Jan. 13, 2011 -- 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.

Leopard Cub and Dog Forge Friendship

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 and Dogs Become Playmates

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.

Elephant and Dog Become Best Buddies

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.

One day, while exploring property near their sanctuary, Buckley and her partner, Scott Blais, came across a white chow mix that appeared to have a bizarre fascination with big pieces of equipment, such as tractors.

The two brought her home and watched as that attraction extended to the hulking creatures at their park, including one in particular: Tara the elephant.

"Elephants are incredibly careful around their young. Even when they're running and a baby is running around in and around their legs, family members never trip up over a baby. They never nick the baby, they never step on the baby, so I never was concerned for Tara and Bella," said Buckley.

In 2007, when an injury partially paralyzed Bella and forced her indoors, Tara held a vigil outside her window and called to her until caretakers brought her outside. Bella's injury eventually healed and the two now roam the refuge together again.

"It's 24/7. It is a very loving and compassionate relationship they have," said Buckley. "They play together and they never want to be separated."

Orangutan and Hound Dog Become Best Buds

One was an outgoing orangutan, the other an underfed stray dog.

But since the moment they met, Suryia and Roscoe have been inseparable.

Bhagavan "Doc" Antle, founder of The Institute of Greatly Endangered and Rare Species (TIGERS) in Myrtle Beach, S.C., said Roscoe, a Bluetick Coonhound, followed him and Suryia, one of TIGERS' orangutans, through the park's gate one day in 2006.

As soon as Suryia saw Roscoe, he ran over to him and started playing, Antle said. After a few unsuccessful phone calls to find an owner, Antle decided to let Roscoe stay.

Now the pair has a ball frolicking around the park. For a few hours each day, they swim or roll around in the grass. Suryia will even grab Roscoe's leash and take him for walks around the enclosure, Antle said.

"To me, they seem like long-lost friends. They would make you believe in reincarnation," he said.

Aging Tortoise Nurtures Abandoned Hippo

After a tsunami washed away his herd and stranded him, a young hippo found himself a most unusual ally: a 130-year-old tortoise.

The 2004 tsunami that devastated Indonesia also hit the eastern shores of Kenya. And after the storm left Owen, a small hippo, all alone, conservationists found a home for him in Haller Park, an animal sanctuary in Mombasa, Kenya.

They hoped that he could be a companion for a female hippo whose previous companion had died. But before they could introduce him to the other adult hippos (which can be aggressive and territorial), they wanted to give him a quiet place of his own to adjust.

And it was there that he met his buddy Mzee, an Aldabra giant tortoise whose name means "old man" in Swahili.

"He ran straight for the tortoise," said Sabine Baer, of Lafarge Eco Systems, which manages Haller Park. "The color is somehow similar, like a hippo color. It was somehow round-shaped, so he must have associated it with an adult hippo and with his mother and his family."

At first, it was Owen who made the friendly overtures. But after a while, Mzee appeared to warm up to the heavyset hippo.

They slept together and ate together and even appeared to cuddle together. Owen would nuzzle Mzee's foot when he wanted to eat, and Mzee would nibble Owen's tail to steer him.

"[Mzee] taught him to eat the cut leaves, he taught him to eat carrots, so for us, it was an absolute blessing to have that relationship to help us raise Owen," said Baer.

But though the pair became even closer and quite famous, inspiring a Web site and children's books, Baer and others worried that as Owen grew, he would put his smaller friend at risk.

So they decided it was time to move Owen into an enclosure with Cleo, the female hippo who had been waiting for him.

At first, the two eyed each other with suspicion. But it wasn't long before it seemed as though Cleo slowly replaced Mzee in Owen's heart. And occupied with a new tortoise companion of his own, Mzee didn't seem to mind at all.

"They became inseparable like Owen used to be with Mzee, and now they are together, living together happily ever after," said Baer.

The second installment of "Unlikely Animal Friends" will air Friday January 14 at 5 p.m. on the National Geographic channel. For more information, click here.