Alexandra Mlejnkova / AP

The phrase is “man’s best friend,” full stop. Not “man’s best friend unless maybe a better offer comes along, in which case, well, it’s been fun.” But Marc Bekoffs’s dog, being a dog, was unfamiliar with this particular saying. And so when a better offer came along, he took it.

When Bekoff, now a professor of ecology and evolutionary biology at the University of Colorado, was a graduate student, he brought home a fox for a temporary stay in his apartment. The attraction between this new animal and his dog was instantaneous: “They became the best of friends,” he said. They played together; they slept side by side. When Bekoff crated the fox during the day, the dog would “sit in front of it and whine so he could see his fox friend.” Once, when he tried to separate them at night, “the fox basically gnawed through one of those baby fences” to get closer to his pal.

Bekoff, in other words, had become a third wheel in his own home.

I melted when he told me this story. I held back squeals on the phone, because they say squealing during an interview is unprofessional. There’s a reason that interspecies friendships are the subject of so many books and calendars and even television shows: They seem like such a pure, against-the-odds type of love.

For about as long as humans have been studying animal behavior, though, they’ve been considering the question of whether such a love is really possible. Is it fair to call these pairings friendships, or is there something else that’s drawing them to one another?

Gordon Burghardt, a psychologist and ecologist at the University of Tenessee, dismisses the argument that animal friendships are just humans anthropomorphizing other behaviors. “Mother-infant bonding, no one has a problem extending that from a human to a chimpanzee,” he says. “I think if you’re careful, it’s pretty reasonable to extend behavioral similarities across species.”

And in recent years, the case for animal friendship has solidified: Chimpanzees choose their companions based on personality, elephants offer one another emotional support in times of stress, bats form cliques within their larger colonies—all elements of the bond that we call friendship when it’s shared between two humans.

But researchers aren’t as sure what’s going on with the star-crossed lovers of the animal kingdom—especially when these interspecies pairings are a predator and its prey. Over the past few weeks, for example, a tiger at the Siberian Zoo named Amur has been palling around with a goat, Timur, which handlers originally left in his enclosure as a meal. Amur had attacked and eaten all his previous goats; this one, though, spends his days romping through the tiger enclosure with his would-be killer. Here’s a video of the two animals at play:

“They are inseparable,” Dmitry Mezentsev, the director of the park, told The Siberian Times. Amur, who had never before been aggressive with park staff, has begun hissing at anyone who gets too close to Timur.

One possible explanation: At the moment that Timur entered his enclosure, Amur was lonelier than he was hungry. Animals in captivity have their food presented to them; they don’t need to worry about marking their territory or looking for mates the way an animal in the wild would. “All those activities take time and energy, and if those needs are removed, the animals get bored,” Burghardt said. Depending on the context, a playmate—even an unorthodox one—can be more satisfying than a meal: “In this particular situation, the animal’s motivation to engage socially and playfully maybe was higher in its need hierarchy than eating.”

A zookeeper from a different Siberian wildlife park recently told The Siberian Times that there’s “an 80-85 percent chance” that Amur will end up eating his new friend. But there’s also a chance, Burghardt explained, that the tiger has come to permanently think of Timur as something other than food, and that once a bond has been established, it can often preclude a predator from reverting back to its natural state. (To hedge their bets—or maybe to spare Timur the trauma—the zookeepers have switched Amur to an all-rabbit diet.)

“I have a friend who raised chickens, and he thought of them like family,” Burghardt said. “He couldn’t eat them. But his neighbor raised the same kind of chickens, and felt the same way about his chickens.” So they switched, and ate each other’s birds without guilt.

The moral of the story: “It’s not inconceivable,” he said, “that the same animals can be viewed quite differently.”

And then there’s personality—possibly the simplest explanation, even if it’s an incomplete one. Maybe Amur liked Timur better than any of the other goats. Maybe Bekoff’s dog and his fox were exceptionally compatible. “Some dogs are very open to other dogs, and some are very closed,” Bekoff said. “It would be the same with a predator and prey. There’s going to be individual differences.”

So far, most of the interspecies relationships researchers have observed have happened in captivity—possibly because the probability of seeing one among zoo or household is just higher than happening upon it in the wild, but also because animals living in the world of humans are just more likely to interact with other species from a young age. There’s a reason so much of the Internet’s interspecies-friendship porn focuses on baby animals: The strongest bonds form early. Studies have shown that geese and ducks raised together will view each other as members of the same family; kittens raised with baby rats would never harm them. If a relationship takes root early enough in an animal’s social development, it can overrule instinct or later learned behavior. (The San Diego Zoo assigns “puppy buddies” to each of its cheetahs from birth, to help the cats learn to be more playful, and other zoos have similar programs.)

Some researchers have made the case that predator and prey, stripped of the rules of the natural world, are actually well situated for friendship. “Predator and prey animals are already set up to know how to read each other,” said Donna Haraway, the author of When Species Meet. “Predators read prey animals incredible well, because it’s how they get dinner. And prey animals read predators very well, because it’s how they avoid becoming dinner.”

At this point, though, these theories are all just that: theories. “We don’t know” why animals form cross-species friendships, Bekoff said. But less mysterious is why our particular species is so fascinated by them.

“It goes back to the idea of why we’re so interested in pets,” Burghardt said. “That interest in other species and having relationships with them is meaningful to us, so the idea that other species may also find them meaningful is, I think, pretty interesting.”

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