It's long believed that tears of joy or sadness are unique to humans. No other animal is known to spill emotional tears.

But now, researchers in Japan have taken on that assumption with a contentious series of experiments involving man's best friend. Our canine pets, they argue, have eyes brimming with feelings, too.

When reunited with their owners after a day-long absence, dogs do more than simply whimper with joy; they also tear up with excitement, the study suggests.

Among 18 dogs, the researchers found a 10 percent increase in tear volume (compared with usual wetness) when the dogs greeted their owners.

But, the pooches didn't well up when they met a familiar non-owner.

The teardrops were measured by placing an absorbent slip of paper against a dog's eye for 60 seconds and recording how far the wetness crept.

In our own species, a greater volume of tears is linked to greater arousal of emotion, and the current findings suggest that it is positive emotions eliciting the extra tears in dogs, too.

When a solution containing the 'love hormone' oxytocin (also associated with feelings such as trust, empathy, and relationship-building) was added directly to the eyes of 22 dogs in another experiment, the researchers found more tears were produced.

An elevation in oxytocin (say, from greeting a loved one) is possibly what is triggering extra tear production in dogs, the researchers conclude.

"We had never heard of the discovery that animals shed tears in joyful situations, such as reuniting 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.

The discovery of happy tears in our canine companions would certainly be a revolutionary find. But other scientists are not convinced by the new experiment or its methodology.

While humans are the only creatures known to cry as an emotional reflex, other animals use the moisture in their eyes to flush out dirt or clear their vision. It's therefore possible that adding oxytocin to a dog's eyes may simply have produced irritated tears, not happy ones.

Kikusui doesn't believe this is the case, as the control substance used in his experiments did not prompt more tears among dogs.

Yet clearly, the difference between an eye-cleaning tear and an emotional tear is a weight not easily measured. And one study is hardly enough to settle the debate.

More research needs to be done to determine if dogs cry tears to reflexively express emotions or if their watery eyes are just an adaptive way to appeal to our own species, similar to the special muscles that make 'puppy dog eyes' so darn irresistible.

When Kikusui and colleagues showed 10 photographs of dogs to 74 human participants and asked them to rate how it made them feel, they found that the dogs with damp eyes elicited greater feelings of care than those without moist eyes did.

"Dogs have become a partner of humans, and we can form bonds," Kikusui says.

"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."

Our species is obviously attracted to dogs. But are we only seeing what we want to see in their eyes?

After thousands of years of co-evolution, there's still so much more to learn about our closest furry friends.

The study was published in Current Biology.