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The fighting was in full swing by the time Alisa Schulman-Janiger arrived on the scene. In the water off Monterey Bay, California, a pod of roughly 10 killer whales was working to isolate a lone gray whale calf from its mother. Blocking their advance, an army of no less than 16 humpback whales was working in shifts to defend the calf and its mother from the attack.

Within 45 minutes, the calf was dead, and the mother left the scene soon after. But the battle carried on for at least another seven hours. The humpbacks chased the orcas, thrashing them with their fins and tails, and preventing them from feeding on the dead gray whale.

“It was a really crazy scene to watch,” says Schulman-Janiger, a researcher with the nonprofit California Killer Whale Project. The humpbacks didn’t seem to be acting in defense of themselves, or even of one of their own. In fact, the humpbacks forsook nearby food, sometimes for hours at a time, to thwart the orcas’ efforts to feed on the gray whale.

In a new review study, Schulman-Janiger and her colleagues have gathered numerous accounts of humpback whales acting with seeming altruism to defend members of other species, including whales, seals, sea lions, and even fish, from killer whale attacks.

“We think that what’s going on for them is they just hear killer whales attacking, and they go in and break it up,” says Robert Pitman, who works for the US National Marine Fisheries Service and coauthored the report.

Pitman first noticed this unusual behavior along the Antarctic Peninsula, south of Argentina and Chile, when humpbacks cut into the middle of an orca attack on a Weddell seal. Pitman says that during this fight, a humpback flipped around and lifted the seal onto its chest in an effort to protect it from the killer whales—a sight that was captured by a BBC film crew that had been on location with the researchers. According to Pitman, the footage shows that the humpback’s protective maneuver was not accidental—at one point the humpback nudged the seal back into safety when it was about to slide off.

“These [humpbacks] are definitely interested in preventing these killer whales from harming these other animals,” Pitman says.

After his Antarctic encounter, Pitman started gathering other similar accounts. Not all humpbacks display this behavior. Out of 72 encounters reviewed between humpbacks and orcas attacking mammals, Pitman and his colleagues found that 61 involved the orcas preying on species other than humpbacks.

What exactly is motivating humpbacks to intervene on behalf of other marine life is difficult to determine. It could be territorial, but Pitman says that even if the whales are not defending other species deliberately, it still amounts to “inadvertent altruism.”

There’s also a chance that some humpbacks may be reacting to previous trauma, says Schulman-Janiger. Two of the 16 whales she identified as exhibiting interventionist behavior had the marks of previous attacks by killer whales on their fins. These scars could have come from being attacked when they were calves themselves, or from injuries sustained while defending their offspring.

In watching the attacks, Schulman-Janiger says she sees what looks like emotions on display. The humpback whales “are definitely bothered by what’s going on.”

Pitman says this behavior could be a remnant of the interspecies conflict that occurred with greater frequency before industrial whaling decimated both populations. “As whales continue to recover, I think we should expect more surprising things like this,” he says.