In many ways, dogs appear and behave more like wolf puppies than wolf adults. Scientists have theorized that these "paedomorphic" features are the byproduct of selecting dogs against aggression. But a new study suggests this is no byproduct at all, but rather the result of a human preference for puppy-like facial expressions.
How wolves were first domesticated is still somewhat of a mystery. It's likely they self-domesticated themselves by adjusting to humans and taking advantage of scavenging possibilities. As for their juvenile characteristics, such as floppy and large ears, curly tails, big eyes, rounded forehead, and shortened snout, those came later — likely the result of selecting dogs that were less aggressive; puppy-like physical traits were a side-effect of this preference.
The Inner Brow Raiser
Perhaps. But a new study by Bridget Waller and colleagues now suggests we have been steadily reshaping the faces of dogs owing to our preference for juvenile expressions.
Specifically, we seem to like dogs that exhibit a particular muscular contraction — the inner brow raiser which lifts the medial portion of the brow. This increases the apparent size of a dog's eyes in relation to the face, thus enhancing a feature associated with human infants (i.e. we find big eyes really, really cute). Another theory is that the same contraction in humans indicates sadness, which can be a sign of vulnerability. It also exposes white sclera, which contributes to gaze following abilities; we are more likely to cooperate or behave altruistically when being watched.
Regardless of the reason, we seem to really like this particular feature in dogs.
A Powerful Selective Force in Domestication
The researchers came to this conclusion by running an interesting experiment at a dog re-homing shelter. While tracking each dogs' inner brow movements, the researchers took note of how quickly the dogs were chosen by adoptive owners. Their results showed that juvenile-looking dogs were in fact more preferred.
"This finding further supports the growing evidence that indirect manipulation of human sensory preferences (particularly a preference for juvenile facial characteristics) has been a particularly powerful selective force in domestication, even more so than genuine indicators of temperament," conclude the authors in their study.
What's more, this preference may have also been at work during the early stages of dog domestication.
It's worth noting that tail wagging and close proximity to humans were not strongly associated with the speed of selection by adopters.