There are some friendships that defy explanation.
One of nature's fiercest cats isn't supposed to fall for a good-natured Golden Retriever. Polar bears aren't supposed to play with Eskimo dogs.
But sometimes, friendships are forged under the most unusual circumstances.
On Friday, Jan. 14 at 5 p.m., the National Geographic Channel will air its latest version of its "Unlikely Animal Friends" special, but if you want a look at some of the friendships they feature (as well as a look back at some of the pairs included in the channel's first animal odd couples show), check out the below.
She is a baby leopard, he is a Golden Retriever. You might think that the cat and the dog would be at each other's throats but, instead, they're inseparable friends.
They met at Glen Afric, a reserve in South Africa, when Jenny and John Booker, professional animal wranglers, adopted the tiny leopard cub, named Salati.
In the National Geographic special, the Bookers said that the two animals fell for each other the instant they met.
Their then 2-year-old dog Tommy licked Salati and treated her like a sister within hours of meeting her. The pair walk together several times a week and play together like fast friends.
But as they grow older and Salati becomes more powerful, their owners worry that the animals' instincts will overpower their bond.
"My worry now is, Would she go for Tommy? It's a big question," said Jenny Booker. "As big of friends as they are, I still believe that animals are 80 percent instinct.
But her husband said that though nature may pull them apart, their bond will last.
"I don't believe that she'll ever lose that bond with Tommy," John Booker said.
Polar bears are the king carnivores of the arctic – they're big and don't back down easily.
But in the 1980s, when dog breeder Brian Ladoon set up his camp off the Hudson Bay in Manitoba, Canada, he observed an interaction between polar bears and his Canadian Eskimo dogs that was among the most mysterious he'd ever seen.
When local polar bears encountered his sled dogs, they didn't pummel, they started to play.
"It's like the lions and the sleep with the lamb story," he said. "It happens and it happened naturally. It was not something that was planned and I worked with it instead of fighting it, you know."
For years every winter, the bears and the dogs would renew their friendship. But the relationship wasn't without controversy, as wildlife authorities urge zero contact between bears and human camps.
In 1996, one polar bear ran through Ladoon's camp and killed and wounded several of his dogs.
But he maintains that it was a young rogue and, despite the attack, continues to let the animals figure out their unorthodox friendship.
"The bears and the dogs, they have an amazing relationship that happened the natural way and I'm not going to destroy it for them, but I am cautious of it, you can be sure," he said.
If bonds were based on size, these two wouldn't even be acquaintances.
But somehow Bella, a 30-pound dog, and Tara, a 4-ton Asian elephant, have become the best of friends since meeting in 2005.
"They are close. They are always together. They play together, they talk. They sleep, they eat – everything together," said Carol Buckley, founder of the Elephant Sanctuary in Hohenwald, Tenn.