Most pet owners probably know what it’s like to cave to those “puppy dog” eyes—no matter the age of their canine. When your dog looks at you with that curled brow and doleful stare, it’s difficult not to give it a loving scratch or meaty treat. And why not: You and your furry friend have been conditioned by thousands of years of evolution for this moment, according to a growing body of research by biological anthropologists like Anne Burrows.
“Dogs are our closest companions,” she says. “They’re not closely related to us [as a species], but they live with us, they work with us, they take care of our children and our homes. So investigating different aspects of the dog-human bond, I thought, would help me understand human evolution and human origins.”
From disarming looks to alarming barks, Burrows and her team at Duquesne University in Pittsburgh investigate the ways dogs have evolved to express themselves in order to earn the title of “man’s best friend.” The research group is taking a detailed anatomical approach to understand how dogs and their wild relatives, wolves, evolved to have different traits, such as facial expressions and vocalizations. Burrows presented preliminary data from the lab’s latest canine facial-muscle studies at the American Association for Anatomy annual meeting in Philadelphia on April 5. These traits are also little windows into the evolutionary history of both dogs and humans.
“The story of dogs is the story of humans,” she says. “It helps us understand how we got here, and what we were doing in terms of technology, social behavior, over thousands of years.”
The ancient relationship between people and canine companions can also give anthropologists a window into human evolution, says Burrows. The timing is still contested, but some 15,000 to 35,000 years ago, early Homo sapiens in parts of Europe and Siberia began to change their relationship with local wolf populations. One theory to how that started is that a few bolder wolves began to cooperatively hunt with people for larger land game, allowing increased success for both parties. Another is that nomads left behind remains from butchered mammals that wolves would then dine on, causing the canines to become more domestic (but scholars have widely debated this narrative). “It could be something completely different, but the main hypotheses are that it somehow involved food,” says Burrows. Looking at the evolution of dogs in the context of these millennia-old interactions can show how human ancestors lived and survived in the past.
“Dogs are by far the earliest species that humans domesticated,” Burrows says. “In general, understanding dogs better will help us understand ourselves better, and where we came from.”
Burrows decided to focus on how dogs communicate with humans through their faces—a unique trait that’s rare between unrelated species, she says. This was inspired by her previous work studying facial muscles in primates. Chimps have displayed the ability to understand the facial expressions of other members of their species, similar to how humans depend on faces for context clues. In 2019, Burrows decided to hunt for similar signals between humans and dogs, and compare them to wolves.
“Whether we know it or not, dogs and humans are constantly looking at one another’s face, and trying to understand what the other one is feeling, and what the other one intends,” Burrows says. “So, facial expression is our proxy for understanding the relationship between dogs and humans.”
Previous studies have demonstrated that dogs can read and respond to human facial expressions—and even synchronize their emotions to match. “Dogs are watching us very closely—some of this is based on our gaze and body language, but also on the sounds we make and the scents we give off,” Monique Udell, an animal behaviorist and associate professor of animal sciences at Oregon State University in Corvallis, told National Geographic in 2021. Another study in July 2021 in the journal Current Biology found that dog puppies make more eye contact with humans than wolves, even when the wolf pups were reared by humans nearly from birth.
The evolutionary differences could be explained by fine-grained face muscles in both wolves and dogs. Burrows, along with collaborators Juliane Kaminski and Bridget Waller, found that a large range of dog breeds had defined musculature around the eye that lifts the brow up. The wolves they studied did not have the same attribute. This suggests that as humans domesticated wolves, they selected individuals with friendlier behaviors and traits, like a persuasive brow raise or smaller teeth and snouts. Since publishing these findings, Burrows has continued to collect data on other muscles that control facial expressions, known as mimetic muscles.
“We know dogs’ facial expressions, but we don’t really know how their muscles work in the actual contractions,” she explains.
Humans mostly have fast-twitch muscles in their faces, but do have more slow-twitch muscles than chimpanzees’, likely in part to form sounds for speech, Burrows says. Currently, her group is applying this reasoning and methodology to dog and wolf muscles by looking at the amount of fast- and slow-twitch fibers that control the duration and speed of contractions. Fast-twitch fibers allow for more spontaneity, but also tire more easily (think of smiling for a long period of time); slow-twitch fibers take longer to start contracting, but are better for endurance (think of sustained walking or running).
For their latest research, Burrows and her graduate students sampled cross sections of facial muscles in humans, dogs, and wolves and determined the amount of each fiber type. Burrows notes that the sample size of the preliminary data is small, with six wolf specimens and 10 specimens of different dog breeds. From this initial data, the team expected that the muscle profiles of dogs and humans would look similar, while the wolves would be distinct. However, they found that humans and wolves were actually more alike with more slow-twitch fibers overall, while dogs had more fast-twitch fibers.
“At first we were horrified,” says Burrows. “But as we thought about what muscle fibers of the face do, it kind of began to make a little bit more sense. Humans use speech, and that means we have to slow down our lips so that we can clearly articulate speech sounds. Wolves use howling and it’s a protracted vocalization—they kind of make a funnel out of their lips.” Meanwhile, a dog’s bark is a much shorter vocalization, so it doesn’t require them to hold their lips in one position for an extended amount of time.
The findings have led Burrows to suspect that humans may have favored wolves that had shorter staccato vocalizations during the process of dog domestication. Anthropologists have suggested that as humans domesticated dogs, they sought animals that could guard or warn them of any sudden threats. This alarm call—or bark—could have been important in the process of dog domestication. Now, “dogs just bark for a living,” Burrows says.
And while both animals do exhibit a range of vocalizations, they have a tendency to stick to their specific styles, Burrows says. Wolves only bark occasionally when they want to alert a nearby pack. And with the exception of certain breeds like huskies and hounds, dogs are much less partial to howls.
“We seem to have kind of created this weird creature, this dog that uses vocalizations very differently than the way wolves use them,” Burrows explains.
The team is planning on completing another year’s worth of data collecting before publishing their next study. But these initial findings are helpful in guiding the group’s next questions, Burrows says. Personally, she would like to investigate how the facial muscles of ancient dog breeds, like huskies, malamutes, and chow chows, compare to wolves, as well as younger dog breeds. The older breeds might be able to help anthropologists really break down the relationships that turned wolves into dogs.
“Our evolutionary history of becoming human is intimately tied to the process of dog domestication,” Burrows says. “When we look at a dog today, we see what was important to Upper Paleolithic people, 30,000-plus years ago. Dogs just get us in a way that no other animal does.”