For many of us, pets aren't just casual companions. They are cherished members of the family. (Sometimes it even feels like we're the pets, and our pets are the masters.) So it’s worth asking: Are humans the only animals that keep pets? Or do other animals also keep pets and form deep companionships with other species?

The answer might surprise you. Not only do some animals display a great capacity to look after and bond with a member of another species, they also appear to form these bonds for no reason other than companionship. To prove it, here's our list of animals with pets of their own.

Koko the gorilla and her cats

Koko the gorilla is best known for being a sign-language speaking ape, believed by her handlers to know more than 1,000 signs. But perhaps her most humanizing characteristic has been the love and affection she has demonstrated toward her pet cats.

Koko was first allowed a pet cat in 1985 after she requested one for her birthday. She was even allowed to choose a kitten from a litter; a gray male Manx that she named "All Ball." Koko's gentle care and affection for All Ball was astounding to those on the outside who had never seen another animal treat another species as a pet before, but to Koko's handlers, who knew her well, it wasn't surprising at all.

Tragically, later that same year All Ball was struck by a car and killed while exploring the world outside of Koko's enclosure. Koko's mourning process after being told of the cat's death showed just how deep her emotional bond to the cat was. The following year, Koko was given two kittens. She named them "Lipstick" and "Smokey."

Tarra the elephant and her pet dog Bella

The Elephant Sanctuary in Tennessee has been home to one of nature's most unexpected odd couples: Tarra the elephant and her pet dog, Bella. The two first bonded several years ago when a stray dog wandered onto the sanctuary's property. Rather than scare off the intruder, one elephant in particular, Tarra, immediately welcomed the stray with open arms. Before long, the two became inseparable. In fact, Tarra seemed to spend more time with Bella than she did with other elephants.

The bond became especially apparent when Bella suffered a spinal cord injury and lost the use of her legs. Caretakers took her indoors to receive medical assistance. For three weeks Bella was bed-bound, and for the entire three weeks Tarra stood just outside the building holding vigil, refusing to leave Bella's side. When the two were finally reunited, their embrace made it clear to everyone involved just how special their bond was. (Watch this video about Tarra and Bella and judge for yourself.)

It goes to show that even a giant animal like an elephant can have a gentle heart.

Amy the deer and her pet dog

Amy the deer and Random the dog met at an animal rehabilitation center in Oklahoma. PBS

This PBS report about animal odd couples showcases many touching stories of animal bonds that cross the species barrier, but perhaps none are as astounding as the story of Amy the deer and her pet dog, Ransom. The tale takes place at an animal rehabilitation center in Oklahoma, Wild Heart Ranch, which cares for thousands of animals every year.

Though many of the animals at Wild Heart are released back into the wild, Amy is a permanent resident since she is a non-native species to the region. She's a welcome resident, however, because of her strong mothering instinct, as she assists in raising many of the orphaned deer that the ranch takes in. But her mothering skills go beyond other deer.

When the ranch took in Ransom, a golden retriever born blind, Amy immediately took to raising him as well. She regularly grooms the dog, plays with him and has demonstrated remarkable patience and compassion in helping Ransom adapt to a world he cannot see. Meanwhile, Ransom is bonded to Amy in a way indistinguishable from the way a dog bonds to its human companions. It's truly a touching and inspiring story!

Capuchins and their pet marmosets

A family of capuchin monkeys adopted a marmoset and treat it as one of the family. Jeanne Shirley

This remarkable story has even gotten the attention of those who are skeptical of reports about animals and their pets. A group of capuchin monkeys in Brazil have been witnessed adopting and caring for a baby marmoset, another type of monkey entirely. The baby marmoset was raised as a regular member of the capuchin family, though the capuchins seemed to understand that the marmoset (named Fortunata) was not a member of their own species. For instance, when they played together, the capuchins treated the marmoset gently, as if they comprehended that she was more delicate than members of their own species.

This case of animals keeping pets is particularly astute because it occurred among animals that were all living in the wild. Also, the marmoset that was kept as a pet was not a human-domesticated animal.

A crow and its pet cat

This remarkable story of a crow that raised a pet kitten shows that its not just mammals that can keep pets. (You may have to see the video for yourself to believe it.) According to the report, the kitten was a stray that probably couldn't have cared for itself without assistance. But the only assistance it could have received was from a mysterious crow that never left the kitten's side. Before long, local witnesses got their proof: the crow was seen regularly feeding the cat with worms and other prey that it had collected.

The two animals would often play together innocently, and the crow would protect its pet from dangers (it would even squawk so that the kitten wouldn't wander into the road).

It's a remarkable story that shows how other animals can display a compassion and bond toward other species in a way that many researchers never believed possible before.

Tonda the orangutan and her pet cat

Koko isn't the only great ape that has shown the capacity to care for a pet. Tonda, an orangutan that lived at ZooWorld in Florida, took in a stray cat named T.K. (for "Tonda's kitten"), and kept it as a pet and companion animal. The bond between the two was particularly special because T.K. was a true stray that had to be fostered gently by Tonda over time before the cat opened up to the concept. Meanwhile, zookeepers credit Tonda's relationship with the cat as reason why the orangutan was able to live to such an old age.

The bond between ape and cat was also noteworthy as a contrast to Koko's relationship with her cats because Tonda was not taught to sign. So it goes to prove that the bond between pet and pet owner runs deeper than what can be communicated through language.