Deep Blue Sea

Why do sharks swim in circles? Science has finally solved it

There’s a logical explanation.

Illustration of sharks forming a circle under a boat in water

It is the stuff of nightmares. The classic image of a shark circling its prey is enough to strike terror into the hearts of even the most landlocked among us — whether we see it in a sharksploitation horror movie, or, even more terrifyingly, in real life.

Sharks are the most famous circlers, but they aren’t the only marine animals that do this. Other, more benign, animals in the ocean also display similar circling behavior, including sea turtles and penguins. Now, a new study blows the idea that only sharks exhibit this behavior out of the water, and sheds light on why so many marine creatures sometimes swim in circles.

What’s new — A new study published Thursday in the journal, iScience, shows certain marine animals will occasionally circle around for different purposes. Some, like the menacing, circling shark of your nightmares, do it as part of foraging for food. Others circle for more romantic reasons, including courtship and oceanic navigation.

The large marine animals in the study include:

  • Tiger sharks (Galeocerdo cuvier)
  • Whale shark (Rhincodon typus)
  • Green turtles (Chelonia mydas)
  • King penguins (Aptenodytes patagonicus)
  • Antarctic fur seals (Arctocephalus gazella)
  • Cuvier’s beaked whale (Ziphius cavirostris)

That some sea creatures circle around is not new information (back to the sharks), but this is the first study to use emerging 3D technology to unlock the secrets of this behavior across a wide variety of animals in the ocean.

Tomoko Narazaki is the lead author of the study and a researcher at the University of Tokyo. Narazaki and her team offer an intriguing, unifying theory to explain why these animals’ puzzling, circular motions. The sea creatures can detect geomagnetic fields in the ocean, which may help direct their movements.

“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 tells Inverse.

In effect: the animals use the geomagnetic field to draw a map of their surroundings.

An illustration of circling ocean animals from the study.

Narazaki et al. / iScience

In this study, the researchers specifically included animals that will circle around at least twice as part of the behavior. The repetitive, circular movements may improve the animals’ ability to detect these geomagnetic fields.

“Many animals are known to have [the] ability to detect geomagnetic fields — and to detect direction,” Narazaki says.

“Animals might also be able to improve measurement accuracy by taking multiple samples by circling several times,” she says.

How they did it — Using new developments in a multi-sensor marine technology known as “dead reckoning,” the scientists reconstructed the animals’ underwater movements in 3D. This new technology enables researchers to analyze small-scale movements that previous studies may have overlooked.

The researchers paid attention to different variables, such as animal speed and change in ocean depth, to study the animals' movements in three dimensions.

A figure illustrating the circling movements recorded in each species. (A) A tiger shark, (B) a whale shark, (C) a king penguin, (D) an Antarctic fur seal, (E) a green turtle, (F) a Cuvier’s beaked whale. G) Movement of a submarine during geomagnetic measurements (H) 3D figure showing how a Cuvier’s beaked whale circled during the final ascent phase of a deep dive.

Digging into the details — According to Narazaki, swimming in certain circular patterns enable the marine animals to scan the sea around them, and determine the maximum and minimum strength of surrounding magnetic field lines.

One interesting finding is that marine mammals circled more often during the day than at night. Some — like penguins and seals — appear to use circling primarily to find food. This ties in with older research, in which scientists observed humpback whales encircling their prey and ensnaring them in a net of air bubbles.

Other marine creatures, like sea turtles, appeared to use circular swimming patterns to detect geomagnetic fields and use them to find their way home. In this study, one sea turtle circled around a staggering 76 times before getting on its way.

Why it matters — The findings suggest that some marine animals get directional cues from geomagnetic fields, allowing them to manage “navigationally challenging situations.” These situations can include homing behavior in sea turtles or foraging trips for penguins and seals.

Because circling behavior occurs across a wide spectrum of diverse ocean megafauna, the researchers theorize it may be the result of convergent evolution.

Convergent evolution refers to the idea that distinct species independently evolved similar behaviors to adapt to shared environmental pressures.

A figure from the study showing GPS tracks of homing green turtles.

What’s next — Scientists still have a long way to go before we can fully understand the meaning behind these circling patterns — and how they might differ across the animal kingdom.

For example, some animals, like seals, gently circle down in the water, like “like falling leaves,” according to the study. Quite a different mental image to the fast-paced, menacing sharks.

“We have been studying behavior and ecology of marine megafauna, but there [is] still lots of mystery,” Narazaki says.

From here, the researchers hope to study the different animals’ emotional states, as well as environmental factors which might influence their movement patterns.

Future research could reveal “circlings in more species — or even more mysterious movements! — that have otherwise been overlooked,” Narazaki says.

Abstract: Advances in biologging technology have enabled 3D dead-reckoning reconstruction of marine animal movements at spatiotemporal scales of meters and seconds. Examining high-resolution 3D movements of sharks (Galeocerdo cuvier, N = 4; Rhincodon typus, N = 1), sea turtles (Chelonia mydas, N = 3), penguins (Aptenodytes patagonicus, N = 6), and marine mammals (Arctocephalus gazella, N=4; Ziphius cavirostris, N = 1), we report the discovery of circling events where animals consecutively circled more than twice at relatively constant angular speeds. Similar circling behaviors were observed across a wide variety of marine megafauna, suggesting these behaviors might serve several similar purposes across taxa including foraging, social interactions, and navigation.
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