Humans domesticated dogs about 30,000 years ago. Since then, we've worked with them, hunted with them, played with them, and come to rely on them for companionship. And, in the process, we've bred them for everything from general cuteness to the ability to guard and fight for us. Figuring out who's manipulating whom and who's getting more out of the relationship is a hopeless task.

But that doesn't mean that some aspects of the changes dogs have undergone aren't amenable to study. After studying the facial muscles of dogs and wolves, a US-UK team of researchers has now found that dogs have two muscles that wolves mostly lack. These muscles control the movements of the face near the eyes, and the researchers suspect that the muscles' presence helps the dogs make a sad-eyed face that we find appealing.

A “take me home” look

The new work arose from an earlier paper done by several of the same researchers (Juliane Kaminski, Bridget Waller, and Anne Burrows). In it, they looked at what's technically called a "paedomorphic facial expression" in dogs. Paedomorphic means that adults retain features that are commonly associated with young animals—we tend to view these as cuter. In this case, the expression was raising the skin above the eyes, closer to the bridge of the nose. This expression, shown above, has been interpreted as "sad-eyed" and is thought to tug on humans' heart strings.

The earlier study showed that it worked. Dogs that showed this expression in a shelter were more likely to be adopted by humans.

That set the stage for the new work. The fact that humans preferred dogs who could flash them this look suggests it could provide a selective pressure that would favor the trait, even if humans weren't consciously aware they were selecting for it. So the researchers hypothesized that this selection could change the musculature of dogs' faces, making it easier for them to flash sad eyes at unsuspecting humans.

To test the hypothesis, the researchers did something that undoubtedly horrified the dog-lovers among them: they dissected the heads of deceased dogs and wolves in order to identify all the muscles present. They did not have a lot of samples (four wolves, six dogs), so the results should be viewed with that limitation in mind. And, for the most part, there was no difference, as you'd expect from species that have only been separated for 30,000 years.

But they did find differences in two sets of muscles. One, called the retractor anguli oculi lateralis muscle, pulls the side of the eyes toward the ears. That was present in some wolves and all dogs but was more developed in dogs. But the key finding was that the levator anguli oculi medialis muscle, or LAOM, is nearly absent in wolves. The muscle is present in dogs, where it's used to pull the upper inside corner of their eyes up, creating a sad expression. So, this provides some significant support for the author's hypothesis.

It’s all in the eyes

Facial expressions aren't the only paedomorphic features that we favor in dogs; there's plenty about puppies we find appealing and often like to see retained into adulthood. One of those features is large eyes, and there's no shortage of evidence that these are a key avenue of communication with humans. Dogs will try to establish eye contact with humans when confronted with a problem, and they will often not follow human gestures if they can't see the eyes of the person doing the gesturing.

The authors also note that humans are one of the few primates that shows off the whites of its eyes most of the time. This allows us to communicate in part by making it apparent where our eyes are directed. To an extent, both of these muscle changes also allow dogs to show more of their eyes, possibly allowing them to take a greater part in this sort of unspoken, subconscious communication.

Whatever the driving force behind the changes in eye musculature, the researchers confirmed they have consequences. Dogs are far more likely to use the LAOM muscle to pull their eyes up than wolves are, and they are able to make a far larger range of facial expressions using it. Wolves, by contrast, appear limited to a minimal movement by their small muscles.

While the evidence provides support for the idea that humans have shaped the facial muscles of their longstanding companions, there's a complication here that may mean the shaping is somewhat indirect: muscles can change in size based on frequency of use. So, there's a chance that the muscles' size are, in part, a downstream effect of behavioral changes. Those could still be genetic (and thus subject to evolutionary change), but the relationship may be more complicated than simply whether or not there's a muscle there.

PNAS, 2019. DOI: 10.1073/pnas.1820653116 (About DOIs).