Marine researchers in Japan and elsewhere have discovered another mystery of the water world. In a new article set out Thursday, they discuss their finding that different species of large marine animals, from turtles to sharks to seals, swim in circles for no apparent reason. This circular work can have several purposes for the animals, such as helping their speed or looking for food, the researchers say.
According to the researchers, scientific advances in recent technology have enabled scientists to get a better picture of how animals in the sea move through their environment with much better precision than before. Lead author Tomoko Narazaki, of the University of Tokyo, and his colleagues decided to make good use of this technology by watching the movements of green sea turtles during their nesting season, when female turtles return to their birthplace to lay eggs.
They moved turtles from their nesting place to elsewhere so they could see how they had sailed back to the original place. But once they did, they noticed a peculiar pattern: the turtles often circle at a relatively constant speed at least twice and then return to their normal swim while venturing home.
Curious, Narazaki told others in her field about the discovery. Eventually, he works with some of these researchers to look back at movement data previously collected about a series of other marine animals across different branches of the evolutionary tree. And sure enough, they made the same kind of circular behavior appear repeatedly. These circulating animals included fish (tiger sharks), birds (king penguins) and mammals (Antarctic fur seals and Cuvier’s jellyfish).
Their job is published in iScience.
‘All the data used in our study were initially collected for different purposes (eg to study shark feeding behaviors, etc.). The data of each species were analyzed by different co-authors for different aspects, ”Narazaki told Gizmodo in an email. “So it took a while before we realized that this circle is a common behavior in many species – until we worked together.”
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At first glance, circular work is almost practical for the survival of these animals, as the most energy-efficient way to travel anywhere in the ocean is usually a straight line. This therefore probably means that it has one or more important features that are worthwhile. Right now all the team has is some good guesses about what’s going on, which could vary between different types.
For example, sharks seem to mostly roam around where they get food, suggesting that it offers some advantage in hunting. Meanwhile, other research showed that some species of whales circle in groups as a way to create “bubble nets” to catch their small fish prey. But nutrition is probably not the only goal to circle.
In at least one male tiger shark, the team found evidence that circling in front of a female was part of its courtship ritual. Seals and penguins apparently circulate mostly near the water surface or outside their normal feeding hours, both of which indicate that it is not part of their feeding technique. The team also cited earlier research that found that northern elephant seals would circle during their drift dives – lazy, passive dives that help them rest or process their last meal.
In turtles, the circle can help them reorient their navigation skills, relying on smell, sight and magnetic fields. The turtles regularly circle just before the last stretch of their journey, and also for a while. One turtle was observed to circle 76 times before proceeding.
“Since similar circular behavior has been observed in a wide variety of marine megafauna taxa, it is possible that it is a behavioral convergence that has similar purposes,” Narazaki said. “But for now, the purpose and function of this behavior is unknown.”
Of course we know that many animals in the country circle for different reasons (just ask you nearest dog before pooping). But the darkness of the vast ocean means that there are probably all sorts of behaviors common among these animals that we have not yet seen. By studying the how and why of marine circulation up close, the researchers hope to illuminate this almost alien world a little more.
“For the next step, we want to investigate animals ‘movements in relation to animals’ internal condition and environmental conditions to investigate why they circle,” Narazaki said. “Some hypothesis-testing experiments are necessary to understand the function and mechanism of circular motions.”