Dogs are marvelous. Regardless of whether they’re asking for your food, woofing at their leash to persuade you to go for them on a walk, or essentially welcoming you when you return home, dogs do all the little things that put smiles on faces around the globe.
The human-dog bond is unique and diagnostic of the evolution of human cultures. Dogs have domesticated over 33,000 y ago and, during that time, selection processes have shaped both their anatomy. Most remarkable among dogs’ behavioral adaptations, because of choice during training, is their capacity to peruse and utilize human correspondence. And behavior and turned them into human’s best friends.
Dogs are more skillful in using human communicative cues. A recent study conducted by the University of Portsmouth scientists has suggested that dogs have evolved new muscles around the eyes to better communicate with humans.
When scientists compared the anatomy and behavior of dogs and wolves, they found that the dog’s facial anatomy has changed over thousands of years explicitly to enable them to communicate with humans more readily.
Scientists also discovered that the facial musculature of both species was similar, except above the eyes. Dogs have small muscles, enabling them to raise their inner eyebrow, which wolves don’t strongly.
The inner eyebrow-raising movement triggers a nurturing response in humans because it makes the dogs’ eyes appear larger, more infant-like, and resembles a movement humans produce when they are sad.
Comparative psychologist Dr. Juliane Kaminski, at the University of Portsmouth, said, “The evidence is compelling that dogs developed a muscle to raise the inner eyebrow after they were domesticated from wolves. We also studied dogs’ and wolves’ behavior, and when exposed to a human for two minutes, dogs raised their inner eyebrows more and at higher intensities than wolves.”
“The findings suggest that expressive eyebrows in dogs may be a result of humans’ unconscious preferences that influenced selection during domestication. When dogs move, it seems to elicit a strong desire in humans to look after them. This would give dogs, that move their eyebrows more, a selection advantage over others and reinforce the ‘puppy dog eyes’ trait for future generations.”
In the past study, scientists have discovered that dogs raise their inner eyebrows significantly more when humans are looking at them.
Dr. Kaminski said, “The AU101 movement is significant in the human-dog bond because it might elicit a caring response from humans but also might create the illusion of human-like communication.”
Lead anatomist Professor Anne Burrows, at Duquesne University, Pittsburgh, USA, co-author of the paper, said, “This is a striking difference for species separated only 33,000 years ago, and we think that the remarkably fast facial muscular changes can be directly linked to dogs’ enhanced social interaction with humans.”
Professor Bridget Waller, also at the University of Portsmouth, said, “This movement makes a dogs’ eyes appear larger, giving them a childlike appearance. It could also mimic the facial movement humans make when they’re sad.”
“Our findings show how important faces can be in capturing our attention, and how powerful facial expression can be in social interaction.”
Co-author and anatomist Adam Hartstone-Rose, at North Carolina State University, USA, said: “These muscles are so thin that you can see through them—and yet the movement that they allow seems to have such a powerful effect that it appears to have been under strong evolutionary pressure. It is remarkable that these simple differences in facial expression may have helped define the relationship between early dogs and humans.”
Co-author Rui Diogo, an anatomist at Howard University, Washington DC, USA, said: “I must admit that I was surprised to see the results myself because the gross anatomy of muscles is normally very slow to change in evolution, and this happened very fast indeed, in just some dozens of thousands of years.”
An alternative reason for the human-dog bond could be that humans have a preference for other individuals who have whites in the eye and that intense AU 101 movements expose the white part of the dog’s eyes.
It is not known why or precisely when humans first brought wolves in from the cold and the evolution from wolf to dog began, but this research helps us understand some of the possible mechanisms underlying dog domestication.
- Evolution of facial muscle anatomy in dogs. Juliane Kaminski, Bridget M. Waller, Rui Diogo, Adam Hartstone-Rose, and Anne M. Burrows. PNAS July 16, 2019 116 (29) 14677-14681; first published June 17, 2019; DOI: 10.1073/pnas.1820653116