A taxon of dolphin that hunts prey surviving 600 metres beneath the aboveground spins its assemblage arsenic it dives truthful it tin drill down done the h2o rapidlyLife 1 December 2021
Risso’s dolphins dive rapidly and efficiently to drawback prey hundreds of metres heavy by twisting done the h2o astatine precocious speeds.
The round-faced dolphins exhale the aerial successful their lungs past dive connected a near-vertical trajectory, making arsenic galore arsenic 3 afloat twists arsenic they “drill” done the seawater successful what researchers person named a rotation dive. The method rapidly gets them to a dense furniture of squid, food and crustaceans with optimal usage of vigor and oxygen, making the dives highly profitable, says Fleur Visser astatine the University of Amsterdam.
“They are air-breathing mammals, truthful it’s costly for them to dive deep,” says Visser.
Visser and her colleagues had already noticed Risso’s dolphins (Grampus griseus) turning astir astatine the h2o aboveground earlier diving. Curious astir what the animals were doing, they equipped 7 dolphins adjacent Portugal’s Terceira Island with biologgers that recorded information astir sound, 3D question and depth, and gathered accusation astir a full of 226 dives ranging from 20 to 623 metres deep.
For the deeper dives, the dolphins started with an aggravated changeable of the fins that rotated their bodies – usually towards the close – combined with a beardown exhalation, presumably to trim their buoyancy. They past turned downward astatine astir 60 degrees and entered a high-speed, twisting descent followed by a rotating, free-gliding phase, achieving an mean velocity of 9 kilometres per hr and reaching an mean extent of 426 metres.
They lone started to echolocate to find prey successful the acheronian h2o erstwhile they had finished spinning, which happened astir 36 seconds into the dive connected average. This suggests the dolphins “planned” the dive knowing nutrient would beryllium there, says Visser. The dolphins past stayed underwater for astir 10 minutes, including hunting time.
By contrast, the shallower dives didn’t impact immoderate twisting oregon turning. The dolphins dropped down to an mean of 178 metres astatine a velocity of 7 kilometres per hour, clicking to echolocate astir arsenic soon arsenic they began diving. These shallower dives were shorter connected average, with dolphins returning to the aboveground for aerial aft astir 6 minutes.
In some cases, it took the dolphins astir the aforesaid magnitude of clip aft the dive began to scope their prey, says Visser. The deeper rotation dives occurred during daylight hours, whereas the shallower dives happened chiefly betwixt dusk and dawn.
This behaviour whitethorn beryllium explained by the dolphins’ preferred food. They people prey that are portion of the alleged deep scattering layer, a mixed-species wide of marine beingness that comes adjacent to the aboveground successful the evening to provender and drops backmost down into the darker depths of the water astatine dawn, presumably to fell from predators, she says.
Whereas different cetaceans thin to provender connected the prey successful the heavy scattering furniture lone erstwhile they are adjacent to the aboveground aft sunset, Risso’s dolphins person evolved a unsocial maneuver that allows them to provender connected these animals astatine immoderate clip of time oregon night.
“[The dolphins] truly cognize successful beforehand wherever they’re going, and what benignant of dive they request to employment to get there,” says Visser.
Journal reference: Royal Society Open Science, DOI: 10.1098/rsos.202320
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