“Form equals function” is a fundamental principle of biology. It means that the physical characteristics of a body part are associated with its purpose and that we can predict the function of something by paying attention to what it looks like. When we see the very long, lean muscular hind quarters of a Greyhound and compare them to the same body parts on a Bichon Frise, we can figure out which of these dogs would win a race between the two. Similarly, when we compare the large ears of bats with those of squirrels, it’s easy to conclude that bat hearing is far more acute than squirrel hearing.
A study of canid eyes found that their form correlates with their function in an unexpected way. The researchers behind the study “A Comparison of Facial Color Pattern and Gazing Behavior in Canid Species Suggests Gaze Communication in Gray Wolves (Canis lupus)” concluded that the more visible canine eyes are in a particular species, the more that species uses their eyes to communicate with one another. Species with highly visible eyes are able to communicate the direction of their gaze effectively with other members of the species. This “gaze signaling” is prevalent in highly social species.
For the study, researchers used photographs of 320 adults from a total of 25 species to analyze the color patters of their face and eyes. After careful measurements, they divided all of the species into three general categories of eye visibility: 1) Both the position of the pupil in the eye and the position of the eye within the face were conspicuous. 2) Only the position of the eye within the face was conspicuous. 3) Neither the pupil nor the eye had a conspicuous appearance.
For each species studied, they relied on previous work to sort out a couple of aspects of their social behavior. Do they hunt alone or in pairs or do they hunt in groups of three or more? In the non-breeding season, do they live solo or in pairs or do they live in groups of three or more during this part of the year? The data revealed a pattern: the more social species tended to have more visible eyes than the less social species.
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The scientists studied a representative species with each type of eye visibility: grey wolves, fennec foxes and bush dogs. The found that the grey wolves (high eye visibility) spent more time gazing at other members of their social group than fennec foxes (medium eye visibility) which in turn spent more time engaged in this gazing behavior than bush dogs (low eye visibility). The more visible the eyes are, the more time animals spent gazing directly at each other. The longer duration of this gazing behavior in more social species suggests that they are using them to communicate with one another more.
There may be a trade-off related to eye visibility in predators. Many species that hunt in pairs or solo have eyes that are not very visible. It has been hypothesized that such eyes keep prey from being alerted if the predator has them in their gaze. For group hunters and social species, the benefits of communication within the species may outweigh that disadvantage.
This study suggests that we should be able to predict the level of gazing behavior and sociality of other canid species based on the visibility of their eyes.