The blue wildebeest, or brindled gnu, migrates annually from Kenya to northern South Africa. Along their migratory route the wildebeests stop at watering holes on the Grameti River, where they become the chief source of food for Nile crocodiles. Scientists speculate that the crocodiles of the Grameti River may feed only once a year, when blue wildebeests arrive during their annual migration.
Being able to move gives animals many advantages, but it also generates its own demands. For any animal, random movement can be unhelpful or even dangerous. To be useful, movement has to be carefully guided.
Animals are guided by their senses, which provide feedback about their changing surroundings. In animals that have radial symmetry (symmetry around a central point), such as jellyfishes, sensory nerves are arranged more or less evenly around the body. This arrangement makes the animal equally sensitive to stimuli from any direction. In bilaterally symmetrical animals (ones made of equal halves), sensory nerves are concentrated in the head. They convey signals to the brain from organs such as ears and eyes, telling an animal about the surroundings that it is about to encounter.
These sensory systems help animals to move toward food and away from possible danger. On a longer time span, they also guide them through much more complex patterns of movement that are essential for their survival. These movements include special kinds of behavior needed to locate a partner, and also seasonal movements or migrations.
The monarch butterfly, Danaus plexippus, is known for its extraordinarily long migrations. During the summer months, monarchs can be found throughout the continental United States and parts of Canada, and they migrate to the California coast and central Mexico for the winter. The longest recorded flight for a tagged adult is 2,900 km (1,800 mi). A large number of monarchs spend their winters in the mountains west of Mexico City. Scientists speculate that the mountainous climate provides a favorable mix of moist air and cool, but not freezing, temperatures. These conditions keep the butterfly from drying out and keep its metabolism low enough to conserve fat stores but high enough to maintain life.
Some of the shortest migrations are carried out by microscopic flatworms that live on sandy shores. These worms migrate up to the surface of the sand at low tide and back into it at high tide—a total distance of about 20 cm (about 8 in) roughly twice a day. In the open ocean, many planktonic animals carry out larger daily migrations, rising to the surface at dusk and then sinking at sunrise. By doing this, they reduce the chances of being eaten.
The longest migrations are annual ones, undertaken by animals in response to the changing seasons. By carrying out these journeys, animals can breed in places where food is abundant for just a few months each year. Long-distance annual migration is seen in some plant-eating mammals, such as wildebeest and caribou, and also in whales, but it is most common in animals that fly. Some birds, such as terns and shearwaters, migrate over 32,000 km (20,000 mi) each year. Research has shown that during these epic journeys, they use a variety of cues to help them navigate. These include familiar landmarks, the position of the sun and stars, and the also the orientation of the earth's magnetic field (see Animal Migration).