Navigating the Atlantic as a giant turtle.

By JW Dowey - 11 Mar 2015 5:0:0 GMT
Navigating the Atlantic as a giant turtle.

The size of an adult male(800lb or 363kg) leatherback is shown well here, tagged for this research. They are by far the largest and the most distinctive turtles alive, reminding us of the huge animals that have existed in the past.Leatherback image; Credit: © Kara L. Dodge

Dermochelys coraicea is the giant turtle that roams worldwide,even in the colder seas with its famed leathery back instead of the more-normal shell. Because of their advanced physiology and abilities, their migrations are extensive, crossing all oceans such as the North Atlantic subtropical gyre many times in their careers. The use of an obvious orientation and navigation sense was tracked in 15 adults and subadults over 2 years to investigate how turtles in general and leatherbacks in particular can achieve precision in their long-distance navigation.

The perpetrators of the study, from the University of New Hampshire, the Large Pelagics Research centre in Massachusetts and Integrated Statistics from Woods Hole, all in the US, were Kara L. Dodge, Benjamin Galuardi and Molly E. Lutcavage. They publish today in the Proceedings of the Royal Society B under the title, Orientation behaviour of leatherback sea turtles within the North Atlantic subtropical gyre.

Turtle hatchlings rely on wave direction to orientate themselves, but soon have to adopt the adult senses that utilise many different cues in the environment. Geomagnetic cues are well researched in many juvenile turtles, as well as insects, birds, and fish. This research is the first to use adult (and subadult) turtles. Their movements from temperate to tropical latitude, beginning off the coast of Massachusetts, were tracked using GPS-linked STDRs and computers.

Southward headings were maintained by the animals studied, without any topographic features or similar cues. Bermuda is the only area where shallower water intrudes on the Atlantic deeps with few currents within the sub-tropical gyre. Visual cues, given the turtles’ limited eyesight, would be an unlikely stimulus, leaving the earth’s geomagnetic field and the sun’s position as two of the very few possible stimuli. Leatherback hatchlings have been shown to use the earth’s magnetic field as a compass while these 15 individuals were using consistent headings for both day and night. Calibration of the geomagnetic clues in birds has been shown to be achieved using sunrise and sunset as visual references.

The argument in favour of this combined navigation system is the aptitude of adult leatherbacks to remain at the surface during the day when traversing the subtropical gyre. Speculation will continue however, with little opportunity to track more turtles, given the difficulty of catching and equipping them. The consistent directional moves, especially by both male and female adults, gives clues about how they all manage to achieve landfall when they migrate to the nesting beaches where they hatched.

Preliminary work by the same authors was covered here as Leatherback Orientation Behavior.