Within its local environment, an animal can organize its behavior spatially by orienting to landmarks, as honeybees and digger wasps do. But what if the destination is a considerable distance away? In this section we examine several modes of long-distance navigation.
Piloting animals orient themselves by means of landmarks
In most cases an animal finds its way using simple means: It knows and remembers the structure of the environment through which it moves. Navigating by means of landmarks is called piloting. Gray whales, for example, migrate sea sonally between the Bering Sea and the coastal lagoons of Mexico. They find their way by following the west coast of North America. Coastlines, mountain chains, rivers, water currents, and wind patterns can all serve as piloting cues. But some remarkable cases of long-distance orientation and movement cannot be explained by piloting.
Homing animals can return repeatedly to a specific location
The ability of an animal to return to a nest site, burrow, or other specific location is called homing. In most cases, homing is merely piloting in a known environment, but some animals are capable of much more sophisticated homing.
Marine birds provide many dramatic examples of homing over great distances in an environment where landmarks are rare. Many marine birds fly over hundreds of miles of featureless ocean on their daily feeding trips and then return directly to a nest site on a tiny island. Albatrosses display remarkable feats of homing. When a young albatross first leaves its parents' nest on an oceanic island, it flies widely over the southern oceans for 8 or 9 years before it reaches reproductive maturity. At that time, it flies back to the island where it was raised to select a mate and build a nest (Figure 52.16). After their first mating season, the pair separate, and each bird resumes its solitary wanderings. The next year they return to the same nest site at the same time, reestablish their pair bond, and breed. Thereafter they return to the nest to breed every other year, spending many months in between at sea.
Homing pigeons can be transported to remote sites where they have never been, and when they are released, fly home. Data on departure directions, known flying speeds, and distances traveled show that homing pigeons fly fairly directly from the point of release to home. They do not randomly search until they encounter familiar territory.
Scientists have used homing pigeons to investigate the mechanisms of navigation. One series of experiments tested the hypothesis that the pigeons depend on visual cues. Pigeons were fitted with frosted contact lenses so that they
52.16 Coming Home A pair of black-browed albatrosses engage in courtship display over their partially completed mud nest. Many albatrosses return to the site of their own birth to find a mate and breed.
could see nothing but the degree of light and dark. These pigeons still homed and fluttered down to the ground in the vicinity of their loft. Thus, they were able to navigate without visual images of the landscape.
Migrating animals travel great distances with remarkable accuracy
For as long as humans have inhabited temperate and subpolar latitudes, they must have been aware that whole populations of animals, especially birds, disappear and reappear sea-sonally—that is, they migrate. Not until the early nineteenth century, however, were patterns of migration established by marking individual birds with identification bands around their legs. Being able to identify individual birds in a population made it possible to demonstrate that the same birds and their offspring returned to the same breeding grounds year after year, and that these same birds were found during the nonbreeding season at locations hundreds or even thousands of kilometers from their breeding grounds.
Because many homing and migrating species are able to take direct routes to their destinations through environments they have never experienced, they must have mechanisms of navigation other than piloting. Humans use two systems of navigation: distance-and-direction navigation and bicoordi-nate navigation. Distance-and-direction navigation requires knowing the direction to the destination and how far away that destination is. With a compass to determine direction and a means of measuring distance, humans can navigate. Bicoordinate navigation, also known as true navigation, requires knowing the latitude and longitude (the map coordinates) of both the current position and the destination.
The behavior of many animals, such as the albatrosses mentioned above, suggests that animals are capable of bico-ordinate navigation. It is possible that these species could use their circadian clock information about time of day and the position of the sun to determine their coordinates—much as sailors did in the days before global positioning satellites. However, there is no strong scientific evidence for such mechanisms to date—though, of course, it is not easy to do experiments on world-traveling animals such as albatrosses. The best evidence for mechanisms of animal navigation comes from studies of distance-and-direction navigation.
Researchers conducted an experiment with European starlings to determine their method of navigation. These birds migrate between their breeding grounds in the Netherlands and northern Germany and their wintering grounds to the west and southwest, in southern England and northern France (Figure 52.17). The researchers captured birds on their breeding grounds, marked them, transported them to Switzerland—south of their breeding grounds—and released them. The researchers expected that if the starlings were using dis-tance-and-direction navigation, the marked birds would be recovered in southern France and Spain, to the southwest of where they were released. Naive juvenile starlings did use distance-and-direction navigation and ended up in Spain, but experienced adult birds were less disrupted by their geographic displacement.
How do animals determine distance and direction? In many instances, determining distance is not a problem as long as the animal recognizes its destination. Homing animals recognize landmarks and can pilot once they reach familiar areas. Evidence suggests that circannual rhythms play a role in determining migration distances for some species. Birds kept in captivity display increased and oriented activity at the time of year when they would normally migrate. Such migratory restlessness has a definite duration, which corresponds to the usual duration of migration for the species. Because distance is determined by how long an animal moves in a given direction, the duration of migratory restlessness could set the distance for its migration.
Two obvious means of determining direction are the sun and the stars. During the day, the sun can serve as a compass, as long as the time of day is known. In the Northern Hemisphere, the sun rises in the east, sets in the west, and points south at noon. As we have seen, animals can tell the time of day by means of their circadian clocks. Clock-shifting experiments have demonstrated that animals use their circadian clocks to determine direction from the position of the sun.
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