Sternotherus odoratus

(Latreille, in Sonnini and Latreille, 1802)
Common musk turtle

The deep carapace (to 13.6 cm) is highly arched, elongated and narrow. Younger individuals may have a prominent middorsal keel, but this is lost in adults; two low dorsolateral keels are also present on hatchlings and small juveniles. The posterior carapacial rim is unserrated, and the vertebral scutes do not overlap. The 1st vertebral is long and never touches the 2nd marginals. The other four vertebral scutes are usually broader than long, and the 5th is expanded posteriorly. In adults, the carapace is plain gray-brown to black, but juveniles have a pattern of scattered spots or radiating dark streaks. The unmarked plastron ranges in color from yellow to brown. There is only a single gular scute present, and an indistinct hinge lies between the pectoral and abdominal scutes. Posteriorly, there is a shallow anal notch. The plastral formula is: abd > an > pect >< fem >< hum > gul. The head is moderately elongated with a protruding snout and a nonhooked upper jaw. A posteriorly furcate rostral shield is present, and there are one or two pairs of chin barbels. Barbels also occur on the throat. Skin is gray to black, and the sides of the head and neck usually have a pair of conspicuous yellow or white stripes, which begin on the snout and extend backward, passing above and below the eye; these stripes may be faded or broken, and absent in some Florida specimens.
The diploid chromosome number is 56: 26 macrochromosomes (14 metacentric, 8 submetacentric, 4 telocentric) and 30 microchromosomes (Stock, 1972; Killebrew, 1975b).
Males have longer, thicker, spine-tipped tails with the vent behind the carapacial rim, and roughened patches of scales on their thighs and crura.

Sternotherus odoratus ranges from New England and southern Ontario south to Florida and west to Wisconsin and central Texas. There are also scattered records from south-central Kansas and western Texas; a record from Chihuahua, Mexico is considered invalid.

Geographic Variation
No subspecies are recognized, but differences exist between populations in maximum carapace length, the degree of carapacial melanism, and the size of the head and development of the jaws. Florida S. odoratus are shorter and much darker than those from more northern localities, and certain populations is which mollusks are the chief prey have larger heads and broader crushing jaw surfaces. Seidel et al. (1981) reported high overall heterozygosity in S. odoratus, but found little difference between populations. Similarly, Reynolds and Seidel (1983) found much intrapopulational variation in 19 morphological characters, but relatively little divergence among populations. Walker et al. (1997), however, found pronounced and hierarchical mtDNA geographic variation, ranging from genetically shallow differences among local populations to genetically deep distinctions among regional assemblages of haplotypes. This pronounced geographic population structure effectively overturns prior hypotheses that extensive dispersal and gene flow account for the morphometric similarities among geographic populations.

This musk turtle occurs in almost any waterway with a slow current and soft bottom: rivers, streams, lakes, ponds, sloughs, canals, swamps, bayous, and oxbows. It may, however, occasionally occur in almost any sort of stream; for example, we have taken them in a gravel-bottomed, fast-flowing stream in northwestern Arkansas. Tinkle (1959) showed that the fall line usually limits the distribution of this species; it is found above it only in the rivers draining into the Gulf of Mexico.

Natural History
Although, males may begin to develop sexually dimorphic characters when 5.1 cm carapace length in their second year (Mitchell, 1988), most males do not mature until they are 6-7 cm long in 3-7 years (Risley, 1933; Tinkle, 1961; Mahmoud, 1967). Females mature in 5.7-6.5 cm in 2-11 years (Risley, 1933; Tinkle, 1961; Mahmoud, 1967; Mitchell, 1988). Spermatogenesis begins in stinkpots with multiplication of spermatogonia in May or June. Primary spermatocytes and maturation divisions appear from mid-June into July with the peak in sperm production occurring from mid-July to mid-August. Spermiogenesis begins as early as late July, is in full progress by late August or early September, and is completed by late September or October. Testes are of maximum size and weight, and seminiferous tubules of greatest diameter in late August and early September, followed by an increase in the diameter of the epididymides as sperm enter them. Restoration of germinal epithelium occurs primarily from March to early May, with some recovery in the winter (Risley, 1934, 1938; Mahmoud and Klicka, 1972; McPherson and Marion, 1981a; Mitchell, 1985b, 1988). The female stinkpot cycle begins in mid- to late July after the nesting period has ended. The follicles are of minimum size, but vitellogenesis begins in late July and continues to December, after which further increased is slowed or stopped during the winter. Some additional yolk material may be added to the ova in the spring until they reach maximum size just prior to ovulation (Edgren, 1960; Mahmoud and Klicka, 1972; Iverson, 1977d; McPherson and Marion, 1981b; McPherson et al., 1982; Mitchell, 1985b, 1988). Eggs may be retained in the oviduct 20-35 days while the albumen and shell are formed; Edgren (1960) calculated that the life of the egg in the oviduct is 5-8 weeks.
Courtship and mating occur sporadically throughout the year, with peaks in the spring and fall. Most mating occurs in April and May, before the nesting period. A second period of mating occurs in September and October but may extend into December. There is evidence that sperm from the late matings may be retained through the winter in viable condition in the oviducts. Mating occurs under water, in shallows, at night or in the early morning. Mahmoud (1967) studied the sexual activities of Sternotherus odoratus, S. carinatus, K. subrubrum, and K. flavescens; he has given a most detailed composite account of their courtship and mating. There were three phases: tactile, mounting and intromission, and biting and rubbing. During the tactile phase a male with head extended approached another turtle from behind and felt or smelled its tail, apparently to determine sex. Courtship usually proceeded no further if the approached turtle was a male; if a female, the male, with head still extending forward, moved to her side and nudged the region of her bridge with his nose. This movement apparently was directed at the musk glands there. If the female was not receptive she moved away. The male responded either by giving chase or by going elsewhere. If chasing occurred, the male, with head fully extended, persistently attempted to nudge or bite the female about the head as he followed. The chase was either by walking or swimming and was sometimes followed by mounting, a few seconds later. If initially receptive, the female remained immobile while the male, with head fully extended, gently nudged her just behind the eye and a few seconds later assumed the mounting position. This tactile phase varied in length from a few seconds to three minutes. The mounting phase usually followed the tactile phase. Males approached females either from behind or from the side. The male positioned himself with his plastron directly over the female's carapace by grasping the margins of her carapace with the toes and claws of all four feet. By flexing one knee the male held the female's tail between the two scaly patches on the opposing posterior surfaces of the upper and lower leg throughout coitus. The male's tail was looped so that the terminal nail was in touch with the skin at one side of the female's cloaca; this brought the vents together, and insertion of the penis followed. The male's head extended forward to gently touch the top of the female's head and neck. These actions occurred simultaneously. When the coital position was attained the rubbing-and-biting phase began. The time between mounting and penial insertion was 5-10 sec.
Nesting season varies with latitude: in the south, egg laying lasts from February through July, in the north, from May through August. Nesting takes place from early morning into the night, and nests have been dug as far as 45 m from the water. Some females lay their eggs on the open ground; others dig well-formed nests as deep as 10 cm. Most nests, however, are shallow and are formed by scraping away debris such as decaying vegetable matter, leaf mold, or rotting wood. Many eggs are laid under stumps and fallen logs and in the walls of muskrat houses. Females are noted for sharing nesting sites; often several oviposit at the same place.
Eggs are elliptical (24-32 x 14-17 mm), with a thick, white, brittle shell that appears slightly glazed when dry. One to nine eggs (usually two to five) constitute a clutch, and four clutches may be laid each year. There is a correlation between clutch size and the body size of the female: the largest females lay the most eggs per clutch. Hatchlings emerge in August and September in Florida and in September and October in the north after an incubation period of about 75 or 80 days.
The hatchling carapace (20-22 mm) is rough and has a prominent vertebral keel and two smaller lateral keels. It is black, with a light spot on each marginal. The dark, light-mottled plastron lacks hinges and is rough in texture. The skin is black and the two light stripes on the head are prominent.
Sternotherus odoratus is omnivorous. Those under 5 cm in carapace length feed predominantly on small aquatic insects, algae, and carrion, whereas those over 5 cm feed on any kind of food. They are bottom feeders, often walking about, with head extended, in search of food. They probe soft mud, sand, and decaying vegetation with their heads, apparently looking for food, and prefer slow-moving prey. They are known to eat earthworms, leeches, clams, snails, crabs, crayfish, aquatic insects, fish eggs, minnows, tadpoles, algae, and parts of higher plants.

Sternotherus odoratus seems more closely related to S. carinatus than to S. depressus or S. minor (Seidel et al., 1981).

IUCN Red List Status (1996)
Not listed.