The evolution of 'puppy dog' eyes

Analysis by Dr. Karen Shaw Becker

puppy dog eyes

Story at-a-glance -

  • Recent studies suggest that selection processes and behavioral adaptations may have given dogs the ability to perceive human cues to communicate with them in ways other animals can’t
  • The “inner eyebrow raising movement” in dogs may have developed in their eyebrow muscle over thousands of years, but only after being domesticated from wolves, and wolves do not have the same muscle
  • Previous studies show that eye contact is seen as a cue to dogs which human communication is specifically relevant to them, and they move their eyebrows more when they know humans can see them
  • Dogs’ expressions change depending on whether they’re looking at another dog or a human, especially if the dogs whose “viewing behavior” is being observed feel threatened
  • According to researchers, the newly acquired eyebrow movement makes dogs’ eyes seem larger and gives them an almost childlike appearance, which may mimic the facial movements made by humans when they’re sad

When you see dogs in different situations, it's uncanny how their eyes seem to reflect the atmosphere of the moment. But perhaps as dogs were domesticated more than 33,000 years ago, studies conjecture, selection processes and behavioral adaptations gave dogs the ability to perceive human cues in ways other animals can't.

Scientists say it's possible the expressions your sweet dog flashes in your direction, say, when you're telling him what a good dog he is, or when he sees your reaction to the hole he dug in your front yard, may be easier to read now than it may have been a few thousand years ago, according to a recent study.

Published in the journal Proceedings of the National Academy of Sciences (PNAS), the review1 compares the anatomy and behavior of both dogs and wolves over millennia, suggesting that their facial structure, specifically the muscles around their eyes, has functionally "evolved" so dogs could "better communicate with humans."

The research team at the University of Portsmouth in the U.K., in collaboration with behavioral and anatomical experts from Duquesne University in Pittsburgh, was led by comparative psychologist Juliane Kaminski, Ph.D., with anatomist professor Anne Burrows, Ph.D., evolutionary psychologist Bridget Waller, Ph.D., anatomist Rui Diogo, Ph.D., from Howard University in Washington D.C. and anatomist Adam Hartstone-Rose, Ph.D., from North Carolina State University as co-writers. According to Science Daily:

"In the first detailed analysis comparing the anatomy and behavior of dogs and wolves, researchers found that the facial musculature of both species was similar, except above the eyes. Dogs have a small muscle, which allows them to intensely raise their inner eyebrow, which wolves do not."2

It may be significant that according to the researchers, the Siberian husky, one of a few ancient dog breeds, is the only dog species they studied that didn't possess the relatively recent eyebrow-moving muscle, given the name Action Unit (AU) 101, according to Waller, who was responsible for mapping out the facial muscular structure of dogs.

It's all about the eyebrows

Kaminski says there's compelling evidence that the "inner eyebrow raising movement" in dogs may have developed in the muscle over thousands of years, but only after being domesticated from wolves.

In fact, after studying the behavior of both species — domestic dogs of the Canis lupus familiaris species and Canis lupus being the scientific name for wolves — the researchers found that when exposed to the presence of a human for two minutes, the inner eyebrows of dogs raised more and with greater intensity than those of wolves. Kaminski says:

"The findings suggest that expressive eyebrows in dogs may be a result of (humans') unconscious preferences that influenced selection during domestication.

When dogs make the movement, it seems to elicit a strong desire in humans to look after them. This would give dogs, that move their eyebrows more, a selection advantage over others and reinforce the 'puppy dog eyes' trait for future generations."3

In a previous study in Germany in 2011, also led by Kaminski, eye contact is seen as a cue to dogs which human communication is specifically relevant to them; interestingly, when humans gestured to dogs, but the dogs couldn't see the humans' eyes, the gestures were ignored.4 It also appeared that the number of times dogs moved their eyebrows when they knew they were being observed by humans was significantly more often than when they didn't.

Using "eye gaze tracking" to observe the expressions of both dogs and humans, the researchers noted that dogs generally looked at humans' eyes first, and for longer than at their nose or mouth areas. According to another one of Kaminski's studies at the University of Helsinki in 2016:

"Threatening faces evoked attentional bias, which may be based on an evolutionary adaptive mechanism: the sensitivity to detect and avoid threats represents a survival advantage.

Interestingly, dogs' viewing behavior was dependent on the depicted species: threatening conspecifics' faces evoked longer looking but threatening human faces instead an avoidance response. Threatening signals carrying different biological validity are most likely processed via distinctive neurocognitive pathways."5

Further, "the social gazing behavior of domestic dogs resembles that of humans: dogs view facial expressions systematically, preferring eyes."6 Additionally, their expression depended on whether they were looking at a dog or a human, especially if the dogs (the ones whose "viewing behavior" was being observed) felt threatened.

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Observations and conclusions regarding dogs' anatomy

According to Kaminski, Burrows and Waller, domestication is what enabled wolves to be transformed into dogs, and it wasn't just about their socialization, but in the anatomy of their facial structure, "specifically for facial communication with humans."

"Based on dissections of dog and wolf heads, we show that the levator anguli oculi medialis, a muscle responsible for raising the inner eyebrow intensely, is uniformly present in dogs but not in wolves …

This movement increases paedomorphism and resembles an expression that humans produce when sad, so its production in dogs may trigger a nurturing response in humans. We hypothesize that dogs with expressive eyebrows had a selection advantage and that 'puppy dog eyes' are the result of selection based on humans' preferences."7

According to Waller, the newly acquired ability of eyebrow movement makes dogs' eyes seem larger and gives them an almost childlike appearance, which may be an imitation of the facial movements made by humans when they're sad. She adds that their research exhibited how important facial expressions can be in capturing our attention, and how powerful it can be in social interaction.

Burrows observed that the eyebrow muscles in wolves were "a scant, irregular cluster of fibres." Hartstone-Rose weighed in, saying that in dogs, they were so thin you could see through them, but with a powerful enough effect that they seem to have been "under substantial evolutionary pressure." Diogo also noted:

"I must admit that I was surprised to see the results myself because the gross anatomy of muscles is normally very slow to change in evolution, and this happened very fast indeed, in just some dozens of thousands of years."8

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