12 Remarkable Interspecies Relationships That Prove Adoption Isn't Just For Humans

Humans aren't the only creatures that adopt needy babies -- and they certainly aren't the only ones that adopt animals of other species.

While scientists aren't exactly sure why interspecies adoptions crop up in the animal kingdom from time to time, some speculate that genetics play a role. Jenny Holland, author of the 2011 book Unlikely Friendships, told National Geographic that empathy may be at the core of this behavior, saying that one animal "may take in another to relieve its pain, hunger or loneliness."

"Mammals have the same brain structures, the same system, related to emotion that we have, so why not?" she said. Here are 12 of the most fascinating -- and touching -- animal adoptions:

1. Tortoise and baby hippo

Credit: Getty A baby hippopotamus orphaned by the Indian Ocean tsunami found a new "mom" in a 130-year-old giant male Aldabran tortoise named Mzee. The hippo, named Owen, was extremely frightened after he was rescued, stranded on a reef after the tsunami. Luckily for Owen, Mzee happened to be nearby when he was released at a nature park in Kenya, and immediately began to cling to him like he no doubt would to his own hippo mother. And Mzee wasn't unhappy about the new tag-along either -- he soon began to return signs of affection to Owen. They spent many days thereafter patrolling the pond together.

2. Koko the gorilla and her kitten

Credit: YouTube, San Francisco Zoo Koko the gorilla, famous for learning approximately 2,000 words of sign language at the San Francisco Zoo, was given a tiny kitten to care for after she became unsatisfied with her stuffed animals. In an episode that surprised biologists and made international news, Koko cared for the kitten, named "All Ball," as if it were her own baby, even trying to nurse him.

Even more remarkable -- when All Ball escaped from Koko's enclosure and was killed by a car, Koko was devastated. When her handler signed her the news of All Ball's death, Koko reacted by signing the words, "bad, sad, bad, frown, cry-frown, sad," and later mourned in her cage by wailing and bellowing.

3. Sperm whale pod adopts deformed dolphin

Credit: YouTube, Alexander Wilson and Jens Krause of Berlin's Leibniz-Institute of Freshwater Ecology and Inland Fisheries

A bottlenose dolphin born with a serious spinal deformity that gave its back an s-shaped curve was taken in by an empathetic family of sperm whales. Researchers were shocked to spot them all swimming together in the North Atlantic: "It really looked like they had accepted the dolphin for whatever reason," the researchers said last year, after they observed the dolphin nuzzling and rubbing against the much larger whales.

4. Macaque adopts stray kitten

Credit: YouTube

A tourist took the footage of a lifetime when she stumbled across a wild long-tailed macaque (also called a crab-eating macaque) monkey nuzzling and grooming a small kitten at the Monkey Forest Park in the Ubud region of Bali, Indonesia. The bond isn't so surprising though, considering that crab-eating macaques are highly social animals and babies learn from interacting with both their mother and other females in their large groups of up to 60 individuals.

5. Dalmatian adopts spotted lamb

Credit: Associated Press

It's not clear whether it was the spots or the lamb's cute face that drew her puppy-less surrogate mother to her -- but whatever it was, the bond couldn't be stopped. The dalmatian, a pet on a farm in southern Australia, was in season when the lamb was born, and immediately took the lamb in. (The lamb's spotted coat is a rare genetic occurrence -- but it worked out well for the lamb, and her new mother.)

6. Monkey and wild boar

Credit: Getty A boar and a macaque, both babies and both orphaned and rescued in the wild, found in one another forever friends -- and the monkey found a surrogate mom. Miwa, a Japanese macaque, and Uri-bo, a wild boar, are at Fukuchiyama City Zoo in Japan, where they spend all of their time together. A keeper introduced them after Miwa was constantly crying out, and he quickly took to Uri-bo's back, as baby monkeys do with their mothers in the wild. The boar now roams around the zoo's grounds freely, tiny tot in tow.

7. Cat adopts bunny

Credit: YouTube

When Snaggle Puss the cat (real name) met Bubbles, the orphaned baby rabbit, the entire relationship between predator and prey was turned on its head. Snaggle Puss took Bubbles in and nursed her alongside her own biological kitten.

8. Husky cozies up to rehabilitated meerkat

Credit: YouTube

A Namibian resort and rehabilitation center for meerkats played host to an unusual pair of guests when one of the meerkats cozied up to a Siberian husky named Bond for comfort.

9. Lioness and antelope calf

Credit: YouTube

In what was called a "completely unprecedented" event, a wild lioness came across a baby oryx (a type of antelope) and took in the antelope like one of her own. Conservationists suspected that the lioness had recently suffered the traumatic loss of a cub, and was therefore eager to fawn over a baby animal (regardless of species).

10. Dogs adopts bunnies

Credit: YouTube

Dogs, traditionally fierce rabbit hunters, aren't particularly known for their friendliness to bunnies. But Ellie, a King Charles Spaniel living in the English countryside, discovered a litter of orphaned baby rabbits in her owner's garden. Ellie quickly adopted them, sleeping with them every night and even trying to nurse the cute little bunnies.

11. Dog adopts owl

Credit: John Picton, Ringwood Raptor and Reptile Centre As a pair, Torque and Shrek are just about as unusual as their names would suggest. Shrek, a baby owl taken from her mother for her own protection at just three days old at the Ringwood Raptor and Reptile Center in the UK, was carefully introduced to Torque the dog. Before long, Torque had adopted the tiny owl, and now the two play together and even watch some TV together while relaxing and cuddling.

12. Cat adopts squirrel

Credit: YouTube

Emmy the cat's mothering instincts are pretty powerful. When she found a baby squirrel that had fallen from the tree in her yard, she picked him up and brought him to her bed to join her other kittens. She quickly became protective of the squirrel, named Rocky by Emmy's owners. Rocky soon became comfortable in a feline family -- In fact, Rocky became so astoundingly integrated with the family of cats that he even started to purr just like a kitten.