When you weigh 36 tonnes (40 tons), you don’t have to worry too much about getting eaten by predators. However, not content to simply luxuriate in their own untouchability, humpback whales have continually been spotted looking out for other marine animals by stepping in to protect them from killer whale attacks. Given that this does not directly benefit the humpbacks, researchers are stumped as to why they bother to put themselves in danger by picking fights with one of the ocean’s most notorious killers, with some suggesting that they may be acting out of altruism.

In 2009, marine ecologist Robert Pitman observed an amazing scene in the waters off Antarctica, where a humpback whale came to the rescue of a Weddell seal that had been knocked from an ice floe by a pod of orcas and appeared destined to become their dinner. Amazingly, however, the humpback placed the seal on its upturned belly, protecting it from its attackers until it was able to scramble to safety on another piece of drifting ice.

After witnessing this incredible rescue act, Pitman and some of his colleagues began collecting accounts of humpback whales interacting with orcas, recorded by 54 different observers between 1951 and 2012. A total of 115 interactions were reported over this period, details of which have now been published in the journal Marine Mammal Science.

Of these 115 events, 57 percent were initiated by the humpbacks, suggesting they actively seek out killer whales in order to engage them in battle. In just over 87 percent of these cases, the orcas were in the process of hunting their prey when the humpbacks showed up.

Yet most remarkably of all, in 89 percent of incidents, the humpbacks intervened as the killer whales were attacking a species other than humpbacks, indicating that these marine giants don’t only protect their own kind, but have also got the back of other creatures.

Among the animals that the whales helped to protect were the likes of California sea lions, ocean sunfish, harbor seals, and gray whales. The humpbacks used a variety of tactics to fend off the orcas, including slapping them with their flippers and flukes, bellowing at them, and chasing them.

Though the researchers aren’t entirely sure what possesses the humpbacks to act so selflessly, they suspect it may be because they are wary of killer whales attacking their own calves. As such, they automatically intervene whenever they hear orcas hunting, before they even know what species their victim is. Why they continue with the confrontation even after learning that the killer whales are not attacking a humpback, however, is still a mystery.