The other day I saw a picture of a dead dog on Twitter. Gross, right? Not at all, for this wasn’t just any old dog: This was Hachiko, perhaps the most famous dog in the world, and certainly the most famous in Japan.

The tweet announced that it was the last photo ever taken of Hachiko. He is shown lying on a pallet, surrounded by the staff of Shibuya Station and his owner’s wife. It is 1935, and every day for the last 11 years Hachiko has been turning up at Shibuya Station to meet his master, a University of Tokyo professor called Hidesaburo Ueno. But unbeknownst to the dog, the professor died in 1925, so for 10 of those years Hachiko has waited in vain.

Even when he was alive, Hachiko was a famous dog. People would come to see him, pat him and give him treats. But when he died (of cancer, not from a yakitori skewer as some legends have it), the fame just got bigger. A year before his death, a statue was erected outside the station in his honor.

It’s obvious why he is so famous. It’s a touching story. People like to feel the sad warmth that comes from contemplating a story of death, loyalty and love. When I searched Twitter to find the photo I’d seen, I found lots of mentions of Hachiko, lots of stories about him and lots of retweets of “the last photo.”

Dog lovers are a fierce tribe, and it may be reckless to even ask why dogs are so loyal, or seem coldly scientific to investigate the underlying cause. But getting on for 100 years since Professor Ueno’s death, fellow University of Tokyo scientists are doing just that.

Teresa Romero, Miho Nagasawa and colleagues — all current or one-time dog owners — are looking at how the hormone oxytocin is involved in the way dogs form bonds with humans and other dogs.

If Hachiko is the world’s most famous dog, oxytocin may be the most famous hormone. (Though adrenalin would put up a good fight.) Often introduced as “the cuddle chemical,” “the love hormone” or even “the moral molecule,” it is known to be involved in the formation of the bond between mother and child (the contractions of the uterus stimulate its production). It is behind the monogamous pairing of prairie voles, and it can also make people more trusting.

Most research on oxytocin has been on monogamous animals, and in humans in a reproductive context. Romero and Nagasawa wanted to explore its possible role in other types of social relationships.

Being dog people, they naturally wondered about dogs — canine relations with humans and with other dogs. How do dogs — which are pack animals, remember — form social bonds with their group members?

“Oxytocin was an excellent candidate, since previous research has shown that it plays an important role in parental bonding and mating in both humans and other animals,” says Romero. And if oxytocin is involved, it makes sense to investigate its role in the relationship dogs form with humans.

The team recruited 16 dogs. (It would be nice for my story if one of the subjects was an Akita dog, the same breed as Hachiko, but no: nine standard poodles, four Labrador retrievers, one German shepherd, one Shetland sheep dog and one border collie.)

The animals were given oxytocin in a nasal spray, and their behavior was recorded for the next half hour. The team found that compared with dogs given only a saline nasal spray, the oxytocin-dosed dogs showed more positive social behavior toward other dogs and humans. They approached them more, looked toward them more and initiated social contact more.

“Oxytocin plays an important role in how dogs form and maintain friendly relationships with both their owners and dog partners,” says Romero.

The team conclude that oxytocin might be the way enduring cooperative bonds evolved. Dogs or other animals don’t need to understand that it will be in their genetic best interest to cooperate with each other, or even with other species — oxytocin does it for them.

Dog people will often assert that their dogs have moods, and even that dogs are capable of love. Sometimes it’s hard to deny the idea. I read about a dog with a lifelong hatred of water, but which jumped into a lake to drag a fallen child to safety.

But oxytocin can explain bonding without invoking love. Since the hormone enhances the feeling of reward that the animal receives, a dog automatically learns to associated the good feeling it gets from positive interactions with another dog or with a human. A positive feedback loop means that if it repeats something that has given it a rewarding feeling — such as waiting for its master at the train station for the pat on the head it receives — it will continue to perform that behavior.

Or as Romero puts it: “Our results provide new insight into the mechanisms that facilitate the maintenance of close social bonds and complement a growing body of evidence that identifies oxytocin as one of the neurochemical foundations of sociality in mammals.”

Her work is published this week in the journal Proceedings of the National Academy of Sciences (DOI reference: 10.1073/pnas.1322868111).

It makes me wonder about Hachiko. Why did he wait so long? Was he stuck in a hopeless feedback loop, waiting every day for a reward that never came? “It’s unknown if differences in the type and distribution of oxytocin receptors in dogs modulate their behavior,” says Romero.

Surely in normal cases the loop wears out — dogs stop waiting, people stop loving. If you never stop loving and you never stop waiting, is something wrong?

Actually, I think that’s something that no experiments on oxytocin will ever answer.

Rowan Hooper (@rowhoop on Twitter) is the news editor of New Scientist magazine. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru (The Evolving Human).”

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