After laying their eggs on Diego Garcia island each winter, hawksbill turtles swim dozens of miles to foraging sites in the Indian Ocean, where they feast on sea sponges and other marine plants and animals. But as it turns out, they’re not really sure where they’re going during this relatively short migration.
A new study published this month in the Journal of the Royal Society Interface finds that, although they eventually reach their target destination, hawksbill turtles often take a meandering, circuitous route to get there, suggesting that they have a relatively crude map sense of the ocean.
Maps the researchers created to show the turtles’ routes look like the scribbles in a child’s coloring book. Many random loops and double-backs occurred before the turtles reached their intended destination. One turtle swam more than 800 miles to reach a spot that was a little more than 100 miles away, according to the researchers.
"Really long migrations are surprisingly easy for turtles to complete from a navigation perspective,” Graeme Hays, a marine scientist at Deakin University and one of the study’s co-authors, tells the Australian Associated Press’ Tracey Ferrier. “It's the journeys to small isolated targets that are the tricky ones."
To better understand hawksbill turtles’ navigational skills, the researchers attached GPS tracking devices to 22 individuals who had completed nesting on Diego Garcia in 2018 and 2019. Then, they tracked the reptiles’ journey to their foraging grounds on submerged banks in the nearby Chagos Archipelago.
Hawksbill turtles typically weigh between 100 and 150 pounds and they’re 2 to 3.5 feet long. For decades, people have hunted this endangered species for its colorful, patterned shell, a practice that is now illegal.
Compared to animals that migrate on land, who have lots of good navigational markers to help them get where they’re going, animals that migrate in the ocean have much less information to go on. How exactly sea turtles, fish, birds and other marine wildlife reach their target destinations has stymied scientists, including Charles Darwin, for decades.
Past research has suggested that sea turtles can perceive the Earth’s magnetic field and use those cues to reach specific sites in the ocean. But, until now, the precision of the turtles’ map sense has remained a mystery. In other words, are they traveling in roughly the right direction, or are they able to pinpoint exactly where they want to go?
Now, researchers have more evidence to help answer that question. Since the turtles don’t nest and forage in the same locations, researchers had assumed the hungry turtles might take the most direct path to food (the turtles tracked in the study likely hadn’t eaten in four or five months, per the researchers). But as it turns out, hawksbill turtles in the Indian Ocean traveled more than twice the beeline, or direct, distance to their foraging grounds, on average, the scientists find.
At times, the turtles seemed to understand that they were off the most direct route and corrected their course. They often did this in shallower water, the researchers find, which suggests that they gleaned important navigational information from the seabed.
“They could probably just recognize the seafloor, just like you would recognize visual landmarks in the area where you live,” Hays tells the Guardian’s Donna Lu.
Once they got close enough to a particular foraging site, they were able to sniff out the rest of the route, per Hays. And though getting lost isn’t ideal for hungry turtles, the off-course reptiles unknowingly helped “solve a more than century-old riddle” in the process, the researchers write in the paper.