Loggerhead sea turtles frequently return to the same foraging sites during migrations, using a mental map of Earth’s magnetic field to do so. Photo: ImageSelect
Earth’s magnetic field enables loggerhead sea turtles to recognise locations.
Loggerhead sea turtles can learn to recognise places based on the local properties of Earth’s magnetic field, and in this way remember where food is to be found. That is the conclusion reached by American biologists on the basis of conditioning experiments published in Nature on 12 February. Loggerhead sea turtles (Caretta caretta) frequently return to the same foraging sites during migrations; their mental magnetic-field map is likely to prove highly useful for this purpose. According to the American researchers, the map relies on a different mechanism from that of the sea turtles’ ‘compass’, which they use to determine the correct swimming direction based on Earth’s magnetic field.
In the laboratory, the Americans allow young loggerhead hatchlings to swim around in two different water tanks. In one tank the turtle is given food each time; in the other it is not. The difference between the tanks lies in the artificial magnetic field, which has been set up according to parameters of known ocean regions. In this way the turtles learn to distinguish between a tank whose magnetic field resembles that of the Gulf of Mexico and one that resembles a location off New Hampshire. That they succeed in doing so after two months of training is demonstrated at the test: do the turtles perform a kind of dance in the correct tank? This dance – lifting the head out of the water, opening the mouth, and waving the front flippers – is well-known behaviour in turtles that are anticipating food. In the laboratory, the animals turn out to perform the dance only in the tank they have learned to associate with food. Other simulated locations can also be successfully taught. Moreover, the turtles are able to learn additional areas on top of these, and still recognise the locations accurately after four months. Both the strength of the field and the angle of the magnetic field lines must be correct; when the researchers mix the parameters, the turtles become confused.
In a separate experiment, the Americans show that loggerhead sea turtles use what is known as chemical magnetoreception to determine their swimming direction – a process in which a series of chemical reactions enables animals to detect Earth’s magnetic field. The presence of a radiofrequency field disrupts this mechanism. In the laboratory, turtles consistently swim westward when placed near a tank with a field resembling a region near the Cape Verde Islands, unless chemical magnetoreception is disrupted. By contrast, they are perfectly capable of distinguishing between two locations even when disruption is present. The animals apparently use a different mechanism for their mental map than for their compass, the Americans conclude. Precisely how this works is a question for future research.