For some dog owners, a leisurely walk can turn stressful the moment their canine companion sees another pup walking by. Dogs with what is known as “leash aggression” may bark, growl or lunge at other dogs during walks, setting the scene for a tense and potentially dangerous interaction. So why do some dogs lash out on the leash while others don’t? Hormones may be to partly to blame, according to new research led by the University of Arizona’s Evan MacLean. Although a number of studies have looked at the role of testosterone and serotonin in aggression in dogs and other mammals, those hormones may be only part of the story, according to MacLean’s findings, which are published in a special issue of the journal Frontiers in Psychology. MacLean and his collaborators looked specifically at oxytocin and vasopressin — hormones that are also found in humans — and found that they may play an important role in shaping dogs’ social behavior. Better understanding the biology behind canine aggression could help with the development of interventions, said MacLean, an assistant professor of anthropology and director of the Arizona Canine Cognition Center in the UA School of Anthropology. “Dog aggression is a huge problem. Thousands of people are hospitalized every year for dog bites, especially kids, and aggression is one of the main reasons that dogs get relinquished to shelters,” MacLean said. “If there are ways to intervene and affect biological processes that produce aggression, that could have a huge benefit both for people and dogs.” MacLean was interested in oxytocin and vasopressin — sometimes thought of as “yin and yang” hormones — because of the growing research on their role in the biology of social behavior. Oxytocin, which is significant in childbirth and nursing, is sometimes called the “love hormone,” as its levels in humans have been shown to increase when we hug or kiss a loved one. Vasopressin is a closely related hormone involved in water retention in the body. In contrast to oxytocin, it has been linked to aggression in humans, with previous research suggesting that people with chronic aggression problems have high levels of vasopressin. For the current