inter-species adoption?

This is really adorable.  Its a video of a chimpanzee taking care of tiger cubs.

The caption says that the chimp has "adopted" the cubs, like a parent.  It doesn't say the cubs are like pets. But remember Koko's kitten? Everyone said that was the gorilla's pet kitten...not her adopted child.

When humans take care of animals, the animals are always called pets, not adoptive children.  But when apes take care of animals, is the relationship more ambiguous?  (To be fair, I guess plenty of humans do call their pets "children." But to my ears, it sounds quite strange when people call me "mom" when referring to my cats). 

What about the hippo and tortoise in this video? I think that should just be called companionship, since the tortoise can't really take care of the baby hippo, like the chimp and Koko took care of their baby felines. But the lionness who adopts baby antelopes is a different story.  She's actively protecting the antelopes. 

I think what we have here is a case of inadequate language to describe a general phenomenon.  We are sub-categorizing needlessly.  So maybe we should drop the term "pet" altogether and just start calling it inter-species adoption.

Here is my instance of inter-species adoption: