Lobsters Navigate by Magnetism
The animal world has its share of celebrated navigators, from flocking geese to spawning salmon. A rather unlikely character, however, may soon take its place among the best of them.
New research suggests that Caribbean spiny lobsters, despite their limited intelligence, may be among the animal kingdom's top navigators. Their homing abilities could also provide scientists with new clues to the long-debated role of the Earth's magnetic fields in animal movements and migrations.
Larry C. Boles and Kenneth J. Lohmann, researchers at the University of North Carolina in Chapel Hill, reported their findings in the January 2003 issue of Nature. Their research suggests that spiny lobsters are able to determine their location on Earth even when transported to an unfamiliar area. The lobsters are the first invertebrates to display this ability known as true navigation.
Animals capable of true navigation can determine their position without relying on recognizable surroundings, cues that originate from a destination, or information collected on the journey to a given location. Only a few animals have been shown to possess true navigationand all but the lobster are vertebrates. Birds such as the homing pigeon comprise most of the short list. However, there is some evidence that sea turtles and at least one type of migratory salamander also use true navigation.
In previous research, Boles and Lohmann found that Caribbean spiny lobsters used an internal magnetic compass that enables them to determine the four cardinal directions: north, south, east, and west. "That's not very unusual in the animal world," Boles said, "but it's one important tool you need to be good navigator."
"We know that lots of animals use the earth's magnetic field as a compass," said Charles Walcott, a professor of neurobiology and behavior at Cornell University in Ithaca, New York, and a longtime homing pigeon researcher. "But if you become lost, a compass cannot tell you where you are. What's exciting about this new work is that it provides pretty strong evidence that [Caribbean spiny lobsters] use this field not just for direction but to know where they are on the Earth."
Boles explained that many considered the lobsters unlikely candidates to possess advanced navigational skills like true navigation.
"I think that a big issue is the general thought that invertebrates, because of their relatively simple nervous systems, might not have the necessary mental capacity to do this kind of thing," Boles told National Geographic News. "They are doing the most sophisticated kind of navigation with a much simpler nervous system than other animals."
Test Designed to Disorient Lobsters
The Caribbean spiny lobster (Panulirus argus) is commonly found in the western Atlantic Ocean in an area stretching from Brazil to Bermuda. Some populations are migratory, but most spend their daylight hours inside coral reef dens, emerging at night to forage before returning to their homes.
To test the lobsters' navigation abilities, researchers Boles and Lohmann developed complicated measures to disorient and confuse the animals. The researchers were careful to ensure that lobsters were not able to determine their location from sensory information gathered while being moved.
Lobsters used in the study were displaced 12 to 37 kilometers (7.5 to 23 miles) from their point of capture. Divers gathered the lobsters and placed them in containers partially filled with seawater. The containers were covered and transported to testing sites via circuitous routes by truck and boat. Further disorienting tricks were employed, such as suspending the containers by ropes so that they swung erratically, and lining them with magnets strong enough to alter the directional functions of compasses. Once at the test site, the lobsters, even with their eyes covered, invariably determined the direction of their capture site and began moving homeward.
"The physical displacement, with all the measures we took to confuse them, was to test the hypothesis of true navigationthat animals are not lost when taken somewhere completely new to them," Boles explained. "It's precisely the test that most animals fail. Once they pass, it implies that they somehow know where they are all the time, that something is built-in."
Exactly how some animals are able to perform true navigation is not known. A leading theory suggests that the ability is based on magnetic mappingan animal's detection and use of the Earth's magnetic fields. Not all scientists subscribe to the concept, but the lobster study suggests that it may play a role.
Boles and Lohmann's experiments also tested the hypothesis that the lobsters were able to determine their locations by measuring even minimal differences in the Earth's magnetic field. The researchers wanted to learn how captive lobsters would react if they altered the Earth's magnetic fields to suggest to stationary lobsters that they had in fact been moved.
"We made extremely small changes in the magnetic field, subtle changes that most animals would ignore," said Boles. "[The captive lobsters] acted as if they were at the location that the magnetic field represented and ignored their actual physical location."
"That's very exciting, because many animals can navigate, like butterflies, whales, and salmon," said Walcott. "We know bits and pieces of what they do, but in no case do we really understand how they find a particular place. This is one of the first bits of evidence that the Earth's magnetic field plays a role in that ability."