But when we do it it's "adopting pets"

At first, the researchers thought it was a rare, one-time case of adoption. Monkeys have been known to “adopt” abandoned infants of the same or other species. But Joker wasn’t caring for the howlers — he was just carrying them on his back, with no clear benefit to himself, until the infants eventually perished of starvation without access to breast milk.

Wow, that is really distressing. I hope the howler monkeys notice what's going on and better protect their young, as the article hopes at the very end.