There are approximately 78.2 million pet dogs in the United States, approximately 86.4 million pet cats in the United States, and 5.3 million house rabbits. The two most popular pets in most Western countries have been cats and dogs. Now, here is a telling statistic: In 2013, pets outnumbered children four to one in the United States. Do we love our pets four times as much? Not quite. But these numbers do tell us something about our connection to our pets. Part of the answer lies in the very definition of “pets”: companionship animals.

 

When did it all begin?

The best records we have come from archeological digs. In northern Israel, the skeletons of a puppy in the arms of a human were unearthed under a 12,000-year-old home. This makes it the first animal to be domesticated when humans were still hunter-gatherers before the advent of agriculture. It also suggests that by that time dogs were not just work animals, but actual pets.

The skeletons of a human and dog were discovered underneath a 12,000-year-old home in northern Israel. (Photo from "Solving the mystery of dog domestication" Science/AAAS | News via Pinterest)
The skeletons of a human and dog were discovered underneath a 12,000-year-old home in northern Israel. (Photo from “Solving the mystery of dog domestication” Science/AAAS | News via Pinterest)

The prevailing theory about the process of domestication is that dogs, the offspring of an extinct species of wolf, were scavengers. And what better source of garbage then what humans leave wherever they go? The smarter and more daring individuals got closer to humans, got more of the food, had more offspring, and so on until after several generations of natural selection, humans realized they can use them for hunting, herding, and guarding.

Sounds good, except that it still doesn’t explain the deep emotional ties between dog and man. Why don’t we have such ties with cows or camels, for instance?

 

The psychology of pet lovers

Part of the answer is that our attraction to animal companionship is probably hard-wired in our brains. Vanessa LoBue, a developmental psychologist, and her colleagues at Rutgers University, showed that toddlers one to three years old, when given a choice, spend more time interacting with live animals (including fish, hamsters, snakes, spiders, and geckos) than they do with inanimate toys. Other psychological studies have shown that humans are drawn to images with characteristics of human infants, like wide eyes, broad forehead, and large head-to-body ratios.

Lassie and Jimmy from the 2005 Lassie movie
Lassie and Jimmy from the 2005 Lassie movie (Photo courtesy of TV Tropes.org)

Of course, it’s never all genetics; you don’t have to be a behavioral scientist to notice the cultural influences on our love of pets. Investigators found that movies featuring specific dog breeds would boost the popularity of that breed up to 10 years. Collies saw a 40% bump in registrations through the American Kennel Club after the 1943 release of Lassie, according to research published in Plos One, though researchers conceded that may have been assisted by its many sequels. But the study also found that registrations of Old English Sheepdogs went up 100-fold after Disney’s 1959 release of The Shaggy Dog, and 101 Dalmatians had a significant impact on the breed after its 1985 premiere.

All these observations are interesting, even suggestive of some love affair with pets innately etched in our psyche. But such studies are not capable of showing a neurobiological foundation for pet loving. And only the latter kind of studies can demonstrate a cause-effect relationship.

 

Neurobiology to the rescue

Kristoff Koch of the Allen Institute for Brain Research found neurons in the amygdala that respond preferentially to animal images. This is significant because the amygdala is an ancient brain structure (even lizards have it) that is involved in emotions.

fMRI study measured brain activity in 14 mothers while they were gazing at pictures of their children and their dogs or at pictures of other people’s children or unfamiliar dogs. The Massachusetts General Hospital researchers found that the brain activation patterns evoked by images of the women’s own children and dogs were similar and that those patterns were different from those elicited by unknown children and dogs. What this suggests is that maternal feelings may extend to animals. More generally, it suggests that pets may help fill a human need to nurture other living beings.

All these are strongly suggestive. But anatomical location is not a definitive proof of mechanism. To close the loop, we need the molecules that actually perform the function.

 

The oxytocin connection

Oxytocin has long been implicated in social bonding, such as mother-infant. But what’s the relationship to dog-owner bonding? Nagasawa and her colleagues published an important paper in 2009. She and her co-workers focused on the role of long eye contact in oxytocin levels. They found that owners and dogs sharing a long mutual gaze had higher levels of oxytocin in their urine than owners of dogs giving a shorter gaze. The Nagasawa paper concluded that their finding was “a manifestation of attachment behavior.” The researchers postulated that gazes between a dog and human (particularly a known human) will share similar properties to mother–infant relationships.

beagle looking into woman's eyes (991x483 px)

Nagasawa’s group published a new study in Science that investigates whether a dog’s gazing behavior is affected by not just the owner’s oxytocin concentrations but the dog’s as well. In Experiment 1, the researchers collected urine from 30 dog-and-owner pairs before and after a 30-minute interaction. Owners whose dogs showed the most gazing behavior had a significant increase in oxytocin concentration, confirming the 2009 paper. But in a notable addition to the last paper, they found that the owner’s urinary oxytocin rose as well.

In Experiment 2, another set of 30 dogs was given an intranasal spray of either oxytocin or saline prior to interacting with people. They found that female dogs that sniffed oxytocin gazed longer at their owners than when given saline. This gazing also stimulated oxytocin secretion in the owner recipients of the gaze. The mutual effects were not seen between dogs and unfamiliar humans; and for reasons that require further investigation, they were not seen in male dogs and their owners. These sex differences were not observed in Experiment 1.

What about the wolf, a cousin of our beloved dog? Nagasawa and colleagues also investigated whether the increased oxytocin observed in dogs appears in hand-raised wolves that have interacted with a known human. The wolves, however, rarely held a gaze with the humans for more than a few moments. This divergence led the researchers to postulate that “dog-to-owner gaze as a form of social communications probably evolved during domestication” with humans. A note of caution: This experiment included only five wolf-owner interactions. More such pairings are needed to confirm the result.

 

The neurobiology is still incomplete

Many things are coming together. Recall the paper by Kristoff Koch cited above, showing that the amygdala containing cells that preferentially respond to animal images? Well, amygdala cells carry lots of oxytocin receptors as well.

Remember the famous documentary of female penguins returning from a long, grueling march and locating their pups out of hundreds of young penguins? Obviously, it would be hard to do research in Antarctica, but consider this: Oxytocin enables pup retrieval behavior in female mice by enhancing auditory cortical pup call responses. Retrieval behavior—required in the left but not right auditory cortex—was accelerated by oxytocin in the left auditory cortex, and oxytocin receptors were preferentially expressed in the left auditory cortex. Mice are not as cute as penguins, but they, and us, respond similarly to hormonal signals. The reason is that evolution is notoriously conservative (in a non-political sense): whatever works tends to be conserved. That’s the biological version of “if it ain’t broke, don’t fix it, use it again, and again”.

Much has to be discovered still. Oxytocin is not the only hormone that is associated with socialization. Endorphins and dopamine participate as well, and the interactions between them remain largely unexplored. But the latest discoveries are already very promising. For instance, scientists can now explore oxytocin response in psychiatric conditions such as schizophrenia and autism, both of which are characterized by difficulty in socialization. Maybe my dog averting his eyes when I gaze at him is the first case of canine autism. And maybe one day a puff of intranasal oxytocin (yes, it’s available) will be mandatory for our politicians before they mouth off.


Updated 01/24/16 in honor of Valentine’s day.

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.

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