Magnetic compasses have been vital to navigation, as they enable people to use the earth’s magnetic field to tell direction. Now, recent experiments have demonstrated that some organisms also navigate with their own “compasses”.
At different places on the earth, the strength of the earth’s magnetic field and its inclination (the angle at which it intersects the earth’s surface) are different. So if organisms could sense these changes, they would have something like longitude and latitude readings on a map.
This is important to young Loggerhead Turtles (Caretta caretta) which must stay within the North Atlantic Gyre, the circular ocean current system that surrounds the Sargasso Sea. Researchers Kenneth and Catherine Lohmann of the University of North Carolina have shown that the turtles use magnetic measurements to stay in the gyre.
They placed turtles in water tanks surrounded by computerized electric coils generating an artificial magnetic field. When the field’s inclination was the same as that of the northern boundary of the gyre, the turtles would swim south, as if back into the gyre. Conversely, when the inclination was the same as the southern boundary’s, the turtles swam north-northeast, again as if away from the danger boundary.
In other experiments, they also kept the inclination constant but varied the magnetic field strength in the tank. When the strength was the same as that of the western boundary of the gyre, the turtles swam east, again as if into the gyre and away from the danger boundary. And they swam west when the strength was the same as the eastern boundary’s.1
The famous British evolutionist (and communist) J.B.S. Haldane claimed in 1949 that evolution could never produce “various mechanisms, such as the wheel and magnet, which would be useless till fairly perfect.”2 Therefore such machines in organisms would, in his opinion, prove evolution false. These turtles which use magnetic sensors have indeed fulfilled one of Haldane’s criteria. Also, the “simple” bacterium propels itself by a filament called a flagellum, which is propelled by a rotary motor - a type of wheel,3 thus fulfilling Haldane’s other criterion. I wonder whether Haldane would have had a change of heart if he had been alive to see these discoveries …
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