The Magnetic Map of Juvenile Green Turtles

 

A juvenile green turtle from the Indian River Lagoon, Florida.  Turtles

of this size are several years of age and typically live in coastal feeding grounds.

Photo by Univ. of Central Florida Marine Turtle Research.

 

From a navigational standpoint, an important difference between hatchling turtles and older turtles is that hatchlings remain within broad oceanic regions but do not navigate to highly specific geographic sites.  In contrast, juvenile and adult turtles in inshore habitats often exhibit strong fidelity to particular feeding areas and return to such locations after long migrations and experimental displacements.  An ability to pinpoint specific geographic targets is also evident during reproductive migrations in which adult female turtles travel hundreds or thousands of kilometers to lay their eggs in specific stretches of beach.

How sea turtles navigate to specific geographic targets has remained an enduring mystery of animal behavior.  Such an ability implies that turtles possess both a compass sense for maintaining headings and a positional or "map" sense to determine their position relative to a goal.  Many animals possess diverse compasses based on stars, the position of the sun, patterns of skylight polarization, and the Earth's magnetic field.  Until recently, however, little was known about the mechanisms that underlie the map sense in sea turtles and other migratory animals.  In this series of web pages we describe a recent experiment providing the first evidence  that the map of sea turtles is based at least partly on information derived from the Earth's magnetic field.


Homing in Sea Turtles

Research on sea turtle navigation began with the writings and musings of Archie Carr, a professor of zoology at the University of Florida, who was among the first to take a scientific interest in sea turtles.  In 1956, Carr published a book titled The Windward Road, which detailed his travels throughout the Caribbean.  In a chapter called "The Captains", he described conversations with turtle fishermen in the Cayman Islands, including a surprising anecdote told to him by a captain of a turtle boat.  The incident involved green turtles that were captured in their feeding grounds along the coast of northern Nicaragua (see map below).  The turtles were branded with the initials of the fishermen, as was the custom at the time, and then loaded onto a boat bound for market in Key West, Florida.  As the boat neared the Florida Keys, a violent storm struck.  The boat capsized and the turtles escaped back into the ocean.  Some months later, turtle fishermen working in northern Nicaragua were astonished to recapture two of the turtles back in the same location where they had been caught initially.  The turtles could be identified unambiguously because the initials of the fishermen were still clearly visible on  their shells.  Somehow the turtles had returned home from more than 1000 km (600 miles) away!

                

This incident and others like it convinced Carr that sea turtles have remarkable navigational abilities.  In The Windward Road, Carr wrote: "It is safe to suppose that green turtles do in fact have some sort of extra sense ... that lets them make long, controlled journeys in trackless seas."  Subsequent experiments by Carr and by others confirmed that sea turtles are indeed capable of navigating to specific geographic targets after long migrations or experimental displacements.  For example, it is now known that if a juvenile  turtle is captured in its feeding area and released some distance away, the turtle is likely to home to the same area where it was captured originally (Ireland, 1980; Avens et al., 2003).  A recent study also demonstrated that, if juvenile turtles are captured in their feeding sites and transported to a distant site, the turtles will attempt to swim in the direction of home even if placed into an arena where they cannot make progress toward the goal (Avens and Lohmann, 2004).

Although the ability of juvenile and adult turtles to return to specific geographic areas has been well-documented, how turtles navigate to specific locations remained a mystery for many years.

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References for this Section:

Avens, L. and K. J. Lohmann. 2004. Navigation and seasonal migratory orientation in juvenile sea turtles. Journal of Experimental Biology 207: 1771-1778. [Download pdf]

Avens, L., Braun-McNeill, J., Epperly, S., and K. J. Lohmann. 2003. Site fidelity and homing behavior in juvenile loggerhead sea turtles (Caretta caretta). Marine Biology 143: 211-220. [Download pdf]

Ireland, L. C. 1980. Homing behavior of juvenile green turtles, Chelonia mydas. In: A Handbook on Biotelemetry and Radio Tracking (editors J. D. Amlaner, Jr., and D. W. MacDonald), pp. 761-764 (Pergamon Press, Oxford).

Lohmann, K. J., Lohmann, C. M. F., Ehrhart, L. M., Bagley, D. A., and T. Swing. 2004. Geomagnetic map used in sea turtle navigation. Nature. 428: 909-910. [Download pdf]



last edited 04/28/2004