Marine researchers from Japan and elsewhere have discovered another enigma of the aquatic world. In a new paper released Thursday, they detail their finding that several species of large marine animals, from turtles to sharks and seals, swim in circles for no clear reason. This circle could have several purposes for the animals, such as helping them navigate or searching for food, according to the researchers.
According to researchers, recent technological advances have allowed scientists to get a more complete picture of how ocean animals move around their environment much more accurately than before. Lead author Tomoko Narazaki, of the University of Tokyo, and her her colleagues decided to take advantage of this technology by observing the movements of green sea turtles during their nesting season, when females return to their birthplace to lay eggs.
They moved the turtles from their nesting location to other places, so that they could observe how they returned to the original site. But once they did, they saw a peculiar pattern: turtles often spun at a relatively constant speed at least twice around, and then returned to normal swimming as they ventured back home.
Curious, Narazaki told other people field on discovery. Finally, it partnered with some of these researchers to look back at the movement data that had been previously collected on a number of other marine animals through different branches of the evolutionary tree. And they certainly found that the same kind of circle behavior appeared repeatedly. These animals surrounding the circle included fish (tiger sharks), birds (king penguins), and mammals (Antarctic-skinned seals and Cuvier-billed whales).
His job is published a iScience.
“All data used in our study were initially collected for different purposes (e.g., to study the feeding behavior of sharks, etc.). Different co-authors analyzed data from each species for different aspects,” he said. tell Narazaki to Gizmodo in an email. “So it took us a while to realize that this circle is a common behavior in many species, until we collaborated.”
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On the surface, circling is not practical for the survival of these animals, as the most energy efficient way to travel anywhere in the ocean is usually a straight line. So that probably means it has one or more important functions that deserve the extra effort. Right now, though, the whole team has some educated guesswork about what’s going on, which can vary between different species.
Sharks, for example, often seem to move around where they receive food, indicating that it provides some advantage in hunting. Meanwhile, there is other research is shown that some whale species will use the circle in groups as a way to create “bubble nets” to catch their small prey. But eating is probably not the only purpose of going for a walk.
In at least one male tiger shark, the team found evidence that circulating was part of their courtship ritual in front of a female. Seals and penguins appear to move near the surface of the water or outside of their typical feeding hours, indicating that it is not part of their feeding technique. The team also cited previous research that found northern sea elephants will circulate during their drift dives: lazy, passive dives that help them rest or process their last food.
In turtles, the circle can help them reorient their navigation skills, which depend on smell, sight, and sensory magnetic fields. Turtles would circulate frequently just before the last leg of their journey, and for a while as well. It was observed that a turtle made a circle 76 times before continuing.
“Given that similar circle behavior was observed in a wide variety of marine megafauna taxa, it could be possible that this is a behavioral convergence with similar purposes,” Narazaki said. “But for now, the purpose and function of this behavior remains unknown.”
Of course, we know that there are many animals on earth for different reasons (just ask your nearest dog before it poops). But the darkness of the vast ocean means there are probably all sorts of common behaviors among these animals that we just haven’t been able to see yet. By studying the how and why of the closest circulating marine, researchers hope to illuminate this almost alien world a little more.
“For the next step, we would like to examine the movements of the animals in relation to the internal state and environmental conditions of the animals to examine why they surround them,” Narazaki said. “Some hypothesis testing experiments would be needed to understand the function and mechanism underlying lap movements.”