All about box turtles

0

Who won the race — the tortoise or the hare? Most of us know that in Aesop’s famous fable the tortoise won. But, why isn’t it a “turtle” that won or even a “terrapin?” Aesop lived more than a thousand years ago, but in the present day, he probably would say a “turtle” won the race, but never a terrapin, and probably not a tortoise. Today “turtle” is the word most people use for any such animal. To experts, though, there is a difference.

U.S. environmentalists and scientists make a distinction between turtles that live in the sea (called turtles or sea turtles), turtles that live on land (called tortoises), and turtles that are semi-aquatic or prefer brackish water (called terrapins). Some other parts of the world use these terms differently. For instance, in Australia, only sea turtles are called turtles, all others are tortoises. In Britain, saltwater species are turtles and freshwater species are terrapins.

When experts want to refer to the three groups as a whole, they simply call them chelonians, which derives from their taxonomic superorder Chelonia (kell-OWN-ee-uh). So, whenever you see a turtle-tortoise-terrapin you can’t identify, call it a chelonian and you’ll never be wrong! (Chelonia comes from the Greek word chelone, meaning tortoise. The word turtle originates from Old English turtla and Latin turtur.)

Chelonians have been around for eons. A fossil found in China in 2008 was discovered to be a 250 to 200 million years old predecessor to turtles. That places it in the Triassic Period when the first of the dinosaurs began to appear. In 2015 the oldest sea turtle fossil was described. It’s at least 120 million years old.

Man standing beside a Giant Cretaceous turtle fossil, more than twice his height.

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



Today there are around 300 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 — even Alaska if you count the sea turtles that occasionally visit its southern coastal waters.

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 chelonians aren’t considered to be amphibians. There are several reasons, most noticeably because reptiles have tough outer scales and claws (excluding snakes), while amphibians do not. Also, reptiles breathe air through their lungs and amphibians breathe through their skin.

Now…on to box turtles
(Note: Some of what is described below is true for all chelonians, but do more research before assuming a fact applies to turtles other than box turtles.)

Box turtles are in the scientific family Emydidae and genus TerrapeneThere are four species and 13 subspecies of box turtles native to North America, including Mexico. They’re classified into two groups: common box turtles and ornate box turtles. The Common Box Turtle and its subspecies are 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 the south-central and southwestern U.S. and Mexico. The Coahuilan Box Turtle and Spotted Box Turtle inhabit Mexico.

Size
Adult box turtles typically range in size from 4 to 7 inches (10.2 to 17.8 cm) across the carapace, depending on the species. For example, the Eastern Box Turtle measures about 4.5 to 5 inches (11.4 to 12.7 cm), the Three-toed Box Turtle is 4.5 to 6 inches (11.4 to 15.2 cm), the Gulf Coast Boc Turtle is about 5 to 7 inches (12.7 to 17.8).

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

Both males and females have an opening in their tail, called a vent. Males have a long, thick tail and their vent is located farther away from the back edge of their shell. Females have a short, skinny tail and their vent is 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. And, there are other, more subtle clues which vary with the species and even between individuals of the same species.

Head and neck
Taxonomists divide chelonians into two suborders, Cryptodira and Pleurodira,¹ based primarily on the way they fold their necks. Box turtles belong to Cryptodira, hidden-necked turtles, because they retract their neck into their shell, folding it into an S shape along the spine for protection. Then they close their door.

Box turtles have a brain, and you may wonder if they have any intelligence beyond mere instinct. There doesn’t seem to be any research about that and so reports are anecdotal. People who keep them as pets say they show sociability. Also, that they know to go to their food bowl when they see a human approach. One owner reports that her turtles would rattle small stones in their food bowl if they weren’t fed on time. Some say their box turtles like to be held, enjoy swimming in kiddie pools, like to roam around and dig in the dirt, and like to sample different kinds of food. Watch a turtle play with a ball 

Senses
Box turtles have superior vision. Not just in the daytime, but also at night. And, they can see colors. Their eyes have lids, unlike some other reptiles, such as snakes. They close them when sleeping and when they’re happy. Their eyes are placed to look downward, more toward the ground (compared to some aquatic turtles that have eyes positioned to look upward.)

Box turtles don’t have external ears. Instead, there’s a layer of skin located on each side of their head well behind the eyes, which forms a tympanic membrane and 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 limited range, so while they do hear some sounds, it’s speculated their ears may be used mostly for balance. Many of the low-frequency sounds they hear also happen to cause ground and water vibrations, which they know how to interpret.

Their nose is just above their mouth. It has two openings called nares (comparable to our nostrils). They have an excellent sense of smell, which they use to help them find food, mates and territory. They don’t have teeth. Instead, they have a rigid beak with upper and lower jaws covered by sharp, horny ridges they can use to chew the tough, fibrous vegetation they eat. 

Their legs are short, stout and round, with rounded feet designed for walking on land. They also use their feet for digging and ripping food into pieces. You’ve noticed that turtles don’t run, they walk. Slowly. They’re capable of bursts of “speed,” but 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. 

Internal organs
Box turtles have a three-chambered heart, whereas humans and other mammals have four chambers. And, their blood flows differently, too — it flows from the hind limbs into the kidneys through the renal veins and in mammals it moves out of the kidneys through those veins.

Color illustration of the turtle's internal and external anatomy, with labels.

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 (art: Titimaster / Wiki; CC by-sa 3.0)



Their lungs are large and located above all the other organs. They breathe differently from mammals. Where a mammal’s ribs move in and out with each breath, the turtle’s ribs are fixed in place. So they breathe by employing their limbs, odd as that sounds: Some of their internal muscles are affected by the movement of their legs and depending on whether the muscles are pushing against the lungs or pulling away air is forced out or sucked in, in a bellows-like fashion.

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 (kloh-AY-kuh. Solid waste passes through the cloaca and out their “anal vent,” located partway down the tail.

The shell
“Box” turtles got their name because they can withdraw their head, tail and legs into their shell and tightly close the lid (something most other turtles can’t do). The shell is permanent, a part of their skeleton — they can’t shrug it off or crawl out of it. It’s their armor, a protection from predators. It’s a shield against extremes of heat and dryness. It protects them sometimes from a fire. Watch how tightly it closes against predators.

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 top scutes of an Eastern Box Turtle's carapace.

Eastern Box Turtle’s carapace. (Kenneth Bader / EOL; cc by-sa 3.0)

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.

2. Plastron (PLASS-tron). This is the bottom shell. It consists of the clavicles (shoulder blades), the bones between the clavicles and portions of the ribs. The plastron of females is relatively flat. Males generally have a more concave plastron, presumably for easier mating.

Underside of a box turtle with labels naming the largest scutes and showing the concave center of a male.

Plastron of a box turtle. Notice the hinge which has allowed the turtle to tightly close its “door.” And, the center is concave, indicating a male is hiding inside. (Ineta McParland / Flickr; cc by 2.0)

The plastron has a hinged joint located between the abdominal and pectoral scutes (or scales), which allows them to close their shell tightly after pulling their head inside. (Not all chelonian species 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 ribs and other bones forming the shell’s shape have three layers covering them. First is a layer of numerous membraneous bones called thecals (THEE-calls). The thecals are covered with a layer of osteoderms (or epithecals), which are fused plates of bone. The final layer is made up of scutes, the turtle’s “skin.”

Scutes are made of beta-keratin, a protein also found in bird feathers and beaks, porcupine quills, reptile claws, human fingernails and the like. 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.

Front view of a female box turtle with her head pulled mostly inside the shell.

Female Ornate Box Turtle, Terrapene ornata ornata. (Tara Allison / WW; cc by nc-sa 3.0)



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.

Female box turtle with neck extended out of her shell.

Box turtles have long, flexible necks. (Frank Boston / Flickr; cc by 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.

Reproduction
Courtship typically occurs in the spring. But, other seasons, too, if a male and female happen to encounter each other. Sources differ on when box turtles reach sexual maturity. Some say it’s at 4 or 5 years of age, while others variously report it’s somewhere between that and 20 years! At any rate, when the time comes, the turtles find each other through scent and sight. They also can make sounds. Males will fight over a female by shoving, butting, biting and flipping the other over.

With both males and females, the sex organs are located in their cloaca. Males have a penis and females have a vagina. Courtship, as such, begins with the male circling and shoving the female and biting her carapace. 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. His penis is long, so at this point, the male leans back nearly to vertical, with the back edge of his carapace touching the ground while they’re joined together. There’s no guarantee box turtles will find a mate every year and breed, so females can store sperm for up to four years!  Watch an awkward courtship and an overturned box turtle upright itself.

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 those in the north. The eggs are left on their own; their mother 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. That 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 (22°C to 34°C). 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. 

Florida Box Turtle hatching out of its egg. Its head and one front leg can be seen.

Florida Box Turtle hatching. (Srs01 / Wiki; cc by-sa 3.0)



While developing inside the shell, the embryos are attached to a yolk sac by something like a human’s umbilical cord. The yolk is vital; it’s the nourishment they need for growth. When the time comes to exit their hard shell, the hatchlings use what’s called an egg tooth. The egg tooth is a hard, sharp protuberance located at the tip of their upper beak between their nares (nostrils). It will fall off in a few days, but for now, the little turtles will use it to peck their way through the shell. It’s a hard job and can take from a few hours up to two or three days to fully emerge. When they hatch, the turtles are 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.

Eastern Box Turtle baby with cord and egg sac attached.

Baby Eastern Box Turtle, Terrapene carolina carolina, with cord and egg sac attached. (Nolan Eggert / iNaturalist; cc by-nc 3.0)

The tiny turtles are vulnerable. Their carapace is somewhat pliable and the hinge doesn’t function until their ribs fuse together over time. 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.

Behavior
Like all reptiles, box turtles are ectothermic. That means they can’t regulate their body temperature; it’s affected by the temperature of their environment. They don’t like to overheat and usually stay in the shade of plants to keep their body cooler, especially in the hottest part of the day. They’re more active at dawn and dusk. They’re more apt to be seen after a rain, too.

In warmer months, they’re often found near water, probably to stay cooler. Those box turtles that live in desert areas burrow underground to stay cool.

Hibernation
Box turtles in cool 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. Several may hibernate together.

If the ground is soft enough, turtles may dig themselves down deeper and deeper as temperatures fall closer to freezing. In the spring, as temperatures rise, little by little the turtles move upward. During hibernation, their heart rate slows from about 40 beats per minute to about one beat every five to 10 minutes.

Communication
Box turtles, as well as other turtles, can vocalize, but they don’t do it often. They may call 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. The turtles may also make a hissing noise, but it’s produced by the way they breathe. Listen to a box turtle vocalize.

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 its diet.

Habitat
Box turtles are mostly terrestrial. They live across the U.S., in most habitats, including open woodlands, forests with moist, well-drained soil, thickets, grasslands and even semi-arid areas.

Lifespan
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 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)
*http://www.nytimes.com/2006/12/16/science/15askscience.html
¹Pleurodira are “side-necked” chelonians; they fold their neck sideways and tuck their head near their shoulder under the edge of the shell for protection. They are mainly freshwater turtles of the Southern Hemisphere.

More reading:

Where does wildlife go over the winter?
Wild animals don’t make good pets  
Turtles: frequent questions    
‘The Evolution of the Turtle’s Shell: It Wasn’t for Protection’   
Reptiles   

Share.