NEW YORK (NYTIMES) - If you've felt your eyes welling up when your dog greets you with licking and tail-wagging after a long day away, the feeling may be mutual.
Dogs produce more tears when reunited with their owners than with other humans, researchers reported on Monday (Aug 22) in a small study in the journal Current Biology.
If that is true, it would be the first evidence that emotions cause tears not just in dogs, but in any non-human animals. Scientists who were not part of the study are not sure this conclusion is justified.
"If we accept the evidence of this paper, this is one of the most stunning discoveries in animal expression of emotions of all time," said Dr Clive Wynne, a canine behaviour specialist at Arizona State University. But, he added, "it would take a lot to convince me".
Sceptics were less concerned with another of the study's findings - that humans more favourably rated pictures of dogs with artificial tears in their eyes than those without tears.
The study's authors did not suggest that dogs weep with emotion the way humans do. But when dogs exhibit "watery, shiny eyes", it "facilitates human caregiving", said Professor Takefumi Kikusui, an animal behaviour and veterinary medicine specialist at Azabu University in Japan who is one of the study's authors.
Dogs have shown keen awareness of human emotion in previous studies.
Scientists have also established that dogs have emotional categories such as, "You are somebody I care about, therefore, I'm pleased to see you", and "You are somebody I don't care about, so I can ignore you most of the time", said Dr Daniel Mills, a veterinary behavioural medicine specialist at the University of Lincoln in England who was not involved in this study.
In the new study, scientists measured dogs' baseline tear volume and then compared it with dogs' tear volume when the animals reunited with their owners after several hours of separation. They also measured tear volume after the dogs interacted with a day care staff member. Twenty dogs participated in the comparison experiment.
Veterinarians quantified the canine waterworks with a Schirmer's test, which is used to diagnose dry eye syndrome in dogs, Dr Mills said. This involves placing a piece of filter paper for 60 seconds between a dog's lower eyelid and cornea. The farther the tears travel on the paper, the greater the tear volume.
Dr Wynne was sceptical that this method could prove a link between emotion and tear volume. If a dog is excited by reuniting with its owner, it might move around more, and the paper might rub against its eye more, leading to more tears. "So I don't buy it," he said.
Prof Kikusui, however, said the dogs' eyes experienced the same amount of paper rubbing in the different experimental conditions.
Scientists have also investigated the role of the hormone oxytocin in human-dog bonding. In research published in Science in 2015, Prof Kikusui and colleagues showed that a dog would experience a rise in oxytocin when reunited with its owner. In fact, oxytocin levels rise in both dogs and their human owners while they gaze at each other, they found.
For the new study, researchers administered oxytocin to the dogs' lacrimal gland, which is responsible for tear secretion. Then, they measured tear volume and found more tears in dogs' eyes with oxytocin than in those given a control substance. This indicates oxytocin plays a role in the watery-eye phenomenon, the study's authors wrote.
To throw a wrench - or maybe a bone - into it: Delivering oxytocin or the control solution to the eye directly could have irritated the dogs' eyes, said Dr Lauren Bylsma, a clinical psychologist at the University of Pittsburgh. Still, Prof Kikusui said the control substance did not change the baseline level of tears, while oxytocin did.
Dr Bylsma is a co-author of a 2018 paper called "Why Only Humans Shed Emotional Tears", a statement she still stands by. Dr Wynne does, too.
Still, Dr Mills supports the authors' broad conclusion that tears may play a role in human-dog bonding, despite also having concerns about their methods.
"It might be that things like a more glossy eye or the presence of tears do encourage nurturing tendencies in us," Dr Mills said, "in the same way that short-nosed breeds of dogs with high foreheads make us want to care for them more."
Perhaps the most famous early story of dog-human reunification is in Homer's Odyssey.
After Odysseus has been away for years, he returns home but no one immediately recognises him except his dog, Argos, who wags his tail and pricks his ears.
Odysseus, wanting to stay undercover, sheds a secret tear, Homer wrote.
Did Argos? More research is needed.