Male lizards possess two distinct reproductive organs called hemipenes. These structures are a characteristic feature of squamates, the order of reptiles that includes both lizards and snakes. Unlike the single external organ found in many other animals, hemipenes are typically concealed within the lizard’s body when not in use. They are specialized for sperm transfer during mating.
What Are Hemipenes?
Hemipenes are sac-like, intromittent organs located internally at the base of a male lizard’s tail, often creating subtle bulges. They are not true penises in the mammalian sense, as they lack erectile tissue and do not serve a urinary function. Instead, these organs are everted, or turned inside out, from the cloaca for reproduction.
Their development differs fundamentally from mammalian penises; while mammalian genitalia arise from embryonic tail-bud tissue, squamate hemipenes originate from the same embryonic cells that form the limbs. Each hemipenis is connected to a single testis, ensuring a direct pathway for sperm.
Hemipenes exhibit a wide variety of shapes and sizes across different lizard species, ranging from bilobed to cylindrical forms. Many are adorned with intricate ornamentation such as spines, ridges, hooks, spicules, calyces, or papillae. These structural features play a role in species recognition and copulatory efficiency. A visible groove called the sulcus spermaticus runs along the outside of each hemipenis, serving as the channel for sperm transport.
How Hemipenes Are Used
During copulation, a male lizard uses only one hemipenis at a time, extending it from its inverted position within the tail base. The eversion process is often compared to a sock being turned inside out. The ornamented surfaces, such as spines and ridges, help anchor the male firmly within the female’s cloaca during mating, aiding sperm transfer. This secure fit can contribute to prolonged copulation, increasing the likelihood of successful fertilization.
The presence of two hemipenes offers several advantages in reproduction. One benefit is redundancy, providing a “backup” organ if one hemipenis is injured or if sperm from one side is depleted after a recent mating. This allows the male to alternate between the two organs across different mating instances, potentially enabling more frequent or successive breeding opportunities. Having two separate organs also provides flexibility, allowing the male to approach and mate with a female from either side, which can be advantageous in varying mating postures or environments.
Similar Structures in Other Reptiles
The presence of hemipenes is not exclusive to lizards; snakes, which belong to the same taxonomic order Squamata, also possess these double reproductive organs. This shared anatomical feature reinforces the close evolutionary relationship between lizards and snakes. Hemipenes are a key characteristic that sets squamates apart from other reptile groups.
In contrast, other major reptile groups, such as turtles and crocodiles, possess a single, non-paired copulatory organ known as a phallus. This difference in reproductive anatomy highlights the evolutionary path taken by squamates in developing their hemipenes. While the phallus in turtles and crocodiles serves a similar reproductive function, its singular nature distinguishes it from the double intromittent organs found in lizards and snakes.