(ORDO NEWS) — For a long time it was believed that tears of joy or sadness are inherent only to people. No other animal sheds emotional tears.
But now researchers in Japan have challenged that assumption by conducting a series of experiments involving man’s best friend. Our canine pets, they argue, also have eyes full of feelings.
When dogs reunite with their owners after a one-day absence, they not only whine with joy, but also cry with excitement, the study says.
Among 18 dogs, the researchers found a 10 percent increase in tear volume (compared to normal sputum) when the dogs greeted their owners.
But when meeting with a familiar non-owner, the dogs did not increase their tears.
Tear volume was measured by applying an absorbent piece of paper to the dog’s eye for 60 seconds and recording how far the moisture seeped through.
In our species, more tears are associated with more emotional arousal, and the current results suggest that in dogs, more tears elicit more positive emotions.
When a solution containing the “love hormone” oxytocin (also associated with feelings like trust, empathy and relationship building) was added directly to the eyes of 22 dogs in another experiment, the researchers found that there were more tears.
An increase in oxytocin levels (for example, when greeting a loved one) may be what causes additional tear production in dogs, the researchers conclude.
“We had never heard of the discovery that animals shed tears in joyful situations such as being reunited with their owners, and we were all excited that this would be a world first!” says animal behaviorist Takefumi Kikusui of Azabu University in Japan.
Finding happy tears in our canine companions would certainly be a revolutionary find. However, other scientists are not convinced by either the new experiment or its methodology.
While humans are the only creatures that cry as an emotional reflex, other animals use the moisture in their eyes to wash away dirt or clear their vision. So it’s entirely possible that adding oxytocin to the dog’s eyes may have simply caused irritated rather than happy tears.
Kikusui does not believe this is the case, as the control substance used in his experiments did not cause more tears in the dogs.
However, it is clear that the difference between an eye-cleansing tear and an emotional tear is a significant amount that is not easy to measure. And one study is hardly enough to resolve the dispute.
More research needs to be done to determine if dogs cry tears to express emotion reflexively, or if their watery eyes are just an adaptive way to get our species’ attention, like the special muscles that make “puppy eyes” so damn irresistible.
When Kikusui and his colleagues showed 10 photos of dogs to 74 people and asked them to rate their feelings, they found that dogs with wet eyes evoked greater feelings of care than dogs without wet eyes.
“Dogs have become partners with humans and we can form bonds,” says Kikusui.
“In this process, it is possible that dogs that show watery eyes when interacting with their owner will receive more care from the owner.”
Obviously, our species is attracted to dogs. But do we see in their eyes only what we want to see?
After thousands of years of co-evolution, we still have a lot to learn about our closest furry friends.
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