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Research of the month: Dogs evolved a special muscle because we’re suckers for ‘puppy dog eyes’

Lucy Goodchild van Hilten, 25th July 2019

In another of our monthly blog post written by Science Wordsmith Lucy Goodchild, we explore the findings and attention around a piece of research published in the previous month that caught the public’s attention. Listen to the accompanying podcast here.

We know the power of ‘puppy dog eyes’ – that irresistible look that dogs use to wind their humans around their paws. Now scientists have shown that the facial movement is peculiar to dogs, and it has evolved since domestication as a result of their interaction with people.

The idea for the study came about several years ago, when comparative psychologist Dr. Juliane Kaminski was studying dogs at a shelter. She videotaped dogs at a shelter when they were meeting new people and then analyzed their behavior, looking at things like how often they barked, how often they wagged their tails, how willing they were to approach new people and what facial movements they made. They then asked the shelters how quickly the dogs would be adopted.

There was only one behavior that seemed to have an effect: a particular eyebrow movement that resulted in ‘puppy dog eyes’. Dogs that produced that movement more often were adopted more quickly than those that didn’t.

They were interested in understanding this movement better, so Dr. Kaminski and her colleagues dug deeper and compared dogs with wolves. In a study published in PNAS, they looked at the anatomy of wolf heads and dog heads to see if there were any differences, then they looked at the behavior of wolves and dogs.

Read the study: “Evolution of facial muscle anatomy in dogs

They found that anatomically, the two are almost identical, except for a muscle found in dogs but not in wolves that produces a certain eyeball movement. The behavioral data confirmed the difference: dogs produce that eyeball movement significantly more often and more intensely than wolves do.

“It’s an eyebrow movement, where the eyebrows are moved upwards and inwards, which gives the dog this kind of puppy-dog-eye look,” explained Dr. Kaminski, who is a reader in Comparative Psychology at the University of Portsmouth. “We think that dogs that presented this eyebrow movement might have triggered a natural caring response in humans – when people saw this movement, unconsciously, they wanted to care for that dog in front of them.”

They believe that dogs that produced the eyebrow movement – called the AU101 movement – had a selection advantage over dogs that didn’t. This suggests that the dogs’ interaction with humans directed the evolution of this muscle in domesticated dogs.

“We think that humans have unconsciously selected dogs to look a bit like infants, or present infant-like traits,” added Dr. Kaminski.

Caption: Dr. Juliane Kaminski, University of Portsmouth, with a Rhodesian Ridgeback

This finding was a surprise: dogs are thought to have become domesticated about 30,000 years ago, which is very little time in evolutionary terms.

“It surprised me that in such a short amount of time we suddenly have the presence of a new muscle and structure in the dog’s face and compared to the wolf,” said Dr. Kaminski. “It’s quite exciting from an evolutionary perspective, and it means that selection pressure on this must have been quite strong. So we humans have unconsciously favored dogs that that produce this movement, and we did so quite strongly.”

Irresistible to journalists

The irresistible eyebrow movement that fascinates Dr. Kaminski and her colleagues pulled in the press: with coverage by more than 300 news outlets, the article has an Altmetric attention scoreof more than 3600.

“Puppy dog eyes evolved to melt a human’s heart,” said the Straits Times. “Dogs’ eyes evolve to appeal to humans,” said the BBC. And “Humans Are Probably Behind the Evolution of ‘Puppy Dog Eyes’,” said Discover Magazine. It was sparked by a press release issued by the University of Portsmouth, helping Dr. Kaminski and the team get the right message out.

Discover the coverage:

“I was really quite impressed with the feedback and with level of the coverage,” said Dr. Kaminski. “People are always excited about dogs, but I think people are also really interested in the research and the science behind this, so that’s good to see.”

It’s not the first time Dr. Kaminski has hit the headlines with her work; as a PhD student in 2004, she published a paperabout a dog called Rico, who could identify 200 different objects by name, resulting in lots of news coverage.

There’s plenty more to study; Dr. Kaminski and the team plan to explore the findings further and to test the hypotheses they raised in the paper. One thing they believe might be happening is that humans are making decisions based on unconscious preferences they might have; another is that the eye movement is more attractive to humans because it exposes more of the white parts of the eyes. This kind of engagement with the public will be vital to their ongoing research.

“It’s always good to see if our research really reaches the people, and it’s good to get this direct response to see that people are actually interested in what we are doing,” said Dr. Kaminski. “The dogs that we are working with are family dogs, and so I see this as a way of giving back some information to the dog owners about why we’re doing this – why we want their dogs to participate in our study. I definitely think it’s really important to engage with these things.”

Dr. Kaminski’s top tip for promoting research

Be open to really engage, but do the things that you’re comfortable with. I definitely think it’s important to engage with the public about research, especially the kind of research we’re doing with dogs.

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