Darkening of the iris of domesticated dogs
Domestication of dogs leads to darkening of their irises.
The research team wondered if modern dogs had developed darker eyes. Almost all dogs had darker eyes than wolves.
A team of animal scientists at Teikyo University of Science, along with a colleague from Showa University, all in Japan, found that most dogs have darker irises than their wolf relatives. This result suggests that darker eye color has evolved due to domestication. In their study, published in the Royal Science journal, the group compared the eye colors of several wolves and dogs.
Previous research has shown that all modern domestic dogs descend from a single ancient ancestor, a species of wolf that is now extinct. Their closest modern relative is the gray wolf, which has the famous colorful irises that give them the reputation of yellow eyes. The extinct relative is believed to have similar eyes.
In this new study, the research team set out to see if modern dogs have evolved darker eyes. And if so, is it because of the human perception that darker eyes are friendlier? Because if that's the case, then that makes it possible for dogs to be raised as pets.
To learn more, the researchers compared photos of 22 gray wolves to 81 images of domestic dogs of various breeds, looking specifically at their iris color. They found that almost all dogs had darker eyes than wolves, and in many cases significantly darker.
They believe this points to the possibility that dogs with darker eyes have been selected for over thousands of years, due to certain traits associated with them.
Previous research has shown that humans see other humans and other animals with darker eyes as friendlier than those with lighter or blue eyes.
The team conducted another experiment in which they asked volunteers to rate the friendliness of dogs in photos. They found that most of the time, humans rated dogs with darker eyes as more likely to be friendlier.
The research team notes that one reason humans may find dogs with darker eyes friendlier is that it is more difficult to determine the size of the pupils. Because changes in the size of the pupils cause negative emotions in people.
The study was published in Royal Open Science.
