All about box turtles

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Background  •  Physical characteristics, Senses  •  Intelligence  •  Shell, Scutes  •  Reproduction, Eggs, Sexes  •  Lifespan, Behavior, Communication, Hibernation  •  Foods, Habitat, Predators, Environmental threats  •  How we can help turtles

Who won the race—the tortoise or the hare? Most of us know Aesop’s famous fable and that 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 the “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.

United States environmentalists and scientists make a distinction between turtles that live in the sea (called turtles or sea turtles), turtles that live on land (called turtles or 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 just 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. Terrapin is from Algonquin torope.)

Background

Turtles are reptiles and belong to the same scientific class, Reptilia, as crocodiles, alligators, lizards, worm lizards, snakes, caimans, and the Gharial and Tuatara. Based on a fossil found in 2008 they’ve existed for 200 to 250 million years. That places them back in the Triassic Period when the first of the dinosaurs began to appear. The oldest box turtle fossils,  found in Nebraska, essentially resemble those of today and date back about 15 million years to the Miocene Epoch.

Today there are around 300 turtle species in the world, all belonging to the order Testudines (tes-TUDE-uh-neez). They’re found on all continents, except Antarctica. The US has 57 native species, more than any other country.

Box turtles are in the scientific family Emydidae and genus TerrapeneThere are four species and 13 subspecies native to North America, including Mexico. They’re classified into two groups: common box turtles and ornate box turtles. The Common Box Turtle, Terrapene carolina, and its subspecies are the most widely distributed, inhabiting south-central, east and southeast regions, and Mexico. The Ornate Box Turtle, Terrapene ornata, along with its two subspecies inhabits the south-central and southwestern US and Mexico. The Coahuilan Box Turtle, Terrapene coahuila, and Spotted Box Turtle, Terrapene nelsoni, inhabit Mexico.

Physical characteristics

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.

Male or female?

That is the question—they’re 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. So, how’s one to know? Well, there are clues. The easiest to see are these.

    • The shell, face and foreleg colors on males may be brighter than on females.
    • Often, but not always, a male’s irises are red or orange, and a female’s are brown or yellow.
    • Males usually have a slightly concave bottom shell.
    • Males have a claw on each hind foot that curves inward.
    • 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 the shell.
    • Females have a short, skinny tail and their vent is located closer to their shell.

And, there are other, more subtle differences which vary with the species and even between individuals of the same species.

Head, senses

The box turtle has two eyes, a nose with two nostrils, tympanic membranes, and hard upper and lower jaws that form a beak with a hooked tip.

The eyes are slightly on the side of the head and positioned to look downward, more toward the ground (some aquatic turtles have eyes placed to look upward.) Some other reptiles, such as snakes, don’t have eyelids, but box turtles do, and they close them when sleeping and when they’re happy. They have superior eyesight, including color vision—not just in the daytime, but also at night—although their shell interferes with their peripheral vision.

Their nose is located just above the mouth and has two nostrils you can easily see. They have an excellent sense of smell, which they use to help them find food, mates, and territory.

As for their ears, they aren’t external. Instead, box turtles have a layer of skin, located on each side of the head well behind the eyes, that forms a tympanic membrane and protects the middle and inner ears. 

Their hearing is in the low-frequency 50 to 1,500-hertz range. That’s very limited. A human’s, by comparison, is 20 to 20,000 hertz. So, while they do hear some sounds, scientists speculate their ears may be used mostly for balance. Also, many of the deep sound waves they can hear happen to cause ground and water vibrations, which they know how to interpret.

Their upper and lower jaws are hard and covered by sharp, horny ridges they use to chew the tough, fibrous vegetation they eat. The upper jaw forms into a hook at the center.

They have a sense of taste, but it isn’t known how sensitive it is. They also have a sense of feel, even through their shell.

Intelligence

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 them specifically. Tests with wood turtles have shown they’re better than white rats at finding their way through mazes. Laboratory tests of the Florida Red-bellied Cooter showed they could hold a memory for at least seven-and-a-half months. Clever tests on “Moses,” a female Red-footed Tortoise showed she used memory and intelligent planning rather than smell to find food in a maze. Why wouldn’t box turtles be just as smart?

People who keep them as pets say they’re sociable. They recognize their owner’s appearance and voice. They know to go to their food bowl when they see a human approach. One owner reports that her turtles rattle small stones in their food bowl if they aren’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 enjoy sampling different foods. They can also be playful. Watch one play with a ball and a dog

Neck

Taxonomists divide turtles into two suborders, Cryptodira and Pleurodira,¹ based primarily on the way they fold their long, flexible necks. Box turtles belong to Cryptodira, or hidden-necked turtles, because their method of retraction is to pull it straight back into their shell rather than fold it sideways as the other group does.

Legs

Box turtles have legs that are short, stout and round, with broad 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 them make various claims about speed, but their measurement is in feet-per-hour, not miles-per-hour. 

Internal organs

Box turtles are different internally from humans and other mammals in many ways. They have a three-chambered heart, instead of four. Their blood flows from the hind limbs into the kidneys through the renal veins, while 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)



They breathe differently, too. Their lungs are large and located above all the other organs. Where a mammal’s ribs move in and out with each breath, the turtle’s 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), through which solid waste passes to the “anal vent,” which is located partway down the tail.

Shell

“Box” turtles are named for their ability to 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 vital, an armor against predators, a shield against extremes of heat and dryness, and, sometimes, from a fire. Watch how tightly it closes against predators.

It has three parts:

1. Carapace (CARE-uh-pace)

The top shell. The inner part of it is composed of about 50 bones, including the ribs and vertebrae, plus cartilage. All this forms the domed 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. Those that are pets can feel their owners petting them. (Their leathery-looking skin is very sensitive, too.)

2. Plastron (PLASS-tron)

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 one, 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.” The center is concave, indicating there’s a male within. (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, legs and tail 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 membranous 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

The arrangement of scutes (which are made of beta-keratin like our fingernails) roughly corresponds to the position of the turtle’s bones and body parts and are scattered to help give the shell more rigidity. They don’t overlap. There are 38 on the carapace and 12 to16 on the plastron.

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-sa 3.0)



Scutes also provide the patterns and colors of the shell. Different species have different patterns and designs, and there are also differences between individuals of the same species. Colors and patterns help box turtles blend into their environment.

The shell grows throughout the turtle’s life and develops growth rings. If it’s 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 movements there.

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 four or five years of age, while various others report it to be somewhere between that and 20 years! At any rate, when the time comes, the turtles find each other through scent and sight. Males will fight over a female by shoving, butting, biting and flipping a competitor onto his back.

Their sex organs are located in the cloaca. Courtship 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 the turtles will find a mate every year, but not to worry, females can store sperm for up to four years!  Watch an awkward courtship and an overturned box turtle right himself.

Eggs

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, and called a clutch), then covers them up and goes away. Females inhabiting southern areas may have more than one clutch per year. The eggs are left on their own; their mother never returns to them or her hatchlings.

Incubation, male-female determination

Incubation takes 70 to 90 days. Unlike humans and other mammals, the embryos lack sex chromosomes that would turn them into males and females. The sex they’ll become falls to the vagaries of the ambient temperature around them (especially during a critical stage of embryonic development), through a process called Temperature-Dependent Sex Determination. Here’s what happens: The ideal range of temperature for, say, incubating Eastern Box Turtle eggs is between 72°F and 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, Terrapene carolina bauri, hatchling. (Srs01 / Wiki; cc by-sa 3.0)



Embryonic development

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. 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. 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 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.

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 of age.

Behavior

Box turtles are diurnal, meaning they’re active in the daytime. Their days are spent foraging and eating, and sometimes mating. Like all reptiles, they’re ectothermic—they can’t regulate their body temperature and it’s affected by the temperature of their surrounding environment. They don’t like to overheat and usually stay in the shade of plants to remain cooler, especially in the hottest part of the day. They’re most active at dawn and dusk. In warmer months, they’re often found near water, probably to stay cooler. Those that live in desert areas burrow underground to stay cool.

Communication

Box 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 make sounds. Turtles may also make a hissing noise, but it’s produced by the way they breathe.  Listen to a box turtle’s trill.

Hibernation (brumation)

Box turtles in colder areas “hibernate” from fall to spring. This is the more widely used term for their period of dormancy. Technically, though, they experience brumation, which differs in the metabolic processes involved. They do several things to prepare for it. They stop eating to empty their digestive system. Usually they move into wooded areas and edges—sometimes traveling several hundred feet from their summer grounds. There, they burrow down into loose soil or under decaying vegetation, sometimes in the same place year after year. Several may hibernate together. Their heart rate slows from about 40 beats per minute to about one every five to ten minutes.

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.

Food sources

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

Habitat

They’re mainly terrestrial and tolerant of most habitats, including open woodlands, forests with moist, well-drained soil, thickets, grasslands, and even semi-arid areas.

Predators, environmental hazards

Young turtles, whose shells are not yet hardened, are at greatest threat of predation. Common predators are minks, skunks, raccoons, large birds, and rodents.

These turtles should be flourishing in urban areas. City yards offer up everything they need—bog gardens, vegetable gardens, shade and fruit trees, insects, snails, and worms, and hiding places. However, habitat destruction, automobiles and illegal poaching for the pet trade are a constant threat. Habitat fragmentation caused by urban development is preventing many from locating mates, so they’re living out their lives without reproducing.

The International Union for Conservation of Nature reports that box turtle populations have probably declined 30 percent over three generations in many states. (More broadly, more than 40 percent of all turtle species are classified as vulnerable, endangered or critically endangered.)

What we can do to help all turtles

The Maryland Zoo lists2 these steps to help protect turtles.

    • Never remove a turtle from the wild.
    • Never relocate a turtle in the wild, unless you see one trying to cross a road. Help a turtle cross a road only if you can do so safely, and be sure to point it in the same direction that it was headed.
    • Never return a pet or rescued turtle to the wild without first contacting [your state’s Department of Natural Resources].
    • Educate friends and family about the importance of observing—but not touching, disturbing or collecting—turtles in the wild.
    • When visiting wetlands, tread lightly and stay on designated paths.
    • Use pesticides and other hazardous materials sparingly and dispose of them properly to insure that they do not end up in waterways.
    • Recycle in order to reduce waste and reduce the need for landfills.
    —————————————–

*Top photo: Eastern Box Turtle, Terrapene carolina carolina. (Jim Lynch, NPS; cc by-sa 2.0)
*http://www.nytimes.com/2006/12/16/science/15askscience.html
¹ Pleurodira, or “side-necked” chelonians, fold their neck sideways and tuck their head near their shoulder under the edge of the shell for protection. They’re mainly freshwater turtles of the Southern Hemisphere.
2 Maryland Zoo

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   

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