A beady-eyed shark encircling its prey is a classic scene known to many ocean lovers. Except this captivating circling behavior is not all it seems, according to a new study that has observed whales, penguins, and sea turtles swimming in circles, too.
"We've found that a wide variety of marine megafauna showed similar circling behavior, in which animals circled consecutively at a relatively constant speed more than twice," says marine biologist and lead author Tomoko Narazaki from the University of Tokyo, Japan.
Tracking their movements in three dimensions and looking at where and when tagged animals tended to circle, the researchers have come up with a host of possible reasons why. But of course, the research throws up more possibilities to ponder than it has answers, so let's dive in.
The researchers first noticed these mysterious circling 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 circles so constantly, just like a machine," says Narazaki, who has also studied why sea turtles swim so slowly.
"When I got back in 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 showed more or less the same circling movements as well.
To study this broad range of marine animals, the researchers teamed up 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 some help from the British Antarctic Survey also.
They tracked 19 animals in total, including tiger sharks, king penguins, Antarctic fur seals, one goose-beaked whale, and a whale shark.
Before this, marine life has been tracked far and wide, but either down to high-pressure depths or horizontally across the ocean surface, using depth recorders or satellite tags.
Multi-sensor data loggers, the likes of which this study team used, have now made it possible for researchers to measure fine-scale animal movements across depth, latitudes and longitudes, and down to a timescale of seconds – an impressive feat in Earth's expansive oceans.
You'd think swimming in a straight line would be the most efficient way to move about - and it is, from the perspective of saving energy. But in an open ocean that's blue in all directions, animals making a beeline from one place to another might be swimming straight past a feast of an opportunity, so best be curious and circle.
Some of the circling displays were recorded in areas where the animals typically forage for food. For instance, four tiger sharks (Galeocerdo cuvier) tagged off the coast of Hawaii circled up to 30 times and down to nearly 130 meters (426 ft) in their feeding grounds.
However, the Antarctic fur seals (Arctocephalus gazella) circled mainly during the day even though they primarily feed at night; meanwhile, a group of playful king penguins (Aptenodytes patagonicus) mostly circled at the surface in between deep food-gathering dives - so there's clearly more to the story than food.
That said, other marine creatures do use extraordinary circling motions to capture prey.In 2019, humpback whales (Megaptera novaeangliae) were seen creating a 'net' of bubbles as they swam in rings to trap fish. So each to their own.
Mating rituals might be another possible explanation, with one tagged male tiger shark trying to court a female partner by swimming around her in circles.
But the most surprising finding to Narazaki was seeing a couple of homing 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, selecting the correct direction to swim in after intense deliberation.
This observation has the research team thinking that circling behavior might also play some role in navigation. Their hunch is that migrating sea turtles may swim in circles to detect gradients in Earth's magnetic fields which they use to navigate across the oceans and to find their way home.
There's likely not just one answer to this aquatic puzzle, as the animals could be circling for a multitude of reasons.
"For example, some animals might move in circles to enhance prey search while simultaneously collecting geomagnetic information," the researchers write.
"Others, such as elephant seals drifting down like falling leaves, might maintain directional sense by geomagnetic scanning while resting in seemingly featureless mesopelagic depths."
All in all, this study is big on collaboration but only small in numbers, and it lacked information on the presence of nearby animals that could have influenced the animals' behavior.
Still, marine scientists around the globe 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 shipping traffic - on animals' movements, behavior and navigational abilities.
The research was published in iScience.