River Turtles
If I had to pick one
category of animals to represent the River Trail, turtles
would win hands down. During a summer hike down the trail, I can
nearly always hear the plop of turtles sliding off their basking logs
and into the water as I pass by. One September, I was lucky
enough to stumble across recently hatched turtle eggs on Bryce
Beach. The white, leathery shells were scattered amid loose soil,
marking the spot where hatchling turtles burrowed their way out of the
ground and crawled into the river.
Like many of
our river creatures, turtles have a dubious future, but not because of
the usual combination of habitat loss and pollution.
Instead, the root of the problem is all about sex. As you may
have learned in high school, the sex of most animals is determined at
the instant of fertilization, when the sperm and the egg cells unite to
create a fertilized egg. In humans, all egg cells have an X
chromosome, while sperm cells can have either an X or a Y
chromosome. If an X chromosome sperm joins with the egg, the
resulting XX fertilized egg is female. On the other hand, if a Y
chromosome sperm joins with the egg, the resulting XY fertilized egg is
male.
This cut and
dried version of sex determination that works for humans gets shaken up
when you enter the turtle world. Many turtles, like the Eastern
Painted Turtles and Snapping Turtles that you often see in the Clinch, have
temperature-dependent sex determination --- a complicated phrase for a
complicated concept. When a mother turtle lays her eggs, the
offspring have not yet been designated as male or female.
Instead, the sex of the young turtles is determined by the temperature
of the surrounding soil during the months that the eggs sit in the
ground. Cooler temperatures result in male offspring while warmer
temperatures result in female offspring.
Temperature-dependent
sex determination seems like something a science fiction writer might
dream up to make his aliens more interesting, but scientists have
discovered that the process may help turtle hatchlings survive the
first critical years of their lives. Hatchlings that come from
nests composed of all female or all male turtles tend to survive better
than those from nests of mixed sex turtles, though the reason for the
hatchlings’ increased survival is unclear. We do know that mother
turtles are able to determine the sex of their offspring to some extent
by laying eggs in shady areas under vegetation to create males or in
sunny areas to create females. Many human expecting parents would
have been thrilled to be given such an option!
Unfortunately,
the temperature-dependent sex determination that has worked so well for
turtles in the past may be the cause of their downfall. As global
warming changes the earth’s climate over the next century, some
scientists predict that the earth’s temperature will rise by 4 degrees
Celsius --- enough to make Eastern Painted Turtles produce completely
female offspring even if the eggs are laid in the shade. Although
older male turtles will be present for a few decades, eventually the
female turtles will have no one to mate with and our familiar river
turtles will fade away. I find it impossible to imagine the River
Trail without the quiet plop as turtle after turtle slides into the
water, but the twenty-second century residents of Russell County may
walk a much quieter trail than the one I know.
Not
pictured:
Eastern
Painted Turtle
Scientific
Name: Chrysemys picta picta
Family:
Emydidae (Box and Water Turtle Family)
Habitat:
Shallow water in ponds, marshes, ditches, lakes, streams, and rivers
Common
Snapping Turtle
Scientific
Name: Chelydra serpentina
Family:
Chelydridae (Snapping Turtle Family)
Habitat:
Permanent bodies of water of any type
|
Want
to be notified when new comments are posted on this page? Click on the
RSS button after you add a comment to subscribe to the comment feed.
|
