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May 18, 2015 | By:  Sedeer el-Showk
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How Dogs and Humans Grew Together

Dogs are commonly called our best friends and truest companions, but what makes our relationship with them so remarkable? According to new research, the answer may lie in how we've evolved to co-opt one another's social machinery.

Like humans, dogs use gaze to communicate, a trait not shared by their closest relatives, the wolves. In fact, dogs and humans excel at socializing with one another. Wolves and chimpanzees lack a similar relationship and can communicate only poorly with each other, raising the question of how our special bond with dogs originated.

To find out, researchers at several Japanese universities teamed together to study the role of the social hormone oxytocin in the relationship between a dog and its caretaker. Oxytocin is a "social reward" hormone; it's important in regulating the bond between a mother and her infant, family members, or monogamous partners. When a caretaker and dog touch, oxytocin levels go up in both, but the team wanted to know if the same thing could happen when the two simply looked at each other. Specifically, they wanted to test whether gaze could set off a postive oxytocin feedback loop in a human-dog pair, deepening their bond.

The team asked caretakers to interact with their dog for 30 minutes and measured how much of the time they spent touching, talking to, or mutually gazing at their dog, and then measured urinary oxytocin levels in the dog and caretaker. They also tested hand-raised wolves and their caretakers as a control. As expected, the wolves hardly gazed at their human, and there wasn't a significant change in oxytocin level in either of them. In human-dog pairs, oxytocin levels in both the human and dog were correlated with how long the dog gazed at the human, but not the amount of time spent touching or talking.

Next, the team wanted to know whether the increase in oxytocin affected the dogs' behaviour. To find out, they gave the dogs a nasal spray of oxytocin (or, as a control, saline solution) and then led them into a room where their caretaker sat with two unfamiliar humans. To avoid social interactions that might confound the study, the humans were forbidden from talking to each other or voluntarily touching the dog. The researchers once again measured how long the dogs spent gazing at or touching their human, and then measured urinary oxytocin levels after the interactions.

The oxytocin treatment increased the amount of time spent gazing, but only by female dogs. Oxytocin can also regulate aggression and its effect is known to depend on sex, so the male dogs may have responded to the treatment with increased vigilance in the presence of two strangers. Another possibility is that the females were more sensitive to oxytocin. However, the important result is that oxytocin levels also went up in the caretakers, though only of the female dogs. In other words, female dogs given oxytocin spend longer looking at their human, leading to more oxytocin in the human, which would lead to friendly interactions and thus more oxytocin — a joyful oxytocin-mediated feedback loop of bonding, bringing the two ever closer.

Sometime during their evolution, after they and the wolves parted ways, dogs began a coevolutionary journey with humans. The two species share a remarkable degree of social tolerance and flexibility. As we've evolved together, we co-opted one another's social attachment systems to create a powerful interspecies bonding mechanism — or, more simply, to become best friends.
Nagasawa, M, Mitsui, S, En, S, et al. Oxytocin-gaze positive loop and the coevolution of human-dog bonds. Science 348(6232):333-336. (2015) doi:10.1126/science.1261022
All images are © Hannele Luhtasela El-Showk and are used with permission.

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