Snakes possess a male reproductive organ structurally distinct from the single organ found in mammals. Male snakes are equipped with a pair of organs known as hemipenes (singular: hemipenis). This specialized anatomy is a defining feature of the entire group of reptiles called Squamates, which includes both snakes and lizards.
The Anatomical Reality: Hemipenes
The hemipenes are mirror-image, tubular structures that function as intromittent organs for sperm delivery. When a male snake is not mating, these organs are stored completely inverted, or turned inward, within the base of the tail. They are held in place by a retractor muscle that pulls them back into the body after use.
Each hemipenis features a visible furrow running along its length called the sulcus spermaticus. This groove allows sperm to travel externally from the male’s cloaca into the female’s reproductive tract during copulation. The lack of an internal tube means the organ’s function is solely reproductive, not excretory.
The external appearance of the hemipenes is remarkably varied across different snake species, often displaying intricate morphology. They may be covered in spines, hooks, ridges, or various fleshy lobes, giving each species a distinct “lock-and-key” structure. This ornamentation helps secure the male to the female during mating and prevents successful copulation between different species. Herpetologists frequently use the morphology of the hemipenes as a tool for classifying and identifying snake species.
Location and Function in Reproduction
The pair of hemipenes is located in the tail section of the male, just behind the vent, which is the external opening to the cloaca. When the snake is ready to mate, the hemipenes are everted, or turned inside out, through the vent by an influx of blood and lymph fluid. This hydraulic action causes the inverted organs to become functional.
During a mating session, the male snake typically inserts only one of the two hemipenes into the female’s cloaca. The two organs are functionally independent, with each one connected to a separate testis. Using only one organ at a time allows the male to alternate between the two during multiple mating events, ensuring a fresh supply of sperm. Their function is strictly to transfer sperm to the female for fertilization.
Why They Look So Different From Mammals
The fundamental difference between snake hemipenes and the mammalian penis lies in their developmental origin and function. Mammalian penises serve a dual purpose, acting as a conduit for both urine and sperm. In contrast, snake hemipenes are reserved exclusively for reproduction, with waste elimination occurring separately through the cloaca.
This functional divergence is traced back to differences in embryonic development. In mammals, the external genitalia develop from a single tissue mass called the genital tubercle. Hemipenes, however, arise from paired swellings originating from the same embryonic tissue that forms the hind limbs in other reptiles.
The location of the embryonic cloaca dictates which tissue receives the molecular signals to form genitalia. Because the cloaca in snakes and lizards is positioned closer to the prospective hind-limb tissue, it recruits those cells to form the paired hemipenes. This distinct developmental pathway results in the inverted, paired organs, which are structurally unique from the single, multi-purpose genital organ of mammals.