Scientists have long known that leatherback sea turtles travel thousands of miles each year through open ocean to get from foraging habitats to nesting beaches and tropical wintering grounds, but how the wanderers find their way has been "an enduring mystery of animal behavior," says marine biologist Kara Dodge. "Adult turtles can pinpoint specific nesting beaches even after being away many years," she notes.
Sea turtles' ability to identify and maintain appropriate headings affect migration distance, duration and, for reproductively-active adults, breeding schedules, so understanding migratory orientation and potential cues is an important step toward understanding how sea turtles optimize travel routes and minimize energy costs of migration, Dodge adds.
She and oceanographer Ben Galuardi, led by doctoral advisor Molly Lutcavage, fisheries ecologist and director of the University of Massachusetts Amherst's Large Pelagic Research Center (LPRC) in Gloucester, Mass., now document these turtles' remarkable navigation abilities with state-of-the-art, GPS-linked satellite tags in Proceedings of the Royal Society B. They believe theirs is the first analysis of migratory orientation in adult and subadult leatherbacks.
Dodge, now a postdoctoral researcher at the National Oceanic and Atmospheric Administration (NOAA) in Woods Hole, Mass., says, "We found that leatherback turtles maintain remarkably consistent compass headings in this deep, offshore realm. The striking consistency of the turtles' headings throughout the gyre suggests a common orientation cue or cues. We speculate that leatherback turtles may use a magnetic and/or solar compass to find their way in open ocean."
With the help of Harwich lobsterman Mark Leach, spotter pilot George Purmont and others who helped to locate and bring 20 adult or subadult leatherback turtles aboard research or commercial fishing vessels, the scientists fitted the animals with satellite time depth recorders from August 2007 to September 2009 on their feeding grounds off Massachusetts. Fifteen turtles were tracked long enough to contribute data to the analyses.
Because they were interested in the turtles' southern migration, Dodge and colleagues limited their analyses to track segments recorded in the western Atlantic subtropical gyre, part of a huge circle of ocean currents circulating from the equator to near Iceland and from the east coast of North America to Europe and Africa. They found leatherback behavior at this time and place characterized by "rapid, highly directed travel consistent with migration."
To analyze the turtles' true headings, Dodge and colleagues corrected tracks for current drift, but they found that currents within the gyre had little impact on the turtles' trajectories. The leatherbacks traveled for an average of 32 days over distances of 686 to 1,372 miles (1,105 to 2,290 km) while in the gyre.
This study demonstrates, the authors say, that "adult and subadult leatherback turtles can consistently maintain southward headings while traveling with the subtropical gyre." They can navigate and orient without topographic features, landmarks or other cues that might be used nearer shore.
Dodge says, "After several years of collecting data on their migration routes, we noticed that these turtles were doing something very interesting. Migrating turtles swam directly offshore into the gyre and although they followed widely-spaced paths, these paths appeared be parallel, as though they shared the same directional orientation despite being in different places at different times. Understanding migratory orientation of leatherback turtles brings us one step closer to solving the mystery of how these ancient mariners navigate their watery world."
Dodge and colleagues point out that the turtles' poor eyesight above water probably means they don't use stars to navigate, and current or wind-borne cues are unlikely to keep these turtles on track over such long distances, but the turtles may well be able to orient to some aspect of the Earth's geomagnetic field or the sun's position on the horizon.
"Solar and magnetic features are ubiquitous and vary in a predictable way from north to south in this region," says Dodge, making them potentially useful for compass orientation. Overall, she adds, "Individual leatherback headings were remarkably consistent throughout the subtropical gyre with turtles significantly oriented to the south-southeast. Adult leatherbacks of both sexes maintained similar mean headings and showed greater orientation precision overall. The consistent headings maintained by adult and subadult leatherbacks within the gyre suggest use of a common compass sense."
Lutcavage adds, "Over the years, our satellite tracking studies have revealed that leatherbacks, along with other large Atlantic voyagers including bluefin and bigeye tuna, sailfish and swordfish, are fantastic navigators. We hope to uncover more of the sensory systems that allow them to find their way across the deep blue."
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