At least two different types of information can potentially be derived from the Earth's magnetic field. Directional or compass information enables an animal to maintain a consistent heading in a particular direction such as north or east. Positional, or "map" information as it is sometimes called, assists an animal in assessing its geographic position, so that it can move in an appropriate direction along a migratory route or toward a specific destination such as a home area.
Magnetic compasses are known to exist in diverse animals (Wiltschko and Wiltschko, 1995a
), and the list of marine animals known to possess such compasses now includes sharks (Kalmijn, 1978), spiny lobsters (Lohmann et al., 1995a), sea turtles (Lohmann, 1991; Lohmann and Lohmann, 1993), isopods (Ugolini and Pezzani, 1995), and salmon (Quinn et al., 1981). Much less is known about the use of magnetic positional or map information. During the past decade, however, evidence has accumulated that at least two marine animals, sea turtles and spiny lobsters, are able to derive positional information from the Earth's field.
Hatchling sea turtles and regional magnetic fields
Young loggerhead sea turtles (Caretta caretta) perform one of the longest and most spectacular marine migrations. Hatchlings along beaches of the east coast of Florida, U.S.A., emerge from nests, scramble across the beach to the sea, and migrate offshore to the Gulf Stream and North Atlantic gyre, the circular current system that encircles the Sargasso Sea (Fig. 1a; Carr, 1986; Lohmann and Lohmann, 2003; Musick and Limpus, 1997). Young loggerheads evidently remain for at least several years in the gyre system, during which time many cross to the eastern side of the Atlantic Ocean (Bolten et al., 1993, 1998) before returning to the waters of the southeastern United States to take up residence in coastal feeding grounds (Carr, 1987; Sears et al., 1995; Musick and Limpus, 1997).
The waters of the North Atlantic gyre provide a favorable, food-rich environment for young turtles, but straying beyond the latitudinal extremes of the gyre is often fatal. As the northern edge of the gyre approaches Portugal, the east-flowing current divides. The northern branch continues past Great Britain and the water temperature decreases rapidly. Loggerheads swept north in this current soon die from the cold (Carr, 1986, 1987; Hays and Marsh, 1997). Similarly, turtles that venture south of the gyre may be swept into the South Atlantic current system and transported far from their normal range. An ability to recognize the latitudinal extremes of the gyre, and to respond by orienting in an appropriate direction, might therefore have considerable adaptive value.
How might a young turtle with no previous experience in the ocean determine when it is approaching the northern and southern boundaries of the gyre? Several features of the Earth's magnetic field vary in a predictable way across the surface of the Earth and might, in principle, be used in position-finding (Skiles, 1985; Lohmann et al., 1999). For example, at each location on the globe, the geomagnetic field lines intersect the Earth's surface at a specific angle of inclination. Because inclination angle varies with latitude, an animal able to distinguish between different field inclinations might be able to approximate its latitude (Skiles, 1985; Lohmann et al., 1999). Experiments have revealed that hatchling loggerheads can indeed distinguish between different inclination angles (Lohmann and Lohmann, 1994). They can also distinguish between different field intensities corresponding to those that they encounter in different locations along their migratory route (Lohmann and Lohmann, 1996). Thus, hatchling loggerheads evidently emerge from their nests already capable of detecting two different magnetic field elements that vary across the Earth's surface and might provide positional information useful in guiding a long-distance migration.
To investigate further whether loggerheads can exploit positional information inherent in the Earth's magnetic field, hatchlings were subjected to fields replicating those found in three widely separated locations along the perimeter of the North Atlantic Gyre (Fig. 1a; Lohmann et al., 2001). Turtles tested in a magnetic field replicating one that exists offshore near northern Florida swam east-southeast (Fig. 1b). Those exposed to a field like one found near the northeastern edge of the gyre swam approximately south (Fig. 1c). Turtles exposed to a field replicating one found near the southernmost part of the gyre swam west-northwest (Fig. 1d). Thus, the results demonstrate that loggerhead turtles can distinguish among magnetic fields that exist in widely separated oceanic regions.
In addition, the orientation behavior elicited by each of the three fields is consistent with the interpretation that these responses have functional significance in the migration. Near northern Florida, orientation toward the east-southeast would lead turtles away from the coast and farther into the Gulf Stream. The Gulf Stream veers eastward soon after passing Florida. When it does, turtles positioned safely away from the gyre perimeter are presumably less likely to stray into fatally cold water that lies to the north. In the northeastern region of the gyre, the Gulf Stream divides. Southward orientation in this area is likely to help turtles remain in the gyre and avoid the North Atlantic Drift, the north-flowing current that can carry turtles into the cold oceanic regions of Great Britain and Scandinavia (Carr, 1986, 1987; Hays and Marsh, 1997). Near the southernmost boundary of the gyre, orientation to the west-northwest is consistent with the migratory route of the turtles. Such orientation may prevent turtles from straying too far south and may also help them remain in favorable currents that facilitate movement back toward the North American coast, where most Florida loggerheads spend their late juvenile years (Musick and Limpus, 1997).
Thus, specific magnetic fields characteristic of widely separated oceanic regions elicit orientation responses that are likely to help turtles remain safely within the gyre and progress along the migratory route. The results imply that young loggerheads in effect exploit such fields as navigational markers.
Magnetic navigation of spiny lobsters
The finding that young sea turtles use regional magnetic fields as navigational markers demonstrates that animals can exploit positional information in the Earth's field during long-distance migrations. Recent results, however, suggest that magnetic field information can also be used in position-finding by marine animals that move over much smaller distances.
The Caribbean spiny lobster, Panulirus argus, lives on hard bottoms and coral reefs throughout the waters of the Caribbean and the southeastern United States. P. argus is a nocturnal forager and spends the day protected within crevices and holes (Herrnkind, 1980). During nightly foraging trips, lobsters often travel significant distances from the den, and at the end of the trip return either to the same den or another one nearby (Herrnkind and McLean, 1971; Herrnkind, 1980). Tag and recapture studies have shown that lobsters are capable of homing after being displaced several kilometers from a capture site (Creaser and Travis, 1950).
Spiny lobsters have a magnetic compass sense (Lohmann et al., 1995a), but the apparent ability of these animals to return to specific areas after experimental displacements suggests that they also have an ability to determine their position relative to a geographic target area. Animals are said to be capable of "true navigation" if, after displacement to a location where they have never been, they can determine their position relative to a goal without relying on familiar surroundings, cues that emanate from the destination, or information collected during the outward journey (Phillips et al., 1995; Phillips, 1996). Until recently, those few animals shown to possess true navigation were all vertebrates (Phillips et al., 1995; Phillips, 1996). In contrast, those few invertebrates that had been carefully studied had been found to return to specific sites by using path integration, landmark recognition, compass orientation, and other mechanisms that cannot compensate for displacement into unfamiliar territory (Wehner, 1996; Wehner et al., 1996; Wehner, 1998; Collett and Collett, 2000; Collett et al., 2002; Graham and Collett, 2002).
Recent experiments, however, have clouded the tidy dichotomy that was once thought to exist between vertebrate and invertebrate navigational abilities. Spiny lobsters were found to orient reliably toward capture sites when displaced to unfamiliar sites over distances of 12–37 km, even when deprived of all known orientation cues en route (Fig. 2a; Boles and Lohmann, 2003). Thus, lobsters are the first invertebrates known to fulfill the criteria of true navigation.
Little is known about the sensory cues and mechanisms that underlie true navigation. To test the hypothesis that lobsters derive positional information from the Earth's magnetic field, lobsters were exposed to fields replicating those that exist at specific locations in their environment. Lobsters tested in a field that exists north of the capture site oriented southward, whereas those tested in a field like one that exists south of the capture site oriented northward (
Fig. 2b; Boles and Lohmann, 2003
). These results provide strong evidence that spiny lobsters possess a magnetic positioning system that is capable of helping them navigate to specific geographic areas. Thus, true navigation in lobsters, and perhaps in other animals, may be based at least partly on a magnetic map sense. Indeed recent experiments with sea turtles have suggested that, as these animals mature, they become capable of true navigation (Avens
et al., 2003
; Avens and Lohmann, 2004
) and acquire a magnetic map similar to the one used by lobsters (Lohmann
et al., 2004
).
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