| Exactly
what is a turtle?
En
Español
Turtles belong to an ancient group of vertebrate (backboned) animals called
reptiles. Fossil turtles date back to the Triassic Era some 100 million
years ago. The most obvious characteristic of this group of reptiles is
the presence of protective shell. The shell is composed of three parts
-- an upper shell, or carapace, attached to the lower plastron by a bony
bridge.
Turtle
ribs are outside of the girdles of the limbs and do not hinder movement
of the legs. The ten dorsal vertebrae are expanded and fused with the
carapace. There aren't any sternal ribs or sternum.
The
Basics
Instead
of teeth, turtles are equipped with a horny bill.
All
turtles belong to the order Chelonia (or Testudinata). This large group
is subdivided into two major groups. One group, the Cryptodira, are able
to draw their head and neck into their shell for protection. The Pleurodira,
or side-necked and snake-necked turtles are not able to do this, but instead
turn the head and neck to the side.
Turtles, like all reptiles, are cold blooded (ectothermic). Their
body temperature is the same as their surroundings. All present day turtles
lay eggs. The land living tortoises produce hard-shelled eggs like a chicken,
while others like sea turtles lay eggs with a leathery or parchment-like
shell.
Names
Several non-scientific names are used for turtles. The term "tortoise"
in North America is used for high-domed, land living turtles with elephant-like
legs and feet. In England and Australia, the term tortoise is applied
to water living turtles. Biologists use scientific names, unique to each
plant or animal species worldwide, to avoid this type of confusion. Another
pesky term is "terrapin." In the United States, the term terrapin is reserved
for the diamondback terrapins of the genus Malaclemys. These turtles live
in coastal marshland and are comfortable on land or in shallow water.
The
common name 'turtle' is applied to the more aquatic types, including:
musk turtles, mud turtles, snapping turtles, sliders and the subject of
this book, the sea turtles. Sea turtle limbs have evolved into flippers
and their body shape is streamlined (aquadynamic) or torpedo shaped.
Growth
Rate
A turtle's growth rate is determined by the temperature of its body
and amount and frequency of food availability. Some sea turtles are herbivores
while others are carnivores. The giant leatherback feeds primarily on
jellyfish and the hawksbill prefers sponges.
Mating
& Reproduction
Sea turtles typically mate at sea a few weeks prior to nesting and
only the females come ashore to nest. With the rare exception of occasional
basking by male olive ridley's (Lepidochelys olivacea) or the Australian
flatback sea turtle (Natator depressus), male sea turtles virtually never
come ashore. Sea turtle females excavate a body pit and then begin digging
the next cavity, using alternating scoops of their hind flippers. Depending
on the species and size of the female, they then deposit 50-180 leathery-shelled
eggs. These eggs will typically incubate from 48 to 62 days, depending
on nesting beach temperatures. The eggs are close packed and in contact
with one another. Metabolic heating within the nest helps to speed up
the hatching process in the final stages of incubation.
Imprinting
For many years sea turtle biologists have stated that female sea turtles
return to their natal beach to lay their own clutches of eggs. This ability
to return to their own hatch site is called imprinting. In recent years,
genetic research has proven this hypothesis to be true. Individual turtle
populations are genetically distinct. But how are sea turtles able to
navigate great distances from their normal feeding grounds to their nesting
grounds?
Navigation
Sea turtle hatchlings usually emerge from their underground sandy nests
under the cover of darkness. Then, when a majority of hatchlings are on
the surface, they make a synchronized frenzied rush to the awaiting surf
through a gamut of predators including ghost crabs, raccoons, coyotes
and feral dogs. Scientists have demonstrated that hatchling turtles crawl
toward the horizon and that they are able to detect subtle differences
in light intensity. Artificial light sources like automobiles or street
lights close to hatchling beaches tend to disorient the babies and many
die as they crawl away from the ocean and onto roads and highways. According
to biologists Salmon and Lohmann (1989), once in the water, the hatchling
turtles orient toward the direction of the waves. Dr. Kenneth J. Lohmann
has recently made a strong case for the turtles having a built-in magnetic
compass which enables them to orient themselves using earth's magnetic
field to guide their long distance marine journeys. Placing newly hatched
loggerhead sea turtles in a circular experimental tank, Dr. Lohmann mechanically
reversed the magnetic field, causing the turtles to swim in the opposite
direction. Dr. David Owens of Texas A&M University likewise has made
a strong case for turtles responding to a particular water chemistry through
their sense of smell (olfaction). Yet, other biologists believe that sea
turtles utilize celestial, or "star maps." Perhaps a combination of these
theories explains how these turtles are able to navigate their way back
home some 15 to 40 years after they hatched.
Once
the baby turtles enter the water, they swim aggressively toward the origin
of the waves, surfacing approximately every two minutes to breathe.
Young
Sea Turtles
Regardless of the species of sea turtles, no one knows where the
young spend the first year of their lives, or "the lost year." Many biologists
speculate that they float in and among rafts of sargassum weed, feeding
on the rich variety of small creatures that seek refuge in it. Only a
small percentage of baby turtles will survive to become adult animals
and make their homeward journey to perpetuate their kind.
Interesting
Sea Turtle Links
- The
Anatomy of Sea Turtles by Jeanette Wyneken, PhD. view
the book.
- A
selection of informative video clips are available at the NOAA website.
View video page
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