A new study found that the domestication of dogs led to a change in their eye color, giving them a friendlier brown color that feels less threatening to humans.
The study, led by a team of animal science specialists at Japan’s Teikyo University of Science, found that “the irises of dogs were significantly darker than those of wolves and that this dark color was perceived as friendlier by humans.”
It’s natural for human to favor dogs with darker eyes. This selective pressure could have led to favoring animals “whose eyes are perceived as friendlier for humans,” reported the study that was published Wednesday in the journal Royal Society Open Science.
Dogs were domesticated from grey wolves at least 15 000–50 000 years ago.
Most of today’s canines have one eye with light-colored, yellowish iris and a dark pupil in the middle.
The study also involved about 30 breeds of dogs that have large, dark reddish iris with a hard-to-distinguish pupil.
This difference could remarkably affect eye contact between humans and their loyal friends. This exchange, which dogs know how to use to catch the attention of their owners, could trigger an oxytocin release, similar to mother–infant bonding in humans.
Another recent study showed that this interaction, in dogs, could also be facilitated through the development of their facial muscles which gives them more expressive eyes, unlike wolves.
Teikyo’s researchers used former studies that involved mammals, mostly humans, that linked a widened pupil to more positive emotions than a smaller pupil.
According to the researchers, a larger pupil could be linked to a more fragile and peaceful creature, like a child whose pupil size shrinks with age.
But when a dog’s iris is so dark that it cannot be distinguished from the pupil, people might feel that they are seeing a very wide pupil. A dog with a dark iris could be seen as “fragile and in need for protection,” reported the study.
To test their theory, the researchers used two copies of the pictures of 12 dogs, one with dark eyes and one with light eyes, and showed them to human participants.
The participants were then asked to describe every animal as more or less friendly, and whether it’s young. They were also asked to report whether they would like to interact with a dog, or even adopt one. The same experiment was repeated with another group of participants.
“Pictures of dogs with darker eyes were perceived as being friendlier and younger. But although this character facilitated interaction, it was not enough to spark an adoption desire,” The study concluded.
The Teikyo University researchers acknowledge that there are limitations to their study, such as “familiarity,” as dogs with dark eyes are favored because they are simply more numerous.
Akitsugu Konno, the first author of the research, told AFP that “this is the first study that looks into the eye color difference between dogs and wolves. There are factors other than human preference that can contribute to dogs having dark eyes,” noting that he hopes to see more studies on the topic to prove “the universality of this phenomenon.”
