This article was originally published in the June 1991 National Geographic.
Up to my hips in the dark swirling waters of Idaho’s Salmon River this frigid November morning, encased in thick neoprene waders and pelted by an insistent rain, I feel like a fisherman disguised as a snow tire. I am casting flies to entice a silver bullet of a fish, between two and three feet long, called a steelhead because its forehead is gunmetal blue. Though its brain is about the size of a peanut, this Pacific salmon is completing an epic round-trip voyage of several thousand miles entailing wondrous feats of navigation.
Three years ago myriads of yearling steelhead home to spawn. From widely scattered locations they head for the American coast, joining steelhead from Oregon's Deschutes and other rivers in running a gantlet of drift nets, killer whales, sea lions, and seals.
Entering the Columbia River estuary, the fish must escape humans with rods and gill nets and traverse fish ladders—a series of water-filled concrete stairs—at Bonneville and seven other huge hydroelectric dams. Ignoring the mouths of the Deschutes and a score of other tributaries, the Salmon River steelhead enter the Snake and turn left into the Salmon, fighting their way up the raging torrents to the place of their youth.
Now with a rude smash one of them takes my fly and tears downstream with my line arrowing behind. About 80 yards away the steelhead lunges from the river, and I have a freeze-frame picture of a flash of writhing silver that hangs in my mind’s eye. My line is limp. The brute has escaped. But I feel privileged to have observed a fish accomplishing this marvel of endurance, survival, and homing accuracy.
The steelhead is only one of many creatures whose exploits of navigation daunt the mind. For centuries people have marveled that fish, birds, insects, and other animals find their way over incredible distances to preordained destinations.
“What is most peculiar is that each salmon searches the stream to the place where he was born,” wrote Norwegian clergyman Peder Claussøn Friis in 1599. “From a little narrow fjord at Egersund two rivers flow. There is not a bowshot between the river mouths, yet each river has its distinct salmon, so that one can know the salmon on the one river from that of the other.”
Automotive-age scientists studying the little blackpoll warbler’s fall migration from Nova Scotia to South America—in which the bird loses half its weight in the four-day-and-night, 2,400-mile flight—calculate a fuel efficiency equal to 720,000 miles a gallon.
Monarch butterflies stream from winter roosts in fir trees on a volcanic plateau in central Mexico to summer in northern latitudes, copulating and laying their eggs atop milkweeds to foster new generations along the way. With the old monarchs gone and all ties to the ancestral site ostensibly cut, an incredible thing happens—butterflies that have never been to Mexico roost there the next winter.
The fabled albatross can teach even a U. S. Navy navigator a thing or two. In 1957 scientists banded 18 Laysan albatrosses on Midway atoll in the Pacific and put them aboard Navy aircraft bound for Japan, the Philippines, the Mariana, Marshall, and Hawaiian Islands, and the state of Washington.
Released at these locations, 14 birds returned to Midway. The albatross from Whidbey Island, Washington, 3,200 miles distant, averaged 317 straight-line miles a day. The bird from the Philippines made its 4,120-mile return in 32 days—or about 130 miles a day. Even more remarkable, some of the birds would have had to fly circuitous routes to avoid strong head winds, leading researchers to conclude that “existing theories of bird navigation do not fully explain their homing behavior.”
Indeed. Preoccupied with the fact that animals migrate, science was slow to approach the mystery of how they do it. Even when the migration pathways of many creatures became well documented in the 1900s, some observers continued to speculate about a mysterious “sixth sense” by which migrants divined routes.
Then, in mid-century, German scientist Gustav Kramer showed that birds use the sun as a compass. Austrian Nobel laureate Karl von Frisch discovered that honeybees take directional cues from polarized light patterns in the sky. American scientist Donald Griffin proved that bats use sound echoes to detect prey. The common thread in these finds was that animals possess sensory capabilities more varied and keener than our own.
Recent researchers, using techniques of the neurosciences, microbiology, and bioacoustics and such fundamentals of physics as electricity and magnetism, are demonstrating that the senses of the creatures of land, sea, and air are incredibly acute. Imagine:
• A homing pigeon senses changes in altitude as minute as four millimeters. Pigeons also see ultraviolet light and hear extremely low-frequency sound that emanates from wind coursing over ocean surf and mountain ranges thousands of miles distant.
• A honeybee detects infinitesimal fluctuations of the earth’s magnetic field that only the most sensitive magnetometers can measure.
• A shark recognizes an electric field on the order of five-billionths of a volt per centimeter.
• Some animals may be able to “see” the earth’s magnetic field, a proposition about as staggering as “seeing” the force of gravity.
Scientists are quick to point out, though, that the existence of a sensitivity does not prove it is used for navigation. Melvin Kreithen of the University of Pittsburgh, who discovered the homing pigeon’s remarkable sensors, says: “Detection is just the first step.” Scientists can observe the behavior of homing pigeons and infer that they use earth’s magnetic field as a reference. But then they must ask: “Where is the animal’s receptor for this information? Does the animal actually use it to navigate?” Finally, concludes Kreithen, “We must go into the field and prove how the sensitivity is used to navigate.”
As any Scout knows, a navigator needs a map and a compass. The map helps tell you where you are, and the compass indicates the direction to your destination.
But consider a steelhead in mid-Pacific, a monarch butterfly in Vermont, or an albatross released 4,000 miles from its island. What is its map and compass? What senses does it use? For those juvenile birds that migrate alone the first time, how do they know when they've arrived?
Such feats of navigation have long baffled students of animal behavior. But now scientists who ask such questions are, with ingenuity and dedication, piercing some of the veils of mystery to reveal answers that surprise and, in some cases, amaze.
Let’s observe the ant that sprints. Near Maharès on the Tunisian coast, Rüdiger Wehner, a biologist at the University of Zurich, introduces me to Cataglyphis bicolor, a black desert ant no longer than my thumbnail that investigates the palm of my hand and tries to pierce my skin with its mandibles.
When the midmorning sun begins to sear the sands with temperatures that can reach 160ºF at ground level, these ants range from their burrows, Wehner explains, searching for the corpses of other insects less heat tolerant than they. When it wants to, this long-legged specimen still trotting around my palm can cover a meter in about one second.
“An ant follows a truly tortuous outbound route as far as 200 meters from home, turning and stopping frequently,” says Wehner. “But once it has found prey, it immediately takes up a straight course for home—despite all the zigs and zags outbound.”
Since few landmarks dot this landscape, the ant’s feat was one of the great puzzles of animal navigation. Wehner demonstrated that the ants use skylight as a compass cue and that their visual systems are especially sensitive to the patterns of polarized light in the sky.
These patterns are created when rays of sunlight entering earth’s atmosphere collide with air molecules and other particles and scatter in all directions. Because most scattering takes place in the blue and ultraviolet wavelengths, we perceive the sky as blue.
The scattering causes polarization—light that was vibrating in many planes now vibrates primarily in one. Distinct patterns of polarization are created: The most intense is always 90 degrees away from the sun.
The eye of the desert ant has a thousand lens elements; a human eye has but one. In their Zurich laboratory Wehner and his colleagues revealed that each of the ant’s eyes has 80 lenses dedicated to receiving polarized light in the ultraviolet range of the spectrum, each from a different point in the sky. “One lens from 180 degrees, another from 270 degrees, and so on,” says Wehner.
He went on to convince his peers that this lens arrangement gives the ant a sort of celestial map keyed to the pattern of polarization. “The ant turns its head when it stops, to enable its eyes to lock into the pattern. This allows the animal to compute the compass heading back to its burrow. It does this constantly. If an ant misses its burrow, it begins a search pattern, a series of loops that usually locates it.”
To test the process at Maharès, Wehner built a device that looks like a power mower with a slot for movable glass plates. These enabled him to control the amount and direction of polarized light streaming to an ant beneath. With a smile Wehner recalls the spectacle of “scientists chasing ants that were trying to sprint for home beneath the device. Somebody wrote that we were mowing the desert."
“What we’re trying to sort out now is how the ant measures distance, perhaps by keeping track of how many steps it has taken.” Its technique of compass direction and distance traveled is actually dead reckoning—short for “deduced” reckoning—practiced by human navigators for centuries. For the ant, however, misreckon and the sun will kill you in less than an hour.
The concept that animals might navigate by earth’s magnetic field, first proposed by the Russian naturalist Aleksandr Middendorf in the 1850s, is one of the most persistent and controversial in the history of navigational theory.
“To get an idea of the magnetic field,” says Charles Walcott of Cornell University in Ithaca, New York, “think of a bar magnet stuffed in an orange.” The earth is the orange, and the magnet is the fluid iron moving in the earth’s core. The field is represented as a pattern, lines of flux extending from one end of the core to the other in oval fashion through the earth, its oceans, and its atmosphere.
The flux lines, horizontal at the geomagnetic equator, intersect the earth’s crust at progressively steeper angles—called dip angles—the farther north and south one goes. Where the angle is 90 degrees defines the north and south magnetic poles—to which compass needles point—each hundreds of miles from the geographic poles. Intensity of the field is much greater at the poles than at the Equator.
Theoretically, to an organism that can detect field intensity and dip angles, the field offers navigational information. The problem is that the field is weak at the earth’s surface—one-thousandth that at the poles of a child’s toy magnet. What creatures could detect it?
On an August day in 1975 Richard Blakemore, a graduate student in microbiology at the University of Massachusetts at Amherst, looked into his microscope and found millions of them. Studying bacteria in mud taken from a pond at Woods Hole, Blakemore had placed some specimens on a microscope slide. He observed a remarkable scene—the bacteria consistently swam toward the north end of the slide. Blakemore joked to his colleagues that he had discovered “north-seeking bacteria.”
Puzzled and skeptical, Blakemore covered the microscope to rule out the influence of light on the bacteria, turned it around, and then moved it to another room to try to confuse them. No matter what he did, the tiny horde—15 million bacteria can inhabit a drop of water—congregated in the same orientation.
A magnet was brought close to the glass and rotated. Incredibly, the horde “swerved in unison,” he remembers, attracted by one end of the magnet and repelled by the other. Blakemore realized he was seeing something no one else had ever recorded seeing.
Nobel laureate Edward M. Purcell of Harvard suggested an experiment: Remagnetize the bacteria with a short magnetic pulse to see if they orient in the opposite direction. Adrianus Kalmijn, an expert in bioelectricity and magnetism now at the Scripps Institution of Oceanography, helped conduct the experiment. The bacteria reversed their direction. They also made U-turns when magnetic polarity was reversed. Even dead bacteria continued to orient!
When an electron micrograph revealed a tiny chain of dense material inside a bacterium, Richard Frankel at the Massachusetts Institute of Technology identified it as magnetite, or lodestone, the mineral once used in compass needles.
“They are swimming magnets,” explains Blakemore, who christened the organism Aquaspirillum magnetotacticum. “The magnetite literally torques the bacteria into alignment with the magnetic field.” Since the lines of flux dip progressively toward the poles, “the bacteria are oriented where they want to go, down to the mud.” For locomotion they use flagella, tiny filaments at each end that whirl at 300 revolutions a minute.
Richard Blakemore and his colleagues had made the first unequivocal demonstration of an organism orienting to earth’s magnetic field, a discovery of such interest that editors of new physics and biology textbooks seldom pass up the opportunity to include a picture of the bacterium. I propose an addition to the picture caption to help students remember its marvelous qualities:
Lacking a good cerebellum,
Aquaspirillum uses magnets to tell ’im,
Which way to yield,
In the magnetic field,
But ’e needs a flagellum to propel ’im.
The limerick points up a fault of the bacterium, at least from a navigational point of view: Aquaspirillum is relatively passive. Navigators on the move, such as birds and fish, should have some active means of detecting the field. Kalmijn has been pursuing this for a long time with sharks, stingrays, and skates, a group of fishes known as elasmobranchs.
While dissecting a shark, a 17th-century Italian anatomist named Stefano Lorenzini puzzled over the function of globular structures connected to pores on the animal’s head by canals filled with a jellylike substance. Lorenzini first thought he was dealing with glands, but the thickness of the canal walls “makes us suspect that they are intended for another, more hidden function, since nature never acts casually,” he pronounced.
Because of their shape, the structures became known as the ampullae of Lorenzini, but their function remained hidden until 1958, when Kalmijn, then a graduate student at the University of Utrecht in the Netherlands, tackled the question as his thesis project.
In Kalmijn’s experimental tanks small sharks called dogfish prowled until, galvanized by their encounter with the electric field generated by a flounder hidden under the sand, they lunged into the sand and grabbed the prey. When Kalmijn buried electrodes in the sand that mimicked the field of the flounder, the sharks attacked the electrodes with fantastic accuracy, biting between electrodes just two inches apart.
Kalmijn proved that sharks and other elasmobranchs, using their ampullae of Lorenzini, can detect electric fields as weak as five-billionths of a volt per centimeter—the most sensitive electric-detection apparatus known in the animal world.
To illustrate the shark’s capability, Kalmijn offers this: “Plant electrodes 2,000 miles apart on the ocean floor and power them with a 1.5 volt flashlight battery. That is a very weak electric field. But every shark in between those electrodes will know what you're up to.”
Kalmijn later put stingrays in a seawater tank and trained them to find food in the eastern part of the tank. Next, he encircled the tank with a wire apparatus called a Helmholtz coil. Sending electric current through the wires enabled him to cancel out the earth’s magnetic field and substitute another. When he changed west to east, the stingrays homed to the new magnetic east, no matter what their location was in the tank.
Kalmijn is now engaged in experiments to verify whether an elasmobranch, in this case a leopard shark, actually relies on its electric sense when orienting to earth’s magnetic field.
At Bodega Marine Laboratory in California, marine biologist A. Peter Klimley offers another hypothesis: Hammerhead sharks that he studies at a seamount in the Gulf of California seem to cruise along minute geomagnetic gradients—magnetic highways—that originate from deposits of iron in the earth’s crust. These are local magnetic fields, as distinguished from earth’s main field.
Klimley free dives 90 feet into a school of hammerheads and, with a spear, implants into a shark’s back a dart tethered to a transmitter—an exercise the former college swimmer shrugs off as “routine.” The shark leaves at dusk to feed and returns in the morning, telemetering its location to a boat in hot pursuit.
“My eureka moment was the first track, when the animal went out 12 miles, turned around, and came back,” he says. “Shark tracks are not always straight, but they’re very directional. Sharks all seem to go out and back along the same paths. That indicates they are orienting to some simple, fundamental features, which I think are the magnetic gradients in the seafloor.”
Klimley rates the shark’s ability to detect the tiny gradients as “extraordinary, perhaps so keen that it cannot be measured with any device that we know of.”
To prove his hypothesis, Klimley wants to enlarge his research on the geomagnetic properties of the seafloor and also to capture and relocate sharks to see if they seek out areas of strong gradients—the “highways.” Fearful of injuring a shark through capture by hook and line, Klimley wants to find a better way.
“People think I’m joking when I say I’m going to lasso a shark.” But he has already checked it out by grabbing a cruising hammerhead by the tail—prospective target of the lasso—and hanging on for a ride. “The animal accelerated a bit but did not appear to mind.”
If some animals do possess a magnetic sense, why not humans? The possibility excites Robin Baker of the University of Manchester in England, a theorist of animal navigation and perhaps the most voluminous writer in the field. To test this, Baker blindfolds students and takes them on a long, circuitous trip away from the university. Then he stops and asks the students to locate it by pointing. Baker concludes that enough students point with sufficient accuracy to suggest that humans have such a magnetic sense. Intriguing as this idea may be, other scientists have not been able to replicate the results of Baker’s experiments.
Volunteering as an experimental subject during a visit to the university, I was blindfolded and seated up front in Baker’s car. I resolved to keep track of the direction and turns—“Most people try that,” Baker said—but soon gave up because of his relentless turning (three times around one traffic circle).
After 20 minutes or so, Baker stopped the car and instructed me to point to the university. Then he asked, “Point to north.” That startled me. Unfamiliar with Manchester, I had no idea of north on this overcast day even when we started. But I had a feeling and pointed.
Baker told me that I had pointed 15 or 20 degrees to the right of the university and that I had indicated south instead of north—“That’s not insignificant,” he said. “You were on the same axis—only you picked the wrong pole.”
Emerging from its nest on the beach, a turtle hatchling is confronted with a life-or-death question: Where is the water?
“The conventional hypothesis holds that hatchlings head for the brighter half of their world, the horizon out to sea,” says biologist Michael Salmon of Florida Atlantic University at Boca Raton.
Investigating this, Salmon built an arena in the laboratory to simulate the light and dark areas of the beach environment. He tested the responses of green and loggerhead turtle hatchlings to these cues, as well as to the slope of the beach.
“The most important factor is the dark silhouette of vegetation and dunes to landward,” says Salmon. “Hatchlings simply crawl away from these objects, a response that directs them toward the ocean.” He also found slope to be a secondary cue for greens but not for loggerheads. Brightness came into play only when slopes were slight and silhouettes were weak.
There is no explanation for a turtle’s navigational prowess in the open sea, including the astounding 2,800-mile round-trip journeys that greens make between Ascension Island and Brazil. Salmon theorizes that they detect wave motion—nearly constant in direction in this belt of trade winds—and may use that in connection with earth’s magnetic field to navigate. Another idea is that the animals use their sense of smell, which brings us again to the salmonids.
Like Peder Claussøn Friis, the 16th-century Norwegian clergyman, U. S. scientist Arthur Hasler had a persistent curiosity about homing in salmon, but it took a combination of circumstances to satisfy it.
At the end of World War II, Hasler’s proficiency in German led to a job with the U. S. Strategic Bombing Survey in southern Germany. One weekend he finagled a jeep to seek out Karl von Frisch, discoverer of the honeybee compass, in his summer home near St. Gilgen, Austria. Bombs had destroyed the scientist's home and laboratory in Munich.
As von Frisch records in his book, A Biologist Remembers, he was anxious when the American jeep drove up. But when Hasler “asked after me and my honeybees,” the scientist relaxed. That summer they became fast friends.
Von Frisch told Hasler about his discovery of Schreckstoff—fright substance—a chemical emitted when the skin of a minnow is broken by a predator. Scenting it, the other members of the school immediately disperse. That fish could have a keen sense of smell impressed Hasler.
On vacation later in Utah, Hasler led his family to a favorite waterfall of his boyhood. “As we approached, the waterfall was hidden by a cliff,” he recalls. “Suddenly I experienced the wonderful fragrance of mosses and columbines growing near it that I had not smelled since I was a boy. The names of my school chums whom I had not seen for 20 years flashed back. And then it occurred to me: Maybe a salmon does this!”
On Issaquah Creek in Washington State, Hasler and colleague Warren Wisby showed in 1954 that migrating coho salmon whose noses had been plugged with cotton missed a crucial turn in the stream while the other fish did not. Hasler concluded, “Smell is important for salmon to find their way home,” and each river has a peculiar odor from its own soil and vegetation.
Responding to other scientists’ criticism that the nose stuffing influenced the cohos’ behavior, Hasler sought to expose smolts—young fish undergoing physiological changes that prepare them for migration—to a chemical to see if they would later home to a river containing that chemical.
“I needed something that wasn’t toxic or polluting and was stable and available,” he says. Hasler also needed something the salmon could detect. One substance, which smelled like horse urine, repelled the fish. He finally settled on morpholine.
Hasler exposed smolting coho in Wisconsin hatcheries to morpholine, then trucked them to Lake Michigan. “They didn’t have home rivers to return to,” Hasler says, so he simulated these by putting morpholine into several rivers flowing into the lake. “The coho, identified by distinctive fin clips, homed to those rivers by the thousands.”
During a lecture trip to Germany, Hasler met Nobel laureate Konrad Lorenz, a specialist in imprinting, the rapid and irreversible learning during a critical period in a creature's early life that determines behavior later on. Enthusiastic on hearing about the salmon, Lorenz termed their homing behavior a “wonderful example of landscape imprinting.”
Another theory surfaced in 1968 when British scientist Roy Harden Jones argued that the imprinting of a migrating smolt is not a one-shot occurrence but a sequential process that takes place continuously as the fish moves downstream.
In Norway another biologist was coming to his own conclusions about salmon homing. As a young man, Hans Nordeng fought against crack German troops in the 1940 battle for Narvik. After the war he took a degree in biology, returning to his home near the Salangen River in northern Norway to study the migration of salmon, trout, and char, a fascination since boyhood.
While tagging fish, Nordeng observed that soon after the smolts descended the river, the adult fish would return. Something about the smolts must be triggering the adults’ return. That something, Nordeng claims, is an odor distinctive to a genetic stock of salmonids, a pheromone that the adults recognize innately.
To test his hypothesis, Nordeng took adult char from the Salangen River system and flew the fish 600 miles to a hatchery at Voss in southern Norway, where their progeny were reared. “Thus nobody could say that the progeny had imprinted on the Salangen,” explains Nordeng.
Four years later, when the offspring were ready to migrate, Nordeng released them in the Salangen Fjord, near the mouth of another river. His theory predicted the char would nevertheless return to the stream of their kin, a place they had never been, because they would smell the distinctive pheromone. Leaning across a table at the University of Tromsø, where we were talking, Nordeng said emphatically: “In spite of everything, they found the place, they came back to the stream of their parents.”
With salmon aquaculture burgeoning in Europe and North America, Nordeng shares a growing concern among researchers that man may be interfering with the navigational abilities of wild fish. “Along the coast of Norway at least two million hybrid salmon escape from the rearing cages each year. They mingle with the wild salmon and migrate randomly with the wild fish to their home rivers.”
The possibility that these hybrid fish are breeding with wild salmon raises the flag of alarm for Nordeng. “We are interfering with their genetic makeup and, therefore, their ability to navigate,” he warns. “It will be a catastrophe for the wild salmon, because someday their descendants may not be able to return to their rivers.”
These are fighting words in Norway, where the harvest of farmed salmon in 1989 totaled nearly 115,000 metric tons—as against 5,777 tons for the entire world’s catch of seagoing Atlantic salmon—and Nordeng has been harshly criticized.
“The genetic component of navigation is a crucial question,” says Eric Verspoor, a population geneticist at the Marine Laboratory in Aberdeen, Scotland. The salmonids themselves seem to be giving an answer. Scientists report that wild fish return to natal rivers in much greater proportions than their hatchery-bred cousins.
Sea ranchers, who release hatchery salmon in a river and expect to reap a harvest of returning fish, are often disappointed. “Return rates are all over the place,” says Richard Saunders, research scientist at St. Andrews Biological Station, New Brunswick, Canada.
John Bailey of the Atlantic Salmon Federation has been raising and releasing hatchery-stock salmon for 15 years. “Initially the returns were very good; then they fell almost to zero,” he says. “Now we are getting about a one percent return. Our experiments with hybridized fish provide pretty good evidence for a genetic component to navigation.”
At Möggingen, a 14th-century castle in southern Germany, biologist Peter Berthold of the Max Planck Institute is demonstrating that juvenile birds he bred have genetic programs that determine the direction, timing, and distance of migration. Thus the birds get to their destination alone, without the assistance of adults.
Berthold takes me to view “Blackcap City,” a complex of 50 aviaries near the castle where he crossbreeds blackcap warblers. “We have several populations of blackcaps in Europe,” he explains. “Some don’t migrate at all, others fly all the way to central Africa, and still others winter somewhere in between.”
Berthold bred blackcaps whose preferred migratory direction was southeast with others who flew southwest. The progeny flew south, showing the “direction to migrate is inherited, as are timing and distance,” he says. Berthold even succeeded in turning the offspring of sedentary blackcaps into migrants. He concludes: “Almost everything that is necessary for a bird to know to fly from the breeding grounds to wintering quarters is inherited from the parents. Incredible but true.”
Such a capability had been suggested by a ten-year study completed in 1957. Researchers intercepted 11,000 starlings in the Netherlands that were migrating from northeastern Europe to Britain and France. They displaced the birds hundreds of miles to Switzerland. Released, the juveniles continued on the same compass direction and arrived in Spain and southern France. The adults, however, compensated for the displacement and took up a heading for their traditional winter quarters.
The adult starlings met the supreme test of the animal navigator—homing from a place they had never been. Despite being hijacked from the Netherlands to Switzerland, they had a map to show them where they were and a compass to tell them where to go.
To investigate map and compass questions, investigators have typically turned to the homing pigeon. Now, indulge me for a moment and imagine you are one of these worthy birds, a descendant of wild European rock doves that developed a homing ability to return to their nests to feed their young. After training you are taken to a place where you have never been and released. You orbit a couple of times and take up a compass heading for your home loft. You have homed from significant distances, such as a 600-mile flight from southern Germany over the Alps to your base in Italy.
You have truly amazing senses that leave humans far behind. You can use the sun as a compass, compensating for its movement with your internal sense of time. On cloudy days you appear to switch to the earth’s magnetic field for compass cues. Researchers forgive you for your reluctance to fly at night; they have shown that other birds appear to take their primary headings from star patterns.
From anywhere in the United States, it can be argued, your keen ears hear a volcano erupting in Java or winds swirling around the Andes. You have excellent vision, but even when scientists try to confuse you by putting frosted lenses over your eyes at a release site, you still make it to the vicinity of the loft.
Indeed, you even home correctly when researchers transport you to the release site under deep anesthesia or inside a rotating drum. However, magnets placed on your back seem to disrupt your initial orientation under an overcast sky—though you make it home. Interference with your sense of smell also seems to affect your ability to home.
Now tell us, worthy bird: Though researchers have come to some agreement on your compass sense, what do you use for a map?
At this moment a taut silence descends on laboratories around the world. So quiet one can hear the coo of a single pigeon. Not one of all the men and women who have studied your sensory abilities and observed your behavior has been able to answer that question in a manner that convinces a jury of peers.
The principal hypotheses are two—olfactory and magnetic. Although a pigeon’s sense of smell is just average compared with other birds, pigeons seem to have a wonderful memory for wind-borne odors, according to Floriano Papi of the Department of Animal Behavior at the University of Pisa in Italy.
Papi explained to me that a pigeon remembers the direction from which particular odors come and somehow organizes these recollections in a cartographic fashion. Thus when a pigeon is released at a new site, it determines its location by an olfactory map and then uses the sun to take up a compass heading for home. But the chemical cues it presumably receives have not been identified, and Papi’s hypothesis has been met with skepticism.
Those of the magnetic persuasion argue that the earth’s magnetic field, with its varying intensities and dip angles, can give map information to an animal with senses keen enough. But most agree that this would only provide information corresponding to latitude. A navigator also requires longitude.
Wolfgang Wiltschko of the Johann Wolfgang Goethe University in Frankfurt, Germany, has labored tirelessly in magnetic research since his 1965 demonstration, with his mentor, Friedrich Wilhelm Merkel, that European robins use a magnetic compass. But Wiltschko stops short of claiming that magnetic parameters create a map.
James L. Gould, a biologist at Princeton University, says a bird’s “map sense seems likely to retain its status as the most elusive and intriguing mystery in animal behavior.”
Do you remember Richard Blakemore’s magnetic bacteria? They precipitated an energetic search for magnetite in other creatures. Scientists reported finding it in tuna, salmon, honeybees, pigeons, turtles, and even in humans. Papers were written hypothesizing the use of magnetite to navigate.
Robert C. Beason of the State University of New York at Geneseo has been studying the bobolink. In the fall bobolinks from North America rendezvous on the coast of the Carolinas, gorged to twice their ordinary body weight—fuel for the 1,600-mile flight over the Atlantic to Venezuela. The birds work their way south as far as Argentina. There the male, in molt, acquires the bright black and white colors that earn him his Spanish name, "charlatán", or trickster.
Beason and a German co-worker, Peter Semm, report that cells in the ophthalmic nerve of the bobolink are sensitive to magnetic field changes—generated by a Helmholtz coil—of as little as 0.5 percent of the earth’s main field. They hypothesize that this capability “may be involved in the detection of the magnetic map used for navigation.”
Beason plans to remagnetize bobolinks, as Blakemore and Kalmijn remagnetized Aquaspirillum. “If the remagnetized birds orient differently from normal birds,” he says, “that points to a transducer in the bird that utilizes magnetite, or a similar compound, to convert magnetic information into nervous impulses that are involved in the orientation.”
In 1977 Michael J. M. Leask, an Oxford University physicist, published a complex theory with the startling suggestion that a sensory basis for magnetic information may lie in a photoreceptor in an animal’s eye. In other words, an animal may “see” a magnetic field. Leask’s theory excited John B. Phillips, a biologist now at Indiana University.
After 12 years' work, Phillips believes that he has established a link between the visual system and magnetic field sensitivity in the blowfly and a migratory salamander, the red-spotted newt. But the goal remains. “I haven’t yet tapped into the receptor that has actually given up vision to do magnetic reception,” he says.
Some observers feel that Beason, Semm, and Phillips are bringing a promising dimension. But Donald Griffin of bat-echolocation fame speaks for many critics of the magnetic school: “There have been some interesting results, and we may very well be at the dawn of a magnetic age, but the experiments have been exceedingly difficult to replicate.”
Could animals be using some “factor X” sensory capability not yet recognized?
Answers Melvin Kreithen: “If we are going to understand animal navigation, we must discover a new sensory channel. Existing ones are not sufficient to explain the behavior.”
In 1942 Henry L. Yeagley, a Pennsylvania State College physicist, proposed that a homing pigeon could tune into the earth’s magnetic field and, simultaneously, sense the effect of the earth’s rotation on its flight path—the Coriolis effect, named after the French engineer who described it. Yeagley argued that magnetic and Coriolis information would create a “navigational grid work” akin to lines of latitude and longitude, thus supplying the two coordinates for position finding.
Yeagley’s experiments were dismissed by those who thought it farfetched that a pigeon could sense the earth’s rotation.
“Though Yeagley didn’t really prove his case, history is showing that he was asking the right questions,” says Kreithen, who agrees that a pigeon might sense the earth’s rotation. “People on a revolving disk detect rotations as slow as one every 2.4 hours. That’s just an order of magnitude away from detecting the rotation of the earth. So it’s not unreasonable to ask if an animal has that ability.”
James Gould comments: “Given the contradictory results we get in pigeon studies, we probably should go out and do Yeagley’s experiments again.
“At the turn of the century,” Gould continues, “we assumed that animals were color-blind, and it was an incredible shock for some of us to learn that bees had color vision. Later on we discovered that fish could hear, pigeons could see ultraviolet light, and snakes have an infrared sensing apparatus.
“The whole history of animal behavior is the animals taking us by surprise,” says Gould. “Why shouldn’t they have some surprises for us now?”