How a Hemipenis Turns Inside Out
A male snake or lizard’s paired reproductive organs, called hemipenes, are tucked into pockets near the base of the tail. Each is normally inverted—folded inward, much like a finger of a glove pushed inside itself. During mating, tissue pressure and muscular action turn one outward through the cloaca, the shared opening used for reproduction and waste.
The exposed organ can enter the female’s cloaca. A groove along it helps guide sperm, while the organ’s shape and surface structures vary among species. The other hemipenis remains inside; the pair are not used together as a double organ.
Why Having a Pair Matters in Practice
A snake or lizard can use either side, though it typically uses only one during a mating. That paired arrangement gives the animal two organs positioned on opposite sides of the tail, rather than a single one centered beneath it. Which side is used depends on how the animals’ bodies meet and align.
The organs are hidden when not in use, so the animal’s external appearance offers little clue to their structure. Their distinctive mechanics become apparent only during mating: one organ unfolds, makes contact, and later retracts, while its counterpart stays stored.**
