Why do most dogs have brown eyes?

[aum]

Humans may have selected for a color they found friendlier—and less threatening

 

This reporter’s mutt, Neko, has unthreatening, puppylike brown eyes.  SARA REARDON

 

When a cute pooch stares up at you, all you want to do is bend down and scratch their belly. If you caught a wolf staring your way, you’d probably recoil in fear. Eye color may be at least partially to blame, according to a new study. Most wolves sport piercing yellow peepers, whereas most dogs have brown eyes—a hue humans may have selected for because it looks less threatening.

 

The findings, reported today in Royal Society Open Science, fit with existing research on how people have changed the appearance of dogs over our shared history, says Molly Selba, an anatomist at the University of Maryland Eastern Shore who was not involved with the work. “It makes sense that eye color would be just one more place where humans have left their mark.”

 

People have been molding dogs since they were first domesticated—perhaps 20,000 or more years ago—choosing animals that are friendly, obedient, and good at working for us. We may have also changed what they look like, from their big eyes to their large foreheads. Such “neotenous” traits are reminiscent of our own children, transforming dogs not only into companions, but fur babies.

 

But where does eye color fit in? The light-colored irises of wolves may be useful for communication in the wild because they make the size and direction of the pupils more visible, allowing the animals to better convey messages such as gaze direction and dominance. Yet more than 90% of domestic dog breeds recognized by the American Kennel Club, all of which descend from wolves, have dark irises.

 

To figure out why, Akitsugu Konno, a behavioral scientist at the Teikyo University of Science, and his colleagues edited photos of dogs from 33 breeds—everything from corgis to Irish wolfhounds—to make their eyes lighter or darker. They showed selections of the photos to 142 Japanese volunteers, mainly students, asking them to rate each dog on traits such as friendliness, aggressiveness, maturity, and intelligence.

 

The volunteers were significantly more likely to judge a dog as friendly—rating them more highly on traits such as sociability and nonaggressiveness—when the photo showed it with dark eyes, the team found. The participants rated these dogs as less intelligent and less mature, more like puppies. Over the course of dog evolution, having big, friendly dark eyes might have proved more advantageous than light eyes that help communicate with other animals in the wild.

 

Konno thinks humans prefer dark eyes because they make dogs’ pupils seem larger. Human babies have larger pupils than adults, he notes, and dilated pupils are associated with friendliness.

 

Still, Selba says she’d like to see the team look at whether the phenomenon holds true across more of the approximately 350 official dog breeds that exist. It would also be interesting, she says, to see whether dark-eyed dogs are adopted more quickly than those with blue or amber eyes. Anecdotal stories suggest shelters often struggle to find homes for dogs with these rarer eye colors, she notes.

 

Konno says there are notable exceptions: Piercing blue eyes, for example, are common among Siberian huskies and some other breeds. Huskies, he says, might be more closely related to wolves than other breeds, at least in terms of their visual anatomy and communication.

 

“We really like telling stories about dog domestication but it’s been hard to do solid studies,” says Jessica Hekman, a veterinarian and dog geneticist at the nonprofit Functional Dog Collaborative. She says the evidence that people selected for dark eyes is convincing, but it’s not clear whether dogs were originally domesticated that way or it’s a more recent phenomenon. It’s possible that dogs acquired their dark eyes over the past few hundred years as breeders conformed to breed standards, she notes. Pinning down the timing will require further research. 

 

Lisa Gunter, a dog behavior researcher at the Virginia Polytechnic Institute and State University, says the study raises interesting questions about dog evolution. But she questions whether the findings support the idea that dark-eyed dogs look youthful: Because most dogs have brown eyes, people are more familiar with them and may simply prefer what they’re used to. Additionally, she says, dilated pupils in adult dogs often mean that the dog is on edge, not that it’s trying to be cute. So perhaps it’s not always time for a belly rub.

 

Source