In May 2012, researchers observed a pod of killer whales attacking a gray whale and its calf in Monterey Bay, California. After a struggle, the calf was killed. What happened next defies easy explanation.

Two humpback whales were already on the scene as the killer whales, or orcas, attacked the grays. But after the calf had been killed, about 14 more humpbacks arrived—seemingly to prevent the orcas from eating the calf.

“One specific humpback whale appeared to station itself next to that calf carcass, head pointed toward it, staying within a body length away, loudly vocalizing and tail slashing every time a killer whale came over to feed,” says Alisa Schulman-Janiger, a whale researcher with the California Killer Whale Project.

For six and a half hours, the humpbacks slashed at the killer whales with their flippers and tails. And despite thick swarms of krill spotted nearby—a favorite food for humpbacks—the giants did not abandon their vigil.

It’s not clear why the humpbacks would risk injury and waste so much energy protecting an entirely different species. What is clear is that this was not an isolated incident. In the last 62 years, there have been 115 interactions recorded between humpback whales and killer whales, according to a study published in July in the journal Marine Mammal Science.

“This humpback whale behavior continues to happen in multiple areas throughout the world,” says Schulman-Janiger, who coauthored the study.

“I have witnessed several encounters, but nothing as dramatic as [the May 2012 event],” she says. It remains the longest humpback-to-killer whale interaction known to date.

What Is Going on Here?

The most logical biological explanation for the humpbacks’ vigilante-like behavior is that the whales receive some sort of benefit from interfering with orca hunts.

For instance, orcas are known to attack humpbacks, and the whales are most vulnerable when they are young. Once fully grown, though, a single humpback is large enough to take on an entire pod of killer whales.

So perhaps the “rescuing” behavior has evolved as a way to help the species get through its weakest life stage, with humpbacks charging in when they think a young whale is at risk.

There’s also a good chance that the calf under attack is related to the whales coming to its rescue.

Was This Whale Trying to Save a Diver’s Life?

“Because humpbacks calves tend to return to the feeding and breeding grounds of their mothers, humpbacks in a given area tend to be more related to neighboring humpbacks than to the population as a whole,” says study leader Robert Pitman, a NOAA marine ecologist and National Geographic Society grant recipient.

But there’s a wrinkle in this explanation. Of all the incidents the scientists investigated over the last five decades, killer whales targeted humpbacks just 11 percent of the time. The other 89 percent involved orcas hunting seals, sea lions, porpoises, and other marine mammals.

There’s even one incident in which humpbacks apparently tried to save a pair of ocean sunfish from becoming orca hors d'oeuvres.

Perhaps it’s personal. Schulman-Janiger notes that not all humpbacks interfere with orca hunts, and many that do bear scars from being attacked by orcas earlier in their lives, perhaps as calves. Therefore, it’s possible that personal history drives humpbacks to respond to orca hunts.

The study also notes that it’s possible the humpbacks are responding to auditory calls made by the killer whales rather than the animals they are hunting. This would mean that the humpbacks don’t know what species is being attacked until they have already invested energy in swimming to the battle.

Such a behavior could persist in the population because it would occasionally benefit humpbacks—apparently enough to justify benefiting other species the majority of the time.

View Images A Weddell seal rests on the chest of a humpback whale, safe for the time being from attacking killer whales. Photograph by Robert L. Pitman

All for One, and One for All?

Other whale experts see a dose of something even more complex: altruism.

“Although this behavior is very interesting, I don’t find it completely surprising that a cetacean would intervene to help a member of another species,” says Lori Marino, an expert in cetacean intelligence and president of the Whale Sanctuary Project.

Humpbacks are capable of sophisticated thinking, decision-making, problem-solving, and communication, says Marino, who is also the executive director of the Kimmela Center for Animal Advocacy.

“So, taken altogether, these attributes are those of a species with a highly developed degree of general intelligence capable of empathic responses.”

Furthermore, humpbacks are not the only animals that seem to display some sort of regard for another species. Dolphins have been famously depicted as “aiding” dogs, whales, and perhaps even humans—though it should be noted that onlookers, not animal experts, often report such events, and it can be easy to misinterpret animal behavior.

Whether humpbacks are truly performing what amounts to a good deed or are benefiting from the process, it’s clear that we still have much to learn about the minds and motivations of the animals around us.

For the most part, Pitman says animals tend to do what is in their own best interest—even if the motivations themselves aren’t entirely clear to us.

“As biologists,” he says, “that is where we should start our search for explanations.”