September 2019

Where's the evolution?

It’s common knowledge that the distinct traits of the hundreds of different domestic dog breeds are the result of human manipulation of evolution. For the last 200 years, humans have opted to breed animals with traits that better served our purposes, generation after generation. For example, favoring dogs that were better at pulling sleds through Arctic snow led to the evolution of the husky; while favoring dogs better at retrieving wild game from the water and pulling fishing nets led to the Chesapeake Bay Retriever. Still other dog breeds were selected for more whimsical, less practical purposes. The Yorkshire Terrier, for example, resulted from breeders’ preference for long, silky fur. These are classic examples of artificial selection in action.

It wasn’t just outward aspects of dogs’ anatomy that we shaped through artificial selection. In the last 15,000 years, while living alongside humans, domestic dogs have diverged from their closest relatives, grey wolves, in many ways. Unsurprisingly, our ancestors seem to have selected for animals that better fit into human societies, resulting in dogs that were less wary of humans and less aggressive, with smaller teeth and shorter snouts.

Now new research suggests that we were also (likely unintentionally) favoring dogs that reminded us of ourselves and human children — in particular, dogs able to pull their inner eyebrows up to make a face that looks a bit “sad” to us. This eyebrow raise is performed by a little muscle with a big name (the levator anguli oculi medialis, LAOM for short). Researchers studied the facial anatomy of domestic dogs and grey wolves, and found that while most muscles in the face were similar in the two groups, the LAOM was much larger in domestic dogs. This larger size translates into more movement, the scientists also found. Dogs raise their inner eyebrows more often than wolves do, and dogs raise their eyebrows higher than wolves do.

These observations alone would be a suggestive “just-so” story, but probably not enough to convince an evolutionary biologist that puppy-dog eyes evolved through human artificial selection. However, other lines of evidence also support this idea. For example, researchers studied domestic dogs in a shelter and found that those that made puppy dog eyes more often were adopted sooner than others. Thus, among shelter dogs, making big puppy dog eyes provides a survival advantage by appealing to human caretakers. If puppy dog eyes boost survival today, it seems likely that, during domestic dog evolution, the trait also conferred a survival and reproductive advantage — and hence, higher evolutionary fitness.

Comparative studies of wolves and dogs further suggest that puppy dog eyes represent just one of a suite of traits that evolved during domestication and allowed dogs to better communicate with and appeal to humans. Dogs understand human communication cues like pointing better than wolves (or even chimpanzees, our closest living relatives) do. Dogs use human eye contact as a way to ask for help, while wolves do not. And finally, sustained eye contact between dogs and humans (i.e., “mutual gaze”) leads to a positive hormonal response (an oxytocin feedback loop, similar to the one that occurs when a mother and her infant gaze at one another) from each species, but does not occur when wolves and humans look at one another.

Based on all this, scientists think it is plausible that some traits of ancestral domestic dogs happened to trigger human caregiving responses, which originally evolved in the context of parent/child interactions. Dogs that expressed such traits benefited from more human care and perhaps favoritism, and so were more likely to survive and reproduce than dogs that did not. Over time, through the accumulation of small variations in these triggers (extending eye contact for a bit longer, having a larger LAOM and producing slightly more heartbreaking eyes, etc.), the connection between domestic dogs and their humans evolved to be even closer.

This hypothesis suggests that evolution, in some ways, “hijacked” the human caregiving response — i.e., a trait that initially evolved in the context of benefitting human offspring wound up also benefitting another species. However, this does not mean that dogs themselves are intentionally manipulating us or that they evolved any traits “in order” to get closer to humans. Evolution does not occur because of need or effort. It simply retains traits that boost survival or reproduction — in this case, a trait that is easily conflated with intentional manipulation or effort. And of course, humans likely also benefitted from their association with dogs, gaining additional protection and a domestic animal that could be put to use in many ways — as a hunting partner or sled puller, for example. So the next time a puppy looks up at you with those big eyes and tugs on your heartstrings, go ahead with the “awwwwww” and the ear rubbing; your ancient human ancestors probably did the same.