Green sea turtles, Antarctic fur seals, tiger sharks, humpback whales and various other marine animals occasionally swim in perplexing circles. While the event can be dizzying and mesmerizing, researchers did not fully understand why circling behavior occurred—until now.
After observing sea turtle navigation using 3-D tracking tags, University of Tokyo researchers suspect that green sea turtles may be circling to navigate through vast oceans, reports Tara Yarlagadda for Inverse. Other marine animals may use circling to stalk prey or court mates. The study was published in the journal iScience this month.
Tomoko Narazaki, co-author and University of Tokyo researcher, first observed sea turtles exhibiting circling behavior while studying how they navigate in vast oceans. But Narazaki noticed turtles circling before they reached coastal waters near their nesting beaches, reports Donna Lu for New Scientist. One turtle even circled 76 times in one day and then the next day, circled for 37 times before eventually heading in the correct direction, reports Clare Watson for Science Alert.
"We hypothesized that some circling may be related to magnetic-based navigation because circling movements seem to be well-suited to examination of the geomagnetic field," Narazaki told Inverse.
It's previously known that green sea turtles use Earth's magnetic field to navigate the ocean and orient themselves to their preferred course. Female turtles specifically migrate thousands of miles to nest at the beach where they were born, known as natal homing, and may use an internal compass to point them in the correct direction towards the shore. Circling repeatedly may help turtles detect magnetic fields from various directions and send them in the right direction home, reports New Scientist.
The 3-D tags can track an animal's movements in three dimensions by tracking speed, changes in ocean depth, and magnetic information, Inverse reports. This type of tracking tag is frequently used to observe various sea creatures. To see if other researchers noticed the same circling movements in other marine animals, Narazaki reached out to colleagues worldwide. After analyzing the data collection, the researchers found the 3-D tags picked up the same circling movements in many animals, including king penguins, tiger sharks, whale sharks and even a Cuvier's beaked whale, New Scientist reports.
For the most part, in other animals, circling displays were recorded in places where the animals may forage for food, reports Science Alert. Tiger sharks on the Hawaiian coast circled 30 times within their feeding ground rage, and one male shark was tracked courting a female shark by swimming around her in circles, Science Alert reports. However, Antarctic fur seals, king penguins, and the beaked whale swam in circles towards the water's surface—a behavior not related to feeding since they feed at deeper depths, New Scientist reports.
A common reason why these animals exhibit this behavior remains unknown. The researchers plan to further look into environmental factors that may influence the circling behaviors and keep investigating whether other sea animals circle, too.
More research could reveal "circlings in more species — or even more mysterious movements! — that have otherwise been overlooked," Narazaki tells Inverse.