A beaded shark surrounding its prey is a classic scene known to many ocean lovers. Except that this captivating encircling behavior is not all it seems, according to a new study that observed whales, penguins and sea turtles swimming in circles as well.
“We found that a wide variety of marine megafauna exhibited similar circle behavior, in which the animals circled consecutively at a relatively constant speed more than twice,” says marine biologist and lead author Tomoko Narazaki of the University of Tokyo. , Japan.
By tracking their movements in three dimensions and analyzing where and when the labeled animals tended to circle, the researchers came up with a number of possible reasons. But, of course, research offers more opportunities to meditate than it has answers, so let’s dive.
The researchers first observed these mysterious encircling behaviors in green sea turtles, which were tagged and tracked using 3D data logs as part of another study.
“To be honest, I doubted my eyes when I first saw the data, because the turtle moves so constantly, like a car,” says Narazaki, who also studied why sea frogs swim so slowly. .
“When I returned to my lab, I reported this interesting discovery to my colleagues who use the same 3D data recordings to study a wide range of marine megafauna taxa.”
What they discovered was that sea turtles were not alone: whales, sharks and penguins had more or less the same encircling movements.
To study this wide range of marine animals, the researchers teamed up with local scientists and field volunteers in Japan, the Comoros Islands on the east coast of Africa and the Crozet Archipelago further south in the Indian Ocean, with help from the British side. Antarctic Survey as well.
They chased a total of 19 animals, including tiger sharks, king penguins, Antarctic fur seals, a goose-beaked whale and a whale shark.
Prior to this, marine life was tracked long and wide, but either to high pressure depths or horizontally along the ocean surface, using depth recorders or satellite tags.
The multi-sensor data logs used by this study team have now made it possible for researchers to measure the fine movements of animals in depth, latitude and longitude and up to a period of seconds – an impressive feat. in the expansion of the Earth of the oceans.
You would think that straight swimming would be the most efficient way to get around – and it is, from an energy saving perspective. But in an open ocean that is blue in all directions, animals that make a line from one place to another could swim directly beyond a celebration of opportunity, so it’s best to be curious and circle.
Some of the circular displays were recorded in areas where animals usually feed for food. For example, four tiger sharks (Galeocerdo cuvier) labeled off the coast of Hawaii surrounded 30 times and up to nearly 130 meters (426 ft) in their feeding basin.
However, Antarctic seals (Arctocephalus gazella) surrounded mainly during the day, even if they feed mainly at night; meanwhile, a group of playful king penguins (Aptenodytes patagonicus) mostly surrounded on the surface between deep dives of food collection – so it is clear that the story is more than food.
That being said, other sea creatures use extraordinary movements to capture prey.In 2019, humpback whales (Megaptera novaeangliae) were seen creating a “net” of bubbles as they swam in the rings to catch fish. So each to his own.
Mating rituals could be another possible explanation, a marked male tiger shark trying to court a female partner by swimming around her in circles.
But the most surprising discovery for Narazaki was to see some sea turtles swimming in circles as they approached their nesting beaches. A turtle in the study circled 76 times a day and 37 times the next, selecting the right direction to swim after intense deliberations.
This observation leads the team to believe that encircling behavior could also play a role in navigation. Their assumption is that migratory turtles can swim in circles to detect gradients in the Earth’s magnetic fields that they use to navigate the oceans and find their way home.
There is probably no single answer to this aquatic puzzle, as animals could surround themselves for several reasons.
“For example, some animals may move in circles to improve prey search while simultaneously collecting geomagnetic information,” the researchers write.
“Others, such as elephant seals moving down like falling leaves, could maintain directionality by geomagnetic scanning while resting in seemingly deep depths without mesopelagic features.”
All in all, this study is important in collaboration, but only in small numbers and had no information about the presence of nearby animals that could have influenced the behavior of the animals.
However, marine scientists around the world should be able to use this new technology in search of more clues as to why marine animals swim in circles.
The findings could also help establish a clear baseline for later identification of the impact of external factors – such as maritime traffic – on the movements, behavior and navigational skills of animals.
The research was published in iScience.