Because of human choice, dogs now have more expressive faces. They have a muscle in their face that allows them to make the “puppy dog eyes” that make many people fall in love with them. This muscle doesn’t exist in wolves, which are dogs’ ancestors.
Dogs can raise their inner “eyebrow”, which makes their eyes look bigger. This gives them a more childlike and sad look, like the eyes of a puppy dog.
It does make us more interested in dogs. Juliane Kaminski and her colleagues at the University of Portsmouth filmed how dogs interacted with people at a shelter in 2013 to find out what made them more likely to be adopted. This eyebrow movement is the only thing that seemed to make a difference. Dogs who did this more often were taken home sooner.
In 2017, her team showed that dogs do this movement more often when people are looking at them.
The anotomy difference underlying eyebrow rasing expression
Now, Kaminski and a group of anatomists have looked at the faces of 6 dogs and 4 grey wolves to compare the muscles in their faces (they used existing specimens – no animals were killed for this study).
The dog’s eyebrows are moved by a muscle called the levator anguli oculi medialis. It is on the inside of the eye, closer to the nose, and it is above the eye.
The main thing they found was that the facial muscles of domestic dogs and gray wolves were mostly the same and only different around the eyes. The levator anguli oculi medialis (LAOM) muscle was always there in dogs, but in gray wolves, it was usually just a few muscle fibers surrounded by a lot of connective tissue.
In wolves, a tendon was sometimes seen that joined with the fibers of the orbicularis occuli muscle on the medial side. This was near the area where a LAOM would normally be found. So, wolves can’t raise the inner corner of their brows as much as they can when they relax their eyes.
The evolution selection pressure
During domestication, the facial muscles of dogs have been changed by the forces of “human selection.” We’ve known for a long time that the shape of a dog’s body and the way its bones are put together are the result of artificial selection, but this shows that the soft tissues are also different.
This is a surprising difference for two species that only split about 33,000 years ago. Changes in soft tissues are hard to prove because soft tissues don’t fossilize easily. Also, these muscles are changing so quickly that it is clear that they are getting along better with people. The rest of their facial anatomy was the same, so this physical difference means that dogs and wolves act differently because dogs move their eyebrows more often and more dramatically than wolves do.