The way migrating animals like sea turtles navigate hundreds to thousands of miles across the ocean has puzzled biologists since Charles Darwin. But some sea turtles might not really know where they’re going, new research suggests.
Analysis by an international team of scientists mapped the movements of hawksbill turtles as they swam from their nesting grounds in the Chagos Archipelago to feeding sites also in the Indian Ocean.
It found that turtles often take circuitous routes when migrating over short distances, suggesting that the animals’ navigational sense is relatively rudimentary in the open ocean.
Turtles typically traveled twice the required distance to their target locations. One individual swam 1,306 km to reach an island that was only 176 km away – traveling more than seven times the distance in a straight line.
The team tagged and satellite-tracked 22 hawksbill turtles after they finished nesting.
Generally, sea turtles do not feed and nest in the same geographic area. These animals would have already undergone a migration from their feeding grounds, a breeding season, and laid several broods before preparing for the return trip.
Chair in Marine Science at Deakin University and the study’s first author, Professor Graeme Hays, said that if turtles were perfect navigators, they would likely move on direct paths from their nesting sites to areas of food looking for food. “These turtles that we’re tracking – they probably hadn’t eaten in four or five months,” he said.
Previous research has suggested that turtles likely imprint themselves on the magnetic field of their natal area – where they later return to lay eggs – and detect changes in the Earth’s magnetic field as a means of navigating the ocean.
Hays said the new study suggests the turtles “almost certainly use a geomagnetic map, but that’s a pretty coarse resolution.”
“So it doesn’t allow for accurate straight-line migration, but it tells them when they’re straying off the road,” he said.
Hawksbill turtles typically migrate distances of around 150 km, a modest distance compared to the migration of green turtles, Hays said.
“For the green sea turtles that nest in the Chagos Archipelago…we tracked them almost 5,000 km to their feeding grounds,” he said. “They will swim across the Indian Ocean to the mainland African coast.
“Although it’s a long journey, in a sense it’s actually quite an easy navigational task because all the turtle has to do is swim vaguely west and it will eventually reach Africa. “
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Although hawksbills performed much shorter migrations in comparison, they had the tricky navigational task of locating specific small places like remote isolated islands or submerged shores.
The new research suggests that turtles’ geomagnetic map sense is not fine enough to locate specific targets.
When closer to their intended locations, the animals likely use other navigational cues, including smell and visual cues, Hays said. “In the final stages, they can sense an island they are heading towards.”
“When they get some kind of visual cue, for example, the water starts to get a little shallower and they can see the seabed, so they’ve probably gotten some kind of cognitive map of that area. They could probably just recognize the seabed, just as you would recognize visual landmarks in the area where you live.
The research, published in the Journal of the Royal Society Interface.
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