By: Zuri Sullivan
If you’re a dog person like me, you probably feel a special bond with your dog, and may even consider them as a member of your family. Though we’re unable to communicate with our dogs in the same way we communicate with other humans, many dog owners would likely agree that they feel an emotional connection to their pets. Because of these close relationships, scientists have long been interested in the evolution of domesticated dogs. Last week, the journal Science featured a number of research articles addressing longstanding questions about domesticated dogs. One of these articles investigated a possible mechanism by which we form emotional connections with our dogs, through a hormone called oxytocin.
Oxytocin is produced in the brain and acts as a powerful modulator of neuronal processes in mammals. It is specifically important in social behavior, playing crucial roles in the bonding between mothers and infants, as well as between sexual partners in species that exhibit lifelong mating behavior. The role of oxytocin in maternal/infant bonding has been well-studied in humans, and acts through a positive feedback loop. Positive feedback loops are extremely common in biology, and describe a system in which signal A promotes signal B, and signal B reciprocally promotes signal A.
Specific interactions, such as eye contact, between a mother and her infant increase oxytocin levels in the mother, and the increase in maternal oxytocin causes a corresponding increase in the infant’s oxytocin levels, which then amplify oxytocin levels in the mother. While this phenomenon is well-described between humans in an intraspecies manner, the authors of this study asked whether oxytocin-mediated bonding could be observed in an interspecies manner, specifically between humans and their pet dogs. This pairing is a great model in which to observe such interspecies interactions because humans have anecdotally described forming one-on-one relationships with their pet dogs.
To study this, the investigators looked at pairs of dogs and their owners, as well as pairs of hand-raised wolves and their owners. Wolves are the closest living relative of domesticated dogs and share many common biological features. However, because dogs, unlike wolves, have cohabitated with humans for many generations, the authors hypothesized that interspecies bonding behavior with humans would have evolved in dogs, but not in wolves.
In their first experiment, the authors observed interactions between the wolf/owner and dog/owner pairs. They specifically focused on eye contact, or gazing, as this interaction has been well-documented to stimulate oxytocin feedback loops in human interactions. They measured oxytocin levels in the urine of the dogs, wolves, or owners before and after their interactions, and also measured the duration of gazing between the animals and their owners. They found that the longer the duration of eye contact between dogs and their owners, the higher the levels of oxytocin in both the humans’ and the dogs’ urine. Interestingly, this correlation was not observed in the wolf/human pairs, supporting the idea that interspecies oxytocin-mediated bonding has evolved specifically in dogs as a function of their close evolutionary relationship with humans.
While this finding showed that gazing behavior was related to oxytocin levels in dogs and humans, it did directly not address the issue of the positive feedback loop. In other words, they showed that dog and human oxytocin levels were both elevated following extended eye contact, but not that dog oxytocin levels directly affect their owners’ oxytocin levels. To answer this question, they administered oxytocin or saline (salt water) to dogs, and then allowed them to enter a room in which their owner and two unfamiliar human volunteers were present. The investigators observed the interactions between the dogs and the humans, and measured oxytocin levels in the dogs and the humans throughout the experiment.
They found that female dogs who had received oxytocin engaged in more eye contact with their owners than the dogs who had received saline, and that the owners of the oxytocin-treated female dogs had significantly increased levels of oxytocin following the interaction, even though the owners had not been given oxytocin themselves. This experiment demonstrated that in female dogs, oxytocin stimulates dog/owner gazing behavior, which results in elevated oxytocin levels in the owner.
These results were not observed in male dogs, for reasons that remain unclear. Some evidence suggests that in humans, females are more sensitive to the effects of oxytocin than males. Additionally, in a small rodent called the prairie vole, oxytocin may be related to male aggression. Thus, the authors hypothesize that in their experiments, male dogs may have been exhibiting an aggressive response to the strangers in the room, limiting their interactions with their owners. In all species studied, however, the differing role of oxytocin between the sexes remains largely unknown.
Nevertheless, the results from this study indicate the bonds formed between dogs and their owners are mediated by oxytocin, the same hormone that contributes to maternal/infant bonding and lifelong sexual partners. Our feelings of affection for our dogs seem to be driven by bona fide neurological mechanism, in addition to how cute they are and how much fun they are to play with.