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 X 0.6 in (2.3 X 1.4 cm) in the stinkpot (Sternotherus odoratus) to 1.7 X 1.0 in (4.4 X 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.
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