Geomagnetic navigation

Sea turtles carry out long-distance migrations between their feeding grounds and their breeding grounds. For example, some green sea turtles feeding off the coast of Brazil migrate over 2000 kilometers to Ascension Island in the middle of the Atlantic. But how do the turtles navigate the oceans? One hypothesis is that they use Earth’s magnetic field for navigation. This was once tested by attaching powerful static magnets to turtles and seeing if it would impede the turtles’ navigational skills. Results from such studies were the first field evidence for the involvement of geomagnetic cues in sea-turtle navigation.

In a new research published recently in the scientific journal Current Biology, researchers from the University of North Carolina were interested in knowing if marine turtles could imprint on the geomagnetic signature of their natal beach and use that information to return to it year after year. Their starting hypothesis was that if turtles imprint on their natal beach using a geomagnetic signature, the turtles would nest at slightly different locations year after year since Earth’s magnetic field shifts slightly over time. To test this they mapped out 19 years of loggerhead turtle nesting data along the Atlantic coast of Florida. They observed that nesting locations would vary from year to year with some beaches being more popular than others depending on the year. They then examined how Earth’s magnetic field changed at their study site over the same period of time and found that there was indeed a strong association between the spatial distribution of turtle nests and subtle changes in Earth’s magnetic field. So it would seem that turtles are able to imprint on their natal beach’s unique geomagnetic signature and use this information to return there to nest!

James Gould, a professor in Ecology and Evolutionary Biology at Princeton University, reports in Current Biology that this is an “extremely clever analysis” in which the authors of the study took “initially unpromising data” and looked at it in an “entirely new way.” But what is the reason for a turtle to return to the beach where she was born in order to nest? J. Roger Brothers, one of the authors of the paper, says: “The only way a female turtle can be sure that she is nesting in a place favourable for egg development is to nest on the same beach where she hatched. The logic of sea turtles seems to be that ‘If it worked for me, it should work for my offspring.'”

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