Scientists Finally Know Why Massive Sharks in Packs Swim Around in Circles

by Lauren Boisvert

Scientists have finally figured out why basking sharks swim around each other in circles. Basking sharks are the second largest fish in the world, after the whale shark, and they have a strange courtship ritual. The Marine Biological Association (MBA) and the Irish Basking Shark Group have recently published a paper that explains basking sharks’ interesting behavior.

The giant basking shark is usually a solitary animal, but they come together to mate. Scientists have never actually observed them mating. But, they have spotted groups of basking sharks swimming around each other in circles. Until now, it was unknown why they did this. It was always thought to be mating behavior, but the exact reason was unknown.

To come to their conclusion, scientists observed 19 groups of basking sharks that exhibited this behavior. The research was conducted in waters off County Clare, Ireland from 2016 to 2021. During this time, researchers found that the groups consisted of 6 to 23 sharks. Some were swimming slowly at the surface and others swam below, in what seemed to be a cylindrical shape. The shark cylinders went to a depth of about 52 feet and ranged from 55 and 130 feet in diameter.

David Sims, a senior research fellow at the MBA, professor at the University of Southampton, and lead researcher on the paper, explained what the team found in their study. “How usually solitary basking sharks find a mate in the ocean’s expanse has been an enduring mystery,” he said in a statement. “Incredibly we now find that a courtship torus not only forms but acts like a slow motion ‘speed-dating’ event for assessing lots of potential mates in one go.”

Basking Sharks: Speed Daters of the Ocean

A torus is basically a donut shape, made by revolving a circle in a three-dimensional space. The sharks create these toruses by revolving around each other in the water. The research team found that each torus was comprised of equal numbers of male and female sharks. Meaning, hopefully, that no one gets left without a mate.

The males and females would gently touch fins or bodies, testing the waters, so to speak. Almost as if they were gauging compatibility. “Additional unifying characteristics were the presence of breaching behavior, dynamic assorting of females and males within a torus and female–male interactions such as touching and rolling/diving behavior. Collectively, the results strongly suggest a courtship function for the basking shark torus,” the researchers wrote in the paper.

“It is astonishing that this wonder of the natural world has remained hidden for so long,” said Sims, “presumably because circles most often form at depth away from surface observation, which could explain why mating itself has never been seen.”

Maybe we should take a page from the basking sharks’ dating book. Revolve around each other in a torus, gently touching fins. Worth a shot, right?