The question of whether a male snake possesses two penises is one of the most common curiosities about reptile anatomy. Unlike many mammals, which have a single external copulatory organ, the reproductive structures in snakes and other squamate reptiles display a surprising complexity. Understanding this difference requires a look into the unique evolutionary adaptations of the serpent world.
The Definitive Answer: Hemipenes
Male snakes and lizards possess two separate copulatory organs, correctly termed hemipenes. Each organ is a distinct structure, not a single organ split into two tips. The term “hemipenes” is derived from the Greek “hemi,” meaning half, and “penes,” the plural of penis. These organs are intromittent, used for internal fertilization, but are not structurally similar to the mammalian penis.
Crucially, hemipenes are eversible structures, meaning they turn inside out during use rather than projecting outward. They do not contain a urethra, and each hemipenis is functionally distinct, receiving sperm from the corresponding testis.
Structure and Storage
When not in use, the hemipenes are held inverted, like a sock turned inside out, within pockets located in the male snake’s tail base, just behind the cloaca. A retractor muscle keeps each hemipenis tucked away. Male snakes often have longer and thicker tails than females because of the space required to store these organs.
The structure of the hemipenes displays vast diversity in shape, size, and ornamentation across species. Many feature distinct patterns of spines, hooks, or ridges. These specialized features act as a “lock-and-key” mechanism, helping to anchor the male firmly during copulation, and are often used by scientists for taxonomic classification.
Mechanism of Use in Reproduction
Despite possessing two organs, a male snake uses only one hemipenis during a single mating event—specifically, the one closest to the female’s cloaca. Copulation begins with the eversion of the chosen hemipenis, a process driven by vascular pressure and muscle action, which flips the organ outward. This projection is non-erectile, differing from the hydraulic swelling seen in a mammalian penis.
Sperm is transported along an external groove called the sulcus spermaticus, which guides the sperm from the male’s cloaca into the female’s reproductive tract. The presence of two separate organs provides a biological advantage, allowing for rapid re-mating. After one hemipenis is used, the male can often use the alternate organ soon after without a recovery period. This ability to alternate maximizes mating opportunities, especially when males are competing for a single female.