In a paper published in the journal iScience, a team of marine biologists from Japan, the United States and France reports the discovery of circling behavior where various marine animals — including the tiger shark (Galeocerdo cuvier), the whale shark (Rhincodon typus), the green turtle (Chelonia mydas), the king penguin (Aptenodytes patagonicus), the Antarctic fur seal (Arctocephalus gazella), and the Cuvier’s beaked whale (Ziphius cavirostris) — move in a circular manner at relatively constant speed.
Biologging 3D movement data revealed circling behaviors in marine animals. Image credit: Narazaki et al., doi: 10.1016/j.isci.2021.102221.
“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,” said Dr. Tomoko Narazaki, a researcher in the Atmosphere and Ocean Research Institute at the University of Tokyo.
Dr. Narazaki and colleagues first discovered the mysterious circling behavior in homing green turtles during a displacement experiment.
They had transferred nesting turtles from one place to another to study their navigation abilities.
“To be honest, I doubted my eyes when I first saw the data because the turtle circles so constantly, just like a machine,” Dr. Narazaki explained.
“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 came next surprized the researchers even more — they realized that various species of marine animals showed more or less the same circling movements.
This finding is surprising in part because swimming in a straight line is the most efficient way to move about. It suggests there must be some good reason that animals circle.
Some circling events were recorded at animals’ foraging areas, suggesting that it might have some benefit for finding food.
For example, a total of 272 circling events were observed in four tiger sharks tagged off Hawaii.
The sharks circled 2-30 times at wide-ranging depths (0.8-129.3 m) but maintained relatively constant depth during each circling event.
The mean duration of circling events was 5.6 min (range – 0.9-31.4 min), and the mean circle diameter was 9.4 m. Circling events were recorded more during day (88%) than night (12%).
However, fur seals were found to circle mainly during the day even though they primarily feed at night. Other circling events also appeared unrelated to foraging.
For example, the scientists saw a male tiger shark circling to approach a female for courtship, and the evidence in sea turtles suggests circling might play some role in navigation.
“What surprised me most was that homing turtles undertake circling behavior at seemingly navigationally important locations, such as just before the final approach to their goal,” Dr. Narazaki said.
It’s possible the circling helps the animals to detect the magnetic field to navigate; interestingly, submarines also circle during geomagnetic observations.
But it’s also possible that the circling serves more than one purpose.
“Enhancing studies scrutinizing fine-scale movements would reveal circlings in more species that have otherwise been overlooked,” the authors said.
“Further examinations in relation to animals’ internal state and environmental conditions would provide new insights into the function and mechanism underlying the movements.”
Tomoko Narazaki et al. Similar circling movements observed across marine megafauna taxa. iScience, published online March 18, 2021; doi: 10.1016/j.isci.2021.102221