We don't expect to see wildly different species getting along, especially ones that would normally be predators and prey. But these days, the evidence is everywhere, going viral on the Internet, in an Android TV commercial, and in many of our own homes, among our diverse menageries. It's no wonder that "Unlikely Animal Friends," which returns for its fourth season Feb. 27, is one of Nat Geo Wild's most popular programs, and with adorable stories like the one about a two-legged Chihuahua and his silkie chicken pal in the premiere, that’s likely to continue.

The stars of the premiere met at a Duluth, Georgia, animal hospital, brought there by a hospital employee who thought each might like the company of other animals, and from there, they became inseparable. It's a story that makes us smile, but it's not as rare as you might think.

"It is surprising, in the sense, that this kind of thing doesn't happen often in nature," says David Mizejewski, an animal expert, author and National Wildlife Foundation spokesman who appears in the series. "It's not surprising, though, that animals would have the capacity to form such relationships. It's more evidence that we probably have more in common with many other species than we think."

"These friendships almost always occur in captive situations and are usually instigated by people, directly or indirectly. Many of them involve domestic species that have had their natural instinct to fear other species bred out of them," he continues. "Typically, these friendships form in captive situations, and often involve domestic species with reduced predatory drive or prey flight instinct. In captivity the animals have their needs met, reducing predator-prey dynamics."

This clip from the season premiere shows how the silkie chicken named Penny Chicken and the two-legged Chihuahua named Roo are big hits at the local animal hospital.

Mizejewski traces the unlikely animal friends phenomenon to Jennifer Holland's 2011 book "Unlikely Friendships," which struck a nerve: "I think there's an appeal for people since different species don't typically form relationships in the wild, so there's a wow factor," he explains.

The sex of the animals doesn't seem to matter, he says, but age can be a factor in these relationships. "Animals introduced at a young age would probably be more likely to be able to form an interspecies friendship."

Is there a dangerous downside to these relationships? "I think there can be risk to the animals if the pairing is one of natural predator and prey, or if one is much larger than the other," Mizejewski responds. "The animals could become so bonded that separating them would be cruel, so keep that in mind before trying to create such a friendship," he advises caregivers.

This clip shows how a mule and goat have become fast friends despite having very different personalities.

"The thing to remember is that these kinds of friendships are anomalies. Most species simply ignore different species, unless they view them as a threat or a potential food source. So there's no guarantee that introducing two animals of different species is going to result in a friendship or bond. In fact, it could put the animals at risk, so I would recommend people only try to introduce different species carefully and under strict supervision if there's a chance they could injure each other," he notes.

Nevertheless, there's an inherent lesson humans can learn from, concludes Mizejewski. "Animals are just as complex as people, and although it's very rare, they can sometimes look past outward differences and become friends with an individual different from themselves."

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