Altruism: behaviour of an animal that benefits another at its own expense.
This is rare within species (bees and other social animals being the notable exception), but between species? Practically unheard of.
But apparently the raging war between humpback whales and killer whales (which I genuinely did not know was happening), has caused humpbacks to intervene in the killer whales hunting other species.
Read about it over at National Geographic… and puzzle over why humpbacks keep picking on killer whales.