Dolphins Perform Elaborate Spin Dives to Hunt Prey

The Risso's dolphin is very acrobatic. Known for its boxy head and prominent dorsal fin, this marine mammal flaps its flippers and tail on the surface and raises its head vertically out of the water, something known as spyhopping.

But the Risso’s dolphin also performs quite dramatic dives.

They can plunge to 1,000 feet (305 meters) and hold their breath for as long as 30 minutes, as they hunt prey. They also make short dives and “porpoise” by leaping in and out of the water at high speeds, typically while being pursued by predators.

Researchers recently observed Risso's dolphins (Grampus griseus) performing a new kind of dive strategy. They started with a sprint combined with a spin as they plummeted into the water. Dubbed a “spin dive,” this bombastic maneuver takes more energy than simple, slower dives, but helps them reach prey located in deep waters, their research finds.

“A spin dive is characterized by a strong acceleration and associated lateral rotation (spin) at the surface, after which the individual rapidly descends,” Fleur Visser, a leading researcher at the Institute for Biodiversity and Ecosystem Dynamics at the University of Amsterdam and the NIOZ Royal Netherlands Institute for Sea Research, tells Treehugger.

“A non-spin dive is the typical, slower so-called arch-out dive, where the individual curves its body, showing the tailstock and dives down. In sperm whales, for example, this is the dive where they show the tail. Risso's dolphins don't typically do so, but the arch is similar.”

Researchers weren’t sure why the dolphins performed the elaborate dives but believed it was associated with foraging for prey. They just didn’t know why the animals would spend so much energy at the start of the maneuvers.

Analyzing Dives

For their study, researchers temporarily attached biologging devices via suction cups to seven dolphins to record their sound and movement. The animals were studied off Terceira Island, Azores, in Portugal between May and August of 2012–2019.

The team analyzed data from more than 260 dives recorded on the devices. They recorded the depth of dives, sound, and movement dynamics. Researchers then compared this data to information on the depth of prey, particularly their favorite: squid.

Risso’s dolphins are typically covered with scars, received from skirmishes with other dolphins, as well as encounters with prey, including squid, sharks, and lampreys.

“They make the sprints specifically to reach their prey when it is at larger depth, deeper than 300 meters. Because they need oxygen and are limited in their dive time they need a specific strategy to maintain enough time to forage at these depths,” Visser explains.

“To this aim, they perform a rotation sprint at the onset, which enables them to dive down much more quickly, reaching first prey at the same time as in normal dives (even though prey is deeper), thus leaving them enough time to forage at those larger depths.”

During the day, the dense group of prey—called the deep scattering layer—moves up and down throughout the water column. The animals hide from predators in dark water during the day by staying in waters deeper than 300 meters (about 1,000 feet).

At dawn, they move up to forage in the surface layers, then return to the deeper, darker spots at dusk. 

Researchers tracked Risso's dolphins as the animals tracked the movement of this deep scattering layer. The dolphins foraged deep after the prey in the day and followed them in the shallow water at night.

“We were amazed by the stark contrast between when a spin and non-spin foraging dive is used. It is like flicking a switch,” says Visser. 

“And related to that, the really clear tracing of a prey layer, and having multiple strategies to hunt in it, depending on its depth. The Risso’s dolphins have adapted to be able to effectively hunt deep, next to shallow, circumventing the predator-avoidance strategy of their squid prey.”

The results were published in the journal Royal Society Open Science.

Why This Matters

Understanding the relationships between predator and prey is one of the key ways to understanding and protecting the oceans, researchers say.

“Whales and dolphins face potential disturbance from a range of anthropogenic influences, including noise and ocean warming. Effects on foraging behavior are of specific importance because it can affect fitness of the individual, and ultimately the population,” Visser says.

“In order to understand and enable mitigation against effects, we first need to understand the natural behavior. Our work provides an important step forward in understanding how deep divers need to strategize in order to keep a balance between expending significant time and energy into deep and long dives that are physiologically challenging and energetic gain from their prey. We need to understand the prey conditions that render deep diving profitable in order to know what is the potential effect on an individual if it loses foraging opportunity, or is disturbed.”