While humans have long shaped the physical traits of domesticated dogs—such as floppy ears, shorter snouts, and reduced aggression—a new study suggests that eye color may also be a result of human influence. Researchers have proposed that the preference for darker eyes in dogs may stem from the perception that they appear friendlier and more youthful, a trait that humans may have unconsciously selected for during the domestication process.
Eye Color Difference Between Wolves and Dogs
In a study published in Royal Society Open Science, a team of Japanese researchers analyzed eye color in both gray wolves and domestic dogs. The study included 22 gray wolves with various coat colors and 81 domestic dogs from 35 different breeds. The researchers discovered that domestic dogs tend to have significantly darker and redder irises compared to their wolf ancestors, which typically have yellow eyes.
Lead author Akitsugu Konno, an animal scientist at Teikyo University of Science, suggests that this difference may be more than just a biological trait—it could be linked to how humans perceive and interact with dogs. “Dogs with darker eyes are often seen as more friendly and immature, and this finding suggests that humans might unconsciously select for dark eyes because they seem less threatening,” Konno explained. “This could make them more adaptable to living with humans.”
The Perception of Friendliness and Youthfulness
To further explore this theory, the researchers conducted an experiment in which they altered the eye color of dog images and asked participants to rate the perceived friendliness, trustworthiness, and aggression of the dogs. The survey included 12 stock images of dogs, each with eyes photoshopped to appear lighter or darker, and 76 participants who evaluated the dogs’ personalities. The results showed that dogs with darker eyes were consistently rated as more friendly and less mature than those with lighter eyes.
Interestingly, while eye color influenced how friendly the dogs were perceived, it did not directly affect whether participants wanted to interact with or adopt the dogs. The study was repeated online with another 66 participants, yielding similar results.
The Illusion of Larger Pupils
The researchers hypothesize that the darker iris color might make it more difficult to distinguish the size of a dog’s pupil, giving the impression of larger pupils—something that humans associate with youth and innocence. Konno explained that larger pupils are often seen in young creatures that require care, which may explain why darker-eyed dogs seem more infant-like and approachable.
Evolutionary Implications and Future Research
Human pupils tend to shrink with age as the muscles controlling them lose strength, which further reinforces the connection between larger pupils and youthfulness. This, according to researchers, may be one reason why we perceive dogs with darker eyes as more childlike and easier to bond with.
While the study presents compelling evidence that humans have influenced dogs’ eye color, experts note that more research is needed to determine when this selection took place. Jessica Hekman, a dog geneticist at the Functional Dog Collaborative, emphasized that it is unclear whether dogs were originally domesticated with dark eyes or if the preference for this trait emerged more recently as breeders conformed to breed standards. Further studies are necessary to clarify the timeline of this genetic shift.
As Molly Selba, an anatomist at the University of Maryland Eastern Shore, puts it, “It makes sense that eye color would be just one more way humans have shaped dogs throughout the process of domestication.”
This study highlights how the domestication of dogs may have extended beyond physical traits like size and shape to influence subtler features such as eye color, reinforcing the deep bond between humans and their four-legged companions.
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