Compared to its ancestor, the wolf, its domesticated descendants, the dogs, have significantly darker eye colors, a study has confirmed. Researchers have now examined in more detail whether this characteristic only arose by chance during the breeding of domestic dogs or whether human preferences were behind it. As they discovered, most people actually instinctively prefer darker eye colors in dogs. Dark-eyed dogs probably receive more affection from us humans and therefore have a survival advantage.
The first animal that humans domesticated around 15,000 to 50,000 years ago and thus made into their livestock and pet was probably the domestic dog (Canis familiaris). This evolved from the gray wolf (Canis lupus) through a series of evolutionary selection processes and gradually acquired a different appearance. Unlike its closest relative, which has striking light yellow eyes, today’s domestic dogs usually have dark eye colors, ranging from brown to black. But how did this come about?
This was investigated by a team led by Akitsugu Konno from the Teikyo University of Science in Yamanashi, Japan. To do this, they systematically compared the color of the iris in the eyes of 22 wolves and 81 dogs from 35 different dog breeds using photos. The researchers also showed test subjects portraits of twelve different dog breeds and used a questionnaire to examine how people reacted to them and whether they perceived the dogs differently if they were given lighter (yellow) or darker (black) eye colors through image processing. Konno and his colleagues carried out this experiment twice, first with 76 test subjects, then with 66 additional test subjects.
People prefer dark-eyed dogs
The result: The iris of dogs is actually usually significantly darker and redder in color than that of wolves. As a result, the pupil and iris in the dog’s eye visually merge with each other and this area appears larger. In line with this, the test subjects in the surveys perceived the faces of dogs with darker eyes as friendlier, more sociable and immature, while the same faces with lighter eyes as smarter, more self-confident and more mature. Overall, the subjects were more willing to take care of the dark-eyed dogs than the light-eyed ones.
Konno and his colleagues conclude that we humans clearly prefer dogs with darker eyes. The study does not reveal exactly why darker dog eyes appear friendlier to us. However, with darker eyes, it is more difficult for us humans to perceive the contrast between the iris and pupil, as the researchers report. This makes it harder for us to see whether the pupil is dilating or constricting in darker eyes and we overestimate the size of the pupil overall. A dilated pupil indicates joy and is perceived by us as attractive, friendly and trustworthy, as previous studies show. In addition, supposedly wide pupils appear younger because babies have larger pupils than older people. A constricted pupil, on the other hand, means anger and threat. Dogs with darker eyes could therefore benefit from the fact that we humans have difficulty seeing anger in their eyes and think they are younger, Konno and his colleagues suspect.
Dark dog eyes receive more care
Overall, the results suggest that dogs have acquired their darker eyes through evolution because their faces and gaze do not appear threatening to us humans, but rather harmless, weak and childlike. This means they receive more affection and care. Accordingly, the domestication of the dog once contributed to the fact that the irises of today’s dogs have a different color than those of wolves. The study does not reveal whether people deliberately bred dogs with dark eyes during domestication or whether they naturally became established in the human environment because of a survival advantage. However, today’s breeding criteria specifically favor darker eyes. Further studies with a larger number of dog breeds must show exactly when these characteristics developed in the course of dog evolution and domestication. These could also clarify whether wolves benefit from their brighter eyes in the wild.
Source: Akitsugu Konno (Teikyo University of Science) et.al, Royal Society Open Science, doi: 10.1098/rsos.230854