Nature: Risso’s dolphins perform spin dives that allow them to “drill” deep into the water

To gobble up fish, squid and crustaceans that live more than 1,970 feet deep in the ocean, Risso’s dolphins “drill” into the water using a twist-diving technique.

This is the conclusion of experts from the University of Amsterdam, who became curious after noticing that the mammals were spinning on the surface of the water before diving.

To find out why, the team attached special sensors to seven dolphins off the island of Terceira, Azores, and logged a total of 226 dives.

They found that dolphins – whose prey only approach the surface at night – use different types of diving depending on how deep they have to go to feast.

“Spinning diving,” the researchers explained, allows them to hunt deeply while spending minimal energy and oxygen in the process.

The investigation was undertaken by behavioral biologist Fleur Visser from the University of Amsterdam and her colleagues.

“Deep dives are expensive for air-breathing marine predators because the increased time and energy costs of the trip, coupled with physiological restrictions, limit effective foraging time at depth,” the researchers explained in their paper.

“Minimizing travel costs is therefore essential to maintain optimal foraging and deep-diving cetaceans have developed specialized diving, oxygen conservation and biosonar strategies to target and locate prey living at depth.”

In their study, the team used biologgers to record various forms of data, including depth measurements, orientation, and sound recordings on 226 dives taken by Risso’s dolphins between May and August each year from 2012 to 2019. .

According to the team, the dives were approximately 20 to 623 m (66 to 2,043 feet) deep and were divided into deep “spin dives” and shallower dives that did not involve any twisting or turning.

Each spin dive began with a deep exhalation to reduce buoyancy, as well as an intense blow from the dolphins’ fins which made their bodies spin clockwise before entering a rapid, twisting descent to an angle of 60 °.

This was followed by a phase of spinning free flight, with average speeds of 5.6 miles / hour (9 km / h) and up to three full turns.

It was not until they finished turning – at an average depth of 1,398 feet (426 m) and usually about 36 seconds after the start of the dive – that the dolphins began echolocation to help them detect. their prey.

This, Dr Visser explained, indicates that the dolphins “planned” these maneuvers knowing they would find food at the bottom of each dive.

In total, each spin dive lasted approximately 10 minutes, including the time spent hunting at the nadir of each excursion to the depths.

In their study, the team used biologgers to record various forms of data – including depth measurements, orientation, and sound recordings – on 226 dives undertaken by Risso’s dolphins between May and August each year of 2012. to 2019. According to the team, the dives were approximately 66 to 2,043 feet (20 to 623 m) deep and were divided into deep “spin dives” (left) and shallower dives that did not involve no twists or turns (right). In the photo, from top to bottom: the dolphin at the start of the dive, orientation during the initial descent, depth, acceleration, orientation, forward speed and energy expenditure

Shallower, spin-free dives, on the other hand, were typically only six minutes long and saw the dolphins descend to an average depth of 584 feet (178 m) at a speed of about 4.3 miles per hour (7 km). / h), with echolocation starting almost from baseline.

Both types of diving saw mammals reach their prey at about the same time.

However, spin dives only occurred during daylight hours, while shallower dives mostly occurred from dusk to dawn.

According to the team, the way dolphins vary their dives may be a necessity to catch their favorite food, which belongs to the deep diffusion layer.

This part of the ocean – first detected by sonar as a “false seabed” during WWII – is home to an abundance of marine life that moves near the surface in the evening to feed before retreating. in the depths at dawn to avoid predation.

The team found that dolphins – whose prey only approach the surface at night – use different types of diving depending on how deep they have to go to feast. In the photo: the frequency of non-rotating dives (top) and rotating dives (bottom), compared to the depth of the so-called deep-scattering prey layer that Risso’s dolphins feast on

Most cetaceans feed only on prey that is the deep diffusion layer when they come close to the surface after sunset.

In view of this, Risso’s dolphins are distinguished by having developed a tactic that allows them to exploit this food resource at all times.

“They really know in advance where they are going and what type of diving they need to use to get there,” Dr Visser told the New scientist.

The full results of the study have been published in the journal Royal Society Open Science.

For their study, the researchers attached sensors to seven dolphins off the coast of the island of Terceira, in the Portuguese Autonomous Region of Azores, and recorded data from 226 dives.

WHAT ARE RISSO DOLPHINS?

Risso’s dolphins are found in temperate and tropical oceans around the world, usually in deep waters close to land.

They are on average 10 feet (3 meters) long and feed almost exclusively on schools of oceanic squid, hunting mainly at night.

While hunting dives are generally short, lasting less than five minutes, mammals can reach depths of 1,000 feet (300 meters).

Risso’s dolphins are found in temperate and tropical oceans around the world, usually in deep waters close to land. Older individuals appear predominantly white and are often covered in scars from social interactions (artist’s impression)

Older people appear predominantly white and are often covered in scars from social interactions.

Dolphins usually travel in groups of 10 to 50, but these can grow into huge collectives of up to 400.

Smaller social subgroups often form within larger groups, and the species is also known to travel with other marine mammals, including the gray whale.