How do killer whales trap seals on the ice?

Three killer whales swim in tight formation in Antarctic waters, their dorsal fins cutting the surface. Ahead of them, on a floating slab of ice, a Weddell seal is resting. Within seconds the killer whales will coordinate their dive and their return to the surface, and together create a single wave that will sweep over the ice and wash the seal into the water. This technique, known as wave washing, is one of the most sophisticated hunting strategies ever documented in the animal kingdom.

Spyhopping: the search phase

It all begins with the search. In the white immensity of the Antarctic pack ice, spotting a seal on an ice floe is no simple matter. The killer whales use their favoured technique: spyhopping. Several animals raise their heads vertically out of the water, sometimes using their flippers to gain height, and scan their surroundings visually. It is often the young killer whales that practise this watching most intensively, social learning in action.

This observation phase can last several minutes. It is not incidental: it already amounts to genuine planning. The killer whales assess the position of the seal, the orientation of the floe relative to the current, its stability, its size. These are strategic pieces of information for what follows. Weddell seals are the preferred prey, their substantial mass (300 to 500 kg) making the energy outlay worthwhile, whereas crabeater seals, smaller and better at keeping their balance, are generally ignored by type B killer whales.

Acoustic communication before the attack

Once the target has been identified, the killer whales use specific calls to coordinate the attack with the other members of the group. These calls differ from the pod’s “ordinary” calls; they are operational signals specific to hunting. Acoustic biologists have been able to identify several distinct signatures corresponding to different phases: searching, mobilising, executing.

One striking point: these Antarctic calls are produced even with the prey close by. That is consistent with the fact that seals hear poorly in the frequency bands the killer whales use. Conversely, Bigg’s (transient) killer whales of the north-eastern Pacific, which hunt cetaceans whose hearing is close to their own, stay silent during the approach phase.

Wave washing: the choreography

The attack phase is what is called wave washing. Three or four killer whales take up a tight formation, usually a triangle or a V, and swim together towards the floe. At the precise moment before they reach it, they dive simultaneously beneath it, driving their massive bodies forward in a coordinated acceleration.

The combined wake of these dives generates a single wave that passes under the floe, lifts it sharply and washes the seal into the water. The timing is crucial: if the killer whales dive out of step, they produce several small, ineffective waves. If they dive simultaneously, the hydrodynamic effect is maximised. The precision is measured in fractions of a second.

A learned technique, not an innate one

Robert Pitman and John Durban, biologists specialising in Antarctic killer whales, published a major study in 2012 documenting this technique in the type B “pack ice” killer whales of the Antarctic Peninsula. Their work showed that wave washing is specific to this ecotype: the other killer whale ecotypes (Pacific residents, Bigg’s transients, Antarctic type A) do not practise it, even when the opportunity arises.

This cultural specificity implies that the technique is learned through social transmission, not genetically programmed. Young type B killer whales watch the adults’ attacks, take part first as onlookers, then as assistants, then as executants. The learning takes several years. An adult type B killer whale removed from its natal group would probably not have developed this technique on its own.

Sharing the prey

Once the seal is in the water, the attack is quick. Several killer whales converge to stop the prey from climbing onto another floe. The seal is usually killed within minutes. The prey is then shared among the members of the group, a social behaviour that reinforces cohesion and allows the young to benefit from the collective effort even when they did not contribute directly to the capture.

Wave washing is one of the finest documented examples of complex cooperative hunting in the animal kingdom, comparable to the hunts of lions in Africa, of wolves in the forest, or of prehistoric humans. It attests to a social intelligence, a capacity for planning, and a physical and acoustic coordination that place killer whales among the most sophisticated minds on the planet. Understanding this technique is also a way of measuring what is at stake in protecting these exceptional populations.

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