Killer whales make themselves a fridge

Catching up on some Quirks and Quarks podcasts last night, I came across an amazing discovery about orcas (a.k.a. killer whales). Scroll down to "A Killer Diet" on this page to listen.

Here's the research paper and a summary from the Vancouver Aquarium. Essentially, a group of transient orcas near Unimak Island in the Aleutians hunt gray whales each spring, usually separating a juvenile gray whale from its mother, then killing and eating it.

For killer whales, that's not especially unusual: many of their populations around the world hunt other whales much larger than they are (even blue whales), using clever pack behaviour, often in the open ocean. But the problem is that most whale carcasses sink after they die, so the orcas can only get to some of the blubbery whale goodness before it disappears into the abyssal depths. (Interesting stuff can happen after that too, but we're on a different story.)

The Unimak orcas have taken advantage of gray whales' defensive behaviour, which is to retreat to shallow water. Separating the juvenile prey from its adult defenders in the relative shallows (10 to 20 m) has allowed the orcas to invent a refrigerator for themselves. Because even a juvenile grey whale provides too much meat for a pack of killer whales to eat all at once.

So the orcas eat, then let their prey sink. But the rocky bottom is right there, and a day or so later, they come back to dive down for leftovers. And again, and again, for nearly a week—like a human fridge after Thanksgiving. (It's Alaska just after winter, so the water is nice and cold too.) In the meantime, other sea creatures—including sharks, as well as invertebrates—chomp and grind away at the remains. Even local bears get in on the action, salvaging bits that float ashore.

This all shows why killer whales are among the most flexible, ingenious, and successful large predators in the world. While they're most common in temperate oceans near shore, orcas range throughout the world's marine surface ecosystems, from the Arctic to the Antarctic, from the Mediterranean to tropical seas. They are the top predator in all those places: nothing else eats them. In those different habitats, they often have specialized diets, hunted with customized techniques learned, and then passed from generation to generation.

We have the Unimak gray whale fridge. Resident populations near my home in British Columbia eat salmon; some people think the whales stun the fish by focusing their vocalizations through the melons at the fronts of their heads. Transient pods here eat marine mammals instead, mostly seals and sea lions, sometimes tossing them in the air during pursuit. But in one spot in South America, the killer whales ride the surf, emerging from the foam to snatch sea lions right off the beach. Off Norway, orcas trap schools of herring in rings of bubbles. Near San Francisco, scientists recorded orcas hunting a great white shark by flipping it on its back, which immobilized the fish and eventually killed it.

I suspect that no other type of carnivore has such a diversity of hunting behaviour. That's probably because they are mammals (the largest variety of dolphin) like us, with big brains—which they obviously use very effectively. Most predator species, whether on land or in the ocean, have common hunting behaviours, and often highly specialized bodies to accommodate them. But killer whales have evolved—as a species—to be generalists. They are smart, they can improvise, they can plan, and they learn. They teach each other what works and how to do it, and pass the knowledge down through families. You might even say they have culture.