A small-eyed shark circling its prey is a classic scene known to many ocean lovers. Except this captivating circular behavior isn’t all that it seems, according to a new study that has also observed whales, penguins and sea turtles swimming in circles.
“We have found that a wide variety of marine megafauna exhibited similar circular 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 observing where and when the marked animals tended to circulate, the researchers have found a number of possible reasons. But of course, research offers more scope for thought than answers, so let’s dig deeper.
The researchers first noticed these mysterious circular behaviors in green sea turtles, which had been tagged and tracked using 3D data loggers as part of another study.
“To be honest, I doubted my eyes when I first saw the data because the turtle is spinning so constantly, like a machine,” says Narazaki, who has also studied why sea turtles swim so slowly.
“When I returned to my lab, I reported this interesting discovery to my colleagues who use the same 3D data loggers to study a wide range of marine megafauna taxa.”
What they found was that sea turtles weren’t alone: whales, sharks, and penguins also showed more or less the same circular movements.
To study this wide range of marine animals, the researchers partnered with local scientists and field volunteers in Japan, the Comoro Islands off the east coast of Africa, and the Crozet Archipelago further south in the Indian Ocean, with the help of the British. Antarctic Survey too.
They tracked 19 animals in total, including tiger sharks, king penguins, Antarctic fur seals, a beaked whale and a whale shark.
Before this, marine life had been tracked far and wide, but either at high pressure depths or horizontally across the ocean surface, using depth recorders or satellite tags.
Multisensor data loggers, such as the ones this study team used, have now made it possible for researchers to measure fine-scale animal movements at depth, latitude and longitude, and down to a timescale of seconds – an impressive feat in the expansion of the Earth. oceans.
You’d think swimming in a straight line 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 going directly from one place to another may be swimming directly in front of an opportunity, so it’s best to be curious and spin.
Some of the circling displays were recorded in areas where animals typically forage for food. For example, four tiger sharks (Galeocerdo cuvier) marked off the coast of Hawaii circled up to 30 times and nearly 130 meters (426 feet) in their foraging grounds.
However, Antarctic fur seals (Arctocephalus gazella) they circulate mainly during the day although they feed mainly at night; meanwhile, a group of playful king penguins (Aptenodytes patagonicus) mostly circled on the surface between deep food-gathering dives, so there’s clearly more to the story than food.
That said, other sea creatures use extraordinary circular motions to capture their prey.In 2019, humpback whales (Megaptera novaeangliae) They were seen creating a “web” of bubbles as they swam in rings to catch fish. So each one to his own.
Mating rituals could be another possible explanation, with a tagged male tiger shark trying to woo a female partner by swimming around it in circles.
But the most surprising find for Narazaki was seeing a pair of sea turtles swimming in circles as they approached their nesting beaches. One turtle in the study circled 76 times one day and 37 times the next day, selecting the correct direction to swim after intense deliberation.
This observation leads the research team to think that circling behavior could also play a role in navigation. His hunch is that migratory sea turtles can swim in circles to detect gradients in Earth’s magnetic fields that they use to navigate across the oceans and find their way home.
There is likely no single answer to this aquatic puzzle, as animals could be circling for a multitude of reasons.
“For example, some animals can move in circles to improve their search for prey and at the same time collect geomagnetic information,” the researchers write.
“Others, such as elephant seals that glide down like falling leaves, can maintain directional sense through geomagnetic scanning while resting in seemingly featureless mesopelagic depths.”
Overall, this study is highly collaborative, but only a small number, and lacked information about the presence of nearby animals that could have influenced the animals’ behavior.
Still, marine scientists around the world should be able to put this new technology to good use in search of more clues as to why marine animals swim in circles.
The findings could also help establish a clear baseline to later identify the impact of external factors, such as maritime traffic, on the animals’ movements, behavior and navigational skills.
The research was published in iScience.