American mud and musk turtles

(Kinosternidae)

Class Reptilia

Order Testudines

Suborder Cryptodira

Family Kinosternidae

Thumbnail description
Small- to medium-sized turtles with a reduced or hinged plastron that are capable of producing a foul-smelling musk

Size
4–15 in (10–38 cm)

Number of genera, species
4 genera; 25 species

Habitat
Freshwater ponds, rivers, or marshes

Conservation status
Vulnerable: 4 species

Distribution
North and South America

Evolution and systematics

The family is most closely related to the Dermatemydidae. Two monophyletic subfamilies are recognized: the Kinosterninae (including the living genera Kinosternon and Sternotherus) and the Staurotypinae (including the living genera Staurotypus and Claudius). Fossils of each subfamily are known from as early as the Eocene.

Physical characteristics

These turtles are generally small (usually less than 8 in [20 cm]) with oblong, moderately domed shells. The Mexican giant musk turtle (Staurotypus triporcatus) is the largest species, reaching a shell length of 15 in (38 cm). The plastron has one (Staurotypus, Sternotherus, some Kinosternon), two (most Kinosternon), or no hinges (Claudius), and is generally reduced (with 11 or fewer epidermal scutes), although some Kinosternon have a plastron extensive enough to close the shell opening completely. The kinosternine plastron lacks the entoplastral bone found in staurotypines. All members produce a pungent musk from glands located in front of and behind the bridge area between the plastron and the carapace. The head of some species may be greatly enlarged (an advantage for mollusk feeding), and all have sensory barbels on the chin.

Distribution

Eastern and southern North America to Argentina, from sea level to 8,500 ft (2,600 m).

Habitat

Within their ranges, these turtles can be found in nearly any freshwater aquatic system. Because of their relatively poor swimming ability, they prefer slow-moving or still waters (e.g., permanent ponds with lush vegetation or backwaters of lotic systems). However, some species inhabit highly seasonal ephemeral ponds which may only contain water for a few months of each year.

Behavior

In seasonal environments (high latitude or deserts), these turtles have short annual activity periods (three months or less in some cases) and spend the rest of the year hibernating or estivating underground. In wetter, more tropical habitats, they are active year-round. Most are highly aquatic, rarely leaving the water except to nest, although a few species may spend considerable time foraging terrestrially (especially during the wet seasons). Some species are active primarily

during the day, others primarily at night, and still others may be found active at any time.

Feeding ecology and diet

These turtles are all primarily carnivorous, feeding mainly on mollusks (snails and clams), crustaceans, insects, annelids, and even fish (usually as fresh carrion). Some are highly specialized mollusk feeders and eat little else. However, some

species, especially those with extensive terrestrial habits and those in highly seasonal ponds, include plant material (primarily seeds) in their diets.

Reproductive biology

In most species the males are larger than the females, have a long muscular tail with an epidermal "nail" at the end, and have rough patches of scales (clasping organs) on the back of their hind legs. As a result, male courtship is forceful and not very elaborate; females may select mates based on the male's ability to subdue or restrain her movements. All mating occurs in the water. Females lay one to 12 eggs (usually three to six) in each clutch and the clutch size tends to be greater in larger turtles. Many species lay multiple clutches (up to six per year). Eggs are oblong, brittle shelled, and range in size from 0.9 × 0.6 in (2.3 × 1.4 cm) in the stinkpot (Sternotherus odoratus) to 1.7 × 1.0 in (4.4 × 2.6 cm) in the Mexican giant musk turtle. In some species the nests are poorly constructed and the eggs are simply laid among leaf litter; however, others dig more typical flask-shaped nests, and still others dig a body pit or bury themselves completely below the ground before digging a nest chamber.

Incubation is generally quite long (e.g., 75 days to a year) and the embryos of some species exhibit diapause during early development or estivation later in development, presumably as adaptations to avoid inhospitable times of the year for a hatchling turtle. Staurotypus is unique in this family in having heteromorphic sex chromosomes (as in humans). All others that have been studied exhibit temperature-dependent sex determination, with warm temperatures producing females, intermediate temperatures producing mostly males, and still cooler temperatures again producing mostly females.

Conservation status

Most kinosternid species are common, reaching amazing population densities (as high as 1,200 per 2.5 acres [1 ha]).

However, two tropical, one subtropical, and one temperate species are listed as Vulnerable on the IUCN Red List. The two tropical species (K. dunni and K. angustipons) are lowland forms with very restricted ranges and hence are probably affected most negatively by habitat destruction. The subtropical species (K. sonoriense) lives primarily in permanent water systems in the deserts of the U.S. Southwest; human competition for water resources has eliminated most of the habitat for this species. The temperate species (Sternotherus depressus) also has a restricted distribution in the permanent streams of north-central Alabama; habitat destruction associated with coal mining and forest clear-cutting seems to have caused the declines in this species.

Significance to humans

Most species of mud and musk turtles have no significance to humans, except as aquarium pets. A few tropical species may be eaten locally by indigenous people, but not at levels high enough to eliminate populations.

Species accounts

List of Species

Yellow mud turtle

Kinosternon flavescens

subfamily

Kinosterninae

taxonomy

Platythyra flavescens Agassiz, 1857, Rio Blanco, near San Antonio, Texas.

other common names

English: Illinois mud turtle; Spanish: Tortuga-pecho quebrado amarilla.

physical characteristics

This is a small kinosternid turtle (maximum shell length 6 in [16 cm]) with a yellowish chin and throat and a low carapace, with the ninth marginal scute raised distinctly higher than the eighth but equal to the height of the tenth. The plastron has two hinges and is not greatly reduced and the pectoral scutes are triangular in shape and are only narrowly in contact.

distribution

Ranges nearly continuously from southern Nebraska to southern New Mexico and to northeastern Mexico, with relict populations known in northwestern Nebraska, Iowa, Illinois, Missouri, Kansas, and east Texas.

habitat

An inhabitant of grassland habitats, preferring still water, this turtle also is found in permanent to very temporary pools, even those created by humans (e.g., ditches and cattle ponds). It is only rarely found in streams or rivers, and then only in backwaters or cutoffs.

behavior

This turtle spends the majority of each year inactive, buried underground (hibernating or estivating). Activity is stimulated by warm rains (and filling ponds); these turtles will migrate considerable distances from emergence sites to bodies of water, and hence are most often seen moving terrestrially.

The activity season for some populations is as short as for any known turtle—as little as two months in very dry years. As ponds dry up these turtles again bury terrestrially and estivate and/or hibernate until the next warm rainy season. Yellow mud turtles are also frequently seen basking at the edge of the water, even in subtropical locations.

feeding ecology and diet

This species is decidedly carnivorous, and plant matter found in their stomach was probably only accidentally ingested when foraging for small animals such as snails, clams, insects, crustaceans, earthworms, tadpoles, and fish. They often forage on carrion.

reproductive biology

Yellow mud turtles mate rapaciously when they emerge from dormancy and reach the water. Warm rainfall escalates courtship behavior; in captivity males can be induced to court females simply by changing the water in their containers. Females leave the water and may migrate considerable distances to nest in May and June. In Nebraska, gravid females bury themselves completely underground 6–10 in (15–25 cm) and deposit their eggs while buried at these depths. Females remain buried for variable periods once the eggs are laid. Some females may dig out and return to the water the next day after nesting, but some apparently remain in estivation with the eggs through the summer and then dig deeper in the autumn to hibernate for the winter. The eggs are small and elliptical (0.9–1.2 × 0.6–0.7 in [23–31 × 14–18 mm]) with white, brittle eggshells. The clutch size ranges from one to nine (typically four to six), with larger females producing larger clutches. A maximum of a single clutch is produced per year in most populations, and some females do not nest every year. However, two clutches per year may be produced in southern populations. Eggs hatch after 90 to 118 days; at least in Nebraska, hatchlings then dig straight down as much as 3 ft (1 m) below the nest to avoid freezing temperatures during the winter. They dig back out and head for the water during the following spring.

conservation status

Not threatened. This species has a wide distribution and exhibits high densities in many populations, and so is not in need of protection over most of its range. However, most relict populations in northwestern Nebraska, southeastern Iowa, western Illinois, and northeastern Missouri are small, vulnerable to extirpation, and hence in great need of protection.

significance to humans

Exploited only minimally by humans for the pet trade.

White-lipped mud turtle

Kinosternon leucostomum

subfamily

Kinosterninae

taxonomy

Cinosternum leucostomum Duméril and Bibron, 1851, Rio Usumacinta, El Peten, Guatamela.

other common names

Spanish: Chachanya, pochitoque.

physical characteristics

This is a medium-sized kinosternid turtle (maximum shell length 8 in [20 cm]), with a smooth carapace (sometimes with a weak medial keel), a large plastron with two hinges capable of fully closing the carapacial opening, a raised eleventh marginal scute, the axillary and inguinal scutes on the bridge not in contact, and usually with a broad yellowish lateral head stripe extending from the orbit to the neck.

distribution

Ranges from central Veracruz, Mexico, southward in Atlantic drainages to Nicaragua, and then in both Atlantic and Pacific drainages southward to Colombia, Ecuador, and extreme northwestern Peru.

habitat

It inhabits nearly any freshwater aquatic habitat except fast-flowing rivers and streams. It prefers still waters, but also wanders extensively on land in some populations.

behavior

This turtle may be active year-round and is primarily nocturnal. If water levels recede, it often leaves the water and estivates terrestrially under vegetation for up to 80 days.

feeding ecology and diet

This mud turtle is omnivorous, eating mollusks, insects, worms, and carrion, but also seeds, fruits, leaves, and stems of plants. It is not known whether they feed out of the water.

reproductive biology

Courtship and mating have not been described. Although nesting may occur in any month of the year, it appears to be concentrated in August to September and February to March in Mexico. Females produce multiple clutches during the year and lay their eggs (usually at night) in shallow nests or under leaf litter. The eggs are relatively large, elongate (1.3–1.5 × 0.6–0.7 in [34–37 × 16–19 mm]), and have brittle shells. The clutch size ranges from one to five, with larger clutches being produced by larger females. The eggs hatch after 90 to 265 days, and embryos exhibit diapause early in development or estivation late in development.

conservation status

Not threatened. Very little is known about the status of this turtle in the field. However, because of its extensive distribution across huge tracts of undisturbed or minimally disturbed habitats, it is not currently in need of protection.

significance to humans

This turtle is occasionally eaten by humans and can sometimes be seen for sale in food markets within its range. Small numbers are also exported to the pet trade.

Stinkpot

Sternotherus odoratus

subfamily

Kinosterninae

taxonomy

Testudo odorata Latrielle, 1801, Charleston, South Carolina.

other common names

English: Common musk turtle; French: Sternothère odorant; German: Gewöhnliche Moschusschildkröte.

physical characteristics

This is a small kinosternid turtle (maximum shell length 5.4 in [13.7 cm]) with two light stripes on each side of the head (sometimes observed in old individuals) and with barbels on the chin and the throat. The plastron is reduced, has 11 epidermal scutes, a single weakly movable anterior hinge, and a pectoral scute with a quadrangular shape.

distribution

Ranges in North America from New England and southern Ontario to Wisconsin and south through Texas and Florida.

habitat

This turtle inhabits nearly any permanent body of fresh water, but prefers ponds or lakes with muddy bottoms and extensive submergent vegetation.

behavior

It is most commonly observed foraging along the bottom, but is sometimes seen basking as high as 7 ft (2 m) above the water on the boles of trees in wooded (and hence shaded) aquatic habitats. It rarely leaves the water except to bask or nest. It hibernates at high latitudes, but is active year-round in the south.

feeding ecology and diet

These turtles prefer animal food (e.g., earthworms, snails, clams, crayfish, crabs, insects, tadpoles, fish, and fish eggs) and even scavenge on dead animals. However, they also often feed on algae and aquatic plants (particularly seeds). They have perhaps the most generalized diet of all kinosternids.

reproductive biology

Courtship and mating apparently can occur any time the turtles are active, with peaks in the spring and autumn. Nesting occurs in the spring, earlier and longer in the south (February to July) than in the north (May to July). Some females simply drop their eggs in leaf litter, whereas others dig well-formed nests up to 4 in (10 cm) deep. The eggs are very small and elliptical (0.9–1.2 × 0.5–0.7 in [22–31 × 13–17 mm]) with white, porcelain-like eggshells. The clutch size ranges from one to nine (typically two to five) and tends to increase with the female's body size. As many as four clutches may be laid each year in the south, but one or two is the norm farther north. Hatchlings emerge in the summer or autumn after about 65 to 85 days of incubation and move directly to the water.

conservation status

Not threatened. This species is so widespread and reaches such high densities that human impact has mostly been via habitat loss (e.g., draining swamps or ponds).

significance to humans

Exploited by humans only minimally for the pet trade.

Resources

Periodicals

Edmonds, Jonathan H., and Ronald J. Brooks. "Demography, Sex Ratio, and Sexual Size Dimorphism in a Northern Population of Common Musk Turtles (Sternotherus odoratus)." Canadian Journal of Zoology 74, no. 5 (1996): 918–925.

Iverson, John B. "Life History and Demography of the Yellow Mud Turtle, Kinosternon flavescens." Herpetologica 47, no. 4 (1991): 373–395.

——. "Molecules, Morphology, and Mud Turtle Phylogenetics (Family Kinosternidae)." Chelonian Conservation and Biology 3, no. 1 (1998): 113–117.

Morales-Verdeja, S. A., and R. C. Vogt. "Terrestrial Movements in Relation to Aestivation and the Annual Reproductive Cycle of Kinosternon leucostomum." Copeia (1997): 123–130.

Van Loben Sels, Richard C., Justin D. Congdon, and Josiah T. Austin. "Life History and Ecology of the Sonoran Mud Turtle (Kinosternon sonoriense) in Southeastern Arizona: A Preliminary Report." Chelonian Conservation and Biology 2, no. 3 (1997): 338–344.

Vogt, R. C., and O. Flores-Villela. "Effects of Incubation Temperature on Sex Determination in a Community of Neotropical Freshwater Turtles in Southern Mexico." Herpetologica 48, no. 3 (1992): 265–270.

John B. Iverson, PhD