Why Do Most Dogs Have Brown Eyes?

Light eyes aren’t common in domestic canines. A dog with piercing blue eyes might be described as strikingly beautiful or borderline unsettling. Rarely do we call the eye color cute, and apparently past generations of dog breeders agreed. As Smithsonian reports, new research suggests we may have selectively bred dogs to have darker eyes to make them appear younger and friendlier.

For their study published in the December 2023 issue of Royal Society Open Science, a team of researchers based in Japan compared eye color in gray wolves to that of domestic canines. After analyzing photos of 22 wolves and 81 dogs across 35 breeds, they found that the dogs’ irises were notably darker on average. While dark brown is the standard eye color in dogs, wolf eyes are more likely to be yellow. 

It’s possible this difference was intentionally selected for by humans. After analyzing canine eye color, the researchers asked participants to look at pictures of dogs and make assumptions about their personalities. Some images were altered to make the pets’ irises lighter or darker. Dogs with dark eyes scored higher for friendly and nonthreatening traits like kindness, sociability, and trustworthiness, while pale-eyed dogs were more likely to be labeled aggressive and mature. This indicates that humans find brown eyes cute and puppy-like, which could explain why breeders gave preference to the feature. 

It’s not the eye color that people find adorable, but the perceived size of the pupils. Humans are born with relatively large pupils that get smaller as the eye muscles that control them get weaker with age. We associate big pupils with youth, and the darker the iris is, the harder it is to see where the pupil begins. The study authors suspect that brown eyes make pupils appear bigger than they really are, tricking us into thinking our pets are helpless babies that need our protection—even when they’re 85-pound Labradors.

Color isn’t the only trait that makes a dog’s eyes appear bigger and more lovable. Domestic canines have evolved to the ability to raise their eyebrows when looking at their owners, a trick otherwise known as puppy dog eyes. Here are more facts about puppies you should know. 

[h/t Smithsonian]