* Updated 16:50 14 January 2009 by Ewen Callaway
Who needs children when a puppy can provide a similar emotional experience? After playing with their pets, dog owners experience a burst in a hormone linked to infant care, not to mention romantic love and friendship, new research finds.
Nicknamed the "cuddle chemical" and the "love drug", oxytocin has been found to dampen stress, combat depression, and breed trust in humans. Studies of voles, mice and rats also point to oxytocin's role in pair bonding and social memory.
For this reason, biologists Miho Nagasawa and Takefumi Kikusui, of Azuba University in Japan, wondered whether social contact between two different species could boost oxytocin levels, as well.
"Miho and I are big dog lovers and feel something changed in our bodies when gazed [upon] by our dogs," Kikusui says.
Look of love
They recruited 55 dog owners and their pets for a laboratory play session. Owners provided a urine sample to measure oxytocin levels, and then played with their dog for half an hour. Another urine test followed.
As a control on another occasion, some owners sat in a room with their dog and were told to completely avoid the gaze of their pets.
Kikusui's team videotaped the sessions and measured how long a dog spent eyeing its owner. Based on the analysis, the researchers split the pairs that were allowed to play into two groups: "long gaze", who locked eyes for an average of 2.5 minutes during the play session, and "short gaze", who made eye contact for fewer than 45 seconds, on average.
They found that these groupings reflected changes in owner's oxytocin levels. In participants that spent a long time making eye contact, oxytocin levels rose by more than 20% during the play session, on average. In the control group, owners that avoided their pooches' gaze saw their oxytocin levels drop slightly.
Kikusui thinks eye contact is a good proxy for the bond between owner and dog. Long-gaze owners tended to rate their relationship with their pet as more satisfying than short-gaze owners. And even when instructed to avoid eye contact during the control session, these owners experienced a mild boost in oxytocin.
A flood of the cuddle chemical could explain why playing with dogs can lift moods and even improve symptoms of anxiety and depression, Kikusui says.
More speculatively, oxytocin might have played a part in the domestication of dogs from wolves, about 15,000 years ago, the pair suggest. "Maybe during the evolutionary process, humans and dogs came to share the same social cues", such as eye contact and hand gestures, Kikusui says. "This is why dogs can adapt to human society."
One previous study found that humans who are administered oxytocin looked toward the eyes of people in photographs more often and for longer than subjects given a placebo.
However, Clive Wynne, a psychologist at the University of Florida in Gainseville, is skeptical that oxytocin release played a role in dog domestication. "Genetic evidence shows that wolves were turning into dogs thousands of years before anyone could suggest that people were involved," he says.
Still, he thinks that oxytocin could explain why some owners seem more devoted to their dogs than their families. "Think of the Helmsley women who gave a hell of a lot more money to her dogs than to her grandchildren," he says.
Journal reference: Hormones and Behavior (DOI: 10.1016/j.yhbeh.2008.12.002)