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.