Homeward bound

IN A life span of 60-70 years, a turtle may travel thousands of kilometers from its birthplace, but it always finds its way back to the same spot or someplace close to it, whenever it lays eggs. This intriguing and unerring instinct is now being attributed to the fact that turtles rely on the earth's magnetic field to reach their destination. This finding was made by Kenneth J Lohmann and Catherine M F Lohmarni - biologists at the University of North Carolina in Chapel Hill, us - who conducted experiments on just-hatched loggerhead turtles (large-headed sea turtles of warm seas). Earlier, the husband-wife team had demonstrated that loggerheads can determine the inclination (the dip or deviation from the magnetic field of the earth).

According to Kenneth Lohmann, the intensity of the magnetic field and the inclination of the region in which the turtles travel, are both inconstant because they vary with changing directions. They form a sort of a grid, much like the on6 made by latitudes and longitudes. Therefore, any 'point on the earth is marked by both the magnetic intertsity and inclination. Since turtles can make out both these features, in all possibility they must be mentally creating a 'magnetic map' and using it to get around.

The biologists devised a small circular, pool fitted with an electric coil to simulate a magnetic field. The loggerheads which the Lohmanns studied, hatched on the beaches of Florida in the us and would be speAding their lives swimming in the subtropical gyre found between North America and Africa (Wind stress induces a circulation pattern which is similar for every ocean in the world. Wind-driven circulation is divided into large gyres that stretch across entire oceans. Subtropical gyres are those that extend from the equatorial current system to a maximum latitude of 50 degrees. And in this case, the subtropical gyre found in this area is the gulf stream). Two magnetic fields were chosen; one that corresponded to a point off the Carolinas in the Lis on the western side of the gyre, and the other to the gyre's eastern edge. When two or three-day-old hatchlings were placed in the pool and exposed to the field corresponding to the gyre's western side, the turtles swam eastwards. And they paddled off in the westerly direction when the magnetic field was changed and both these movements were made by the turtle in an effort to stay within the gyre. Any aberration and the turtles would move fatally into the icy waters, north of the gyre. it was interesting to note that even these jUst-born turtles that were being exposed to these magnetic intensities for the first time, moved in the manner they did. Lohmano feels that the newborns were "pre-programmed to swim in a particular direction". The couple suggest that on hatching, the magnetic features of the nesting ground are imprinted on the brain of the yourtgone so that it develops a broader magnetic map. The creature reaches the vicinity of the beach with the help of this map, and thereafter other instincts like its sense of smell guide it homewards.

According to Kenneth P Able, a biologist with the State University of New York at Albany, is, and an expert on animal navigation, that the animals use the earth's magnetic field to move in a certain direction, is not news. Rather, what is interesting in the Lohmantis' study, is the ability of the turtle to locate itself on a map. According to him, "This is the first good empirical evidence of the animal doing something sophisticated with the magnetic field".