Contents
TURTLES AND TORTOISES,
common name for reptiles that make up the order Chelonia, (also
known as Testudines), recognized by a hard shell that encloses the
internal organs of the body. Turtles are ancient life forms. The
earliest fossils recognized as turtles date from the Triassic period,
about 200 million years ago; thus, turtles were in existence prior
to the emergence of the great dinosaur groups. Unlike dinosaurs,
however, turtles have continued to adapt and flourish.
The order Chelonia is divided into two suborders: the Cryptodira,
comprising species that pull their heads into the shell by a telescoping
movement of the neck; and the Pleurodira, comprising freshwater
species that hide their heads by bending the neck sideways. About
250 species are grouped into 9 families. Found only in the temperate
or tropical zones, most species are adapted to a freshwater or terrestrial
habitat, and a small group to sea life.
In American usage the term tortoise refers
only to species of a terrestrial, or land-inhabiting, family, belonging
to the Cryptodira; all other members of the order are called turtles.
In Great Britain the term turtle is restricted
to sea turtles of the families Cheloniidae and Dermochelyidae.
Turtles show a wide variation in size, from the 15-cm (6-in) box
turtle of North America to the giant sea-dwelling leatherback, which
may attain a length of about 2.4 m (about 8 ft). The upper shell
of the turtle, under which the head, limbs, and tail can be more
or less completely withdrawn, is called the carapace. The lower
shell, characteristically flat, is called the plastron. The two-part
shell is connected to the vertebrae and ribs. The structure and
size of the carapace and plastron vary among turtles, along with
adaptive changes in behavior and mode of life.
A turtle shell typically consists of two layers: an inner
bony layer, the sections of which are called plates; and an overlapping,
horny layer, made up of so-called shields. Although it is hard,
and in some species quite thick, the turtle shell is a surprisingly
sensitive structure because of the many nerves embedded in it. The horny
outer layer of some turtles—especially the hawksbill turtle—has
been used in making ornamental objects; this material is popularly
called tortoiseshell.
The bones of the turtle skull are immovably connected; the
animals have no teeth, although in an embryonic soft-shelled turtle,
traces of them have been detected. The turtle also lacks a breastbone.
The heart, as in other reptiles except crocodilians, has three chambers,
but it operates almost as if it had four because of the presence
of an incomplete partition in the ventricle.
Although few species of turtles can be considered strictly
carnivorous or herbivorous, some inland habitats prefer vegetation,
and aquatic species are mostly carnivorous. All turtles lay eggs,
which they bury in holes. They are generally long-lived; some species
live for more than 100 years.
The freshwater and wood turtles, of the family Emydidae, constitute
about half the species in the order. These turtles are characterized
by their fully or partly webbed feet and their usually flat, streamlined
shells; both features help in diving and underwater escape. Several
genera in this family, however, have evolved to terrestrial life
and show an arched carapace, more typical of tortoises. Among these
is the American box turtle. Box turtles are further protected by
a hinged lower shield that affords complete closure. Other important
U.S. species of this family include the terrapins, especially the
diamondback; the cooters in the genus Pseudemys;
the map turtles in Graptemys; the painted turtles
in Chrysemys; and the wood and pond turtles in Clemmys.
The flattened, round-shelled turtles of the family Trionychidae
are known commonly as soft-shelled turtles, although in fact only
the marginal bones of the carapace are replaced by soft parts. By
virtue of their thin-edged, flat shell these turtles are well adapted
to a life of concealment on the bottoms of lakes and rivers. Tough
skin, instead of the usual horny layer, covers the shell bones,
giving the turtles a leathery appearance. The family is represented
in the U.S. by the genus Trionyx, which includes
two species and several subspecies. Soft-shelled turtles are known
for their aggressive behavior, and the bite of a large specimen
is capable of severing a human finger.
In the snapping turtles of the American family Chelydridae,
the plastron consists merely of two narrow, crossed bands. As with
the soft-shelled turtles, large, sharp jaws compensate for the vulnerability
of the exposed underside. Examples of these turtles include the
common snapping turtle and the alligator snapping turtle, one of
the largest freshwater turtles in the world; an adult alligator
snapping turtle may reach a weight of 90 kg (200 lb).
The best-known marine turtles are the green, loggerhead, hawksbill,
and ridley turtles of the family Cheloniidae and the huge leatherback
turtle of the family Dermochelyidae. The leatherback is the largest
living turtle, sometimes reaching a weight of about 540kg (1200
lb). Sea turtles live mostly in warm waters around the world and
are graceful swimmers, with limbs modified into long flippers that
enable them to migrate long distances. The green turtle, for example,
migrates from the coast of Brazil to breed on the small island of
Ascension, some 3340 km (1400 mi) out in the Atlantic.
When female sea turtles crawl above the tide line to bury
their eggs, they and their large egg clutches are easy prey to shoreline
predators and especially to turtle hunters, who prize the meat of
the green turtle. Also, because of diminishing nesting grounds due
to shore development and the drowning of turtles in fishing trawls,
the green turtle and the Pacific ridley are close to extinction,
and the leatherback is considered an endangered species.
The reptiles known as tortoises are generally restricted to
the terrestrial family Testudinidae. The carapace of tortoises is
high and domelike. The upper and lower shields of some tortoises, such
as those of the African genus Kinyxis, are equipped
with a hinge, which permits complete closure. The giant tortoises
of the Galápagos Islands belong in this group.
Because their shielding and their heavily scaled limbs afford
complete protection, tortoises generally have mild dispositions.
The forelimbs of burrowing tortoises such as the American gopher
tortoise, are especially adapted for digging deep underground burrows
used as shelters against uncomfortable extremes in temperature.
As human habitation spreads, turtle populations quickly become
depleted, not only because the slow-moving reptiles are easily captured
for food but because the turtle habitats are changed radically.
Many thousands of the giant tortoises on the Galápagos
Islands were slaughtered by whalers and pirates in order to supply
their ships with fresh meat, and the few remaining specimens are
now seriously endangered because goats, which were introduced to
the islands to be raised for food, stripped their habitats of vegetation.
In North America, river channeling, swamp draining, and highway
construction in recent years have diminished turtle habitats and
inhibited movement, especially during nesting.