Kinosternon subrubrum

(Lacepède, 1788)
Common mud turtle

This is a small (to 12.5 cm) turtle with an oval, smooth, unkeeled (in adults) and often depressed carapace. Its sides are straight, the posterior marginals unserrated, and the carapace drops abruptly behind. Hatchlings have three longitudinal keels. The 1st vertebral scute is long and widely separated from the 2nd marginal scutes. Vertebrals 2-5 are usually broader than long, and the 5th may be expanded. The 10th marginal scute is elevated above the rest. The patternless carapace varies from yellowish brown to olive or black. The immaculate yellow to brown plastron has a single gular scute and two well-marked transverse hinges (which may ossify in old individuals). Its forelobe is longer than the hindlobe in two of the three subspecies; there is a posterior notch. The gular scute is short, and much less than half the forelobe length. The plastral formula is: an > abd > hum > fem >< gul > pect. Axillary and inguinal scutes meet on the bridge. The head is medium sized with a slightly protruding snout and a hooked upper jaw. Its rostral scute may or may not be posteriorly furcated. The head is usually brown with some yellow mottling; sometimes there are two light lines on each side of the head and neck. Skin is brown to olive or grayish and may exhibit some markings. The tail is spine tipped.
The full diploid set of chromosomes is 56. Killebrew (1975b), using a squash technique, reported 26 macrochromosomes and 30 microchromosomes, but Sites et al. (1979b), using a banding technique, found 24 macrochromosomes (6 pairs of metacentric or submetacentric chromosomes and 6 pairs of telocentric or subtelocentric chromosomes) and 32 microchromosomes.
Males have longer, thicker tails, and a patch of roughened scales on the posterior surface of each thigh and crus.

Kinosternon subrubrum ranges from Connecticut and Long Island south to the Gulf Coast, west to east-central Texas, and north to Missouri, southern Illinois, and southern Indiana. Isolated colonies exist in northwestern Indiana and in west-central Missouri.

Geographic Variation
Three subspecies are recognized. Kinosternon subrubrum subrubrum (Lacepède, 1788), the eastern mud turtle, ranges from southwestern Connecticut and Long Island to the Gulf Coast and northwest through Kentucky to southern Indiana and Illinois. This subspecies has a wide bridge, a spotted or mottled head, and the anterior lobe of the plastron shorter than the posterior lobe. K. s. hippocrepis Gray, 1856, the Mississippi mud turtle, ranges in the Mississippi Valley from Louisiana and eastern Texas northward to Missouri and western Kentucky. It has a wide bridge, two distinct light lines on each side of the head, and the anterior lobe of the plastron shorter than the posterior lobe. The Florida mud turtle, K. s. steindachneri (Siebenrock, 1906c), is restricted to peninsular Florida. It has a narrow bridge, a plain or mottled head, and the anterior lobe of the plastron often longer than the posterior lobe. See Ernst et al. (1974) for a more detailed analysis of variation.
During a morphometric comparison of K. subrubrum and K. baurii, Lovich and Lamb (1995) found a striking resemblance between K. s. hippocrepis and K. baurii with respect to head striping, and a discriminant function analysis of a limited number of K. s. hippocrepis classified them morphometrically as K. baurii. These results raise the question of the true evolutionary relationship of K. s. hippocrepis. A detailed cladistic analysis of K. subrubrum and K. baurii from throughout the combined ranges of both employing both molecular and morphological characters will be required to determine if the similarities between K. s. hippocrepis and K. baurii reflect phylogenetic closeness or merely convergence. Such a study could also have an important bearing on the status of the subspecies of K. subrubrum.

This mud turtle prefers slow-moving bodies of shallow water with soft bottoms and abundant aquatic vegetation. Frequently it inhabits the lodges of muskrats. We have found it in ditches, sloughs, wet meadows, ponds, marshes, bayous, lagoons, and cypress swamps. Kinosternon subrubrum shows a marked tolerance for brackish water and is often abundant in salt marshes.

Natural History
Males mature at a carapace length of 8-12 cm in 4-7 years (Mahmoud, 1967). Females reach maturity at a carapace length of 7-12 cm in 3-4 to 8 years (Mahmoud, 1967; Iverson, 1979d; Ernst et al., 1973; Gibbons, 1983; Frazer et al., 1991). The male and female reproductive cycles are similar to those described for the stinkpot, Sternotherus odoratus (Mahmoud and Klicka, 1972; Iverson, 1979d, Houseal and Carr, 1983).
Mating occurs from mid-March through May; copulations are earliest in the south. Courtship and mating are as in Sternotherus odoratus (which see). Mating usually takes place under water but sometimes occurs on land.
Most nesting occurs during June but has been observed from February through September in various parts of the range. The nesting site usually is open ground not far from water. Sandy, loamy soils are preferred, but piles of vegetable debris also are used. In some localities mud turtles often nest in muskrat tunnels. Eggs have been found on the surface of the ground and under piles of boards, and, in the south, alligator nests are often used. The completed nest usually is a semicircular cavity 75-125 mm deep and entering the ground at about a 30° angle.
Clutches vary from one to nine eggs; normally two to five, and most commonly three to five eggs are laid. At least three clutches are laid annually. The eggs are elliptical, pinkish white or bluish white, and brittle shelled, 22-29 mm long and 13-18 mm wide. Hatching occurs after about 100 days.
The hatchling carapace (20-27 mm) is shaped like that of the adult but has a vertebral keel and two weak dorsolateral keels, is rough, and is not depressed anteriorly or sharply turned down posteriorly. It is dark brown or black, with light spots along the marginals. The plastron is irregularly mottled with orange or red, and the hinges are poorly developed. Hatchling skin is brown or black, and in the subspecies K. s. hippocrepis there may be two faint yellow lines on each side of the head and neck.
Kinosternon subrubrum is an omnivorous feeder. Mahmoud (1968) reported the following percentages of frequency and volume, respectively, of food items: Insecta 98.3, 30.4; Crustacea 15.0, 1.4; Mollusca 93.1, 31.8; Amphibia 30.0, 2.2; carrion 68.6, 11.9; and aquatic vegetation 89.6, 22.3. Mud turtles under 5 cm in carapace length feed predominantly on small aquatic insects, algae, and carrion; those above 5 cm feed on almost any kind of food. Captives feed readily on canned and fresh fish, canned beef, hamburger, dog food, snails, insects, tomatoes, and watermelon. K. subrubrum occasionally feeds at the surface but is predominantly a bottom feeder, frequently walking along the bottom, probing into soft mud, sand, and decaying vegetation.
Kinosternon subrubrum is quite terrestrial. It leaves the water in early summer, forages on land for a short period, and then burrows to aestivate during the hot weather. Many remain underground until the next spring.

IUCN Red List Status (1996)
Not listed.