Risso’s dolphins use a twisting diving technique to eat fish, squid, and other crustaceans living more than 1,970 feet under the sea.
Experts from the University of Amsterdam came to this conclusion after becoming curious about the mammal’s movements at the surface of the water before they dived.
To find out why this was, the team attached special sensors to seven dolphins off of the coast of Terceira Island, in the Azores, and recorded a total of 226 dives.
They found that the dolphins — whose prey only come close to the surface at night — use different types of dive depending on the depth they need to get to to feast.
According to the researchers, spin diving allows them the ability to search at depth and expend minimal oxygen and energy.
Risso’s dolphins are able to gobble up crustaceans, fish and squid that live over 1,970 feet in the ocean.
Fleur Visser (University of Amsterdam) and colleagues conducted the research.
‘Deep dives are costly for air-breathing marine predators, as the increased temporal and energetic costs of travel, in combination with physiological restrictions, constrain effective foraging time at depth,’ the researchers explained in their paper.
‘Minimising cost of travel is thus essential to maintain optimal foraging and deep-diving cetaceans have evolved specialised diving, oxygen-conserving and biosonar strategies to target and locate deep-dwelling prey.’
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 the May and August each year from 2012–2019.
According to the team, the dives ranged in depth from around 66–2,043 feet (20–623 m) and were divided into deep ‘spin dives’ and shallower dives that didn’t involve any twisting or turning at all.
Each spin dive was found to begin with a deep exhalation — to reduce buoyancy — along with an intense stroke of dolphin’s fins that turned their body in a typically clockwise rotation before they entered a rapid, twisting descent at a 60° angle.
Following this would be a free, rotating gliding phase at a speed of 9 km/h (5.6 miles/hour). Three full turns would be required for each dive.
Only when they had finished spinning — at an average depth of 1,398 feet (426 m) and typically some 36 seconds into the dive — did the dolphins begin echolocation to help them detect their prey.
This, Dr Visser explained, indicates that the dolphins ‘planned’ these manoeuvres knowing that they would find food at the bottom of each dive.
In total, each spin dive lasted for around 10 minutes — including time spent hunting at the nadir of each excursion into the deep.
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 the May and August each year from 2012–2019. According to the team, the dives ranged in depth from around 66–2,043 feet (20–623 m) and were divided into deep ‘spin dives’ (left) and shallower dives that didn’t involve any twisting or turning at all (right). The top-bottom picture shows the orientation of the dolphin during the initial dive, the depth, acceleration and forward speed, as well as the energy expended.
For the shallower dives that were spin-free, the duration was typically only six minutes. The dolphins sank to an average depth (584 feet) and a speed of approximately 4.3 miles per hour (7 KPH). Echolocation began almost as soon as the start of the dive.
Both types of dive saw the mammals reach their prey in roughly the same amount of time — however, the team reported, spin dives were seen to take place only during the daylight hours, while shallower dives mostly occurred from dusk–dawn.
According to team members, dolphins might need to diversify their dives to capture their favourite food. This is the deep scattering layer.
This part of the ocean — first detected by sonar as a ‘false sea floor’ during World War II — harbours an abundance of marine life that moves close to the surface in the evening to feed before retreating back to the depths at dawn to avoid predation.
The team found that the dolphins — whose prey only come close to the surface at night — use different types of dive depending on the depth they need to get to to feast. Below is a comparison of the number of spin (top) or non-spin (bottom), and the depth at which Risso’s dolphins can feast on the prey called deep scattering.
Most cetaceans only feed on prey that make up the deep scattering layer when it comes close to the surface after sunset.
Risso’s dolphins are unique because they have developed a strategy that allows them access to this food source at all times.
The New Scientist was told by Dr Visser that they really do know ahead of time where they want to go and the type of diving they should use to get there.
Royal Society Open Science has published all of the findings.
Researchers attached sensor to seven dolphins at Terceira Island in the Portuguese autonomous region. They also recorded 226 dives.
WHAT ARE RISSO’S DOLLINS?
Risso’s Dolphins can be found worldwide in tropical and temperate oceans, often in shallow water near land.
On average they grow up to 10 feet (3 m) in length and eat almost exclusively oceanic squid shoals, hunting mainly at night.
Although hunting dives typically last for less than five seconds, mammals can reach depths up to 1,000 feet (390 metres).
Risso’s dolphins can be found around the globe in both tropical and temperate seas. They are often located in deeper water closer to shore. The older individuals are mostly white and often have scarring from social interaction (artist’s perception).
Senior citizens are usually mostly white and often have scarring from social interaction.
The dolphins typically travel in groups of 10–50, but these may grow to huge collectives that reach 400.
Sometimes, smaller social groups form in larger groups. The species is also known to travel with gray whales and other marine mammals.