Puppy-Dog Eyes Evolved to Pull Your Heartstrings

Which is harder to resist: a child's big eyes pleading with you for a piece of candy, or a puppy's forlorn expression begging for table scraps? It's a tough call, and according to science, that may be thanks to evolution. A recent study published in the Proceedings of the National Academy of Sciences suggests that dogs specifically evolved puppy-dog eyes to appeal to humans.

Puppy-Dog Eyebrows

The researchers found that dogs, but not wolves, possess a muscle called the levator anguli oculi medialis, which raises their inner eyebrows and helps them produce those classic puppy-dog eyes. Specifically, the researchers note, this muscle enables an "infant-like" expression that's similar to the expression humans make when they're sad.

They figured this out by analyzing the anatomy of six dogs, all of different breeds, alongside four grey wolves. They found that the domestic dogs had this muscle but the wolves didn't, which suggests that the muscle was one of many traits that evolved as humans and dogs bonded over tens of thousands of years of evolution. A 2017 study backs this up: Dogs pretty much only use this look when interacting with humans.

SensorSpot / iStock / Getty Images Plus

That bond between man and dog is just as strong as that between a parent and child.

A 2015 study published in Science found that both dogs and humans experience a surge of oxytocin, known as the bonding hormone, when making eye contact with each other. That's the same thing that happens when humans make eye contact with someone they love, like a mother looking at her child. This discovery surprised the researchers since typically, staring can be seen as a sign of aggression by wild animals. But dogs seem to be an exception.

Maybe that's because humans see their dogs as their own kids. A 2014 study in PLOS ONE found that when women saw a picture of their dog, their brains responded similarly to if it were a picture of a child — but only if it were their own child. The reaction to their dog was stronger than it was to a picture of an unknown child, suggesting that this was more than just a reaction to a cute face. Maybe dogs really are our fur babies!

How Did We Get So Close?

How dogs became man's best friend is a question that's been subjected to a lot of scrutiny in the past few years. One study published in the journal Nature Communications in 2013 suggests that dogs became domesticated around 32,000 years ago. That study also found that humans and dogs evolved many traits in parallel, including certain genes for digestion and metabolism.

Why? Well, the researchers believe that as humans moved into more crowded living environments, their diet changed, and because dogs fed off human scraps, dogs' diets changed too. By analyzing the DNA of 10 different breeds of wolves and dogs and comparing it with human DNA, the researchers were able to find striking similarities in genes dealing with things like cholesterol transport and even the processing of serotonin. We've been bonded with dogs for so long that we've evolved together. Our friendship is written in our DNA.

But how did wolves turn into dogs originally? Researchers at Duke University's Canine Cognition Center think it happened like this: Wolves originally competed with early humans for food. They figured out that they could get more food if they scavenged for humans' scraps, so they started hanging around human settlements. But any wolf that posed a threat to humans risked being killed, so it paid to be friendly. Over time, these ancient good-dogs evolved to be even more appealing to humans, gaining floppy ears, youthful features — and those iconic puppy-dog eyes. Thus was the start of a beautiful friendship.