The facial structure of a dog has changed as they have become domesticated, according to new research.

Researchers claim domesticated dogs have evolved facial muscles that can twitch at a faster rate than wolves. They believe this evolutionary change occurred to help dogs better communicate with humans.

A previous study by the same researchers hypothesized that "dogs with expressive eyebrows had a selection advantage and that 'puppy dog eyes' are the result of selection based on humans’ preferences."

Researchers say a muscle responsible for raising the inner eyebrow intensely is uniformly present in dogs but not in wolves.

"An accurate understanding of how dog and wolf faces differ from one another is fundamental to understanding the processes of animal domestication, evolution of the dog, and the origins of human behavior since the Upper Paleolithic," the researchers said.