The box turtle (Terrapene) or box tortoise is a genus of turtle native to North America (United States and Mexico). The 12 taxa which are distinguished in the genus are distributed over four species. It is largely characterized by having a domed shell, which is hinged at the bottom, allowing the animal to close its shell tightly to escape predators. Box turtles have become popular pets, although their needs in captivity are complex.
Taxonomy and genetics
The genus name Terrapene was coined by Merrem in 1820 as a separate genus from the Emydes for those species which had a sternum which was separated into two or three divisions and which could move these parts independently. He placed in this genus amongst others Terrapene boscii (now accepted to be Kinosternon subrubrum subrubrum) and Terrapene carolina (but under the name Terrapene clausa). Also several Asian box turtles have been (mis)classified within the genus Terrapene: e.g. Terrapene bicolor (now Cuora amboinensis couro) and Terrapene culturalia (now Cuora flavomarginata). Currently four species are classified within the genus and twelve taxa are distinguished:
- Common box turtle, Terrapene carolina (Linnaeus, 1758) (type species)
- Florida box turtle, Terrapene carolina bauri Taylor, 1895
- Eastern box turtle, Terrapene carolina carolina (Linnaeus, 1758)
- Gulf Coast box turtle, Terrapene carolina major (Agassiz, 1857)
- Mexican box turtle, Terrapene carolina mexicana (Gray, 1849)
- Terrapene carolina putnami O.P. Hay, 1906 (extinct) 
- Three-toed box turtle, Terrapene carolina triunguis (Agassiz, 1857)
- Yucatán box turtle, Terrapene carolina yucatana (Boulenger, 1895)
- Coahuilan box turtle, Terrapene coahuila Schmidt & Owens, 1944
- Spotted box turtle, Terrapene nelsoni Stejneger, 1925
- Ornate box turtle (or: Western box turtle), Terrapene ornata, (Agassiz, 1857)
All box turtles have a highly domed carapace. All species are domed, with a first central scute at an angle of more than 50° while the next central scutes are often flattened. While T. carolina species have a prominent medial keel (a ridge over the length of the carapace), this feature is (nearly) absent in the other species. As a result of the hinge in their plastron (between the abdominal and pectoral scales), box turtles can close very tightly to protect themselves from predators. The plastral formula (size relation between the scales) varies between species, but the order of the largest three scutes is anal > abdominal > gular in all species. The ability to close develops when it is one quarter grown and is generally only lost due to corpulence. Based on morphological characteristics, two distinct lineages can be distinguished: the ornata/nelsoni lineage as well as the carolina/coahuila cluster. The ornata/nelsoni cluster is the smallests (maximum carapace lengths of 14–15 cm), while the other cluster is larger (with T.c.major with a carapace length of 20 cm). The radiation pattern varies strongly: from the none-patterned C. coahuila, to the characteristic stripes in C. ornata and the yellow/brown spots in C. nelsoni. C. carolina is the most varied genus with spots, bars and lines which pattern often also varies from scute to scute.
To distinguish between a male and a female box turtle, it is simplest to turn the turtle over and examine its plastron. The male usually has a concave area centered posterior to the hinge. This adaptation apparently aids the male's ability to climb on top of the female during mating. The one pictured above left is a male.
Ecology and behaviour
Life cycle and predation
Once maturity is reached, the chance of death seems not to increase with age. The survivorship curve of box turtles is therefore probably similar to that of other long-living turtles. The average life span of adult box turtles is 50 years, while a significant proportion lives over 100 years. The age of a growing box turtle in the wild can be roughly estimated by counting the growth rings on the scutes; the plastron is the best place to do this because it also allows examination of wear pattern. However, the rate of 1 ring per growth season has not been fully confirmed, and estimates beyond 20 years are unreliable because the scutes is usually worn smooth. Box turtle eggs are flexible, oblong and are (depending on the taxon) on average 2–4 cm long weighing 5-11 g. The normal clutch size is 1-7 eggs. In captivity and in the southern end of their range, box turtles can have more than one clutch per year, while the average clutch size is larger in more northern populations. Turtles can defend themselves from predation by hiding, closing their shell and biting. The risk of death is greatest in small animals due to their size and weaker carapace and plastron. While the shell of an adult box turtle is impenetrable to virtually any animal, they do not always close in a case of threat. Common predators are mammals like minks, skunks, raccoons, dogs and rodents, but also birds (e.g. crows, ravens) and snakes (e.g. racers, cottonmouths) are known to kill box turtles.
North American box turtles are omnivores with a very varied diet as box turtles "basically eat anything it can catch". Invertebrates (amongst others insects, earth worms, millipedes) form the principal component, but the diet also consists for a large part (reports range from 30-90%) of vegetation. The diet is amended with fruits (amongst others from cacti, apples and several species of berry), gastropods (Heliosoma, Succinea). While reports exist that during their first five to six years, box turtles are primarily carnivorous, while adults are mostly herbivorous, there is no scientific basis for such a difference.
Distribution and habitat
Box turtles are native to North America. The widest distributed species is the common box turtle which is found in the United States (subspecies carolina, major, bauri, triunguis; South-Central, Eastern and South Eastern parts) and Mexico (subspecies yukatana and mexicana; Yucatán peninsula and North Eastern parts). The Ornate box turtle is endemic to the south-central and South Western parts of the U.S. (and adjacent Mexico) while the spotted box turtle is endemic to North-Western Mexico only. The coahuilan box turtle is only found in Cuatro Ciénegas Basin (Coahuila, Mexico).
Because box turtles occupy a wide variety of habitats (which both vary on a day-to-day, season-to-season, but also species-to-species basis), a standard box turtle habitat can not be identified. Mesic woodlands are a habitat where box turtles are generally found. T. ornata is the only species regularly found in grasslands, but its subspecies the desert box turtle is also found in the semidesert with rainfall predominantly in summer. The single location where Coahuilan box turtles are found is a 360 km2 region characterized by marshes, permanent presence of water and several types of cacti.
Prior to hibernation, box turtles tend to move further into the woods, where they dig a chamber for overwintering. Ornate box turtles dig chambers up to 50 centimeters, while Eastern box turtles hibernate at depth of about 10 centimeters. The location for overwintering can be up to 0.5 km from the summer habitat and is often in close proximity to that of the previous year. In more southern locations, turtles are active year-round, as has been observed for T. coahuila and T.c.major'. Others box turtles in higher temperatures are more active (T.c.yukatana) or only active during the wet seasons.
Box turtles appeared "abruptly in the fossil record, essentially in modern form". The absence of strong changes in their morphology might indicate that they are a generalist species. It is therefore complicated to establish how their evolution from other turtles took place. The oldest finds of fossilized box turtles were found in Nebraska (U.S.), date from about 15 million years before present (in the miocene) and resemble the aquatic species T. coahuila most, which indicates that the common ancestor was also an aquatic species. Fossilized specimens of T. ornata and T. carolina were dated circa 5 million years before present and indicated that those main lineages also already diverged within the miocene. The only recognized extinct subspecies (T.c. putnami) dates from the pliocene and was with a carapace length of 30 cm much larger than any other species.
Interaction with humans
As the conservation status is defined for species and not for a genus, differences exist between the different genera. Terrapene coahuila is -as it is endemic only to Coahuila- classified as endangered. While the range reduced with 40% to 360 km2 in the past 40–50 years, the number of species reduced from "well over 10,000" to circa 2,500 in 2002. The most widely distributed species Terrapene carolina is classified as vulnerable, while Terrapene ornata is in a lower category as near threatened. For Terrapene nelsoni not sufficient information is available for classification.
Sniffer dogs have been trained to find and track box turtles as part of conservation efforts.
Box turtles as pets
Most turtle and tortoise societies recommend against box turtles as pets for small children. Box turtles are easily stressed by over-handling and require more care than is generally thought. Box turtles can be injured by dogs and cats so special care must be taken to protect them from household pets. Box turtles require an outdoor enclosure, consistent exposure to the sun and a varied diet. Without these, a turtle's growth can be stunted and its immune system weakened.
A 3-year study in Texas indicated that over 7,000 box turtles were taken from the wild for commercial trade. A similar study in Louisiana found that in a 41-month period, nearly 30,000 box turtles were taken from the wild for resale. Once captured, turtles are often kept in poor conditions where up to half of them die. Those living long enough to be sold may suffer from conditions such as malnutrition, dehydration, and infection.
Indiana and other states have laws against collecting the turtles from the wild. In many states, it is illegal to keep them without a permit. Collecting box turtles from the wild may damage their populations, as these turtles have a low reproduction rate.
Box turtles are official state reptiles of four U.S. states. North Carolina and Tennessee honor the eastern box turtle. Missouri names the three-toed box turtle. Kansas honors the ornate box turtle.
In Pennsylvania, the eastern box turtle made it through one house of the legislature, but failed to win final naming in 2009. In Virginia, bills to honor the eastern box turtle failed in 1999 and then in 2009. In opposition one legislator had asked why Virginia would make an official emblem of an animal that retreats into its shell when frightened. What may have mattered most in Virginia, though, was the creature's close link to neighbor state North Carolina.