Source: The Washington Post (Extract)
Posted: Jun 17, 2019

The mournful, pleading, eyebrows-up expression dogs make is so familiar that we have made it into an idiom: puppy-dog eyes.

A study suggests it’s yet another example of dogs’ remarkable ability to communicate with people, one that evolved as the animals were domesticated from ancient wolves at least 15,000 years ago.

A specific muscle causes the inner-eyebrow raise that makes dogs’ eyes appear bigger and more infant-like and produces a look similar to one humans make when sad. The researchers found that while six deceased dogs they dissected uniformly possessed this muscle, four dissected wolves either did not have it or barely did. They also found that 27 shelter dogs in the United Kingdom did the eyebrow-raise far more often and intensely when interacting with strangers than did nine wolves at two U.K. wildlife parks.

This suggests ancient canines with expressive eyebrows might have elicited nurturing from humans, the authors write, and that care would have given the animals a selection advantage that allowed them to pass on puppy-dog eyes to their descendants. The study was published in the journal Proceedings of the National Academies of Sciences.

“What is so provocative about this finding is the likelihood that our unconscious biases shaped the evolution of the dog’s eye musculature,” said Brian Hare, a Duke University evolutionary anthropologist and canine cognition expert, who edited the study but was not involved in the research. “The presence of these anatomical differences between wolves and dogs is a smoking gun for the role of our desire to cooperate and communicate with dogs being a driving force in dog evolution.”

The new research built on the authors’ previous work examining this muscle movement in dogs. One study found shelter dogs that performed the inner-eyebrow raise were adopted more quickly than those that did not, in a sort of modern-day demonstration of puppy-dog eyes’ power over humans. Another study found that dogs make puppy-dog eyes more when people are looking.

“It’s just that kind of behavior that causes lots of dog owners to think: ‘Oh my gosh, they want something. What do they need?’ ” Anne Burrows, a biological anthropologist at Duquesne University, said in an interview.

When it comes to human-dog connections, eyes are important. Dogs make eye contact with people when a problem stumps them; wolves do not. Researchers have found that both humans and dogs experience a rise in levels of oxytocin – the “love hormone” – when they’re gazing into each other’s eyes. The same thing happens when mothers and babies make eye contact.

The new study identified another facial difference between wolves and dogs. A muscle that pulls the eyelids toward the ears, which Burrows said happens when dogs are panting and almost appear to be “smiling,” was present in all the dogs dissected except one – the Siberian husky, an ancient breed that is more closely related to wolves. Three of four wolves dissected had it, but it was thinner and probably weaker, the study said.

The eyebrow finding raises the possibility that puppy-dog eyes aren’t something humans chose, but instead are a by-product of domestication. A long-running study on silver fox domestication has demonstrated that when people select the tamest animals for breeding, certain physical traits result – floppy ears, curly tails, mottled coat colors. Just why remains unclear. This is called the domestication syndrome, and it’s possible dogs’ eyebrow raise is a part of it, Burrows said. “Domestic dogs were created by humans, and clearly we selected species that weren’t going to bite us or bite our kids,” Burrows said. “So this whole domestication syndrome has features that all link together.”