Generally speaking, the males of most species are smaller than the females of the same species. Their vent (cloaca) openings are positioned farther from the margin of the bottom shell (plastron) than those of females. The tail of male water turtles tends to be relatively long and tapered, but thick at the base. The tail of females is generally short and stubby. The males of some water turtles species also have unusually long claws on their front feet. Certain species of water turtles have been successfully bred in captivity. During mating, the male's penis may protrude during sexual excitement and resembles an "opening flower." Inexperienced observers often regard this structure with bewilderment. Copulation takes place when the male inserts this structure into the female’s cloacal opening.
Eggs can be incubated by burying them in 1-2 inches of sand or dirt kept at 70-85 F. Incubators can be rudimentary to elaborate. The eggs should not be disturbed in any way during the incubation period. The eggs usually hatch in 60-140 days (average 80-110 days). The eventual sex of a water turtle may be influenced by egg incubation temperatures. Red-Eared Slider eggs, for example, incubated at 85 F yield primarily female turtles, while those incubated at 75 F yield primarily male turtles. The numbers of each sex tend to be equal when eggs are reared at 80 F. This interesting phenomenon does not occur in all water turtle species. Among other chelonian species (certain tortoises), higher environmental temperatures produce more male offspring. Some scientists speculate that temperature-induced sex determination is the major factor responsible for the demise of dinosaurs. They theorize that a meteor collision produced a massive dust cloud, blocking out much of the sunlight and greatly reducing the environmental temperature. Such cooling may have resulted in drastic changes in sex ratios of dinosaur offspring. Such an imbalance in the numbers of males and females could have, in turn, greatly compromised the dinosaurs' reproductive success. A specialized sharp projection (called an "egg tooth") on the "upper beak" of hatching water turtles aids them in emerging from the egg. Premature hatching may occur from time to time. When this occurs, the yolk sac is conspicuous as it hangs from its attachment to the lower shell. These hatchlings can be saved as long as the yolk sac is kept moist and not injured. The baby can be suspended with the yolk sac gently wrapped in saline-soaked gauze until the material within the sac has been completely absorbed. The hatching will not eat on its own during this period because of the adequate nourishment it receives from the yolk sac material.