An extraordinary new study, examining the anatomical differences between dogs and wolves, has revealed our canine companions recently evolved a very small facial muscle allowing them to raise their inner eyebrow. This tiny muscle was not found in wolves, suggesting dogs rapidly evolved this extra tissue to better communicate with humans.

The new research followed on from earlier study into the ways dogs facilitate eye contact with humans. Prior work has revealed humans favor dogs that perform certain facial movements, particularly an inner eyebrow raise (dubbed AU101 by researchers).

"This movement makes a dogs' eyes appear larger, giving them a childlike appearance," explains Bridget Waller, co-author on the new research. "It could also mimic the facial movement humans make when they're sad."

One fascinating early study discovered that dogs in shelters performing this particular facial movement were rehomed faster than dogs not utilizing this movement as frequently. Juliane Kaminski, co-author on the study, suggests this specific facial movement may have evolved in dogs to heighten the animals' bond with humans.

"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," says Kaminski.

So, to home in on whether this physical characteristic was truly an object of evolution from domestication, the researchers closely examined both the anatomical differences between dogs and wolves, their evolutionary predecessors, and also behavioral differences between the two in facial communications with humans.

Through close soft tissue examinations the researchers discovered a tiny facial muscle, called the levator anguli oculi medialis, was almost always present in domesticated dogs but almost completely absent in wolves. Behavioral studies then revealed dogs produced these AU101 movements at significantly higher frequencies and intensities than wolves.

"The raised inner eyebrow movement in dogs is driven by a muscle which doesn't consistently exist in their closest living relative, the wolf," says lead anatomist on the project, Anne Burrows. "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."

It is hypothesized that dogs evolved this particular musculature trait as a way to selectively enhance their bond with humans who, either consciously or unconsciously, exhibited a preference for specific animals with these facial movements. One of the more fascinating elements in the study is the detection of such a rapid soft tissue evolutionary change. Obviously, changes in these kinds of tissues cannot be traced in fossil records so it is rare to be able to effectively home in on such a specific musculature adaptation. Even so, the researchers do suggest this is a fast evolutionary change, occurring over as little as 10 or 20 thousand years.

The study was published in the journal PNAS.

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