Not for nothing are Orcas called Killer whales, but reports of their hunting the "great" whales (baleen & sperm whales) have been rare, and there is little consensus as to the importance of such predation. But new evidence suggests that Humpback whale calves are a regular snack for the black and white killers.

At one time, Killer whales were thought of as "top omnivores." That is, they were only opportunistic predators, striking when the circumstances presented themselves. But recent research has shown that the massive marine mammals, at least those swimming in the high-latitude oceans near the poles, can actually be prey specialists. Some feed exclusively on fish, others exclusively on marine mammals like sea lions and seals. It's only in the lower latitudes near the equator that Killer whales are more likely to gobble up whatever they can, perhaps owing to fewer or patchier resources.

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Great white sharks get top billing as the so-called "perfect killing machines" but Killer whales have something that the sharks don't: friends. That is, they're not just big, toothy, and long-lived. They're also social. And put together, that makes them formidable predators.

Still, despite the fact that people have reported witnessing Killer whales attacking each of the "great whale" species, successful kills have rarely been documented. "These events are still so infrequently observed that documentation of even individual attacks continue to appear in the scientific literature," notes NOAA scientist Robert L. Pitman in the latest issue of Marine Mammal Science. "On the other hand," he continues, "the prevalence of presumed killer whale tooth rake marks on the flukes and flippers of nearly every large whale species has been cited as evidence that attacks might, in fact, be fairly common."

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To try to figure out just what's going on, Pitman and his colleagues turned to the oceans off of Western Australia. Over the course of hundreds of hours of observations, the researchers identified nineteen different Killer whales from three different social groups attacking juvenile Humpback whales. In all, they witnessed twenty-two individual attacks in which they could ascertain the outcome. Two thirds of the time, the Killer whales lived up to their name, killing the Humpbacks. (Kills often attracted Great white and Tiger sharks who, in fact, scavenged off the dead whales; one was seen ripping a hunk off a calf while the body was inside the mouth of a Killer whale!)

In addition, they satellite tagged a single adult female Killer whale and followed her group for six days. In that time, the group attacked eight Humpback calves, and identified three successful kills out of the seven attacks in which they witnessed the end result.

Interestingly, the only other prey species that the researchers saw the Killer whales go after was Spinner dolphins. In one instance, the whales managed to successfully kill two dolphins. (And two hours later, that same group successfully took down a Humpback calf!)

What it suggests is that Humpback calves are a "predictable, plentiful, and readily taken prey source" for Killer whales, at least during the summer months when the Humpbacks are in the area.

But the Humpbacks have a few tricks up their sleeves to avoid the Killers. For one thing, when the Orcas approached, the Humpback mothers tried to move their calves into shallower water. If she was able to do it fast enough, that was usually an effective response. And as they moved, mothers and their calves tried to stick near coral reefs, which allowed them some protection from surprise attacks.

In several instances, both in response to attacks from Great white and from Killer whales, the mother actually lifted her calf out of the water on top of her head or on her back!

A mother lifts her calf out of the water on her back in the second photo; the Killer whale on the far left can be seen of another calf's carcass killed just minutes earlier. Photo via S. Wenngren/Marine Mammal Science.

In addition, the traveling pairs - mother and calf - were often seen traveling with an escort. Traditionally, escorts are thought of as adult males that swim together with an adult female - with or without a calf - on breeding grounds. The idea is that he's waiting for her to become sexually receptive, or it could be that he's there to prevent other males from breeding with her.

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However, that doesn't explain what the researchers repeatedly witnessed, which was the escort's role in driving off predators. "The defensive behaviors of the escorts were similar to that of the mothers, including flanking the calf opposite the mother, flailing its flukes or flippers, charging toward the attacking killer whales, and trumpeting" they report. They continue:

The question that remains is how do escorts benefit from this apparent alloparental behavior? To answer this, future research will need to determine if the escort is related to the mother or the calf (kin selection), or if the escort's behavior increases his chances for breeding with the mother once she comes back into estrus (reciprocity).

For now, these findings lend support to the notion that Killer whales routinely hunt and eat the great whales. It's possible that the systematic exploitation of the great whales for their blubber is, in part, why we're now so confused about this predator-prey dynamic.

One hypothesis suggests that prior to industrial whaling, Killer whales were important predators of the large whales. After their food was all but wiped out by humans, those who didn't starved turned to other marine mammals, the ones not profitable to the whaling industry and which had not evolved to account for Killer whale predation. Eventually the Killer whales depleted those populations. Together, this is why this sort of food web implosion has been called the "sequential megafaunal collapse" hypothesis.

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Now, as the large whales have begun to recover from the harms of human whaling, Killer whales have begun once again to view them as a good source of nutrition - at least, that's the theory. That natural experiment may already be unfolding where those whales are recovering, off the coast of Western Australia, in the eastern Canadian Arctic, and in Alaska's Aleutian Islands. We just have to watch and see.

[Marine Mammal Science]

Header image: Flickr/Bugsy