All reptiles practice internal fertilization. In all modern forms except the tuatara the male has an organ kept turned outside in, in the base of the tail, and everted through the opening of the cloaca during erection. In the tuatara the transfer of sperm is accomplished by bringing the genital openings into contact, as in birds. This was probably the method used by the ancestral reptiles – it is clear, in any case, that the penis had separate origin in turtles, crocodilians and mammals on the one hand, and in lizards and snakes on the other.
Thus, male lizards and snakes have not just one, but a pair of hollow structures called hemipenes, which make up their copulatory organs. Located as they are in the tail just behind the opening of the cloaca, the hemipenes often give the tail of the male a thicker, more gradually tapering contour than that of the female, and in many species the sexes can be distinguished by this difference. A groove that serves as a channel for the sperm extends from the opening of the sperm ducts along the inner wall (which is the outer wall during erection) of each hemipenis, and the surface may be pleated or set with spines that keep it in place on the oviduct of the female during mating. Either one of the hemipenes may be used, but only one, the one nearest to the female, is everted and protruded from the cloaca during erection, which is brought about by a combination of muscular action and distension of the walls with blood.
Among different reptiles fertilization is scheduled differently with respect to the time of nesting. In most species it seems to occur, as might be expected, just before the eggs are laid; but in some the sperm may live on in the reproductive tract of the female and continue to fertilize eggs months or even years after copulation has taken place. The longest known periods of such deferment of fertilization are four years for the diamondback terrapin of the southern United States, and five years in the case of the tropical American cat-eye snake. The green turtle, which evidently mates only in the sea off the nesting beach, often does so after the female has gone ashore and laid her eggs.
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