“At first it seemed like the usual, deviously clever attack. Several killer whales were trying to catch a Weddell seal that had taken refuge atop a drifting patch of Antarctic ice. The orcas swam alongside each other, creating a wave that knocked the hapless seal into the water. Death seemed certain.
“Then something amazing happened: A pair of humpback whales turned up. As the panicked seal swam toward them, a lucky wave tossed it onto the chest of the closer, upturned whale. The whale arched its chest out of the water, which kept the seal away from the charging killer whales. And when the seal started to fall off, the whale carefully pushed it back onto its chest with a flipper. Soon after that, the seal scrambled to safety on another ice floe.
“‘I was shocked,’ recalls marine ecologist Robert Pitman, who witnessed the episode in 2009, as it looked like the whale was ‘protecting the seal’ from the orcas.
“Humpback whales will vigorously defend their own calves when attacked by killer whales, of course. But after analyzing other encounters between the two species, Pitman and his colleagues concluded that humpback whales will also launch preemptive attacks on their predators. Sometimes the intent seemed to be protecting another whale's calf. But more often, like with the Weddell seal, the humpbacks for some reason helped a different species.
“This happened in nearly 90% of attacks where the killer whales' prey could be identified. ‘It's pretty mysterious,’ says Trevor Branch, a fisheries scientist at the University of Washington, Seattle, who has studied populations of large whales. ‘We tend to think of altruism as being reciprocal, but there's no way these other species would come back and help the humpback whales.’”
Erik /Stokstad, “Why did a humpback whale just save this seal’s life?” Science, Jul 22, 2016.