The hemipenis is a specialized male copulatory organ that represents a unique evolutionary path in reproductive anatomy. This structure is not a single organ but rather a pair of separate intromittent organs housed within the body. When not in use, the hemipenes are held inverted and protected within a pocket at the base of the tail, near the cloaca. Their unusual nature, often featuring elaborate ornamentation like spines or ridges, distinguishes them from the single, unpaired penile structure found in mammals.
Defining the Hemipenis
The hemipenis is a dual structure, consisting of two distinct organs situated side-by-side in the male’s caudal region. Each hemipenis is essentially a hollow, tubular sac, which remains retracted within a hemipenial pocket until copulation. The structure is characterized by a deep groove on its surface called the sulcus spermaticus, which is the channel for sperm transport.
This groove carries the sperm from the cloaca externally along the surface of the organ into the female’s reproductive tract. This differs markedly from the mammalian penis, where sperm travels internally through a closed tube, the urethra. The outer surface of the hemipenis is frequently covered in species-specific ornamentation, which can include sharp spines, small spicules, or calyces, which are small, cup-like flaps.
These surface textures exhibit remarkable diversity, ranging from smooth forms to deeply bi-lobed structures covered in hooks and rosettes. The specific morphology of these features is thought to function as a “lock-and-key” mechanism, ensuring reproductive isolation between similar species. The ornamentation also plays a role in anchoring the male during copulation, which can be a prolonged process.
The Unique Mechanism of Use
Unlike the mammalian penis, which achieves erection through the engorgement of specialized erectile tissue, the hemipenis deploys through eversion. This process involves the organ literally turning inside out, much like a sock. Deployment is primarily driven by hydraulics, involving the rapid filling of internal lacunar spaces with lymphatic fluid and blood.
The hydraulic pressure forces the soft, inverted sac to fully extend and protrude from the cloaca. Deployment is assisted by propulsor muscles, while a powerful retractor muscle pulls the organ back into its pocket after mating. During copulation, only one of the two hemipenes is inserted into the female’s cloaca at a time. The male often alternates which side is used during subsequent mating events.
The use of a single, alternating organ may allow the unused side to replenish its sperm supply or recover from mechanical stress. External sperm delivery occurs via the sulcus spermaticus. The mechanical advantage provided by the surface spines helps maintain intromission, sometimes for hours.
Which Animals Have This Reproductive Trait?
The presence of the hemipenis is a defining anatomical characteristic of the taxonomic order Squamata, which encompasses all lizards and snakes. This includes geckos, chameleons, iguanas, rattlesnakes, and pythons, all sharing this paired reproductive anatomy. The hemipenis is not found in other major groups of reptiles, such as turtles and tortoises, which possess a single, median penis.
Crocodilians also have a single phallus, meaning the hemipenis is exclusive to the squamate lineage. The evolutionary origin of this dual structure is distinct; the hemipenes develop from the tissue that gives rise to the hind limbs in other vertebrates. This contrasts with the single phallus of mammals, which originates from tail-bud tissue during embryonic development. The hemipenis is an example of divergent evolution, achieving a similar function through a different structural design.