Raccoons Are Showing Early Signs of Domestication

Raccoons show first signs of domestication

By Marina Wang edited by Andrea Thompson

With their dexterous child-like hands and cheeky “masks,” raccoons are the omnipresent bandits of North America. In fact, these creatures are so comfortable in human environments that a new study finds that raccoons living in urban areas change physically in response to living around humans, a first step in domestication.

The study demonstrates that the process of domestication is often wrongly considered to be initiated by humans, with humans selectively capturing and breeding wild animals. But the study authors say the process begins much earlier, when animals become accustomed to human environments.

“One thing about us humans is that everywhere we go, we produce a lot of waste,” says Raffaela Lesch, study co-author and biologist at the University of Arkansas at Little Rock. Piles of human remains provide an endless buffet for wildlife, and to access this bounty, animals must be bold enough to rummage through human waste, but not bold enough to become a threat to humans. “If you own an animal that lives near humans, you have to behave well enough,” says Lesch. “This selection pressure is quite intense.”

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Proto-dogs, for example, would have dug in human trash piles, and cats were attracted to mice that gathered around trash. Over time, individual animals with a reduced fight-or-flight response might forage more successfully in the presence of humans and pass on their non-reactive behavior to their offspring.

Oddly enough, docility has also long been associated with traits such as a shorter face, smaller head, floppy ears, and white patches of fur, a pattern noted by Charles Darwin in the 19th century. The emergence of these traits is known as domestication syndrome, but scientists didn’t have a comprehensive theory to explain how the traits were linked until 2014. That’s when a team of evolutionary biologists noticed that many physical traits that coexist with domestication can be traced back to a large group of cells during embryonic development called neural crest cells. Early in development, these form along the back of an organism and migrate to different parts of the body, where they become important for the development of different cell types. Biologists hypothesized that mutations that impede the proliferation and development of neural crest cells could later result in a shorter snout, a lack of cartilage in the ears, loss of coat pigmentation, and a blunted fear response, leading to a better chance of survival in close proximity to humans.

Lesch says neural crest cells are the most important hypothesis scientists currently have to explain domestication syndrome, but they continue to gather and evaluate evidence for or against it. One piece of the puzzle would be to see if the domestication syndrome was observable in real time in wild animals. For the new study, she and 16 graduate and undergraduate students collected nearly 20,000 photographs of raccoons across the contiguous United States from the community science platform iNaturalist. The team found that raccoons in urban environments had snouts that were 3.5 percent shorter than those of their rural cousins.

The results agree with observations of urban foxes and mice and “indicate that once wild animals start spending time near people, they become a little less frightened and perhaps even begin to show physical signs of domestication syndrome,” says Adam Wilkins, a biologist at Humboldt University in Berlin who first proposed the neural crest cell explanation but was not involved in the new study.

Lesch would like to further his research, perhaps trapping raccoons and comparing genetics or stress hormones between urban and rural animals. She and her colleagues could also test whether the trends hold true for other species such as armadillos and opossums. “I would love to take the next steps and see if our trash pandas in our backyard are really friendlier than the ones in the countryside,” she says.

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