But the marine mammals aren't the only ones that form odd alliances, experts say.

Such adoptions are relatively common among domestic animals, and occasionally seen in the wild, according to Jenny Holland, author of the 2011 book Unlikely Friendships. (Read a Q&A with Holland about her book.)

Some examples include a dog that nursed a baby squirrel as part of her own litter, captive apes that treated cats like infant apes, and a dog that watched over a baby owl, Holland said by email.

And in her forthcoming book, Unlikely Loves, Holland will feature a Dalmatian that adopts a calf that happens to wear Dalmatian-like spots, a goat that helps a young giraffe learn self-confidence, and a hen that sits on "her" pups to keep them warm.

Why Adopt?

But what motivates these adoptive families?

"I wish I could crawl into these animals' minds and ask! But we can make some educated guesses based on what we know about animal brains—and our own," said Holland, a National Geographic contributing writer. (Read about how animals are smarter than you think.)

For instance, in some cases, an animal will adopt one of its own species, which is instinctual.

"Instinctively animals take care of young to help them survive and therefore pass on the family DNA," Holland said. "So I think there's some hard wiring in there that leads them to offer care to another animal in need. If it isn't a relative, there maybe some wires crossed, but I think the behavior comes from the same place."

Mutual benefit is also a motivator, noted Jill Goldman, an applied animal behaviorist based in southern California.

"In order for the relationship to be sustained, I believe both parties will need to benefit in some way," said Goldman, who has studied wolf behavior. (See pictures of animal odd couples.)

"How we define benefit is another matter. Social companionship in some cases may actually be enough of a benefit so long as it is not outweighed by competition [or] threat."

For instance, adding an individual to a group could help secure more food or offer more protection—which is probably what happened in the case of the deformed dolphin, Goldman said.

"No one's going to allow you to hang around if you're not pulling your weight."

Goldman added that a lot of such adoptions occur when a nursing mother takes in a young orphan. (See National Geographic's pictures of animal moms.)