This photograph was taken off the Graham Coast, Antarctica, from aboard the National Geographic Explorer on January 3, 2018. It captures one moment during a two-hour encounter between four cooperatively hunting orcas and a crabeater seal. The seal, known informally as “Kevin”, survived the ordeal despite over three dozen attempts by the orcas to wash him off of various ice floes. The orcas repeatedly aligned four abreast and charged directly at the ice floes in a synchronized fashion. By swimming in close formation just below the surface of the water, the orcas generated enormous bow waves that would wash over the ice floes, dislodging Kevin time and again. Kevin finally escaped by swimming at a 90-degree angle to the course of the whales as they dove under his ice floe a final time. Credit: Wikimedia Commons/Callan Carpenter
A seal resting on an Antarctic ice floe may look safe, at least for a moment. It is out of the water, beyond the reach of most marine predators, perched on a slab of floating ice like a castaway on a raft.
But orcas have found a way around that problem. They do not always need to bite, ram or leap. Sometimes they swim together, build a wave and let physics do the hunting.
A new study in the Journal of Fluid Mechanics takes a close look at how Antarctic orcas, also referred to as killer whales, generate the powerful waves used to knock seals from ice floes. The researchers found that the animals appear to use a precise swimming posture with heads raised, bodies tilted upward and tails pushing downward to create a deep “depression wave” behind them. That wave can crack large ice floes or wash across smaller ones, sending seals into the water.
“We observed that groups of orcas swim rapidly towards ice floes, generating a depression wave within seconds on otherwise calm water,” the researchers wrote.
This video shows a small orca pod producing a depression wave to crack an ice floe on top of which a seal was seeking shelter. The maneuver proved successful and the orcas snatched the seal. Credit: YouTube.
The clever physics trick orcas use to create powerful waves
The behavior, often called “wave-washing,” has been seen in nature before. Pods of orcas swim in formation toward a seal on ice, then dive or pass just under the surface. The wave they create lifts, tilts or breaks the floe. If the seal falls in, the hunt can quickly turn in the orcas’ favor.
What has been less clear is how the animals make such an effective wave. The study suggests the secret is not just speed. It is posture.
To test the idea, the team built a scaled model of an orca pod and pulled it through a towing tank. The model had an oval body and a movable wedge-shaped tail. Researchers changed the angle of the body and tail to mimic different swimming positions, then measured how the water surface changed.
The most powerful waves appeared when the model’s body was pitched upward and the tail angled downward. In that position, the water jet between the model and the surface stayed attached for longer, almost as if the flow were being guided. That attachment helped pull the water surface down into a deeper trough.
×
Thank you! One more thing…
Please check your inbox and confirm your subscription.
“Based on our observations, orcas appear to deliberately raise their heads during hunting,” the researchers wrote. They added that this posture, paired with downward tail slaps, “may enhance wave-generation efficiency.”
Four orcas create a strong bow wave with which they hope to wash the seal off the ice floe. Credit: Wikimedia Commons
That matters because the first part of the wave is not a crest but a dip. Picture a moving dent in the water. When it reaches the edge of an ice floe, the front of the floe drops toward the trough. On a large floe, the back may remain flatter while the front bends downward. If the stress becomes high enough, the ice cracks.
The researchers compared this to a flexing board. Push down hard enough on one part while another part resists, and the board snaps. In the case of orcas, the force comes from the moving water.
For smaller floes, the effect is different. They may not break as easily, but they tilt. Then the following wave crest washes over the top, like a sheet of water thrown across a slippery table. That can be enough to sweep a seal off.
The study’s images and tank tests support both strategies: large floes can flip and fracture, while smaller floes can be overwashed. “The tail-generated depression waves produced by cooperating orca groups can first be used to break up large ice floes, and subsequently to wash seals off smaller ice floes,” the researchers wrote.
The study also helps explain why orcas often hunt in groups. A single animal can disturb the surface, but a pod swimming shoulder to shoulder makes a wider, stronger disturbance. That width matters. The researchers noted that orca waves differ from waves made by wind, ships or submarines. They are more focused and better matched to the size of the seal’s icy refuge.
There are limits to the study’s model. The model was simplified, and artificial ice can never capture every detail of sea ice. Real orcas also do not hold one fixed posture as they swim. Their bodies bend, tails beat and positions shift second by second. The researchers acknowledge that the real behavior is more complicated than a lab model can fully recreate.
Even so, the study offers a striking lesson in animal intelligence and fluid mechanics. Orcas are not just using brute force. They are shaping water into a tool.