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 pinniped 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 and described it and another example in Natural History magazine that year. “It looked like they were trying to protect the seal.”

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 conclude 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.

When prey gang up and harass a predator, it’s known as mobbing. A flock of crows, for example, can drive away a hawk by repeatedly dive-bombing. The behavior is also known among fishes, insects, and terrestrial mammals, but it hadn’t been studied in marine mammals. Because of their large size, humpback whales don’t have to worry about many predators. Killer whales are the only species known to attack, and they target small calves. The mothers will try to scare them off with thunderous bellows. If that fails, they defend their young by smacking their massive tails or swinging their 5-meter-long, barnacle-encrusted fins.

To find out whether the seal rescue in Antarctica was unusual behavior for humpbacks, Pitman, who works for the National Oceanic and Atmospheric Administration’s (NOAA’s) Southwest Fisheries Science Center in San Diego, California, posted a request for information on a marine mammal listserv. He received 115 descriptions of encounters, many from commercial whale-watching trips, which sometimes included photos and videos. In 31 cases of mobbing, humpbacks approached killer whales that were already engaged in a fight. They would chase the killer whales, often bellow, and slap their fins and tails. “The humpbacks were definitely on the offense,” Pitman says. He and colleagues published their findings online this week in Marine Mammal Science .

The conclusions have convinced Phillip Clapham, a NOAA marine biologist in Seattle, Washington, who was not involved in the research. “They make a very good case that it’s a proactive response to killer whales,” he says. “I think they’re absolutely right.”

It’s not hard to imagine why humpbacks would rush to the rescue when another humpback whale is under attack. Because they migrate to and from the same breeding grounds where they were born, humpbacks are likely to encounter relatives. So a threatened calf might share some genes with a rescuer, making the apparently altruistic act of saving it somewhat self-interested.

But what about protecting other 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.”

Pitman suspects that it is inadvertent altruism. The humpbacks might simply rush to the scene of a fight whenever they hear killer whales fighting. “I think they just have a simple rule,” Pitman says. “When you hear a killer whale attack, go break it up.” Clapham adds that the confrontations may teach the killer whales a lesson, making them think twice about messing with humpbacks.