Migration and Navigation
MIGRATION AND NAVIGATION
Among the most intriguing aspects of animal behavior and perception is the tendency to migrate long distances, coupled with the navigational ability that makes this possible. Most such migration is seasonal, a primary example being birds' proverbial flight south for the winter. Sometimes, however, animals widely separated from their home environments nonetheless manage to find their way home. This fact has long fascinated humans, as reflected in a number of true and fictional stories on the subject that have circulated over the years. For example, The Incredible Journey, a 1963 Disney film remade in 1993, is a fictional tale, but there are numerous true stories of dogs and cats making their way home to their masters across thousands of miles. How do animals do this? Scientists do not fully understand the answers, but theories regarding animal navigation abound. In any case, there is no question that animals possess navigational abilities unavailable to humans, for example, echolocation, used by bats, whales, and dolphins for local navigation, requires an ability to hear sounds far beyond the range of the human ear.
HOW IT WORKS
Reasons for Migration
Why do animals migrate? Seasonal temperature changes, of course, are a factor, as in the well-known instance of birds flying south for the winter. But to justify enduring the dangers and hardships of long-distance migration, there must be an underlying cost-benefit equation whereby the benefits of migration outweigh the costs. Or, to put it another way, the physical "cost" of migrating must be less than the cost of staying home.
Human beings would perform such calculations rationally, of course, by thinking through the options and weighing them. Animals, on the other hand, rely on instinct—a word that, like many terms in science, has a somewhat different meaning within the scientific community than it does for the world at large. People tend to think of instinct as a matter of "just knowing" something, as in "I just know he/she is a good/bad person," but, in fact, instinct seems to have nothing to do with "knowing" at all.
On the contrary, instinct can be defined as a stereotyped (that is, largely unvarying) behavior that is typical of a particular species. Instinctive behavior does not have to be learned; rather, it is fully functional the first time it is performed. Though animals do exhibit some problem-solving ability, when a bird flies south for the winter, it has not thought that process through in any way. Instead, it is on "autopilot." This may seem almost magical, but it probably just reflects the processes of natural selection (see Evolution): for a particular bird species, those individuals "hard-wired" with a tendency to fly south were those that survived harsh winters, and therefore this tendency became favored in the gene pool.
Just as circumstances in the creature's home environment present a compelling need for migration, so there are other circumstances in the wintering environment that force the animal to leave as spring approaches. The wintering environment, after all, has its own native species, which most likely remain in the area even as the influx of visitors from up north arrives. Competition for food and shelter thus can become rather intense. Over time, this increased competition creates a situation in which it is advantageous for the migrating creature to return home.
Types of Migration
The idea of birds flying south en masse for an entire winter represents only one of four different types of migration: complete, as opposed to partial, differential, or irruptive migration. Complete migration involves the movement of all individuals within a population away from their breeding grounds at the conclusion of the breeding season. Usually this entails migration to a wintering site that may be thousands of miles away.
Some species practice partial migration, whereby some individuals remain at the breeding ground year-round, while others migrate. Others employ differential migration, in which all members of the population migrate, but for periods of time and over distances that vary as a function of age or sex. For example, herring gulls migrate for increasingly shorter distances the older they get, and male American kestrels remain at the breeding grounds longer than females. Even when the male birds do set out on their journeys, they do not travel as far as their female counterparts. Finally, there is irruptive migration, whereby certain species do not migrate at all during some years but may do so during other years. The likelihood of migration seems to be tied to climate and resource availability: the colder the winter and the more scarce the food, the more likely migration will occur in species prone to irruptive migratory behavior.
Directions of Migration
Though southward migration is the most widely known form of migratory behavior, not all migration is from the north to the south. Actually, this type of movement is more properly called latitudinal migration, since it also takes place in the Southern Hemisphere, where, of course, it is from south to north. (Also, winter in those latitudes occurs at the same time as summer in the Northern Hemisphere.) There is far less habitable land below the equator than above it, however, so latitudinal migration in the Southern Hemisphere is not nearly as significant as it is at northerly latitudes.
There are, in fact, species of bird, such as the prairie falcon (Falco mexicanus ), that travel longitudinally, or from east to west. This type of movement probably is related to seasonal changes in the location, availability, and choice of prey. Nor does all migration involve movements across Earth's surface; there is also elevational migration, which entails a change of altitude or depth beneath the sea.
Animals that live on mountains, for instance, may take part in elevational migration, moving to lower elevations in winter just as other species move to lower latitudes. For zooplankton, tiny animals that float on the waters of the open ocean, migration is a matter of changing depths in the water. During the summertime, when populations of zooplankton are large, these organisms live on the surface and feed on the plant life there. During the cold months, however, zoo-plankton migrate to depths of about 3,300 ft. (1 km) and do not feed at all.
The Process of Migration
When animals migrate, they move along more or less the same corridors or paths each year. For North American birds migrating south for the winter, one of the most commonly used "flyways" is across the Gulf of Mexico, a journey of 500-680 mi. (800-1,000 km). To make it across the open waters of the gulf, birds have to store up fat, on which they can live for some time while out of sight of food sources.
Migrating birds are not like the proverbial parent (usually a father) who will not let the children stop to go to the bathroom on a long road trip. As they make their way south, birds stop regularly to rest and eat, sometimes for days at a time. These stops are particularly frequent and long just before crossing a large expanse of water. Only when the bird has stored sufficient quantities of body fat does it resume the journey.
DAY AND NIGHT TRAVEL.
In North America it is common to see flocks of birds apparently flying south during the daytime in the autumn months. Yet most migrating bird species travel at night. The fewer species that travel by day tend to follow paths that are slower and less direct than those of nocturnal migrants. The reason for this difference has to do with the differences in feeding opportunities.
Nocturnal migrants have the entire day in which to rest, forage for food, and eat, thus building up reserves of energy for the nonstop flight that night. Daytime migrants, on the other hand, have to forage at the same time that they are traveling. For this reason, they tend to stick close to the coastlines, which offer abundant supplies of insect life. This slows them down but offers a dependable food supply.
Feats of Navigation
In making their journeys, some creatures display navigational skills that would put such great mariners as Ferdinand Magellan and James Cook to shame. For example, the arctic tern (Sterna paradisaea ) is a complete migrant in every sense of the word. Not only does the tern engage in complete migration, as defined earlier, but it also literally crosses the globe from pole to pole. In traveling from the Arctic to the Antarctic and back again in a single year, the arctic term completes a round-trip migration of more than 21,750 mi. (35,000 km). This distance, nearly the circumference of the planet, is the longest regular migratory path of any animal.
A specimen of another bird species, the Manx shearwater, was taken via airplane to Boston from its home on an island off the coast of Wales. Released experimentally in Boston, the bird took only 13 days to return to its point of origin, a flight of some 3,050 mi. (4,880 km). In another experiment, an albatross from Midway Island, deep in the Pacific, was released in the Philippines and made its way to its home area, a distance of 4,120 mi. (6,592 km), in only 32 days. There are also many examples of swallows finding their way across great distances. For instance, swallows that winter in southern Africa still manage to get back to their homes in northern Europe each spring. Then there are the swallows of San Juan Capistrano, southwest of Los Angeles, which leave every year on October 23 and return like clockwork on March 19.
Birds are not the only creatures capable of such great navigational feats. Monarch butterflies (Danas plexippus ), which are born in Canada or the northern United States, winter each year in southern California, just as they have done for countless years. Then, when winter is over, they make their way back to their home regions. Likewise, salmon come back from the oceans and fight their way upstream to spawn in the very same spot at which they were hatched.
How Do They Do It?
Based on these and other examples, one is left wondering, how do they do it? Numerous observations and theories have been put forward to answer this question. Salmon, for instance, seem to distinguish their home streams on the basis of smell, whereas some birds appear to use visual signals, primarily the position of the Sun or star patterns. Auditory cues and the sensitivity of migratory species to these cues often have been advanced as a key to migration behavior. Finally, one theory, which we discuss shortly, holds that sensitivity to Earth's magnetic field provides long-distance travelers with the navigational aid they need.
Several experiments have been performed on a variety of creature well known for its navigational abilities: the homing pigeon, which often can return across many hundreds of miles to its home. While they are undergoing training by humans, these pigeons are released from a series of sites, each just a little farther from the birds' home area. This training seems to make them accustomed to traveling long distances and to finding their way back. Thus, once trained, a pigeon released 100 mi. (63 km) or more from its home will begin flying in the correct direction within a few minutes.
BIOLOGICAL CLOCKS AND NAVIGATION.
Several theories regarding pigeons' homing skills cite internal or biological clocks (see Biological Rhythms). One such theory, which is no longer accepted widely, held that the pigeon's perception of the Sun's position in the sky, combined with its biological clock, helped it navigate. Experiments have not proved this to be the case, however.
In one such trial pigeons were kept in a laboratory from which they could see the Sun for only very limited periods of time each day. After an extended period, the pigeons, with their eyes covered, were taken away about 40 mi. (64 km) to the south and released, so that the moment of release was their first unobstructed view of the Sun in weeks. Assuming the theory was correct, the pigeons would have been disoriented, but, in fact, they quickly took stock of their position and began flying north.
One intriguing theory of animal navigation holds that creatures carry in their brains "magnetic maps," or strong sensitivities to Earth's magnetic field, that assist them in finding their way across distances. As reported in National Geographic Today on-line (October 12, 2001), research on loggerhead turtles has shown that hatchlings are sensitive to the strength and direction of Earth's magnetic field and apparently use this in their migratory navigation.
By rigging up harnesses attached to electronic tracking units and outfitting the turtles with these devices, researchers were able to follow the course of their migration. Because these are, after all, turtles (though seaborne rather than terrestrial or land-based), migration is no speedy affair. To complete the trip, an 8,000-mi. (12,900-km) circuit from south Florida around the Sara-gasso Sea in the north Atlantic and back again, takes 5-10 years.
In the experiment, baby loggerheads were tagged with tracking systems just after they came out of their underground nests on the eastern coast of Florida. Still babies, they would normally begin a journey across the Atlantic, past the Canary and Cape Verde islands on the west coast and Africa and then back to Florida. But instead of going on this journey, the turtles in the experiment were placed in a large circular water tank surrounded by an electric coil capable of generating specific magnetic fields.
By turns, the research team exposed the animals to fields simulating those in three key spots along the route: northern Florida, the area off the coast of Portugal, and the region near the Cape Verde Islands. In each case, turtles responded to these magnetic stimuli by turning in the appropriate direction—for instance, south when they perceived that they were in the magnetic field equivalent to that of Portugal and west in the field resembling that of Cape Verde.
The research team, which published its results in the distinguished journal Science, concluded that the turtles were hardwired with magnetic sensitivities. The team leader Kenneth Lohmann, a biologist at the University of North Carolina in Chapel Hill, told National Geographic Today, "These turtles have never been exposed to water, yet they were able to process magnetic information and change their swimming direction accordingly. It seems they inherited some sort of magnetic map."
One final, fascinating example of animal navigation is echolocation, which differs from most of the navigational behaviors we have discussed so far in that this one is not necessarily tied to migration. Rather, echolocation provides a means of local navigation for creatures that lack the ability to see in their environments: bats flying through caves and dolphins, porpoises, and toothed whales swimming beneath the ocean's surface.
The frequency of a sound is related not to volume but to pitch: the higher the note, the higher the frequency. The frequency is measured in Hertz, or cycles per second. Human beings are capable of hearing sounds between 20 Hz and 20,000 Hz, whereas cats can hear sounds up to 32,000 Hz and dogs up to 46,000 Hz. This is why these creatures can hear dog whistles and other sounds in audible to humans. Even the hearing ability of these household pets pales compared with that of bats and oceangoing mammals, which can hear tones up to 150,000 Hz.
Echolocation represents an evolutionary triumph in the form of adaptation to environments—the dark, nocturnal world of the bat and the cloudy realm of sea mammals. In the distant past, bats that hunted for insects during the day would have been at a disadvantage compared with birds, which are nimble of movement and extremely sharp-eyed in spotting their insect prey. Whales, porpoises, and dolphins were in an even worse situation with regard to sharks. Sharks, known for a finely tuned sense of smell, were not only competitors for prey but, as tertiary consumers (see Food Webs), they were and are also potential predators of sea mammals.
Only certain kinds of bats use echolocation. These are the carnivorous varieties that live on frogs, fish, and insects, as opposed to the herbivorous eaters of fruit and nectar. These carnivorous bats fly through the darkness, emitting extremely high frequency sounds and receiving the echoes from these sounds. Contained in the echo is a whole database of information regarding the object off which it has reflected: its distance, direction, size, surface texture, and even material composition.
Interestingly, the volume of these sounds is so great—as high as 100 decibels (dB)—that if people were able to hear them, the noises would be almost literally ear-splitting. (The upper range of safety is 120 dB.) As it is, a person would hear only clicks or chirps. Scientists have long wondered how the bats can both emit these sounds, which would be deafening to the bat, and hear them at the same time. Experimental evidence indicates that when the bat emits a sound, the middle ear (that is, the middle portion of the ear's interior) adjusts in such a way as to momentarily deafen the bat. A split second later, the bat's inner ear readjusts so as to permit it to hear the echo from the previous sound.
IN OCEANGOING MAMMALS.
It is true, as the tagline for the 1979 film Alien threatened, that "In space no one can hear you scream." Another way of putting this is that sound requires a material medium through which to travel, and the more dense the medium, the more efficiently it moves. Thus, sound moves more effectively through water than through air, and for this reason echolocation is even more suited to the deep-sea environment than it is to the world above ground.
As with bats, undersea mammals send out sounds and then listen for the echoes. Owing to their heightened sense of hearing, these creatures obtain far more information from sounds than a human would. Sound, in fact, does for them what sight would do for a human, providing a detailed, three-dimensional image of their surroundings, but the images it offers are even more precise because sound waves are less subject to interference and diffraction than light waves. (For example, sound waves can simply go around a building, whereas the light waves that hit one side of a building are not visible from the other side.)
Toothed whales, dolphins, and porpoises, all of which normally would be disadvantaged by their weak senses of sight and smell, use more or less the same means to navigate by echolocation. A fatty deposit in the mammal's head helps it direct the sound; then an area of the lower jaw called the acoustic window receives reflected noises. A fatty organ in the middle ear transmits vibrations from the echo, which are translated into neural impulses that go to the brain.
As with the bat, the toothed whale has special structures in its head that help it hear even as it sends out noises. Some species have bony insulating structures, which separate the portion of the head where sounds are received from that part where sounds are generated. Likewise, structures in the middle ear assist the whale in distinguishing whether sounds come from the left or the right, thus facilitating the whale's navigation.
WHERE TO LEARN MORE
Caras, Roger A. The Endless Migrations. New York: Dutton, 1985.
Journey North: A Global Study of Wildlife Migration (Web site). <http://www.learner.org/jnorth/>.
McDonnell, Janet. Animal Migration. Elgin, IL: Child's World, 1989.
Monarchs and Migration (Web site). <http://www.smm.org/sln/monarchs/>.
Neuroethology: Echolocation in the Bat (Web site). <http://soma.npa.uiuc.edu/courses/physl490b/models/bat_echolocation/bat_echolocation.html>.
Penny, Malcolm. Animal Migration. Illus. Vanda Baginska. New York: Bookwright Press, 1987.
Trivedi, Bijal P. "'Magnetic Map' Found to Guide Animal Migration" National Geographic Today (Web site). <http://news.nationalgeographic.com/news/2001/10/1012_TVanimalnavigation.html>.
Waterman, Talbot H. Animal Migration. New York: Scientific American Library, 1989.
A mechanism within an organism (for example, the pineal gland in the human brain) that governs biological rhythms.
The use of sound waves, which are reflected back to the emit ter, as a means of navigating.
A stereotyped, or largely unvarying, behavior that is typical of a par ticular species. An instinctive behavior does not have to be learned; rather, it is fully functional the first time it is per formed.
A small cavity that transmits sound waves, via a network of tiny bones, from the eardrum, which lies between it and the outer ear. Bats use the middle ear to separate transmission and reception signals in echolocation.
A pattern of movement, usually regular and seasonal, whereby animals travel (typically guided by instinct) to specific locations.
The process whereby some organisms thrive and others perish, depending on their degree of adap tation to a particular environment.