlin and a co-author of the study.
"So it was tremendously exciting when we realized that the neural crest hypothesis neatly ties together this hodge-podge of traits."
Cells in the neural crest cells form near the spinal cord as it develop in the early embryos of vertebrates. As the development of the embryo progresses, the cells transfer to the rest of the embryo's body and form many different types of tissue, including components of the skull, teeth, ears and jaws. Neural crest cells also form the adrenal glands and have an indirect effect on the development of the brain.
Wilkins explains that when humans were breeding animals to be tame, they could have chosen animals with mild deficits in the neural crest cells. This is because mild deficits in the neural crest cells would lead to smaller adrenal glands, making the animals less fearful and tame compared to the ones in the wild.
In addition to the smaller adrenal glands that develop, the deficiencies in neural crest cells would have create the physical features that are now associated with tame animals. One example are the floppy ears, which despite looking cute on rabbits and dogs, are actually signs of a malformed ear cartilage. Floppy ears also affect an animal's hearing.
Another effect that may have been caused the mild deficits in neural crest cells are the smaller brains that domesticated animals have compared to their counterparts in the wild.
Wilkins adds that the domestication of animals was a crucial moment in human civilizations, as with the help of these animals, societies and economies thrived, leading to their current state today.