In a strange encounter off the coast of western Australia, a pod of orcas seems to free a humpback whale from a rope entangling its tail. But were they really trying to rescue it?

It isn't clear whether the orcas (Orcinus orca) were trying to manipulate the rope or why they approached the humpback in the first place. Observers with Whale Watch Western Australia who caught part of the interaction on drone video initially thought the orcas might attack the hobbled humpback. Whale watchers have witnessed orcas brutally attack humpbacks before. As pack hunters, orcas can take down prey much larger than themselves, though they typically target humpback calves and yearlings rather than full-grown adults. When attacking whales, orcas often try to grab the whales' flippers, turn them over, and hold them under in order to drown them.

For whatever reason, though, the humpback whale (Megaptera novaeangliae) escaped unscathed on Jan. 10, Whale Watch Western Australia staffers wrote in a description accompanying their video footage. It was the first observed interaction between orcas and a humpback in Bremer Bay, Australia, in summertime, they wrote.

It's not clear why the orcas didn't attack the humpback, said Erich Hoyt, a research fellow at the charity Whale and Dolphin Conservation and author of the book "Orca: The Whale Called Killer" (Firefly Books, 2019).

"It could be that the orcas' summer diet is distinctly different and humpbacks are 'out of season' (it is well documented that they do feed on beaked whales here, which would be a different hunting and feeding operation, a different food)," Hoyt, who is not affiliated with Whale Watch Western Australia, wrote in an email to Live Science. "It could be simply that the orcas have just eaten or are in the middle of some other behavior, or that the entanglement somehow put them off."

For the whale-watchers, the encounter seemed almost altruistic. Orcas do have complex social lives and well-developed brain regions that are associated with empathy and emotion (in humans, at least), but there's no way of knowing whether orcas feel a sense of altruism for fellow sea creatures.

An off-season encounter

Humpbacks typically spend the summer months in the Southern Hemisphere, feeding on krill in Antarctica. According to the Australian Department of Agriculture, Water and the Environment, humpbacks are normally seen off the Australian coast between June and August, during their northward migration to their breeding grounds in subtropical waters, and between September and November, when they head back south to Antarctica.

Seeing the humpback in Bremer Bay in January was, therefore, a surprise. Observers on the whale-watching boat soon realized that the whale was in bad shape. It was scrawny and covered with sea lice, parasites that feed on the blood and skin of fish and whales. A snarl of rope was tangled around the whale's tail. It seemed certain that the whale would be pickings for the swift-swimming orcas.

Two male orcas nicknamed Blade and Hookfin approached the whale, apparently displaying curiosity — a normal behavior for orcas, Hoyt said. The whale defended itself by lashing out with its pectoral fins and tail fluke. Next, according to Whale Watch Western Australia, the matriarch of the orca pod, nicknamed Queen, approached in a great flurry of splashing and white water. When the water cleared, a large chunk of the rope binding the whale's fluke was floating away. To the whale watchers' surprise, the orcas then moved on, swimming in the opposite direction of the whale.

Rescue by orca?

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"The incredible fact that the Orca managed to rid most of the rope from this whale before letting him swim away freely was truly fascinating," the whale watch crew wrote in their video description. "[D]id the Orca deliberately rescue this Humpback or was the decision made that due to his ill health the effort of the hunt was not worth the energy reward at the end[?]"

Hoyt said that question isn't easy to answer. "Certainly whales are familiar with fishing gear," he told Live Science. And it's possible that the orcas grabbed the line, either intentionally or accidentally. But it's impossible to say whether they intended to untangle the whale, he said.

After the encounter, the humpback circled the whale-watching boat and then swam on. The orcas, meanwhile, spent the rest of their day socializing and "harassing sunfish," according to the whale-watching team.

The observation makes clear how challenging it can be to understand the perceptions and motivations of marine mammals.

"The range of behaviour by whales at sea is complex and varies depending on many factors," Hoyt wrote to Live Science. "It's difficult to interpret behaviour!"

Originally published on Live Science.

Editor's note: This article was updated to reflect the more recent edition and publisher of the book "Orca: The Whale Called Killer."