Risso’s dolphins: Spin-dive innovation lets them reach deep water for hunting

A species of dolphin that hunts prey living 600 metres below the surface spins its body as it dives so it can drill down through the water rapidly

By Christa Lesté-Lasserre

Risso’s dolphins dive rapidly and efficiently to catch prey hundreds of metres deep by twisting through the water at high speeds.

The round-faced dolphins exhale the air in their lungs then dive on a near-vertical trajectory, making as many as three full twists as they “drill” through the seawater in what researchers have named a spin dive. The technique quickly gets them to a dense layer of squid, fish and crustaceans with optimal use of energy and oxygen, making the dives highly profitable, says Fleur Visser at the University of Amsterdam.

“They are air-breathing mammals, so it’s costly for them to dive deep,” says Visser.

Visser and her colleagues had already noticed Risso’s dolphins (Grampus griseus) turning around at the water surface before diving. Curious about what the animals were doing, they equipped seven dolphins near Portugal’s Terceira Island with biologgers that recorded data about sound, 3D movement and depth, and gathered information about a total of 226 dives ranging from 20 to 623 metres deep.

For the deeper dives, the dolphins started with an intense stroke of the fins that rotated their bodies – usually towards the right – combined with a strong exhalation, presumably to reduce their buoyancy. They then turned downward at about 60 degrees and entered a high-speed, twisting descent followed by a rotating, free-gliding phase, achieving an average speed of 9 kilometres per hour and reaching an average depth of 426 metres.

They only started to echolocate to find prey in the dark water once they had finished spinning, which happened about 36 seconds into the dive on average. This suggests the dolphins “planned” the dive knowing food would be there, says Visser. The dolphins then stayed underwater for nearly 10 minutes, including hunting time.

By contrast, the shallower dives didn’t involve any twisting or turning. The dolphins dropped down to an average of 178 metres at a speed of 7 kilometres per hour, clicking to echolocate almost as soon as they began diving. These shallower dives were shorter on average, with dolphins returning to the surface for air after about 6 minutes.

In both cases, it took the dolphins about the same amount of time after the dive began to reach their prey, says Visser. The deeper spin dives occurred during daylight hours, whereas the shallower dives happened mainly between dusk and dawn.

This behaviour may be explained by the dolphins’ preferred food. They target prey that are part of the so-called deep scattering layer, a mixed-species mass of marine life that comes close to the surface in the evening to feed and drops back down into the darker depths of the ocean at dawn, presumably to hide from predators, she says.

Whereas other cetaceans tend to feed on the prey in the deep scattering layer only when they are close to the surface after sunset, Risso’s dolphins have evolved a unique tactic that allows them to feed on these animals at any time of day or night.

“[The dolphins] really know in advance where they’re going, and what type of dive they need to employ to get there,” says Visser.

Journal reference: Royal Society Open Science, DOI: 10.1098/rsos.202320

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