How Sea Turtles Navigate
As soon as they hatch, sea turtles swim across hundreds of miles
of featureless ocean; as adults} they navigate home to nest. Research
has begun to identify the biological compasses and maps that guide them
by Kenneth]. Lohmann
j\:
ter sunset on summer nights,
A turtle's ability to orient itself seems
tochondrial DNA of Ascension Island
when the heat of Florida's At­
quite sophisticated and reliable. For
green turtles is distinct from that of
lantic coast beaches finally dis­
some sea turtle populations, the initial
other green turtle populations. This ge­
sipates, newly hatched loggerhead sea
offshore migration appears to be just
netic study is additional evidence that
these turtles consistently navigate back
turtles lying dormant in underground
the first step of a transoceanic journey
nests begin to stir. In sporadic bursts
lasting several years. After leaving flori­
to their natal beaches to nest and do
of activity, the hatchlings squirm and
da as hatchlings, for example, logger­
not mix with other populations.
thrash, dislodging sand from the roof of
head turtles (Caretta caretta) are not
Green turtles that nest at Tortuguero,
the nest chamber and trampling it onto
seen again in U.S. waters until their
Costa Rica, demonstrate a similar fi­
the floor. The floor slowly rises, and
shells have grown to more than half a
delity to their home beaches. Turtles
the chamber ascends like a miniature
meter in length. No one has yet tracked
tagged there have been recaptured at
elevator until at last the baby turtles
the tiny turtles through the miles of
feeding grounds dispersed throughout
break through the surface of the sand.
open sea into which they vanish. Cir­
the Caribbean and as far north as Flori­
The hatchlings, each no bigger than a
cumstantial evidence, however, suggests
da, but no turtle in the more than
child's hand, immediately confront a
that the Florida loggerheads eventually
years of tagging at Tortuguero has ever
dangerous world. Pursued by ghost
cross the Atlantic and navigate back.
been found nesting elsewhere.
crabs, foxes and raccoons, the turtles
There is an orderly progression of juve­
A logical first step toward uncover­
scramble across the dark beach to the
nile turtle sizes along the North At­
ing the mechanisms used in navigation
30
ocean and plunge into the surf. They
lantic gyre, the clockwise oceanic cur­
by sea turtles is to determine the orien­
quickly establish a course away from
rent system that encircles the Sargasso
tation cues hatchlings use. Their off­
land and toward the open sea. Buffeted
Sea (the relatively still portion of the
shore migration resembles the long­
by waves and currents, attacked by
central north Atlantic). Thus, after en­
distance migrations of adult turtles in
predatory fish and seabirds, the turtles
tering the Gulf Stream, young turtles
that both require oriented movement
maintain their seaward bearings day
may swim eastward across the Atlantic,
through seemingly featureless ocean.
and night as they swim toward the rel­
then south around the Sargasso Sea
Hatchlings and adults probably have
ative safety of the Gulf Stream. There,
and westward back to the Florida coast.
similar sensory abilities. But hatchlings
they may
Adult turtles navigate even more skill­
are small and easily studied in the labo­
find shelter in floating sargassum sea­
fully than young turtles do. For instance,
ratory, whereas adults may exceed a
weed, as well as food in the form of
green turtles (Chelonia mydas) that fre­
meter in length and
small invertebrates. Years later those
quent feeding grounds off the coast of
(about
that survive to adulthood (perhaps one
Brazil regularly migrate eastward across
with my colleagues Michael Salmon
out of every
more than
some
30 to 50 miles offshore,
1,000) navigate back,
often
returning to the same, speCific beach
each nesting season.
1,400
400
180
kilograms
pounds) in weight. Working
miles of open ocean
and Jeanette Wyneken of Florida At­
to reach their nesting grounds at As­
lantic University, I began to study how
cension Island. This remote speck of
hatchling sea turtles on the east coast
How do the turtles navigate through
land is so isolated and difficult to find
of Florida maintain their seaward ori­
a vast ocean that contains no obvious
that during World War II air force pilots
entation while swimming offshore.
landmarks? Only recently have investi­
required to stop there for refueling
gators, through observation in the lab­
summed up the situation with a rhyme:
oratory and at sea, begun to identify
"If you miss Ascension, your wife gets
the environmental cues the turtles use
a pension."
to maintain their bearings.
Tagging studies at Ascension Island
W
e knew right from the very be­
ginning that many orientation
cues might be available to the
hatchlings. Other migratory animals,
have shown that females remain re­
for example, rely on multiple cues such
markably faithful to their nesting site;
as the position of the sun or stars, po­
no turtle seen at Ascension has ever
larized light, odors, wind direction, in­
KENNETH]. LOHMANN is assistant
been found nesting elsewhere. Instead
frasound (low-frequency sound, such
professor of biology at the University of
the turtles swim to the Brazilian feed­
North Carolina at Chapel HilL He com­
pleted his Ph.D. in zoology at the Uni­
versity of Washington in 1988. His area
of expertise is magnetic orientation by
ing grounds from Ascension after nest­
ing, then repeat the long migration to
the island and back once every two to
marine animals and the neurobiology of
four years. Recent studies by John C.
magnetic field detection.
Avise and his colleagues at the Univer­
sity of Georgia have shown that the mi-
100
SCIENTIFIC AMERICAN January 1992
© 1991 SCIENTIFIC AMERICAN, INC
GREEN TIJRTI..E swims in the waters off
Hawaii. Such turtles may use the earth's
magnetic field and the direction of ocean
waves to navigate between their feeding
grounds and nesting sites.
as that from waves breaking on a
ians, insects and mollusks. We rea­
vided a continuous record of the tur­
beach ) and the earth's magnetic field.
soned that a search for this ability in
tle's direction.
Indeed, the geomagnetic field is one
of the most pervasive and consistent
hatchlings would be a good beginning.
At the start of each experiment, we
We began by testing the orientation
provided a dim light in the magnetic
sources of directional information avail­
of Florida loggerhead hatchlings in a
east so that hatchlings would begin
able to animals. Unlike most other po­
water-filled arena made from a fiber­
swimming in the appropriate seaward
tential cues, it is essentially constant
glass satellite dish about one meter in
direction. (In their natural habitat, tur­
throughout the day and night and re­
diameter. In each experiment, a hatch­
tles emerging from their nests at night
mains largely unaffected by weather
ling turtle was placed in a nylon-Lycra
encounter a similar light cue in the
changes. Although physicists and biol­
harness and tethered to a lever arm.
direction of the ocean, which reflects
ogists alike once ridiculed the idea that
Because the lever arm was free to ro­
more starlight and moonlight than land.
certain animals can sense the geomag­
does.) After
netic field, research during the past
15
tate in the horizontal plane and could
easily be pulled by a swimming turtle,
the light off, plunging the hatchlings
years has shown that a surprising num­
the arm reliably tracked the turtle's po­
into complete darkness.
ber of diverse animals are able to do
sition. Hatchlings could swim freely
Immediately after the light went out,
so. Among these are various migratory
within a radius defined by the tether. In
the hatchlings circled the satellite dish
birds and fish (such as salmon, tuna
an adjacent room, a chart recorder or a
as if confused. After several minutes,
and shark), as well as certain amphib-
computer wired to the lever arm pro-
the turtles usually established a consis-
© 1991 SCIENTIFIC AMERICAN, INC
an
hour or more, we turned
tent course toward a specific direction,
Now we could test the orientation of
But this hypothesis, formulated in
which they held for several minutes be­
hatchlings under two different magnet­
fore circling again. The hatchlings con­
ic field conditions. Each hatchling be­
the laboratory, needed to be tested in
more natural settings. To study the ori­
tinued this curious behavior for hours
gan the new experiment swimming in
entation of hatchlings in the ocean, we
in complete darkness, alternating be­
the earth's magnetic field while a light
devised a floating orientation cage [see
tween brief periods of oriented swim­
shone in the magnetic east. We then
bottom illustration on page 104). By
ming and periods of circling.
turned the light off and let each hatch­
tethering a hatchling to a floating buoy
When we calculated the average di­
ling swim in complete darkness under
in the cage, we could observe the direc­
rection that different hatchlings swam
one of two magnetic fields. Half of the
tion in which the turtles swam.
toward in darkness, it became clear that
turtles swam in the unaltered magnetic
During our next few field experi­
the turtles were not swimming random­
field of the earth (with the coil turned
ments, the turtles paddled vigorously
ly. Instead most hatchlings swam to­
off ). For the other half, we turned the
toward the open ocean, even when test­
ward points between magnetic north
coil on, so that the direction of the mag­
ed nearly
and east, adopting bearings that would
netic field was reversed.
land could not possibly be seen. They
As in the preliminary experiments,
lead them away from the east coast of
Florida and toward the Gulf Stream.
hatchlings tested in darkness in the ge­
We hypothesized that this directional
omagnetic field usually adopted bear­
preference in complete darkness was
ings between magnetic north and east.
based on magnetic orientation. To veri­
Turtles tested in the reversed magnetic
fy the hypothesis, we needed to show
field, however, swam in approximately
15
miles from shore where
swam seaward, we initially presumed,
by detecting the geomagnetic field.
B
ut a few days later, on a hot, un­
usually calm morning with al­
most no breeze, the behavior of
that changing the direction of the mag­
the
data
the hatchlings took a surpriSing twist.
netic field around the turtles changed
demonstrated that hatchling loggerhead
We had run our research boat just a few
the course in which they swam. We
turtles can sense the magnetic field of
miles offshore and tethered hatchlings
therefore constructed a system of five
the earth and orient themselves to it.
opposite
direction.
These
Armed with this new information, we
square coils of copper wire (known as a
to the floating buoy as before. For the
first time, the turtles behaved as if con­
Rubens cube coil) and placed it around
thought we had solved the puzzle of
fused: some swam aimlessly in circles;
the orientation arena. When activated,
how hatchlings maintain their bearings
others established courses in seemingly
the coil generated a weak, relatively
during the offshore migration. We rea­
random directions, including back to­
uniform magnetic field throughout the
soned that soon after entering the
ward shore. We were confused as well. If
area it enclosed. The coil was adjusted
ocean, hatchlings must begin to rely on
the hatchlings were using the magnetic
to generate a field twice as strong as
a
field of the earth to guide them toward
magnetic
compass.
The compass
the horizontal component of the earth's
would continue to function after hatch­
the open ocean, why didn't they behave
field but opposite in direction. Thus,
lings passed beyond sight of shore, en­
like the turtles we had tested before?
we could reverse at will the magnetic
abling them to maintain their steady
field experienced by the turtles.
seaward path to the Gulf Stream.
As we sat in the boat pondering this
unexpected change in behavior, a breeze
slowly
picked
up,
generating small
waves on the glassy ocean surface. Al­
Sea Turtles of the World
most as soon as we noticed these tiny
Most species are found throughout the world's tropical and subtropical waters;
leatherbacks are occasionally sighted as far north as Scandinavia. Some species,
however, do not range far. Kemp's ridley turtles remain in Atlantic waters, and
flatbacks inhabit the South Pacific, near Australia. Some taxonomists consider the
green turtle in the eastern Pacific Ocean to be a distinct species.
waves moving westward toward shore,
the hatchling turtle under observation
turned abruptly and began to swim out
toward the open ocean, adopting a
course straight into the approaching
waves. We continued to test hatchlings
under the new weather conditions; now
that waves were present, all the hatch­
200
lings swam toward the waves and the
open sea, as turtles tested on previous
180
days had done.
Could hatchlings be using wave di-
160
140
CfJ
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120
100
80
60
40
20
0
LEATHERBACK
(Oermochelys coriacea)
102
GREEN
(Chelonia mydas)
SCIENTIFIC AMERICAN January 1992
© 1991 SCIENTIFIC AMERICAN, INC
LOGGERHEAD
(Caretta caretta)
MIGRATORY PATHS of sea turtles
(red)
provide evidence of
the Sargasso Sea a number of times before returning to their
extraordinary navigational abilities. After leaving their Flori­
home area to nest several years later. They are about half­
da nesting beach, loggerhead hatchlings may swim around
grown at that time. Arrows indicate the flow of ocean current.
rection as an orientation cue? To find
or use a combination of cues? If the ge­
Stream to the Florida coast, then teth­
out, we took hatchlings of three species
ering turtles in one location would pre­
of sea turtles-loggerheads, green tur­
omagnetic field is used, then why did
turtles tested on the calm day in the
tles and leatherbacks (Dermochelys co­
ocean fail to swim seaward using their
in different locations, thus precluding
riacea)-to the laboratory and tethered
magnetic compass sense?
detection of a gradient at all. Teth­
vent them from comparing the water
them inside a wave tank. The tank was
The questions were not easy to an­
partly constructed of steel beams, which
swer. On the few occasions when waves
toward waves because other cues they
distorted the local magnetic field. Un­
moved in directions other than toward
might prefer to use were not available.
ered hatchlings might therefore swim
der these conditions, hatchlings did
shore, hatchlings in the floating cage
To examine these possibilities, we
not show directional preferences when
usually swam toward the waves regard­
studied the orientation of unrestrained
waves were absent. When waves were
less of the direction from which the
hatchlings released offshore. In con­
generated, however, the turtles swam
waves came. Suggestive as the results
trast to the floating-cage experiments,
into them, demonstrating that they
were, a problem persisted: the restraint
we made no attempt to monitor the
could maintain a consistent course by
imposed on the turtles by the tethering
orientation of turtles for prolonged pe­
using waves as an orientation cue.
system might conceivably have pre­
riods. We simply dropped each hatch­
vented hatchlings from using other, as
ling into the ocean and recorded the di­
yet unidentified cues.
rection it swam as it paddled away. The
So we knew, at least in the laborato­
ry, that hatchling turtles could orient
themselves by detecting waves as well
For example, nearly two decades ago
hatchlings presumably had access to
as by detecting the earth's magnetic
laboratory studies by Marion Manton
all cues available to them during the
field. But under natural conditions, do
and his colleagues at Columbia Univer­
normal offshore migration.
sea turtles use waves, the geomagnetic
sity showed that juvenile green turtles
In initial tests conducted under typi­
field or some completely different cue
can detect chemicals dissolved in wa­
cal weather conditions, unrestrained
to guide their offshore migration? When
ter. If hatchlings normally migrated
hatchlings oriented themselves into
would they prefer one cue over another
offshore by detecting a chemical gradi­
waves as they swam away. Because
ent (or even a temperature or salinity
waves approached from the seaward
gradient)
direction, however, these results told
stretching
from
the
Gulf
us nothing new; hatchlings might have
been using waves, the magnetic field of
80
60
40
20
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FLATBACK
(Chelonia depressa)
HAWKSBILL
(Eretmochelys imbricata)
KEMP'S RIDLEY
(Lepidochelys kempi)
OLIVE RIDLEY
(Lepidochelys olivacea)
SCIENTIFIC AMERICAN January 1992
© 1991 SCIENTIFIC AMERICAN, INC
103
a
GEOMAGNETIC FIELD
NORTH
b
RUBENS CUBE COIL
they swam. The results were remark­
ably clear: most of the turtles swam to­
ward the waves, even though this ori­
entation led them back toward shore.
We were fortunate enough to be able
EAST
WEST
to repeat this experiment several times
during the next few weeks, as more un­
usual weather temporarily generated
waves in other atypical directions. In
each case, the hatchlings swam toward
SOUTH
approaching waves, regardless of the
REVERSED FIELD
SOUTH
magnetic direction from which the
waves came. These results indicated
that the hatchlings were indeed using
the direction of wave propagation to
guide their migration.
WEST
EAST
Why
might
hatchlings
use
ocean
waves as an orientation cue? Waves en­
tering the shallow waters near the beach
refract until they move toward the
NORTH
MAGNETIC-ORIENTATION APPARATUS demonstrates that Florida loggerhead
hatchlings can detect the earth's magnetic field
(a).
Each turtle is tethered to the
shoreline. Thus, wave movement is nor­
mally a reliable indicator of offshore di­
rection, and swimming into waves usu­
ally results in movement toward the
lever arm, which electronically tracks the turtle's swimming direction. The Rubens
cube coil is used to reverse the direction of the magnetic field. Findings (b) show
open sea.
that most individuals
ly the primary cue used by hatchlings
(dots)
swim in directions between magnetic north and east
under both conditions. The arrow represents the average direction of the group.
Although wave direction is apparent­
during the early phases of the offshore
migration, other cues may simultane­
ously or subsequently be used. The mag­
the earth, a chemical gradient or some
spawned peculiar weather throughout
netic sense, for example, might even­
other cue to establish their offshore
much of the Southeast. One morning
tually supplant wave orientation when
bearings. To determine whether waves
soon after the hurricane passed, we
hatchlings enter deeper water, where
were of primary importance, we needed
awoke to find a strong wind blowing out
waves are a less reliable indicator of off­
a day in which waves moved in an un­
of the west, away from land and toward
shore direction. Partial reliance on other
usual direction-that is, in some direc­
the open sea. Recognizing a unique re­
mechanisms, including chemosensory,
tion other than straight toward shore.
search opportunity, we loaded hatch­
visual or other as yet unidentified cues,
Then the hatchlings would have to de­
lings into a Styrofoam cooler and hasti­
also cannot be ruled out.
cide whether to swim toward the waves
ly ran our research boat out to sea.
or to move seaward using another cue.
Five miles offshore we found what we
were looking for:
O
an
area in which waves
As remarkable as the orientation
abilities of hatchlings seem, adult sea
turtles are capable of even more com­
ur chance to resolve the issue
were clearly mOving eastward toward
plex
finally arrived in the autumn of
the open ocean and away from land.
feats. Unfortunately, it is difficult to
1989,
and
sophisticated
navigational
when Hurricane Hugo
Under these rare and short-lived con­
track these creatures over the immense
swept north along the coast of Flori­
ditions, we released hatchlings one by
stretches of water through which they
da. On its way to landfall, the storm
one and observed the direction in which
migrate,
and their size and weight
FLOATING ORIENTATION ARENA studies the cues hatch­
buoy protects the hatchlings from predatory fish. Because
lings use in the ocean. A harnessed turtle
the buoy rotates easily, a turtle can swim toward any direc­
(photograph)
is
tethered to a partially submerged buoy. The net around the
104
tion. Most hatchlings head directly into approaching waves.
SCIENTIFIC AMERICAN January 1992
© 1991 SCIENTIFIC AMERICAN, INC
make them mostly unsuitable for labo­
ratory experiments. But based on the
limited research done on adults (most­
ly tagging studies) and our own experi­
ence with hatchlings, we can infer the
navigation problems adult turtles face
and speculate about how they might
overcome them.
'\
One important difference between
hatchlings and adults is that adult tur­
tles seem to be able to fix their position
with respect to their destination. Be­
cause the Gulf Stream flows parallel to
the coast, Florida hatchlings need only
maintain a consistent offshore course
to intersect the current. Such orienta­
tion can be accomplished with a simple
compass sense or perhaps just by swim­
ming into the waves. But a compass
sense alone is not sufficient to guide an
adult female turtle back to a specific
nesting beach; she also needs to know
.......
=
'\
WAVE DIRECTION
where she is. Researchers refer to such
WAVE DIRECTION ordinarily provides
an ability in animals as a "map sense."
a good orientation cue: during the hatch­
To understand the difference be­
ing season, waves typically move to­
tween compass sense and map sense,
ward the coast
consider the dismal situation you would
ditions confirm that the turtles use
too enormous to walk out of, given a
flight in an hour at another clearing,
which is located exactly at the center of
The swimming
the ocean during unusual weather con­
by helicopter to a clearing in a forest
compass and told to meet your return
(above).
directions of hatchlings dropped into
confront if you were blindfolded, flown
waves to guide their offshore migration.
WAVE DIRECTION
234 DEGREES
=
WAVE DIRECTION
357 DEGREES
=
They swim into waves even if the di­
rection leads them back to shore
(left).
the forest. By using the compass, you
straight course, but
netic maxima and minima stripes on
this ability alone could not guide you
could maintain
the ocean floor. Such stripes occur in
tersect land, suggesting that cetaceans
directly to your destination in time.
seafloor-spreading zones, that is, areas
may follow such pathways during mi­
You would first need to know where
where continental plates diverge. The
grations. These pathways might also
you were with respect to the central
plates move apart at a rate of a few
provide migrating sea turtles with di­
clearing-whether, for example, you
centimeters per year. Molten material
rectional information.
were located east or north of it. A com­
here continually seeps out along the
Just as the nature of the map sense
pass, then, would not be enough. You
ocean floor. As the material cools, it ac­
remains mysterious, so too are the com­
quires a magnetization parallel to the
pass cues used by adult turtles to guide
direction of the geomagnetic field.
them as they migrate. The magnetic
a
would also need a map, or a map sense.
R
on beaches where magnetic minima in­
elatively little is known about
Because the polarity of the earth's
field is one clear possibility. Wave prop­
the map sense of animals. De­
magnetic field has reversed at irregular
agation direction is another; in many
parts of the open ocean, waves and
tailed investigations have been
intervals over geologic time (at least 2 3
carried out only on birds, and experts
times i n the past five million years
swells provide a consistent directional
do not agree on either the precise na­
alone), stripes of ocean floor formed
cue for much of the year. Ocean swells
ture of the map or the cues used. In
during periods of opposite geomagnet­
reflect wind patterns that prevail over
both birds and sea turtles, however,
ic polarity are magnetized in opposite
huge expanses of open water and en­
the ability to detect magnetic fields is
directions. As the seafloor spreads, the
dure for considerable periods. Thus,
theoretically well suited for determining
plates diverge further, eventually re­
because the propagation direction of
position.
sulting in a series of alternating stripes
swells in a given area is determined
Several geomagnetic parameters vary
along the ocean floor. The magnetic
by prevailing wind conditions hundreds
in a consistent and predictable manner
signal of each stripe either adds to the
or thousands of miles away, swell di­
according to latitude, such as the field­
local geomagnetic field, enhancing the
rection is largely independent of local
line inclination (the angle at which
total field slightly (creating magnetic
weather patterns and varies little over
magnetic field lines intersect the sur­
maxima), or opposes the present earth's
weeks or months. The seasonal con­
face of the earth, sometimes called the
field, decreasing it (resulting in mag­
stancy of swell direction has long been
dip angle) and the field intensity in the
netic minima).
horizontal and vertical directions. Any
recognized; Polynesian navigators, for
These stripes of maximum and mini­
example, employed it in their long-dis­
of these features could be used as one
mum magnetic intensity are detectable
tance voyages.
component of a map for determining
over large regions of the open ocean.
position with respect to a goal.
Studies by Joseph L. Kirschvink and his
Whether adult turtles use waves as
a cue is not known. During migrations,
adults almost certainly swim in direc­
An additional source of magnetic in­
colleagues at the California Institute of
formation potentially available to sea
Technology have shown that whales
tions other than straight toward waves.
turtles exists in the form of the mag-
and dolphins often become stranded
Thus, the tendency of hatchlings to
SCIENTIFIC AMERICAN January 1992
© 1991 SCIENTIFIC AMERICAN, INC
105
HATCHLING SEA TURTLES scamper down Nancite Beach, in
searchers think the few of these olive ridleys that reach
Santa Rosa, Costa Rica, to the relative safety of the ocean. Re-
adulthood will navigate back to the same beach to nest.
means that depleted populations are
swim directly into waves might be sup­
many migratory birds use star patterns
planted in juveniles and adults by the
in orientation, anatomic studies of sea
not likely to be replenished by individ­
ability to hold courses at fixed angles
turtle eyes by Koch, now at Indiana
uals from other areas.
to waves (a wave compass).
University, and his colleague David W.
Some hope exists. Knowledge of the
Chemosensory cues might also be
Ehrenfeld of Rutgers University have
orientation cues hatchlings use now en­
used by sea turtles in at least some mi­
revealed that adult sea turtles are ex­
ables workers to release laboratory­
grations. Several researchers have theo­
tremely myopic when their heads are
reared turtles under conditions that
rized that the Ascension Island green
above water. They are therefore almost
maximize their chances of successfully
turtles might home in on their nesting
certainly unable to discern star config­
migrating offshore. Conservationists are
beach by detecting dissolved chemicals
urations at night.
also trying to exploit the turtles' navi­
gation prowess by moving eggs to pro­
unique to the island (a chemical signa­
ture) from hundreds of miles away.
T
he
extraordinary
navigational
tected beaches. The turtles hatching in
abilities of sea turtles have no
the safe havens may return there to
doubt contributed to their evolu­
nest. Understanding how adult turtles
have suggested that the concentration
tionary success, enabling them to ex­
navigate may one day enable returning
of natural chemicals entering the sea at
ploit feeding grounds far removed from
females to be tricked into nesting on
Ascension Island might dilute surpris­
nesting sites. They are
ancient group
protected beaches. Studying the orienta­
of animals, relatively unchanged in the
tion mechanisms of sea turtles not only
Calculations by Arthur L. Koch and his
colleagues at the University of Florida
ingly little-only about
100
to
1,000
times-before reaching Brazil.
an
fossil record for millions of years.
provides insight into one of the most
Several considerations, however, cast
Yet despite their long history, sea
sophisticated navigation systems ever
doubt on the hypothesis that turtle navi­
turtles now face unprecedented threats
to evolve, it may also help save these
gation is based solely on chemical cues.
to their survival. Years of relentless
animals from extinction.
Green turtles near the coast of Brazil al­
turtle fishing and egg harvesting have
most certainly cannot detect a chemical
wiped out some nesting populations
gradient originating at Ascension with­
entirely. Many other populations-faced
out sampling and comparing the water
with widespread loss of nesting beach­
in locations separated by many miles.
es from human encroachment, growing
Moreover, the ability of green turtles
marine pollution and accidental drown­
that nest at Tortuguero (and elsewhere)
ing in fishing nets-are also in jeop­
FURTHER READING
So EXCELLENT A FISHE: A NA TURAL HIs­
TORY OF SEA TURTLES. Revised edition.
to converge on specific nesting beaches
ardy. Two of the three species of sea
from widely dispersed feeding grounds
turtle that nest in Florida (the leather­
A. Carr. Charles Scribner's Sons, 1984.
RIps, FADs, AND LITTLE LOGGERHEADS.
Archie Carr in Bioscience, Vol. 36, pages
92-100; February 1986.
(both upstream and downstream) ar­
gues for navigational mechanisms that
back and the green turtle) are now clas­
MAGNETIC ORIENTATION BY HATCHLING
are not strictly chemosensory.
loggerhead) is considered threatened.
Orientation based on the positions of
the stars also seems unlikely. Although
Moreover, the strong fidelity for the
106
sified as endangered, and the third (the
nest site exhibited by many sea turtles
SCIENTIFIC AMERICAN January 1992
© 1991 SCIENTIFIC AMERICAN, INC
LOGGERHEAD SEA TURTLES. K.]. Loh­
in Journal of Experimental
Biology, Vol. 155, pages 37-49; January
mann
1991.
SHE'D BE
WAY THE FRENCH
TREAT HER,
IF ONLY SHE
HAD LIPS.
To
the French, a hen's
happiness isn't chicken feed.
And on the next edition of
Scientific American Frontiers,
we travel to France and visit
scientists who are doing
everything they can to make
hens feel good about
themselves. To see if it helps
them lay more eggs. We'll
also take a look at doctors who
use sea coral to replace
human bones. Non-alcoholic
wine. New techniques to
quickly contain oil and chemi­
cal spills. And sailing as
therapy. So put on your beret
and tune into Scientific
American Frontiers. On PBS.*
Sponsored by GTE Corporation.
It'll make you smile. If
you have lips.
·Check local listings for date and time.
SCIENTIFIC
AMERICAN
FRONTIERS
© 1991 SCIENTIFIC AMERICAN, INC
liji4
THE POWER IS ON