While a great many animals, including mollusks, salamanders, turtles and honeybees, are affected by magnetic fields, and other animals are clearly able to obtain compass direction from the Earth's magnetic fields, it is a far different and more complex problem to use our planetary magnetic fields as a sort of biological Global Positioning System. To determine locational information with more precision than `that way is north' requires the ability to detect both magnetic field intensity and inclination angle as they vary across the earth's surface. Until now, the most celebrated cases of true navigation have involved homing pigeons, of course, and sea turtles, which may navigate thousands of miles between feeding and nesting grounds. One well-known investigator of sea turtle navigation and his colleague, however,have now demonstrated for the first time a case of true navigation in an invertebrate, the Caribbean spiny lobster Panulirus argus. The spiny lobster is a crustacean known for both its large nocturnal foraging range and long seasonal migrations, and has been shown to exhibit homing behavior if removed from home territories to an unfamiliar location. This led Larry Boles and Ken Lohmann, of the University of North Carolina Chapel Hill, to question whether lobsters are capable of true navigation.
In their first sets of investigations, lobsters were transported from their capture site in closed boxes, some of which had dangling, swinging magnets,along a circuitous route by boat or lorry to a test site. The animals were unable to obtain useful chemical, visual or magnetic clues during transport. With their eyestalks covered for testing, the juvenile lobsters were then tethered in the center of a circular arena and the direction each lobster walked was recorded. Lobsters captured north–northeast of the test site oriented significantly homeward during the test, while animals captured west–southwest of the test site oriented towards home in a predominantly southwesterly direction. No differences in orientation occurred between animals transported in the presence of constantly swinging magnets, which produced fields strong enough to continuously misalign a compass placed outside the box, and those that did not have magnetic fields disrupted during transport. If the lobsters were navigating by detecting magnetic fields, they were thus clearly not following a `map' of changing fields towards home that had been somehow learned on the route out.
However, showing that lobsters can orient in a homeward direction after displacement is not the same as demonstrating that they may use magnetic cues to do so, so an additional set of experiments placed the animals in externally imposed magnetic fields. With the subjects inside a Faraday cage, Boles and Lohmann generated magnetic fields replicating an area either approximately 400 km north or 400 km to the south of the capture site; lobsters exposed to the field of the northern site walked south–southwest, while animals that found themselves `south' of their home site walked approximately north.
While unable to show in these initial experiments the precise nature of the`map' and the magnetic features used by these lobsters, Boles and Lohmann have demonstrated for the first time the ability of an invertebrate to derive sufficient information from the Earth's magnetic fields to determine the location of `home'. And the next time you see a lobster and only think`dinner!', keep in mind that a group of America's best and brightest (Cornell University undergraduates), subjected to a nearly identical test in the early 80s, were far less successful at finding their way back to campus from the wilds of Ithaca, New York!
