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Frontiers
Footloose On Earth's Invisible Highways How Migrating Animals Find Their Way

September 1996

The majestic "V" of Canada geese flying south is a trusted sign of autumn. It is also one of the more familiar displays of animal migration, the regular movement of birds, mammals, and insects along well-established routes that takes place every day, all over the globe. Only the most begrudging human observer, however, could regard these navigational accomplishments as routine.

Take, for example, the journey of the loggerhead sea turtle in the Atlantic Ocean.

Immediately upon emerging from underground nests on Florida beaches, loggerhead hatchlings crawl toward the moonlit ocean, swim boldly into oncoming waves and out to sea. There they spend the bulk of their estimated 60-year life span, swimming distances up to 6,000 miles. But most impressive and puzzling is the periodic migration of adult females. Every few years they return to nest on the same stretch of Florida beach where they themselves hatched years before.

While scientists and casual observers alike have long known that animals possess navigational skills far superior to ours, the essential mystery remains: How do they find their way?

The search for the key to that puzzle integrates a broad spectrum of disciplines, including animal behavior and physiology, evolutionary and population biology, ocean sciences, and habitat conservation and restoration.

Migratory animals gather information about the world through a rich array of sensory cues: solar and celestial maps, sound and smell, visual landmarks, Earth's magnetic field, and patterns of light. Unraveling this complex guidance system is one step to understanding how different animals read and utilize the information necessary for travel.

Like humans, most migrants refine their navigating skills through experience, enabling them to set and stay the course over longer distances and overcome dangers along the way. Their journeys allow scientists to explore how complex behavior develops, both in individual animals and in species, and help to identify the impact of environmental change on the unique life-cycle needs of different animal species.

A MAP IN THE MAGNETIC FIELD

For over seven years, biologists Kenneth Lohmann and Catherine Lohmann have conducted experiments with loggerhead hatchlings at nesting sites on beaches near Boca Raton, Florida. Supported by the Animal Behavior Program in NSF's Division of Integrative Biology and Neuroscience, the Lohmanns, of the University of North Carolina at Chapel Hill, have charted the sequence of navigational cues that loggerheads use to find their way.

In their initial steps toward the water, the turtles appear to be drawn toward the low light over the ocean and away from the darker dunes. As they start swimming, the loggerheads no longer rely on visual cues but use the direction of the incoming waves to orient themselves.

The Lohmanns' most recent research has focused on the next stage of the journey. As the hatchlings travel into the Gulf Stream and the North Atlantic gyre--the circular system of currents that stretches between North America and Africa--they apparently rely on the earth¹s magnetic field as their road atlas.

READING THAT MAP CLOSELY

Since the late nineteenth century, scientists have speculated that some migrating animals could sense magnetic field lines and use them for orientation. Modern experiments with long-distance migrants have shown that many animals can determine direction from the earth's magnetic field.

Loggerheads seem to take this one step further. Not only are the turtles able to stay on course once launched, but they can pinpoint familiar and distant areas, such as nesting beaches, after several years at sea.

The Lohmanns suspect that the turtles do this by detecting the magnetic field's intensity as well as its inclination, the angle the field lines make in respect to the surface of the earth. Each point in the North Atlantic gyre has a unique combination of intensity and inclination, similar to each point's set of longitude and latitude coordinates. The Lohmanns say the ability to perceive both magnetic features would allow the sea turtles to find a position anywhere in the gyre.

To test the theory, the team placed hatchlings in a small, circular pool of seawater and exposed them to magnetic field intensities that corresponded to different points along the migratory route.

Although the young turtles had never encountered these intensities before, the Lohmanns found that the turtles swam in directions that kept them within the gyre's magnetic field boundaries. The researchers suggest that hatchlings learn the magnetic gradients of their native beaches, and this initial "imprinting" develops eventually into a sophisticated magnetic map for long-distance navigation.

Knowing what turtles can do, however, is not the same as knowing how they do it. The internal compass that the loggerheads use to collect magnetic information and create amap remains a mystery.

MANY TYPES OF COMPASSES

While the magnetic field offers a powerful map, few animals limit themselves to one type of navigational tool. The Savannah Sparrow, a songbird that migrates between the northeastern United States and Mexico, relies on an array of compasses typical of avian migrants, including magnetic fields, stars, the sun, and polarized ligh

In order to understand the development and relationship of orientation mechanisms in individual birds, Kenneth Able and Mary Able collect young Savannah Sparrows from field nests in upstate New York. The birds are hand-raised in special cages where the Ables, biologists at the State University of New York at Albany, are able to manipulate the relevant navigational information the birds encounter.

The experiments depend upon the fact that a bird needs to know true, or geographic, south in order to set off in the right direction at the onset of the first migratory season. This a bird discerns--as we do--from visual cues, such as the apparent rotation of the sun and stars.

But how do the animals find south for the first time? The Ables discovered that young birds that have never seen the sky can, nevertheless, fly in the correct direction.

The birds--it turns out--are innately sensitive to magnetic field inclination. The Albany team discovered that a young bird calibrates its magnetic sense through visual cues, using one compass to program another.

Furthermore, the researchers found that the governing cue for the sparrow is not the position of the sun, as scientists have long assumed. It is rather the patterns of polarized light created when sunlight hits the atmosphere, particularly at sunset, when the Savannah Sparrow normally sets out.

Additional experiments with adult sparrows revealed that the birds recalibrate their magnetic sense as they mature. This enables them to compensate for magnetic and other environmental variations and also makes for more flexible and skillful travelers. Rather than being confined to the earliest stages of development, Kenneth Able says, the crucial interactions of different compasses characterize the navigational system of the Savannah Sparrow throughout its life.

STOPS ALONG THE WAY

Unlike these studies of birds and turtles that focus on the physiological basis for migration, the research of Rob Stevenson and William Haber emphasizes the ecology and conservation of migratory butterflies and their significance for local ecosystems.

With support from the NSF Division of Environmental Biology, Stevenson, a biologist at the University of Massachusetts-Boston, and Haber of the Missouri Botanical Garden are investigating the seasonal east-west migration of butterflies in the mountains near Monteverde, in north-central Costa Rica.

The journey of the butterflies is relatively short, ranging from 10 to 100 kilometers. It is nonetheless a spectacular event: about 80 percent of the fauna--over 250 species of butterflies--of the dry lowlands of the Pacific slope migrate to the wetter forests in the east.

The short traveling distance gives the biologists a chance to study an entire migratory route. Stevenson and Haber, with the help of research assistants and volunteers, identify and record thousands of butterflies at 40 different sites along the migration track. A monthly census allows the team to monitor the ups and downs of butterfly populations and annual changes in migratory behavior.

They have focused their initial efforts on documenting just how many and which butterfly species take part in the migration, comparing the physiological traits of different migrants, and observing the effects of weather and habitat fragmentation on different species.

The study of these short-distance migrants, says Stevenson, provides a "broad brush perspective" on invertebrate migration. As is true in tropical areas all over the world, the Monteverde region has suffered from clear cutting and heavy agricultural use, resulting in a severely fragmented forest and a change in the composition of the butterfly population. Through their comprehensive surveys of both forests and fragments, Stevenson and Haber have increased the list of butterflies known to live in the area from 500 to 600 species. In addition to documenting butterfly biodiversity, the team has established baseline data on the migrating butterflies that can help understand how changes in land use will affect butterfly survival.

These recent investigations on animal migrations enrich the meaning of the term: "bird's eye view." With these skilled guides, we discover much that we cannot perceive on our own.


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