Schmidt and Owens, 1944
Coahuilan box turtle
Recognition
This uniformly colored species is the only truly aquatic American box turtle. Its elongated (to 16.8 cm), narrow carapace is domed, but medially flattened (depth usually less than 40% of the length) with, at best, only vestiges of a medial keel. Vertebral 1 is not straight sided, but instead wedge shaped; 2-4 are very flat dorsally, and all five are broader than long. There may be a hump on the 5th. Posterior marginals are not serrated. The carapace is brown to olive and lacks any pattern. The plastron is large and well-developed with no posterior notch on the anal scutes. The plastral formula is: an > abd > gul > pect > hum > fem. Smith and Smith (1979) found that the interpectoral seam length averages 30% and the interhumeral seam length 20% of the length of the anterior plastral lobe. The short interfemoral seam averages only 11% of the posterior plastral lobe length. A large axillary scute is usually present on each bridge. The plastron is yellow to olive with dark seams and some irregular dark flecking on each scute. The skull contains a heavy postorbital bar. The grayish brown to olive head is large with a strongly hooked and notched upper jaw. There may be some dark mottling on the head. Limbs, neck, and tail are also grayish brown to olive. There are five foretoes and four hind toes, all with little webbing. None of the hind toes is capable of medial rotation. T. coahuila is the only Terrapene with cloacal bursae.
Diploid chromosomes total 50 (Killebrew, 1977a; Bickham and Carr, 1983).
The plastron of the male is concave, that of the female convex or flat. The male iris is brownish and flecked with yellow, that of the female is yellowish and flecked with brown. The female carapace is slightly higher than that of the male, and she has a shorter, less thick tail.
Distribution
Terrapene coahuila is restricted to the intermontane Cuatro Ciénegas basin of Coahuila, Mexico.
Habitat
This aquatic turtle occurs in shallow waters with soft bottoms and abundant sedges, waterlilies (Nymphea), and reeds (Phragmites). It usually is found in water of slow current and has been taken from streams, ponds, and marshes.
Natural History
Captive males at the Jersey Wildlife Preserve first mated when almost five years old. Mating in the wild has been observed in the fall (September and November), winter (December), and spring (March through May) (Brown, 1974). During courtship the male pursues the female with his head extended and frequently bumps her shell with his carapace. Copulation may occur in or out of the water, with the male gripping the female with his claws and at times biting her on the neck and head (Brown, 1974; Tonge, 1987).
Ovulation occurs in April or May (Brown, 1974), with nesting extending from May to September. Two or three clutches may be laid a season (Brown, 1974; Tonge, 1987). The elongated to oval (28.5-33.0 x 17.0-18.0 mm), white to cream colored eggs have smooth, finely granulated shells, and clutches include 1-8 (usually 2-3) eggs (Brown, 1974; Tonge, 1987). Hatching occurs in late summer or early fall.
Hatchlings are more brightly colored than adults with yellow and black radiations or spots on the carapace and a yellow postorbital stripe extending backward across the tympanum; the carapace length is 33-36 mm (Brown, 1974; Tonge, 1987).
T. coahuila is omnivorous and can feed on land as well as in water. In the wild it feeds on Chara and Eleocharis, and various insects, crustaceans, snails, and small fishes. In captivity it accepts a variety of foods such as newborn mice, earthworms, chopped heart, cuttlebone fish, insects, lettuce and fruits (Brown, 1974; Tonge, 1987; Carl H. Ernst, pers. obs.). T. coahuila is an active forager, hunting out its prey, but it probably also scavenges.
Although this species is mostly diurnal, activity may continue into the early night (Tonge, 1987). Brown (1974) estimated the home range of T. coahuila to be only 25.6 m in diameter, with population densities of 133-156 adults per hectare of marsh. The turtles spend much of their time buried in the mud bottom or up under overhanging grasses. Although active all year, cooler winter temperatures may temporarily curtail activities.
While other species of box turtles readily float in water and have difficulty submerging or cannot submerge, T. coahuila is capable both of submerging with ease and remaining under water for considerable periods. It is interesting, in this respect, that Williams and Han (1964) compared the densities of T. coahuila and T. carolina and found T. coahuila to be much more dense (0.95 and 0.96 g/cm3 for T. coahuila versus 0.73 and 0.83 g/cm3 for T. carolina). Also, their cloacal bursae may play a role in removal of excess water after diving. Milstead (1967) speculated that T. coahuila evolved in response to increasing rigors of terrestrial life as the Cuatro Ciénegas basin, in which it was trapped, became increasingly more arid.
IUCN Red List Status (1996)
Endangered (A1acd, B1+2cd). T. coahuila is endangered due to drainage and development in and near its shrinking habitat. In recent years this species was also exploited for the pet trade.