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.
Potpourri: 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.