Dolphin 'invented' spinning diving technique to hunt

The Risso dolphin specializes in hunting at a depth of 600m below the sea surface, rotating its body while diving to be able to rush through the water extremely fast.

The Risso dolphin dives quickly and efficiently to capture prey hundreds of meters deep by rotating its body while swimming through the water at high speed. This round-faced dolphin releases air in its lungs, then dives in an almost vertical line, spinning up to 3 complete revolutions as it penetrates the sea. The researchers call this the swing diving technique. This technique can help Risso dolphins quickly access water rich in squid, fish and crustaceans, while making optimal use of energy and oxygen. As a result, diving is as beneficial as possible, according to researcher Fleur Visser of the University of Amsterdam.

The Risso Dolphin near the Azores Islands in the Atlantic Ocean. (Photo: Alamy)

Visser and colleagues noticed that the Risso dolphin (Grampus griseus) whirled around in the water before diving. To find out what the animals were doing, the researchers attached biometers to seven dolphins near the Portuguese island of Terceira to record sound, 3D motion and depth data, collecting information Information about a total of 226 dives with depths ranging from 20 to 623 m.

For deeper dives, dolphins start pounding their fins and rotate their body from right to left in combination with strong breathing to reduce buoyancy. They then swam down at a 60-degree angle and started spinning at high speed, followed by a quick glide, reaching an average speed of 9 km/h and an average depth of 426 meters. Risso dolphins only echolocate to find prey in the dark water after they stop spinning (about 36 seconds from the start of the dive). This shows that dolphins know where food is available and plan their diving activity. They will stay underwater for almost 10 minutes, including hunting time.

In contrast, diving in shallow water does not require rotation. Dolphins dive to an average depth of 178m at 7km/h, starting echolocation almost as soon as they dive. The duration of shallow dives is also shorter, dolphins return to the surface for oxygen after about 6 minutes.

In both cases, the dolphin took the same amount of time from diving to approaching prey, according to Visser. Deep-water swing dives usually take place during the day while shallow dives take place mostly at dawn or dusk. This behavior can be explained by the dolphin's preferred prey. They target deep-water dispersion, where many marine species come close to the surface at night to feed and dive deeper into the dusk to avoid predators. "Dolphins really know in advance where they need to swim and the type of diving they need to use to get there," concludes Visser.