Common
Name: Box Turtle, Box tortoise – The
retraction of the legs and head into the box-like fortress of the shell yields
the descriptive nomenclature.
Scientific
Name: Terrapene
carolina – In the Algonquian Indian language group, the name for turtle
was a homophone of the Anglicized word terrapin, which, in addition to the
genus name for the box turtle, is the common name for any of several edible
turtles living in fresh or brackish water (the Delaware or Lenape Indians called
the turtle a torope). The species
name indicates that the box turtle was first assigned a taxonomic designation
based on observations in the Carolina colony.
The ponderous, virtually indestructible box turtle
is the epitome of slowness and steadfastness; a metaphor for the
self-sufficiency of carrying one’s house on one’s back. The shell, formed of
the upper domed section called the carapace coupled by bony side bridges to the
flat lower section called the plastron, is a marvel of evolutionary complexity
in providing a bastion against predators. The success of the box design is
evident in survival statistics; absent accident or disease, box turtles live an
average of 50 years while not an insignificant number achieve the century mark,
a distinction held by few other animals, and one that denotes longevity in Homo sapiens.
The shell is the sine qua non of the turtle; one
must have one to be one. It is at the same time a prison and a Palladium,
affording protection at the expense of freedom. The turtle cannot escape from
its shell in an act of reptilian contortion, as the shell is literally its skin
and bones. The dorsal domed carapace is formed by a fusion of the spinal
vertebrae and the costal rib bones and the ventral plastron by the fusion of
the ribs and the clavicles, the whole encasing the shoulder and pelvic girdle;
altogether about 60 bones comprise the shell, a virtual if not actual
exoskeleton. The outer portion of the bone-shell structure is covered with
large horny scales called scutes (from the Latin scutum meaning shield) that are essentially transmogrified
epidermal skin segments. They are made from the same proteinaceous fiber called
keratin that is the primary component of the scales of their brethren reptilian
snakes and lizards. The shell is the refuge of last resort; the water turtles
that can totally enclose head, arms and legs whereas the land tortoises cannot.
The box turtle is, then, an evolved water turtle and not a uniquely talented
tortoise. This is consistent with the fossil record of box turtles, which date
from about 15 million years ago and have taxonomic similitude with aquatic
species of the same Miocene epoch. Box turtles belong to the same family as the
aquatic turtles including the painted turtle and the diamondback terrapin.
Turtles are monophyletic; their evolutionary history
from a single common ancestor has never been seriously questioned. Their highly
adapted physiology is testimony to a linear successor progression. The only
thing that seems to be lacking in agreement is their scientific name which can
be found as either Chelonia (the Greek word for tortoise is chelys), or more frequently as
Testudinata (the Latin word for tortoise is testudo).
The testudines have their origins in the Triassic Period 225 million years ago
when the first dinosaurs appear in the fossil record, though the precise
reptilian ancestry is still subject to legitimate taxonomic conjecture. Turtles
have no cranial fenestration (holes in the temple area of the skull) and are
hence thought to have evolved from reptiles of the Carboniferous Period called
anapsids that also lacked the temporal penetrations. Their origins among the
diapsids which include snakes and lizards, is a hypothesis that has become more
widely accepted due in part to molecular biology. While the diapsids, as their
name implies, have two holes in their skulls in the area of their temples, it
is thought that the testudines may have reverted to the anapsid-type skull
through an involution, or retrograde evolution.
The life cycle of the box turtle is not particularly
robust, a paucity of offspring combined with essentially no parental oversight
of the hatchling turtles absent the selection of a concealed nest location. Courtship
between male and female box turtles takes place early in the spring and
involves some foreplay in the form of the male nipping at the female’s shell in
the course of circling and nudging. Once there is agreement as to intent, the
male mounts the female by gripping the back of her shell with his claws so as
to extend slightly beyond the vertical. This, of course, is necessary because
the shell is not configured to facilitate intercourse. The male box turtle can
be distinguished from the female box turtle by the slight concavity of the lower
shell plastron, which is an adaptation to facilitate the mounting of the
female. The female plastron is flat. Other aspects of sexual
dimorphism are the color of the eyes, the length of the tail and the shape of
the shell. Males have bright orange or red eyes (the females have light orange
eyes), wider and longer tails and flatter shells. The photograph depicts a male
box turtle (right) in the early stages of courtship with a female (left). In that the sexual act is cumbersome at best
and nearly impossible at worst, evolution has provided an answer: the female
can store sperm for up to four years after mating that is still viable for egg
fertilization. Between May and July, the female will excavate a flask-shaped
hole in sandy soil and lay a relatively small number of eggs (estimates range
from 3 to 11), which are thereafter on their own – parenting is not a reptilian
attribute. One of the more interesting observations that have recently been
made is that the sex of the baby turtles is not determined by the genetics of
meiosis, but by environmental factors of the nest such as temperature and
humidity. It is not yet known why (or how) environmental sex selection occurs. After an incubation period of about 75 days,
one-inch long hatchlings emerge to face an unforgiving world of predation to
which the vast majority will succumb. It is estimated that only one in a
hundred box turtles reach the sexually mature age 10 years with a fully formed
and protective 6 inch shell. One can determine the age of a juvenile box turtle
by counting the number of annual growth rings on the epidermal scutes that
cover the shell, though this is no longer possible after growth slows to near
stasis at the age of 20 years.
As the adult box turtle is not constrained to a
protective habitat on account of its keratin aegis, it can and will live in a
wide variety of environments. Mesic woodlands near a source of water is the preferred
location, as heating and cooling alternatives are available - a perennial concern
for a cold-blooded reptile. In the case of the encased turtle, keeping cool is
generally of greater concern, except in the winter months when hibernation
beneath the frost line is practiced to maintain body temperature. The matter of
cooling in the summer has led to some behavioral anomalies; box turtles will
spread saliva on their legs and their head and will urinate on their back legs
to take advantage temperature drop associated with the latent heat of
evaporation, an ersatz sweat. However, there are subspecies of the box turtle
that have adapted to live in grasslands and there is even a desert box turtle
that lives in semiarid conditions. Once a habitat is chosen by an adult box
turtle, it will generally not venture outside a circular area with an
approximate 100 meter radius. If relocated by human intervention, they are
inexorably drawn to their natal grounds without regard to any obstacles like
roads that must be traversed, which is one of the many reasons why turtles
should never be taken home to your garden. This also answers the question of
what you should do if you see a turtle crossing the road – you should move it
to the far side and not move it back to the start as it will just try again.
Habitat diversity is supported by the dietary practices of box turtles; they
are omnivorous and will eat just about anything that they run into. While the
hatchling turtles are thought to be primarily carnivorous in their mostly
insect and worm diet, adults are primarily herbivorous and subsist on leaves,
grass, and fruits. Interestingly, adult box turtles are also mycophagists, they
eat fungi. It is reported in several sources that they consume mushrooms toxic
to humans without apparent distress, not too surprising as mushroom toxins are
very species specific.
The physiology of the box turtle is also strongly
affected by the bastion afforded by the carapace; they have never had the
evolutionary selection driving force to improve on their virtually non-existent
defenses and their somewhat meager sensory perceptions to enhance their
survival. Unlike most other reptiles, box turtles have no teeth – just a beak
with strong jaws. While this is adequate for vegetation and soft bodied animal
prey, it would hardly afford a defense. Their feet have claws for digging and
for holding on while mating, but these are again functional and not defensive.
Most other reptiles use their tongues as either a sensor or as a weapon or
both; the tongue of the box turtle cannot be extended. The auditory capability of turtles is limited
to vibrations without significant directionality as they essentially have no
external ears. To some extent the deficient hearing is offset by what is
characterized as their superior sight and olfactory senses. Breathing inside
the rigid external ribcage of the shell requires some unique adaptations, as
the diaphragmatic mechanism of most other animals will not work. The lungs of
the box turtle are manually inflated by muscles in the leg region and manually
deflated by muscles on the top and bottom of the lung. As this is not all that
efficient, the cloaca, which is the digestive, urinary and reproductive opening
in the tail, can absorb oxygen directly in some species. So, it is a tradeoff; turtles
who survive the gauntlet run of early life to achieve the mobile fortress of
adulthood into which they can withdraw for protection are set for a long life
of wandering around in the forest, eating, sleeping and mating as the seasons
unfold for years on end. Not a bad evolutionary dead-end. The victory of the
turtle over the hare according to the moral probity of Aesop may be allegorical
in terms of speed, but it is categorical in terms of longevity.