Dogs and their owners: A chemical bond
Have dogs hijacked a communication system that bonds mother and infant? Apparently so, says a study in Science this week. The action centers on oxytocin, a hormone first identified as stimulating childbirth, but now known to play many "affiliative" roles in people and animals.
Oxytocin is released into the blood (and can be detected in the urine) in many loving situations: It rises after healthy, happy sex. It rises in mothers and infants after they cuddle or look into each other's eyes. "In humans, 'mutual gaze' is the most fundamental manifestation of social attachment between a mother and infant," wrote Takefumi Kikusui, a professor of companion animal research at Azabu University in Sagamihara, Japan1. Mother's oxytocin level goes up as the mother-to-infant gaze is extended.
And now we see oxytocin rising when owners stare long and lovingly at their dogs. In the owners. And in the dogs.
In the experiment, the owners sat quietly for half an hour as the dogs had free rein in a plain, empty room. Before and after the session, both participants gave urine samples that were tested for oxytocin, and the results were correlated with the time spent gazing, talking and touching.
The biggest rise in owner oxytocin appeared among owners who had the longest eye contact with their dogs. The "change ratio" (the percentage change of final versus baseline levels) showed further harmony between dogs and owners. The owner's oxytocin change ratio correlated with:
their dog's oxytocin change ratio
the duration of their mutual gaze
how long the owner touched the dog
To test the reactions of dogs' ancestors, owners and their pet wolves went through the same procedure. Wolves are threatened by human eye contact, and not only was eye contact minimal, but wolf oxytocin dropped during the half-hour experiment. And here's news: Because the eye-contact interaction was absent in the wolves, the ancestors of dogs, it must have evolved since the domestication of wolves into dogs began.
Cementing the bond
The oxytocin-mediated relationship between dogs and owners resembles the one between mothers and infants, Kikusui says. Although the relationship between puppies and their mothers, "is based on licking, touching and grooming, which stimulate oxytocin release in dogs," the oxytocin communication system could help explain the close bond between dogs and people.
The dog-owner gazes were grouped as short (less than 70 seconds) and long (more than 90 seconds). It's not clear why the long gaze was so much more effective in raising the human oxytocin. The longer time may reflect a stronger bond between the two species, Kikusui wrote. Or, because eye-gaze "attachment behavior" is more common when dogs are anxious, the disparity may reflect an unfamiliar setting, an anxious dog and a stronger reaction (and therefore more oxytocin) from the owner.
Love in a spray bottle?
To prove that oxytocin was a cause, and not just an effect, in the interaction, the researchers spritzed oxytocin into the dogs' noses, then measured an increase in gazing between female (but not male) dogs and their owners. And then came the feedback: after these love-looks, "owners' urinary oxytocin increased even though they were not treated with oxytocin," Kikusui said.
While some of the gaze and oxytocin relationship has previously been published, Charles Snowdon, a professor emeritus of psychology at the University of Wisconsin-Madison who has worked extensively on hormones and behavior, highlights three aspects of the current study:
The discovery of a feedback loop between dogs and their owners.
The use of oxytocin spray. "This part of the study is frankly experimental," Snowdon says. "This has not been done before, and they found an increase in gaze behavior, that in turn increased the owners' level of oxytocin. That's an interesting feedback: you change the hormone in a female dog, it changes the level in the owner. " Snowdon, like the study authors, could not explain why male dogs were not affected. "A lot of studies show that male animals, men and tamarins, show increased affiliative behavior when given an oxytocin nasal spray."
The comparison to domesticated wolves. "Something changed fundamentally as dogs evolved from wolves, and it's plausible that their interaction with humans through the gaze and oxytocin feedback provided rewards to both sides," Snowdon says. "This makes a lot of sense as a factor in the domestication of wolves into dogs."
The eye-gaze-oxytocin interaction is a "tool" that originated in human-infant communication and was recycled by the dogs, Kikusui contends. "Humans instinctively release oxytocin while gazing with affiliative members. The point is that dogs can show the same communicative gazing toward humans."
– David J. Tenenbaum
Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer
- Oxytocin-gaze positive loop and the coevolution of human-dog bonds," by M. Nagasawa et al, Science, 17 April 2015. ↩
- Man’s Best Friend? The mysterious role of oxytocin revealed. ↩
- Is your dog happy to see you? Look at its eyebrows to find out. ↩
- How hunting with wolves may have helped humans outsmart the Neanderthals. ↩
- Did dogs really evolve from wolves? New evidence suggests otherwise. ↩