New research from scientists in Japan affirms a common suspicion held by dog owners: that their pets also deeply miss them when they’re away. The researchers found that dogs’ eyes well up with more tears after being reunited with their owners than with familiar non-owners or when the owners stayed at home. This behaviour in dogs appears to be influenced by the increased production of oxytocin, a hormone believed to play a role in emotional bonding in both humans and dogs.
Study author Takefumi Kikusui was inspired to conduct the research by an incident with his own dogs six years earlier. One of his two poodles had recently given birth, and while she was nursing her puppies, Kikusui noticed that she looked even cuter than usual and that her eyes were filled with tears.
Kikusui knew about oxytocin’s reputation as the “love” or “maternal” hormone, since it helps stimulate labour during pregnancy and the ability to breastfeed in new mothers. Oxytocin also seems to be released in higher amounts during moments of positive emotional connection in humans, such as having sex or even just being hugged, which can sometimes be expressed through tears. Many studies have shown dogs can have human-like social skills, and his team’s earlier work suggested that dogs and humans alike produce more oxytocin when spending time with one another.
“So that gave me the idea that oxytocin might increase tears,” Kikusui, a veterinary researcher at Azabu University in Japan, told Gizmodo in an email. “We previously observed that oxytocin is released both in dogs and owners when interacting. So we conducted a reunion experiment.”
The researchers recruited around 20 dogs for their experiments. They set up different scenarios, with the dogs having the tear volume on the surface of their eyes measured each time (dog tears don’t necessarily flow out as readily as they do in humans). In one scenario, the owners simply stuck around and interacted as usual; in the second, the owners would leave and return after five to seven hours; and in the third, they compared dogs at a day-care centre that interacted with owners as well as familiar non-owners. Compared to the baseline, dog tears were significantly more plentiful after they were reunited with their owners, while no significant differences were observed before and after the dogs saw familiar non-owners.
To further test their hypothesis, the researchers then dosed dogs with eye drops containing oxytocin or a placebo. When dogs received oxytocin, they produced more tears. Lastly, the team showed human volunteers photos of dogs with or without artificial tears in their eyes, asking them to rate how positively or negatively they felt about the dogs. Those with more tears were rated more positively on average, with the humans reporting wanting to touch or take care of them more.
In their paper, the researchers allude to the common sight of dogs overjoyed to see their owners again after a long (or even very short) period of time away, as shown by many a viral video. But as far as the researchers know, theirs is the first to investigate the link between emotional state and tear production in non-human animals like dogs. The findings suggest, Kikusui said, that “dogs shed tears associated with positive emotions, such as a reunion with the owner.” And if that’s the case, then oxytocin is likely to be the hormonal lever driving these tears.
The team’s findings were published Monday in Current Biology.
It’s hard to know what dogs or any animals are truly thinking, since we can’t verbally communicate with them. So there’s always some caution that should be taken when trying to interpret the meaning of canine behaviour. But one strength of the study, Kikusui said, is that they were able to measure a behavioural reaction in dogs (the tears) in response to a stimuli such as reuniting with their owners, and then they were able to demonstrate a possible and relevant physiological trigger (the oxytocin) for that behaviour.
Of course, dog owners hardly need any convincing that their dogs love and miss them. But this sort of research may help garner a deeper understanding of our best friends and the evolutionary path we’ve taken together. The fact that humans seem drawn to teary-eyed dogs might offer an explanation as to why this behaviour arose among dogs in the first place, for instance.
“We found that teary eyes of dogs can facilitate human caregiving. Dogs have become a partner of humans, and we can form bonds,” Kikusui said. “In this process, it is possible that the dogs that show teary eyes during interaction with the owner would be cared for by the owner more.”
There are other unanswered questions that Kikusui’s team or others may hope to explore in the future, he added. It’s not known whether dogs become more teary in response to negative emotions, for example, or whether they show the same tear response when they reunite with long-lost familiar dogs.