The science behind puppy dog eyes

Dogs have evolved muscles around their eyes that allow them to make the so-called puppy eyes that melt humans’ hearts, a UK-US research team has found.

The study - published this week in the US journal Proceedings of the National Academy of Sciences - says that in a process spanning tens of thousands of years, people unconsciously bred dogs with “expressive eyebrows” and an “infant-like” look after domesticating the animals from wolves.

Is this a new discovery?

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Previous studies have shown how such canine expressions can appeal to humans, with dogs with more mobile brows proving most likely to be rescued from animal shelters. What is new is the understanding that an anatomical difference has evolved over time to exploit the human instinct to nurture.

People respond to dogs that can raise their eyebrows in the middle of their faces, creating a look that suggests anxiety, also known as puppy dog eyes. The team behind the new research say mobile brows allow dogs to “create the illusion of human-like communication”.

“When dogs make the movement, it seems to elicit a strong desire in humans to look after them,” says the study, co-authored by Dr Juliane Kaminski, a psychologist at the University of Portsmouth.

The movement creates an expression like that which “humans produce when they are sad”, and also makes dogs’ eyes “appear larger [and] more infant-like”, the study authors explain.

How do they know?

Study leader Kaminski and her colleagues dissected the bodies of dogs and wolves that had been donated to science after dying natural deaths. These dissections revealed that the muscle around the eye that facilitates that movement was “almost completely absent” in wolves but “large and prominent in dogs”, says National Geographic.

A second eye muscle, which makes dogs look more human by exposing more of the white of the eye, was also larger in domesticated animals. With the exception of these eye muscles, dogs and wolves are “very similar anatomically”, the science news site notes.

How did this difference arise?

The bigger eyebrow muscles found in dogs have “evolved” as a result of “human selection”, says Scientific American. In other words, people chose to breed dogs that had more mobile eyebrows, along with other desirable characteristics such as docility.

The new study argues that people have not deliberately chosen dogs with expressive eyebrows, but rather exhibited an “unconscious preference” for those that looked more human.