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.
The team say dark eyes may subsequently have been favoured by humans, possibly unconsciously, during domestication from wolves to dogs.
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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.
