Ever wonder why your furry friend’s eyes seem to speak volumes? Delving into the fascinating world of dog evolution, we explore how our canine companions developed the iconic “puppy dog eyes” that tug at our hearts.
Humans have selectively chosen traits in dogs as they became domesticated, and this includes the ability to make facial expressions. Scientists have shown that dogs developed muscles around the eyes that were not present in their ancestor, the gray wolf. These muscles enable dogs to raise their eyebrows, creating “puppy dog eyes.”
Ongoing research into the evolution of dog facial expressions is shedding light on why these muscles developed, helping us to further understand the human-dog relationship.
Thousands of Years in the Making
The symbiotic relationship between domestic dogs and humans dates back over 30,000 years. Early wolves benefited from humans by consuming leftover meat from hunts, while humans leveraged their wolf companions’ ability to find prey faster, provide protection from predators, and help with carrying supplies.
Wolves that had physical and behavioral traits advantageous to humans were selectively favored, and those traits were passed down through generations. Over time, they became more prevalent, resulting in the domestic dogs we know and love today.
Building the Bond
Although domestic dogs and gray wolves remain closely related genetically, dogs have several characteristics that have evolved to support their bond with humans. Many of these encourage eye contact and mutual gazing.
For example, mutual gazing between humans and dogs produces oxytocin (the “love hormone”) in a feedback loop not observed between humans and wolves. Researchers artificially induced oxytocin in dogs to better understand its link to human eye contact. Dogs who received oxytocin made more attempts to make eye contact with humans than dogs who did not.
Dogs use eye contact with humans to signal that they need assistance. This behavior is much less likely to happen with wolves. For example, dog puppies attempt to make eye contact earlier than hand-raised wolf puppies. They also lack the avoidant and aggressive behavior observed in hand-raised wolves toward humans.
The Evolution of Puppy Dog Eyes
Scientists have noted that this eye contact with humans in dogs is often accompanied by facial expressions. One expression is the raising of the inner eyebrows to make “puppy dog eyes.” And, as any pup parent can tell you, what a powerful expression it is!
A 2013 study involving 27 shelter dogs found that those who gave puppy dog eyes more often were adopted faster. This suggests a possible human preference for dogs who make the expression.
A 2019 study examined facial muscles in six dogs and four wolves. While wolves are capable of raising their eyebrows, the strength and number of times they do so is much lower than in dogs.
Dogs use eye contact with humans to signal that they need assistance.
Researchers found differences in two muscles surrounding the eye that helped explain this observation—the levator anguli oculi medialis muscle (LAOM) and the retractor anguli oculi lateralis muscle (RAOL).
The LAOM is located above the eye toward the nose and raises the eyebrows. This muscle is highly developed in dogs. In wolves, however, it’s only a small set of muscle fibers sometimes hindered by a tendon connecting it to another muscle called the orbicularis oculi muscle.
The RAOL pulls the outer corner of the eye back toward the ears. This muscle was found in five of the six dogs examined and three of the four wolves. When present, the RAOL was made of thinner fibers in the wolf than in the dog. The one dog without a RAOL was a Siberian Husky, an ancient dog breed more closely related to the wolf than the other dogs studied. This may explain why a RAOL was absent.
What Drives Puppy Dog Eyes?
There are several possible reasons why puppy dog eyes were a trait selected by humans.
One of the leading ideas is that puppy dog eyes allow dogs to communicate better with humans.
Humans naturally focus on the eyes and upper half of the face during social interactions. Expressions in this area, such as eyebrow raising, convey information and help to communicate intent and emotions. Humans may have favored movements in similar areas of the face in dogs and selected for them.
One study examined the communicative function of dog eyebrow raising by testing the effect of human attention and food presence on facial expressions in 24 dogs. The results showed that dogs who received human attention made more facial movements than the dogs who did not, regardless of whether or not food was present. This finding suggests that facial movements made by dogs are an active attempt to communicate with humans.
Studies have supported the idea that dogs can express their emotions through involuntary facial expressions. Puppy dog eyes may be a reaction to a dog’s emotional state. Puppy dog eyes resonate with humans because they resemble a similar look made when humans are sad. Dogs who are perceived to be sad may have been selected due to the empathy they received from humans.
While research has found that dogs do not make human-like facial expressions, studies have shown that humans and dogs can correctly interpret each other’s facial expressions of emotion.
To Solicit Care
Research shows that humans have a preference for dogs with a larger forehead and eyes. This may be because these same features are associated with infants. Dogs with a larger forehead and eyes are perceived to be younger and more vulnerable and are more likely to be spoken to in a pitch used for infants.
The puppy dog eyes expression exaggerates the size of the forehead and eyes, increasing the appearance of infant-like features. This may cause humans to select for dogs who make puppy dog eyes and nurture them.
Puppy dog eyes resonate with humans because they resemble a similar look made when humans are sad. Dogs who are perceived to be sad may have been selected due to the empathy they received from humans.
The Cooperative Eye Hypothesis
Visible white space in the eye allows humans to determine gaze direction and infer cues. This improves communication as well as cooperation and has played a large role in human evolution.
In a study comparing stuffed animals with and without visible white space in the eyes, humans showed a preference for those with visible white space. Puppy dog eyes, characterized by an exaggerated white portion, align with this preference and may be another reason why dogs may use this expression.
Facilitating Eye Movement
A 2021 study concluded that eyebrow raising in dogs primarily serves the purpose of aiding eye movement. Researchers counted the number of eyebrow raises in 21 dogs, both in the presence and absence of a human.
Dogs exhibited fewer eyebrow raises when a human was present, and these raises were almost never observed without concurrent eye movement. The authors suggested that the reduced frequency of eyebrow movements in the presence of a human might be attributed to the dogs focusing on the human and refraining from shifting their gaze. Alternatively, dogs shifted their gaze more frequently in the absence of a human, and performed more eyebrow raises as a result.
More Than One Right Answer
All of the possible reasons mentioned above share the common thread of strengthening the human-dog relationship. It’s possible that one factor is most responsible, or that multiple factors have contributed to humans favoring puppy dog eyes over time.
The Co-Evolution of Dog Anatomy with Humans
Learning more about how and why puppy dog eyes came to be can help humans provide better care for dogs and offers insight into human and dog evolution.
It’s possible that one factor is most responsible, or that multiple factors have contributed to humans favoring puppy dog eyes over time.
Studies suggest multiple reasons behind puppy dog eyes. They may be used to communicate with humans, involuntarily express emotions, facilitate eye movement, or a combination of these factors. Knowing the purpose of puppy dog eyes can help humans better understand and respond to their dog, further strengthening the bond between them.
Lastly, the anatomy responsible for making facial expressions does not easily change through evolutionary time. Dogs are an exception due to their co-evolution with humans. Studying the progression of facial muscles in dogs may help scientists better understand the links between emotion, brain processes, and facial expression, which can then be applied to studies in other species.
Research so far continues to support the strong preference of facial expression by humans.
One recent study found that fast-twitch muscle fibers made up 66 to 95% of fibers in dog faces, but only 25% in wolves. Slow-twitch fibers are responsible for endurance movement while fast-twitch fibers are responsible for quick and explosive movement, like those needed for facial expressions.
Regardless of the initial reasons puppy dog eyes resonated with humans, it’s a safe bet that they’re here to stay.
Featured Image: sonsam via iStock / Getty Images Plus
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