At any given moment, a sea turtle is probably swimming in a constant, hypnotic circle somewhere in the ocean — and so are a king penguin, a tiger shark and a Cuvier's beaked whale, among a number of other species.
A new study published March 18 in iScience has, for the first time, linked a puzzling swim behavior across a large group of aquatic species, from various penguins, to sharks, to turtles and seals. Researchers have found that these species will occasionally swim in steady circles, revealing a new and possibly convergent behavior with similar functions across species.
"We never expected this kind of strange behavior," said lead author Tomoko Narazaki of the University of Tokyo. "When I first saw this circling movement I was quite surprised. It's such a steady movement, just like a machine, so I thought there must be something wrong with the data logger, like it got detached from the animal or something."
Narazaki primarily studies sea turtles, and she has examined circling firsthand in her work. When she discussed this behavior with her colleagues, they looked back at their movement data and found that the animals they study were circling, too.
This realization inspired the new study, which compares instances of circling across a range of species that includes mammals, birds, fish and reptiles. The data showed that some of these animals, such as a tiger shark, spent roughly 12 hours circling over the course of a 150-hour period.
Researchers typically track the movement of ocean-dwelling species with a 3D data logger that measures depths; body angle, or pitch and roll; and heading, or forward-facing direction. The logger measures these values every second, which allows researchers like Narazaki to recreate how an animal moves three-dimensionally in water. While the data revealed an exciting connection, not all species circled the same way, and Narazaki can only hypothesize why they do it in the first place.
"This is a big surprise for us, but we don't have any evidence yet that the circling movement has a specific function," Narazaki said in an interview with The Academic Times. "We still don't know."
In prior studies, researchers have posited that circling in individual species could be related to courtship, as researchers have seen in tiger shark footage; foraging, as seen in sandbar sharks; or navigation, as seen in sea turtles.
Whatever the reason, circling is likely a vital behavior because animals don't typically swim in a constant, fluid motion like what occurs in circling's robotic glide. Swimming in a circle is also less efficient than swimming in a straight line, so the benefit of turning must outweigh the cost.
"What surprised me was that the movement was so steady," said Narazaki. "It feels like it's not like natural animal movement."
Informed by her previous research, Narazaki emphatically believes that circling is strongly associated with how animals navigate. In an earlier experiment, she observed circling in displaced sea turtles that were trying to return to their nesting beach. It appeared they began to circle at crucial points where a directional decision had to be made, as if trying to figure out a fork in the road.
If animals do circle to help determine their location, it may be tied to their ability to sense magnetic fields.
Scientists already know that many animals, including sea turtles, can sense magnetic fields to geolocate, according to Narazaki. But they are not sure how the creatures are able to sense the fields. Circling may allow different species to take magnetic readings from different directions, though this hypothesis remains unproven. However, submarines similarly circle when they need to make an accurate geomagnetic measurement from all directions. Animals may be doing the same thing.
Because so little is known about why animals swim in these circles, Narazaki wants to do more research on the physiology of circling to help answer why animals do it and how it works. Investigating the environmental conditions of circling will also help answer these questions, according to Narazaki.
The study, "Similar circling movements observed across megafauna taxa," published March 18 in iScience, was authored by Tomoko Narazaki, Kagari Aoki and Katsufumi Sato, The University of Tokyo; Itsumi Nakamura and Masao Amano, Nagasaki University; Takashi Iwata, The University of Tokyo, Sasakawa Peace Foundation and Kobe University; Kozue Shiomi, Tohoku University; Paolo Luschi, University of Pisa; Hiroyuki Suganuma, Everlasting Nature of Asia; Carl G. Meyer, University of Hawai'i at Mānoa; Rui Matsumoto, Okinawa Churaumi Aquarium; Charles A. Bost, Université de la Rochelle; Yves Handrich, Université de Strasbourg; Ryosuke Okamoto, Ogasawara Whale Watching Association; Kyoichi Mori, Teikyo University of Science; Stéphane Ciccione, Kelonia; and Jérôme Bourjea, Université de Montpellier.
Correction: A previously published version of this article incorrectly referred to the Cuvier's beaked whale as the animal found to circle for 12 hours during a 150-hour period. It is a tiger shark. The error has been corrected.