Sometimes an eyebrow raise is enough to melt our hearts. That is exactly what happens every day to many dog owners as their furry friends look at them with those “puppy eyes.” This seemingly simple facial movement dogs make has taken thousands of years to evolve and cements the human-dog bond.


Humans first prompted the development of dogs as we know them today around 30,000 years ago when wolves began to show a “friendliness” or tolerance towards humans. Indeed, it is now known that dogs evolved from these “friendly” wolves but unlike wolves, dogs can be the sweetest companions, forging a bond with their owners that is many times unbreakable.

Gray Wolf

A major part of this bond is the unique ability that dogs have to read and interpret their human’s actions. Dogs can follow a “communicative cue” from humans, such as pointing to an object. Dogs also pay attention and respond to humans when they look in a certain direction (follow their gaze) or touch an object.

In fact, eye contact between humans and dogs is essential for the human-dog bond. Scientists found that when dogs and humans established eye contact, oxytocin levels, a hormone important in mother-infant bonding and attachment, increased in both humans and dogs. Humans are also able to read and react to dog’s facial expressions, especially when their dog makes an infant-like expression with an “AU101 inner eyebrow raise.”

The Action Unit (AU) 101 movement makes a dog’s eyes appear larger, is a paedomorphic trait that humans prefer and is similar to the facial expression of human sadness, perhaps prompting a nurturing response from humans. To their surprise, comparative psychologist Dr. Juliane Kaminski and evolutionary psychologist Professor Bridget Waller found that dogs with this seemingly small facial movement were are adopted from shelters more quickly, suggesting that humans are indeed responsive to dogs’ facial expressions.

As mentioned earlier, thousands of years ago humans took in friendly wolves, interacted with them, and thus began a domestication process that affected their behavior and anatomy. Scientists have performed multiple studies that show how the human-dog bond works. However, we still don't know specifically how this skill has been shaped by humans and whether wolves can communicate in similar ways. In their recent study, Dr. Juliane Kaminski, Professor Bridget Waller and their team, published in Proceedings of the National Academy of Sciences, asked whether the selection of wolves and subsequent interaction with these animals through the years produced dog’s facial anatomy and behavior, specifically, the AU101 movement.


First, Dr. Kaminski and her colleagues compared the facial anatomy of four wild gray wolves (canis lupus) and six domestic dogs (canis familiaris) by performing dissections of dog and wolf cadavers obtained from various organizations (no animals were euthanized for this research). They found that dogs and wolves’ eyes are different in that dogs, and not wolves, have a levator anguli oculi medialis muscle (LAOM) above the eye, which helps to raise the inner corner of the eye. This explains why only dogs can give the “puppy eyes.”

In addition, they also found that dogs had a more prominent retractor anguli oculi lateralis (RAOL), another muscle around the eye that pulls the eyelid towards the ears. Interestingly, like the wolf, the Siberian husky, a breed that is closest to the wolf than all other breeds studied, did not have the RAOL.


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Syberian Husky

Then, they determined whether wolves and dogs behave differently by measuring the frequency and intensity of AU101 movements in both gray wolves and domestic dogs when a stranger approached their kennel. They found that dogs made more intense and frequent inner eyebrow movements than wolves suggesting that the muscles dogs developed as a result of selection by humans do play a significant role in social interaction.

In our interactions, we are wired to pay attention to and prefer certain facial expressions in other humans AND in their pets. Past research shows that dogs know to make certain facial expressions when they have their human’s attention, in an attempt to communicate. This is not surprising since many times, when many of us look at our pets and they give us that puppy eye look we immediately start thinking “do they need something? Are they sad? Are they hungry?” Since these facial muscles are absent in the wolf, it seems that these muscles evolved in dogs after domestication, suggesting humans played a role in the selection of these traits because of our preferences.


This study not only presents evidence of the dog’s evolution and behavior, but it also indicates we have much to learn about our evolution as human beings and our need to communicate and connect.


References

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Hare, B. (2002). The Domestication of Social Cognition in Dogs. Science,298(5598), 1634-1636. doi:10.1126/science.1072702

Kaminski, J., Waller, B. M., Diogo, R., Hartstone-Rose, A., & Burrows, A. M. (2019). Evolution of facial muscle anatomy in dogs. Proceedings of the National Academy of Sciences, 116(29), 14677-14681. doi:10.1073/pnas.1820653116.

Maclean, E. L., & Hare, B. (2015). Dogs hijack the human bonding pathway. Science, 348(6232), 280-281. doi:10.1126/science.aab1200

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Waller, B. M., Peirce, K., Caeiro, C. C., Scheider, L., Burrows, A. M., Mccune, S., & Kaminski, J. (2013). Paedomorphic Facial Expressions Give Dogs a Selective Advantage. PLoS ONE, 8(12). doi:10.1371/journal.pone.0082686.

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Fernanda Ruiz is a science content writer at Gold Biotechnology. She holds a bachelor's of science in biology from St. Mary's University and a PhD in molecular biology from Baylor College of Medicine.