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How do reptiles reproduce through internal fertilization?

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A painted turtle (Chrysemys picta) with a dark shell and red and yellow markings rests on a rock near water, surrounded by green aquatic plants. — reptile Painted turtle (*Chrysemys picta*).

Internal fertilization is a prerequisite for the development of the shelled egg, which allowed reptiles to become fully terrestrial. This process involves the deposition of sperm into the female’s reproductive tract, which occurs through one of two primary methods:12

Cloacal Apposition: Characteristic of the tuatara, this method involves the close alignment of the male and female cloaca, allowing the male to discharge semen directly into the female's reproductive chamber.1
Intromittent Organs: In all other reptiles, males utilize specialized copulatory organs to deliver sperm. Turtles and crocodiles possess a penis, while lizards and snakes use paired organs called hemipenes.12

During copulation, these organs are inserted into the female's cloaca. Sperm then travel from the cloaca into the oviducts to reach the eggs near the ovaries. Notably, many reptiles can store sperm within their oviducts for extended periods—sometimes months or even years—allowing fertilization to occur long after the initial mating event.13