All about box turtles

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Turtle, tortoise or terrapin?
Most of us tend to call any turtle a “turtle.” U.S. environmentalists and scientists, however, make a distinction between turtles who live in the sea (turtles or sea turtles); turtles who live on land (tortoises); and turtles who are semi-aquatic or prefer brackish water (terrapins). These terms are used differently in other parts of the world. For example, in Australia, only sea turtles are called turtles, all others are tortoises. In Britain, saltwater species are “turtles” and freshwater species are terrapins.

When referring to the three groups collectively, experts call them chelonians. That refers to their taxonomic superorder Chelonia (kell-OWN-ee-uh), from the Greek word chelone, meaning turtle. Whenever you see a turtle-tortoise-terrapin you can’t identify, call him a chelonian and you’ll never be wrong!

Giant Cretaceous turtle

Giant Cretaceous turtle fossil. (“Animals of the Past,” by Frederic A. Lucas / McClure, Phillips & Co., 1900; PD)

Chelonians
Chelonians have been around for eons. The oldest fossil found so far, in China in 2008, lived in the Triassic period. That’s 250 to 200 million years ago, which was also the beginning of the age of dinosaurs. (In the early Cretaceous Period, 145.5 to 65.5 million years ago, sea turtles were dinosaur-sized: 15 feet long!) Today there are seven species of sea turtles and around 293 other chelonian species in the world (experts disagree on the exact number), all belonging to the order Testudines (tes-TUDE-uh-neez). They’re found on all continents, except Antarctica.

The U.S. has 57 native chelonian species, more than any other country. Most are found in the Southeast, but every state has at least one species — including southern Alaska, if you count the sea turtles who occasionally visit its coastal waters.

The U.S. has 57 native chelonian species, more than any other country. Most are found in the Southeast, but every state has at least one species — including southern Alaska, if you count the sea turtles who occasionally visit its coastal waters.

As for box turtles, there are four species and 13 subspecies native to North America, including Mexico. They’re sorted into two groups: the common box turtles and the ornate box turtles. All belong to the genus “Terrapene,” a name that comes from the Algonquin Indian word for turtle. The Common Box turtle and its subspecies is the most widely distributed, inhabiting south-central, east and southeast regions of the U.S. and Mexico. The Ornate Box Turtle and its subspecies inhabit south-central and southwestern aU.S. and Mexico. The Coahuilan Box Turtle and Spotted Box Turtle inhabit Mexico.

Chelonians are reptiles and belong to the same scientific class — Reptilia — as crocodiles, alligators, lizards, worm lizards, snakes, gavials, caimans and tuataras. You may wonder why they aren’t considered to be amphibians. There are several reasons, but most noticeably, because reptiles have tough outer scales and claws (excluding snakes), while amphibians do not. Reptiles also breathe air through their lungs and amphibians breathe through their skin. 

Physical characteristics
Adult box turtles generally range from 4 to 7 inches across the carapace, depending on the species. For example, the Eastern Box Turtle measures about 4.5 to 5 inches, the Three-toed is 4.5 to 6 inches, the Gulf Coast is about 5 to 7 inches.

Male and female box turtles are hard to tell apart (especially when they’re babies or juveniles). Their sex organs are hidden inside their shell. And size isn’t necessarily an indicator, as females may be larger than males. The clues are subtle. Here’s what to look for:

Males have a long, thick tail with the “vent” (the opening through which they make their droppings) located farther away from their shell. Females have a short, skinny tail with the vent located closer to their shell. The shell, face and foreleg colors on males may be brighter than on females. Males have a claw on their hind feet that curves inward. Males usually have a slightly concave bottom shell. Often, but not always, a male’s irises are red or orange and a female’s are brown or yellow. There are other, more subtle, clues which vary with the species and even between individuals of the same species.

The shell
The shell, of course, is what first comes to mind about box turtles. It’s more than just a “house” for them to live in. It’s permanent, a part of their skeleton — they can’t shrug it off or crawl out of it. It’s their armor, protecting them from predators. It shields them from extremes of heat and dryness, helping the animal to preserve moisture. It even sometimes protects them from fire.

The shell has three parts:

1. Carapace (CARE-uh-pace). This is the dome-shaped top shell. The inner part of it is composed of about 50 bones, including the ribs and vertebrae, plus cartilage. These form the shape of the shell. The carapace is very hard, but it isn’t impervious to pain and pressure. It contains nerve endings and, just as humans can feel through their fingernails, turtles can feel through their shell. (Their leathery-looking skin is very sensitive, too.)

Plastron

The flat plastron tells us this is a female. Note how she has closed her hinged joint to hide completely inside. (Slave2TehTink / Flickr; CC BY-NC 2.0)

2. Plastron (PLASS-tron). This is the bottom shell, composed of the clavicles (the shoulder blades), the bones between the clavicles and portions of the ribs. The plastron is relatively flat. Males generally have a more concave plastron than females, presumably for easier mating. The plastron has a hinged joint located between the abdominal and pectoral scales, which allows them to close their shell tightly after pulling their head inside. (Not all chelonians can do this. Mud turtles, for example, can only close the front; hinge-back turtles can only close the rear; and sea turtles and snapping turtles can’t close their shell at all.)

3. Bridge. This is a bony structure that runs between the top and bottom shells, from behind the forelegs to the front of the back legs, on each side of the turtle. It joins the carapace and plastron together.

The shell’s bony inner structure is overlaid with numerous membraneous bones called thecals (THEE-calls). Thecals are covered with a layer of osteoderms (or epithecals), which are fused plates of bone.

Eastern Box Turtle

Note the scutes on this Eastern Box Turtle’s carapace. (Kenneth Bader / EOL; cc by-sa 3.0)

Covering the osteoderms are scutes. Scutes are the turtle’s “skin,” made of beta-keratin, a protein also found in bird feathers and beaks, porcupine quills, reptile claws, human fingernails and more. Scutes are arranged in sections. Box turtles (and most other chelonians) have 38 on the carapace and 12 to16 on the plastron. Scutes don’t overlap; instead, their arrangement roughly corresponds to the position of the tortoise’s bones and body parts and are scattered to help give the shell more rigidity.

Scutes form the noticeable patterns and coloration of the shell. Different species have different patterns and designs, and there are also differences between individuals of the same species. Color and patterns help box turtles blend into their environment. The shell grows throughout the box turtle’s life and develops growth rings. If the shell is injured, it can regenerate — keratin slowly grows beneath the afflicted area and eventually the damage falls off. If a box turtle gets rolled on his back, he can right himself, but it takes great effort.

Box Turtle

Eastern Box Turtle. (WW)

Head and neck
Taxonomists divide chelonians into two suborders, Cryptodira and Pleurodira, based primarily on the way they fold their necks. Box turtles belong to Cryptodira, the “hidden-necked turtles,” who retract their neck into their shell, folding it into an S shape along the spine for protection. Then they close their “door.” (Pleurodira are “side-necked” chelonians, who fold their neck sideways and tuck their head near their shoulder under the edge of the shell for protection.)

Ornate box turtle

Ornate Box Turtle (Terrapene ornata) demonstrating his flexible neck. (Phaedra / Flickr; cc by-nc-nd 2.0)

The box turtle’s vertebrae are elongated and rigid in the central part of the shell, but small and flexible in the neck and tail, allowing for easy movement.

Box turtles don’t have teeth. Instead, they have a rigid beak with upper and lower jaws covered by sharp, horny ridges they can use to chomp the tough, fibrous vegetation they eat. (In comparison, Green Sea Turtles have a serrated beak for cutting up the tough, fibrous sea grasses they eat. And, some carnivorous chelonians have razor-sharp ridges that are scissor-like or hawk-like, for cutting up prey.)

Their legs are short, stout and round, with elephant-like feet designed for walking on land. They also use their feet for digging and ripping food into pieces. You’ve probably noticed that turtles don’t run, they walk. Slowly. However, they’re capable of bursts of “speed,” but for most that just means walking at a faster pace. People who race box turtles make various claims about speed, but their measurement of speed is feet-per-hour, not miles-per-hour. (Note: turtles should never be removed from the wild for racing or other purposes.)

Sounds
Box turtles, as well as other turtles, can vocalize, but they don’t do it often. They may vocalize to find mates or while mating. There are reports that baby box turtles will vocalize to get attention. Distressed or ill turtles may also may sounds. You may also hear a hissing noise, but this is produced by the way they breathe. Listen to a box turtle vocalize. Listen to a Giant South American river turtle vocalize.

Internal organs
Box turtles have a three-chambered heart, whereas humans and other mammals have four chambers. The turtle’s blood system is different from mammals, too, in that blood from the hind limbs flows into the kidneys through the renal veins and in mammals blood moves out of the kidneys through those veins.

Turtle anatomy

1. Eye 2. Nuchal scute 3. Central/vertebral scute 4. Costal scute 5. Marginal scute 6. Supracaudal scute 7. Head 8. Front paw 9. Carapace 10. Claw 11. Hind paw 12. Tail 13. Esophagus 14. Trachea/windpipe 15. Lung 16. Heart 17. Stomach 18. Liver 19. Intestine 20. Urinary bladder 21. Rectum 22. Cloaca 23. Anus. (Titimaster / Wiki; cc by-sa 3.0)

The lungs of box turtles are situated under the shell and above all the other organs. The turtles breathe through a bellows action: Muscles in the abdomen push other organs against the lungs, moving them in and out.

The digestive system is similar to that of most other vertebrates (animals with a backbone). The two main parts are the stomach and the intestines. The digestive system ends at the cloaca. Solid waste passes through the cloaca and out their “anal vent,” located partway down the tail.

Senses
Box turtles and other chelonians have binocular vision. They see only what is in front of them — but they see it very well. Their eyes have a large number of rods (cells which function in low light), which suggests they have excellent night vision. They also have lots of cone cells, which means they see colors. Their eyes are situated to look downward. (Some aquatic turtles have eyes more toward the top of their head, which enables them to submerge, except for their eyes and nostrils.)

All chelonians have eyelids, unlike other reptiles who don’t (snakes, for instance). They close them when sleeping, and also when they’re happy. (Galapagos Tortoises at the San Diego Zoo enjoy having their neck rubbed by the zookeepers and close their eyes in contentment.)

Box turtles and other chelonians don’t have external ears. They have a layer of skin located on each side of their head well behind the eyes, which forms a tympanic membrane. The membrane protects the middle ear. They have an auditory nerve, as well as inner ear structures that hear low-frequency sounds in the 50 to 1,500 Hz range. This is a very limited hearing range and it’s speculated that the ear may be used mostly for balance. In fact, many of the low-frequency sounds they’re able to “hear” also cause ground and water vibrations, which they know how to interpret. Still, they respond to mating calls and emit a few other sounds. You can here the courting calls of several tortoises here.

Box turtles have a good sense of smell, which they use to help them find food, mates and territory.

Habitat
Box turtles are predominantly terrestrial. They live across the U.S., in almost all habitats, including wooded areas, grasslands, even semi-arid places.

Behavior
Like all reptiles, box turtles are ectothermic. This means they can’t regulate their body temperature and it’s affected by the temperature of their environment. They’re less active during the hottest part of the day and more active at dawn and dusk, when the temperature is lower and they can stay cooler. They’re more apt to be seen after a rain, too.

In warmer months, they’re often seen near water, probably to stay cooler. They usually stay in the shade of plants to keep their body from overheating. Box turtles living in desert areas burrow underground to stay cool.

Some box turtles — those in cooler regions — hibernate from fall to spring. To prepare for hibernation, they stop eating in order to empty their digestive system. They tend to move into wooded areas and woodland edges — sometimes traveling several hundred feet from their summer area. There, they burrow down into loose soil or under decaying vegetation, sometimes in the same place year after year. Sometimes several may hibernate together. If the ground is soft enough, turtles may dig themselves down deeper as temperatures fall, and they’re able to withstand freezing temperatures for a few days. In spring, as temperatures rise, little by little the turtles move upward. Their heart rate slows from about 40 beats per minute to about one beat every five to 10 minutes.

Food sources
Box turtles are opportunistic omnivores and eat whatever is available. They especially like earthworms, snails, beetles, caterpillars, fallen fruit, berries, flowers, leafy plants and grasses. They sometimes eat carrion. The Desert Box Turtle includes cactus in their diet.

Reproduction
Courtship typically occurs in the spring. But, other seasons, too, if a male and female happen to encounter each other. Courtship begins with the male circling and shoving the female and biting her carapace. His sex organ is located in his cloaca. Eventually he grips the back of the female’s shell with his hind feet and positions himself over her to mate. If she’s receptive, she’ll use her hind legs to help him grasp. At this point, the male leans back, nearly to vertical with the back edge of his carapace touching the ground. Since there’s no guarantee box turtles will find a mate every year and breed, females can store sperm for up to four years!

Sometime in late-spring to mid-summer, the female digs a hole in sandy or loamy soil and lays her eggs (usually four to six), then covers them up and goes away. Females inhabiting southern areas may have more than one clutch per year, with each clutch consisting of fewer eggs than northern clutches.The eggs are left on their own, the female never returns to them, nor to her hatchlings.

Incubation takes 70 to 90 days, depending on temperature — lower temperatures increase incubation time. The embryos lack sex chromosomes to turn them into males and females. Instead, a process called Temperature Dependent Sex Determination occurs. This means the temperature around the incubating eggs, especially during a critical stage of embryonic development, determines gender. For example, the ideal range of temperature for incubating Eastern Box Turtle eggs is about 72°F up to 93°F. Eggs that incubate in the lower range produce males. Higher temperatures produce females. Eggs incubating mid-range have a 50-50 chance of being male or female.

Bog Turtle hatchling with partially absorbed yolk sac. (Rosie Walunas, USFWS; PD)

While developing inside the shell, the embryos are attached to a yolk sac by something like a human’s umbilical cord. The yolk is the nourishment they need for growth. Hatchlings have an egg tooth (caruncle) at the tip of their beak between the nostrils (nares), which they use to peck their way through the shell — it can take from a few hours up to two or three days for them to fully emerge. (The caruncle drops off within a few days.) When the tiny turtles hatch, they’re still attached to their yolk sac. The last of its contents rapidly absorbs into the hatchling’s abdomen and they can live on it for several weeks.

The tiny turtles are very vulnerable. Their carapace is somewhat pliable and the hinge doesn’t function until, over time, the turtle’s ribs fuse together. They’ll spend most of their time hiding and feeding on any small prey they find. If they can survive the seven to 10 risk-filled years it takes for them to reach adulthood, their shell will become nearly impregnable. Plus, they can protect themselves by closing up inside it.

Life span
If they make it to adulthood, box turtles have an average life expectancy of about 50 years. Many live to be over 100 years.

Predators
Box turtles should be flourishing in urban areas. City yards can offer up everything they need — bog gardens, vegetable gardens, shade and fruit trees, insects, snails and yummy worms. However, habitat destruction, automobiles and illegal poaching for the pet trade are a constant threat. Also, habitat fragmentation caused by urban development is preventing many box turtles from locating mates, so they’re living out their lives without being able to reproduce.

The International Union for Conservation of Nature reports the box turtle’s decline “probably exceeds 30 percent over three generations,” in many states across the turtle’s range. (More broadly, more than 40 percent of all living turtle species are classified as vulnerable, endangered or critically endangered.)

*Top photo: Eastern Box Turtle, Terrapene carolina carolina. (Jim Lynch, NPS; cc by-sa 20)

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