Dogs Have Co-opted Our Physiology to Win Our Hearts

Oxytocin is a neurotransmitter that plays a critical role in social bonds between mammalian mothers and their babies and between pair bonded mammalian mates. Activation of the oxytocin system increases feelings of reward and reduces feelings of stress. When a mother nurtures her infant, the baby will produce more oxytocin, which will cause the baby to feel more attached to its mother. The baby's attachment behaviors towards its mother will then cause the mother to produce more oxytocin, which will encourage her to be even more nurturing towards her baby. A Japanese research team lead by Miho Nagasawa hypothesized that this feedback loop has been hijacked by our furry companions to increase our level of care for them.

The researchers conducted two experiments. In the first experiment, they brought in volunteer dog-owners and wolf-owners and their canine companions. When a volunteer came in, the researchers collected pee samples from both the person and their dog or wolf. Then the owner was told to go into a room and to sit in a chair while his or her dog was allowed to freely explore the room for 30 minutes. The owner was allowed to freely interact with his or her dog during this time, but not give it food or toys. The researchers measured the amount of time the dog gazed at the owner, the number of times the owner talked to the dog, and the number of times the owner touched the dog. After this interaction period, the researchers collected pee samples from both individuals again. The pee samples were used to measure oxytocin levels in the body before and after this social interaction.

Dogs from this first experiment were categorized by how long they gazed into their owners' eyes: A long-gaze group spent more than a minute and a half in the first five minutes gazing at their owners and a short-gaze group gazed at their owners for less than a minute in the first five minutes. Wolves rarely gazed at their owners (most likely because sustained eye-contact is a threatening behavior among wolves). Interestingly, both the owners and the dogs in the long-gaze group increased oxytocin production in response to their social interactions, while neither the owners nor the dogs in the short-gaze group nor the owners with the wolves experienced this increase in oxytocin.

In the second experiment, additional dog owners volunteered themselves and their dogs. They all gave a urine sample, and the dogs were then given either an oxytocin nose spray or a saline nose spray. They then went into a room in which the owner sat in a chair and two unfamiliar people sat in two other chairs. The dog was allowed to freely explore the room and interact with all three people for 30 minutes, during which the researchers measured behaviors again. After the social interaction time, the owners and dogs all gave urine samples again to measure oxytocin.

The female dogs that were given oxytocin spent much more time gazing with their owners than the female dogs that were given saline. However, male dogs did not seem to be affected by the oxytocin nose spray. Furthermore, the owners of female dogs given oxytocin also increased their own natural oxytocin production. This was not seen in the owners of dogs given saline or the owners of male dogs. This sex difference may seem surprising, but remember that oxytocin is critically involved in the bonding between a mother and her infant, in large part through breastfeeding. Although males also produce and use oxytocin, another neurotransmitter called vasopressin often plays a stronger role in attachment in males.

Our furry companions appear to be taking advantage of a physiological system that we developed to drive us to take good care of our infants. Not only that, but their closest living relatives, wolves, do not do this. Most likely, we bred domestic dogs to interact with our physiology in this way: Our ancestors were more likely to breed dogs that were friendly and made us feel good when they gazed into our eyes. In doing that, they bred more dogs that had an active oxytocin system and strong attachment behaviors. And dogs with active oxytocin systems and strong attachment behaviors are more likely to work their ways into our hearts and homes.

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Further reading:
MacLean, E.L. and Hare, B. Dogs hijack the human bonding pathway, Science, 438:6232, 280-281 (2015). DOI: 10.1126/science.aab1200

Nagasawa, M., Mitsui, S., En, S., Ohtani, N., Ohta, M., Sakuma, Y., Onaka, T., Mogi, K.., and Kikusui, T. Oxytocin-gaze positive loop and the coevolution of human-dog bonds, Science, 438:6232, 333-336 (2015). DOI:10.1126/science.1261022

Image Credits:
Stella.jpg by Roberto Ferrari at Wikimedia Commons.