A big-eyed shark surrounding its prey is a classic scene known to many ocean lovers. Except for a new study that has observed that whales, penguins, and sea turtles also swim in circles, except that this captivating behavior in a circle is not all that it seems.
“We found that a wide variety of marine megafauna exhibited circumferential-like behavior, in which animals circulated consecutively at a relatively constant speed more than double that,” says Tomoko Narazaki, marine biologist and lead author at the University of Tokyo, Japan. .
Following their three-dimensional movements and observing where and when the labeled animals used to circulate, researchers have presented a number of possible reasons why. But, of course, research offers more opportunities for reflection than answers, so let’s dig deeper.
The researchers first noticed these mysterious back behaviors in green sea turtles, which had been tagged and tracked using 3D data records as part of another study.
“To be honest, I doubted my eyes when I first saw the data because the turtles are constantly circling, just 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 were not alone: whales, sharks, and penguins exhibited more or less the same back movements.
To study this wide range of marine animals, researchers partnered 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 a little help from the British. Antarctic Survey as well.
They tracked 19 animals in total, including tiger sharks, king penguins, Antarctic fur seals, a goose-billed whale and a whale shark.
Prior to that, marine life has been tracked extensively, but to high-pressure depths or horizontally across the ocean surface, using depth recorders or satellite tags.
Multisensor data loggers, which have used this study equipment, have now allowed researchers to measure large-scale animal movements across depth, latitude and longitude, up to a time scale of seconds, an impressive feat. to the extent of the Earth’s oceans.
I would think that swimming in a straight line would be the most efficient way to move, and it is, from the perspective of saving energy. But in an open ocean that is blue in all directions, animals that make a line from one place to another can swim directly beyond a party of opportunity, so it’s best to be curious and circle.
Some of the circle samples were recorded in areas where animals usually feed. For example, four tiger sharks (Galeocerdo cuvier) labeled off the coast of Hawaii circulated up to 30 times and dropped as low as nearly 130 meters (426 feet) into their feeding grounds.
However, Antarctic fur seals (Arctocephalus gazella) circulated mainly during the day although they feed mainly at night; meanwhile, a group of playful king penguins (Aptenodytes patagonicus) circled mostly to the surface between deep dives of food gathering, so the story clearly has more to eat.
That said, other sea creatures use extraordinary circular motions to capture prey.In 2019, humpback whales (Megaptera novaeangliae) were seen creating a “net” of bubbles as they swam in rings to catch fish. Therefore, each to his own.
Mating rituals could be another possible explanation, with a marked male tiger shark trying to woo a couple by swimming around them in circles.
But the most surprising finding for Narazaki was to see a pair of sea turtles swimming in circles as they approached their nesting beaches. A turtle in the study circled 76 times one day and 37 times the next, selecting the right direction to swim after intense deliberation.
This observation leads the research team to think that circle behavior could also play a role in navigation. Their intention is that migrating sea 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 probably isn’t just one 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 the search for prey while collecting geomagnetic information,” the researchers write.
“Others, such as elephant seals moving down like falling leaves, could maintain directional sense by geomagnetic scanning while resting in seemingly intransigible mesopelagic depths.”
However, this study is important in collaboration, but is only small in number, and had no information on the presence of nearby animals that could influence animal behavior.
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.