The reproductive biology of most included species is very poorly known. Several species exhibit seasonal changes in the color of the head and soft parts that are apparently related to breeding. The production of multiple clutches per year is typical (at least five in some cases). In general, clutch size is positively related to body size. Many smaller species lay only one or two large eggs per clutch, but some of the larger species may lay up to 35 eggs in a clutch. Incubation times are highly variable, ranging from 60 to 272 days. Those with the longer times experience embryonic diapause,
in which development arrests for various lengths of time early during incubation. Two species are known to have dimorphic sex chromosomes, and therefore genetically determined sex. However, five species also are known to have temperature-dependent sex determination, with warm temperatures producing females and cooler temperatures resulting in males. In one of these five species, still cooler temperatures again produce females. These turtles hybridize readily in captivity, even between distant genera, and wild populations of hybrids may exist.
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