Wolves are wild, powerful, and fearsome predators, capable of bringing down even large prey. And yet, tens of thousands of years ago, some wolves started forming close associations with humans. They became more docile. Their bodies changed. They turned into domestic dogs. Today, we share bonds with them that can be as strong as those that tie us to other people. How did this happen? How did we go from fear to friendship? How did dogs start inspiring such genuine feelings of love and affection?
Miho Nagasawa at Azabu University, Japan has a possible answer. It involves oxytocin, a mammalian hormone that draws our attention to social cues. In yesterday’s post, I described mouse experiments which show how oxytocin sensitises an inexperienced mother to the distress calls of her pups, and eventually cements the bond between them. Dogs seem to have hijacked this chemical connection between mother and child, to cement a similarly strong bond with their owners.
Nagasawa showed that a dog’s gaze raises the oxytocin levels in its owner, prompting more social contact. In return, the owner’s gaze raises oxytocin levels in the pooch. This chemical loop unites the brains of two different species. “[It’s] a powerful mechanism through which dogs win our hearts—and we win theirs in return,” write Evan MacLean and Brian Hare from Duke University, in an accompanying editorial.
First, Nagasawa allowed thirty dog owners to interact with their animals for half an hour. She collected urine samples from both parties before and after that period, so she could measure the oxytocin levels in their bodies.
She found that the volunteers whose dogs gazed at them for the longest time experienced the biggest surges in oxytocin. They, in turn, spent more time looking back at their dogs, touching them, and talking to them. And the dogs that received the most reciprocal attention also experienced the biggest oxytocin spikes. Nagasawa had already shown the dog-to-human part of this loop in 2009 but she has now closed it, demonstrating that both species raise oxytocin levels in each other.
The same can’t be said for wolves. Nagasawa did the same experiment with eleven pure-bred wolves that were hand-reared by people. They weren’t pets, but they did have daily contact with their owners, who fed them and occasionally played with them. Despite their dog-like existence, these wolves did not make regular eye contact with their owners, and their gaze didn’t trigger a rise in oxytocin. The cross-species oxytocin loop only works between humans and domestic dogs.
Next, Nagasawa injected 27 dogs with oxytocin and placed them in a room with their owner and two strangers. After the injections, the dogs—but only the female ones—spent more time gazing at their owners, who then experienced a rise in oxytocin. It’s not clear why only the female dogs responded in this way. They might be more sensitive to the hormone, or less wary about the presence of unfamiliar people. Whatever the case, this second experiment confirmed that an oxytocin spike in one species can trigger a similar spike in the other.
These results offer important clues about the events that transformed wild wolves into domestic dogs. “During dog evolution, we have probably selected for a behaviour in dogs that elicits a physiological response in us that promotes bonding,” says Larry Young from Emory University. “That behaviour is eye-gazing.”
Among wolves, eye contact is a threat, which is why they rarely look directly at each other. Young suspects that wolf pups might communicate with their mothers through looks, triggering the same kind of affectionate cycle that exists in humans. “This just goes away as they mature,” he speculates. Perhaps as wolves evolved into dogs, they simply kept this child-like means of communication, just as they also retained some of the physical traits of their younger selves. In this way, they could have tapped into the oxytocin loop that strengthens bonds between human mothers and their babies, and triggered an almost parental affection.
Indeed, brain-scanning experiments have shown that there are overlaps in the brain regions that become active when human mothers look at images of their children or their dogs. “Diverse aspects of our biology appear to be tuned into dogs and children in remarkably similar ways,” write MacLean and Hare.
Reference: Nagasawa, Mitsui, En, Ohtani, Ohta, Sakuma, Onaka, Mogi & Kikusui. 2015. Oxytocin-gaze positive loop and the coevolution of human-dog bonds. Science http://dx.doi.org/10.1126/science.1261022