Japanese scientists have stumbled on a mysterious phenomenon: sharks, turtles, penguins and various marine mammals all swim in circles.
They observed animals repeatedly move in consecutive circles at relatively constant speeds, reporting their discovery in the journal iScience. The circles they swim in vary in size depending on species, with diameters ranging from a few centimetres to several metres.
The researchers were collecting animal movement data using advanced biologging technology and noticed homing green turtles circling during an experiment on a small African island (Mohei Island, Comoro) to transfer nesting turtles to oceanic release sites.
“I was there in the field and the main purpose of the study was to examine navigation ability of sea turtles,” says lead author Tomoko Narazaki, from the University of Tokyo.
“To be honest, I doubted my eyes when I first saw the data because the turtle circles so constantly just like a machine.”
Back in the lab, she reported the discovery to her colleagues who use the same 3D data loggers to study other marine megafauna. “Surprisingly, we found that various animals showed more or less similar circling movements.”
The high-resolution technology revealed the same behaviour in five tiger sharks (Galeocerdo cuvier) one whale shark (Rhincodon typus), three green turtles (Chelonia mydas), six king penguins (Aptenodytes patagonicus), four Antarctic fur seals (Arctocephalus gazella) and a Cuvier’s beaked whale (Ziphius cavirostris), who was seen circling 17 times.
The researchers say that energetically, swimming in a straight line is more efficient, so there must be a purpose for the behaviour.
Clues from other observations suggest there could be multiple reasons. Some circling events were recorded at foraging areas, as seen in four tiger sharks near Hawaii and in king penguins during foraging forays. In contrast, fur seals were seen circling during the day when they mostly forage for food at night.
A male tiger shark was seen circling to get close to a female for courtship, while sea turtles might use it to detect magnetic fields for navigation.
“What surprised me most was that homing turtles undertake the circling behaviour at seemingly navigationally important locations,” says Narazaki, “such as just before the final approach to their goal.”
The researchers recommend further finely tuned observations of animals’ circling in the context of other variables so they can learn more about this behaviour.