Animals can forge bonds across species boundaries if the need for social contact pre-empts their normal biological imperatives. A cat raised with dogs doesn't know it's a cat, the logic goes.
In the PBS film "Animal Odd Couples," a cheetah and a dog are shown as friends. A narrator explains that the two grew up together at Busch Gardens and are a regular attraction at the theme park - they race out from behind a fence at 32 kph to the gasps of the audience.
Tim Smith, curator of behavioural husbandry at Busch Gardens, tells me the cheetah and the dog, both born in 2011, were just infants when they befriended one another. The male cheetah, named Kasi, was the only cub to survive a rather surprising birth from what had been considered a post-reproductive mother. He was paired with the female Labrador retriever mix Mtani when no cheetahs were available.
Smith concedes that there have been growing pains. "Part of learning is going through situations where you learn things not to do," he says. "If Mtani doesn't want Kasi next to her, she will show him signs, such as with her teeth or with a low-toned growl."
It's not clear that this will end well. As Lauren Brent, a post-doc primatologist and evolutionary biologist at Duke University, points out, "These animals are juveniles. I worry that their relationship will change once they became adults, with negative consequences for the dog."
Strong attachments often arise in captive animals, says Bonnie Beaver of Texas A&M's College of Veterinary Medicine. "Two very stressed individuals may lean on each other for comfort." In most cases, cross-species friendships are forged most strongly when animals are young. But in captivity even an older captured animal might seek out a friend, including a member of another species.
Marc Bekoff, professor emeritus of ecology and evolutionary biology at the University of Colorado, who has written several books on animal emotions, believes wholeheartedly in such bonds.
He told me, "I think the choices animals make in cross-species relationships are the same as they'd make in same-species relationships. Some dogs don't like every other dog. Animals are very selective about the other individuals who they let into their lives." Even predators and prey (including his dog and a bunny) can form relationships - which as he points out requires "incredible trust" from the prey animal.
Anthropologist Barbara King from the College of William & Mary says she and other scientists have documented a number of animals, ranging from dogs to hippos to apes, that make bonds with animals of another species. If one dies, the survivor grieves. An elephant named Tarra was close to a dog named Bella and grieved after Bella was killed by coyotes. King says she has also been moved by the relationship between Owen the hippo and a 100-year-old tortoise named Mzee. "In this friendship that formed over years, they worked out a system of cross-species communications," says King. In some cases, the biggest risk in cross-species friendships isn't getting eaten. It's emotional loss.
The story of Tarra and Bella:
Some special moments between Owen and Mzee: