Human preferences for a friendly face may have steered the evolution of canine eye colour, researchers have suggested.
Ever since canines were domesticated, some time between 15,000 and 50,000 years ago, humans have selected – whether consciously or not – particular traits in their dogs.
Among them, it appears humans have a penchant for puppy dog eyes. Scientists previously found that dogs have evolved particular facial muscles that allow them to pull a doleful expression, triggering a nurturing response in humans and so giving such dogs an advantage.
Now research suggests humans may have influenced canine eye colour, too. A study by scientists in Japan found that dark eyes are more common in domesticated dogs than their wild relatives, and that humans perceive dogs with dark eyes as being more friendly.
“I speculate that lighter irises have some evolutionary advantage for wolves, but domestication has lost this selective pressure and darker eyes have emerged in some primitive dogs,” said Akitsugu Konno, the first author of the research, from Teikyo University of Science.
The team say dark eyes may subsequently have been favoured by humans, possibly unconsciously, during domestication from wolves to dogs.
However, Konno noted that the later development of particular breeds could also have influenced canine eye colour: for example, the blue merle coat colour seen in breeds including shelties and border collies is associated with blue eyes.
Writing in the journal Royal Society Open Science, the team describe how they compared iris colour in 22 images of grey wolves of various coat colours and from different locations, and images of 81 domestic dogs, finding that the latter tended to have darker and more reddish eyes.
The researchers then created 12 pairs of images, with one image of each pair showing a domestic dog with light eyes and the other showing the same dog with dark eyes. A selection of these pairs was shown to 76 people who were asked to rate the dogs for various personality traits and how much they would like to interact with or keep the dog.
The team found that dogs with dark eyes were rated higher for friendliness than light-eyed dogs and deemed more easy-going, sociable and dependent, and less aggressive, intelligent and mature. Similar results were obtained when the survey was repeated with another 66 participants.
“We speculate that a darker iris makes it more difficult to distinguish the size of the pupil and thus gives the illusion of a large pupil, which is associated with our perception of being more infant-like,” Konno said.
The researchers add that the association of dark eyes with immaturity could influence humans to protect and care for such canines, although the study found eye colour was not itself directly related to whether participants wanted to interact with or keep the dog.
“Overall, dogs with dark eyes may have evolved the trait largely as means to send non-threatening gaze signal to humans,” the team write.
