Snakes do not possess a single reproductive organ like mammals, but rather a pair of organs known as hemipenes. These twin structures are the intromittent organs of male squamates, the order that includes all snakes and lizards. They are used for internal fertilization and, when not in use, are typically hidden from view, inverted and retracted within the base of the male’s tail.
The Anatomy of Hemipenes
The hemipenes are stored internally in invaginations, or pockets, located just behind the cloacal opening at the base of the tail. Each hemipenis is a sac-like, muscular structure that is normally inverted, similar to a glove finger pushed inward. For copulation, the organ is deployed by eversion, turning inside out to expose its functional surface.
The morphology of these organs is remarkably diverse across species, often featuring elaborate ornamentation that can include spines, hooks, ridges, or calyces (cup-like structures). These species-specific patterns are thought to function in a “lock-and-key” mechanism, ensuring reproductive compatibility only between males and females of the same species. A defining feature of each hemipenis is the sulcus spermaticus, an open, external groove that runs along its length.
The sulcus spermaticus is the channel through which sperm is transported from the male’s cloaca into the female’s reproductive tract. This design differs significantly from the mammalian penis, where sperm travels internally through the urethra. Crucially, the hemipenes are solely dedicated to the transfer of sperm and are not involved in the excretion of waste products.
Deployment and Use in Reproduction
During mating, only one hemipenis is deployed and inserted into the female’s cloaca. The male can choose to use either the left or the right organ, often alternating usage between subsequent copulations. This ability to alternate usage may offer a reproductive advantage, potentially allowing the male to engage in more matings over a shorter period.
Deployment involves the hemipenis turning inside out through the cloacal opening, driven by engorgement with blood and lymph fluid, a process called eversion. The spines or hooks present on the surface of the everted organ are thought to help anchor the male securely within the female during the mating ritual. This secure fit is necessary for successful sperm transfer, especially given the various contortions involved in snake copulation.
Once copulation is complete, the organ is pulled back inside the tail base by a specific set of retractor muscles. These muscles are connected to the base of the hemipenis and are responsible for inverting and withdrawing the organ back into its protective pocket.
Why Two: A Comparative Look at Reptile Anatomy
The presence of paired reproductive organs is a defining trait of the entire reptilian order Squamata, encompassing all snakes and lizards. This anatomy stands in contrast to other reptile groups, such as turtles and crocodilians, which possess a single, centrally located phallus. The evolutionary reason for this anatomical divergence can be traced back to embryonic development.
In mammals, turtles, and crocodiles, the single phallus develops from paired genital swellings that eventually fuse together at the midline. Squamate hemipenes, however, arise from separate embryonic cells that are closer to the developing hind limbs of the embryo. These twin buds remain separate, leading to two independent organs rather than one fused structure.
The dual structure offers functional benefits, such as redundancy, ensuring that a male can still reproduce if one organ is damaged or depleted of viable sperm. Furthermore, having two separate organs allows for flexibility in mating position, as the male can approach the female from either side and still achieve insemination.