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Green sea turtles are one of several marine animals that swim in mysterious circles

Jordi Chias / naturepl.com

Several marine animals, including green sea turtles, tiger sharks and Antarctic fur seals, have been observed swimming in circles, but the reason for the behaviour is a mystery.

Tomoko Narazaki at the University of Tokyo in Japan unexpectedly discovered this circling behaviour while studying the navigation of green sea turtles (Chelonia mydas) off the coast of Mohéli, one of the islands of Comoros in the Indian Ocean, and the Japanese island of Chichijima in the Pacific Ocean.

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She had been tracking the homing capabilities of green turtles when they were moved away from their breeding ground, and noticed that the tracking data showed multiple circling events when the turtles returned to the coastal waters off their nesting beaches. One turtle swam in large circles 76 times, with each loop taking 16 to 20 seconds.

Initially, Narazaki thought that the tracking tag might have been broken. The tags used on these turtles had a much higher spatial resolution compared with most GPS trackers; they are able to trace animals’ 3D movements down to the metre and they update every second by recording depth, acceleration and magnetic information.

Narazaki reported her findings to colleagues who use the same 3D tracking tags on other marine animals. In their own data on other species, they found similar circling behaviour in tiger sharks, king penguins, Antarctic fur seals, a whale shark and a Cuvier’s beaked whale.

For example, 272 circling events were observed in four tiger sharks (Galeocerdo cuvier) tagged off the coast of Hawaii. In each circling event, the tiger sharks circled between 2 and 30 times, swimming at a relatively constant depth. On average, each circle was 9.4 metres in diameter and the behaviour lasted 5.6 minutes.

These underwater loops could be related to foraging, similar to how humpback whales turn to capture prey while bubble-net feeding, says Narazaki. But the fur seals, king penguins and Cuvier’s beaked whale swam in circles near the ocean surface, while they normally feed at depth.

Circling for navigation is a potential explanation. “Homing turtles did the turning behaviour at seemingly navigationally important points,” says Narazaki.

How green sea turtles navigate isn’t well understood, but previous research has shown that they are able to detect magnetic fields.

“This turning behaviour seems to be well suited for this magnetic detection,” says Narazaki. Circling might allow the turtles to detect magnetic fields from different directions, and repeating the behaviour would provide multiple measurements to help calibrate their position.

Narazaki and her colleagues plan to study the behaviour in more animals and in other species, as the sample size was small in this study, and also aim to confirm whether navigation is the reason behind the circling.

Journal reference: iScience, DOI: 10.1016/j.isci.2021.102221

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