Saturday, March 14, 2015

Leatherback turtle navigation mystery deepens

Leatherback turtles leave their hatching beaches as feisty
infants little bigger than a silver dollar.

As long as 10 years later and vastly larger, they are able to return to those very same beaches with pinpoint accuracy to reproduce, despite changing ocean currents, changing weather, changing climate cycle, changing food resources.

(Image: Capt. Mark Leach with satellite tag-outfitted leatherback off Cape Cod, immediately before its release. Credit: LPRC.)

It remains one of the great mysteries of the animal world. What kind of memory and genetics make that kind of navigation possible? 

In a new paper on leatherback navigation, authors Kara Dodge, Benjamin Galuardi and Molly Lutcavage, if anything, expand on the mystery. The three are with the Large Pelagics Research Centre at Gloucester, Mass.  The paper is entitled “Orientation behaviour of leatherback sea turtles within the North Atlantic subtropical gyre.”

Lutcavage is also conducting yellowfin tuna research in Hawai`i, and is a member of the scientific advisory panel of the Western Pacific Regional Fishery Management Council.

“Leatherback sea turtles (Dermochelys coriacea) travel thousands of kilometres between temperate feeding and tropical breeding/over-wintering grounds, with adult turtles able to pinpoint specific nesting beaches after multi-year absences. Their extensive migrations often occur in oceanic habitat where limited known sensory information is available to aid in orientation,” the authors write.

They followed 15 turtles with satellite tags over a two-year period in the Atlantic. The Pacific hosts the same species of turtle. They found the warm-blooded animals were able to maintain consistent direction, day or night, in changing conditions of all kinds.

Leatherbacks are ancient, remarkable creatures. They are the last surviving species of warm-blooded turtle. (Once there were many) They grow from 1.3 ounces to as much as 2,000 pounds over a lifetime. They eat only gelatinous marine life--like jellyfish and salps. And while they are able to relocate their home nesting beaches, for unknown reasons, they are also known to select alternative beaches in some nesting years, Lutcavage said.

The turtles clearly use a range of wayfinding techniques throughout their lives. As hatchlings, light direction and beach slope help them find their ways from the beaches to the ocean, the authors write.

“In deep water beyond the reach of shoreward-propagating waves, hatchlings switch to other cues that may include the Earth’s magnetic field,” they write.

There is evidence of a magnetic compass component to their navigation, but a compass alone won’t get you across a complex, diverse landscape—or seascape. In the case of leatherbacks, visual cues won’t work that well, and checking the stars won’t, because they have poor eyesight.

“Adult female loggerhead turtles appear to use geomagnetic cues to find their natal beaches along continental coastlines through a combination of geomagnetic imprinting and magnetic navigation,” the authors write.

 But other possibilities also present themselves.

“We also cannot rule out the possibility of alternative perceptual cues that have yet to be discovered,” the authors write.

The New York Times covered the research here. 

The Boston Globe here

And lead author Dodge writes a lay version at The Conversation.

© Jan TenBruggencate 2015

Citation: Dodge KL, Galuardi B, Lutcavage ME. 2015 Orientation behaviour of leatherback sea turtles within the North Atlantic subtropical gyre. Proc. R. Soc. B 282: 20143129.

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