Here’s Why Most Dogs Have Brown Eyes

Scientists have long known that domesticated animals tend to share traits like floppy ears, shorter snouts, smaller brains and decreased aggression. Now, researchers have discovered that humans may have influenced another trait in dogs: the color of their eyes. When domesticating our four-legged companions, humans may have selected dogs with darker eyes because we view them as more friendly and youthful, a new study suggests. 

A team of researchers in Japan analyzed irises in photos of 22 gray wolves with various coat colors and 81 domestic dogs of 35 major breeds. In their study, published Wednesday in Royal Society Open Science, they report that domestic dogs tend to have significantly darker and redder irises than their yellow-eyed wolf cousins.

“Not only was there a clear eye color difference between wolves and dogs but also we found that dogs with darker eyes are perceived as more friendly and immature by humans,” lead author Akitsugu Konno, an animal scientist at the Teikyo University of Science in Japan, tells Newsweek’s Robyn White. “This finding suggests that humans may unconsciously select for dark eyes because they seem less threatening and being protected by humans, which might be more evolutionarily adaptable to domestic dogs.”

To test perceived friendliness, the researchers picked 12 stock images of dogs, photoshopped their eyes and created a questionnaire that asked participants to rate the pups’ personality traits (such as friendliness, trustworthiness and aggression), on a scale of zero to five. The survey also asked how much they would want to interact with a specific dog or adopt it. The researchers created four different versions of the questionnaire and asked 76 participants to fill it out. 

Their results showed participants rated dark-eyed dogs as more friendly and less mature than light-eyed dogs. Overall, however, eye color did not directly correspond to whether participants wanted to interact with or keep the dog. The survey was then repeated online with another 66 participants, and the team obtained similar results.

“We speculate that a darker iris makes it more difficult to distinguish the size of the pupil and thus gives the illusion of a large pupil, which is associated with our perception of being more infant-like,” Konno tells the Guardian’s Nicola Davis. 

Human pupil size tends to decrease with age, because the muscles that respond to light lose some of their strength over time. Therefore, we may associate larger pupils with young creatures that require our care. 

“It makes sense that eye color would be just one more place where humans have left their mark,” Molly Selba, an anatomist at the University of Maryland Eastern Shore who was not involved with the work, tells Science’s Sara Reardon. 

Jessica Hekman, a veterinarian and dog geneticist at the nonprofit Functional Dog Collaborative who also did not participate in the research, tells the publication it appears convincing that humans selected for dark eyes. However, she adds, it remains unclear whether our pups were originally domesticated that way, or if the selection is a more recent phenomenon as breeders worked to conform to breed standards. Further research will be needed to determine exactly when the eye color change occurred. 

“We really like telling stories about dog domestication, but it’s been hard to do solid studies,” she tells Science.

Margaret Osborne | | READ MORE

Margaret Osborne is a freelance journalist based in the southwestern U.S. Her work has appeared in the Sag Harbor Express and has aired on WSHU Public Radio.