In keeping with my earlier post about the wolf pups at the Catoctin Zoo, via National Geographic we read this fascinating info about animals who may "adopt" other animals, even those of a different species altogether:
...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."
Greyhound with owl...image credit Nat Geo, here
I've never seen this phenomenon myself, but I'm waiting for the day when I'm running in the backcountry and see a squirrel riding on the back of a deer....