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Terrapins: At well past 100, they're in the mood
(gg)
By
Faye Flam
It sounded incredible. A species that can live
virtually forever, enjoy sex well into its second
century, and bypass menopause completely.
Rumor had it such a charmed life belonged to a humble
turtle that inhabited the marshlands of the Jersey
Shore.
To get the scoop on the secret lives of turtles, I
went to visit international turtle expert James
Spotila of Drexel University. It just happens that
Spotila was taking time off from following the
fast-disappearing sea turtles to catalogue a
closer-to-home species - the marsh-dwelling
diamondback terrapin.
I found Spotila near Waretown, N.J., at a rare
undeveloped stretch along Barnegat Bay. Cormorants
waded and the afternoon sea breeze rustled through the
scrubby trees. The area had been set aside as a camp
for the blind from the 1920s to the 1990s;
environmental activists managed to save it as a
research station.
Spotila doesn't know if the terrapins are endangered.
"No one has counted them," he says. Their habitat is
getting squeezed by condos, and hundreds get crushed
under the tires of careless drivers. So this year he
and his graduate students are tagging the population
here.
In a wooden shack converted into a laboratory, a tank
held some diamondback terrapins, the big ones about
the size of dinner plates. Interlocking pyramids that
rose from their backs displayed the concentric rings
that reveal a turtle's age.
Palm-sized turtles that I assumed were babies flapped
around as if plotting escape. "Those aren't babies,"
said Spotila. "They're males." The females benefit
from being big because they can lay more eggs, he
says. A clutch can have from 3 to 18 depending on the
mother's size. "Males just have to be big enough to
jump on the back of the females."
What made them male or female? He says these turtles
remain in sexual limbo until about two-thirds of the
way to hatching time. After that, they become male if
they're cooler than about 84 degrees, female if
they're warmer.
Scientists still don't know how it all works, though
some suspect heat changes the activity of an enzyme
called aromatase, which converts testosterone to
estrogen.
Wouldn't you get a skewed sex ratio every time you had
a hot or a cold year, I asked. Spotila said it
averages out in the long run, and newly mature turtles
can always mate with turtles many generations older.
That's because turtles don't age the way people do,
says Spotila. It's hard to comprehend this, since we
humans are programmed to grow old and die, but turtles
don't senesce and don't die until they get diseased or
killed by predator, car or fishing net.
They don't go through menopause or lose their sex
appeal but instead continue to mate and reproduce
every year until they die - which for some can go on
well over a century. So there's no problem for a
105-year-old female to make babies with a 5-year-old
male.
And if a warm spell leads to a male shortage, females
can use sperm stored away from trysts as far back as
five years, says Spotila. "They're lucky - they don't
have to deal with the males," he says. Or at least not
every year.
Spotila said there's still much mystery in turtle sex.
Scientists don't know whether females mate with many
males in a season, whether they're picky about mating,
or when they use the leftover sperm.
But there are hazards to letting the weather determine
your sex.
A few years ago Spotila and Penn Veterinary School
paleontologist Peter Dodson wrote a paper suggesting
that temperature may have determined the sex of
dinosaurs.
When a catastrophic event altered the climate 65
million years ago, turtles managed to adjust, perhaps
evolving a lower female-producing temperature, while
dinosaurs could have gone years without hatching
females. If that was the case, Spotila says, "the last
dinosaur was a frustrated male."
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