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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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