A beady-eyed shark circling its prey is a classic scene familiar to many ocean lovers. Except this engaging circular behavior isn’t all it seems, according to a new study that has also seen whales, penguins and sea turtles swimming in circles.
“We found that a wide variety of marine megafauna showed similar circular behavior, with animals circling at a relatively constant speed more than twice in a row,” said marine biologist and lead author Tomoko Narazaki of the University of Tokyo, Japan.
By tracking their movements in three dimensions and looking at where and when tagged animals tended to circle, the researchers came up with a host of possible reasons why. But of course, the research offers more opportunities to think than answers, so let’s dive in.
The researchers first noticed this mysterious circular behavior in green sea turtles, which had been tagged and tracked using 3D data loggers as part of another study.
“To be honest, I doubted my eyes when I first saw the data because the turtle is circling so constantly, just like a machine,” said Narazaki, who has also studied why sea turtles swim so slowly.
“When I got back to my lab, I reported this interesting discovery to my colleagues who use the same 3D data loggers to study a wide variety of marine megafauna taxa.”
What they found was that sea turtles weren’t alone: whales, sharks and penguins also showed more or less the same circular motions.
To study this wide range of marine life, the researchers teamed up with local scientists and field volunteers in Japan, the Comoros off the east coast of Africa and the Crozet Archipelago further south in the Indian Ocean, with some help from the British. Antarctic Survey too.
They tracked a total of 19 animals, including tiger sharks, king penguins, Antarctic fur seals, a goose-beaked dolphin and a whale shark.
Before that, marine life was tracked far and wide, but either to high pressure or horizontally across the ocean surface, using depth recorders or satellite tags.
Multi-sensor data loggers, as this research team used, have now enabled researchers to measure animal movements on a fine scale over depth, latitudes and lengths, and down to a time scale of seconds – an impressive feat in the vast Earth. oceans.
You would think that swimming in a straight line is the most efficient way to get around – and it is from an energy saving perspective. But in an open ocean that’s blue in all directions, animals walking from one place to another can swim right past a celebration of an occasion, so be curious and circle.
Some of the circling displays were recorded in areas where the animals typically forage for food. Four tiger sharks (Galeocerdo cuvier) labeled off the coast of Hawaii circled up to 30 times and up to nearly 130 meters (426 ft) in their feeding grounds.
The Antarctic fur seals (Arctocephalus gazella) circled mainly during the day, although they mainly feed at night; meanwhile a group of playful king penguins (Aptenodytes patagonicus) usually circled the surface between deep dives to collect food – so clearly there’s more to it than just food.
That said, other sea creatures use extraordinary circular motions to catch prey.In 2019 humpback whales Megaptera novaeangliae) were seen they created a ‘net’ of bubbles as they swam in rings to catch fish. So everyone for himself.
Mating rituals may be another possible explanation, where a tagged male tiger shark tries to woo a female partner by swimming in circles around her.
But the most surprising finding for Narazaki was seeing a pair of homing sea turtles swimming in circles as they approached their nesting beaches. One turtle in the study circled 76 times one day and 37 times the next, choosing the correct direction to swim in after intensive consultation.
This observation makes the research team think that circular behavior can also play a role in navigation. Their suspicion is that migratory sea turtles can swim in circles to detect gradients in Earth’s magnetic fields that they use to navigate the oceans and make their way home.
There is probably not just one answer to this water puzzle as the animals can circle for a variety of reasons.
“For example, some animals can move in circles to improve prey search, while at the same time gathering geomagnetic information,” the researchers write.
“Others, such as elephant seals drifting down like falling leaves, could maintain their sense of direction through geomagnetic scanning while resting in seemingly bland mesopelagic depths.”
All in all, this study is large in collaboration, but few in number, and information about the presence of nearby animals that could have influenced the animals’ behavior was lacking.
Still, marine scientists around the world should be able to put this new technology to good use to find more clues as to why marine animals swim in circles.
The findings could also help establish a clear baseline to later identify the impact of external factors – such as shipping traffic – on the movements, behavior and navigational ability of animals.
The research is published in iScience.