Male snakes do have two penises, or more precisely, two independent intromittent organs called hemipenes (singular: hemipenis). These paired structures sit inverted inside the base of the tail and are everted during mating, though a male only uses one at a time. The arrangement is shared across all squamate reptiles, the group that includes snakes and lizards, and it turns out the story behind why they have two, how they use them, and what that means for snake reproduction is far stranger than the basic anatomy suggests.
What Hemipenes Actually Look Like
When retracted, hemipenes are tucked inside pockets at the base of the tail, invisible from the outside. When a male prepares to mate, blood and lymph fill the erectile tissue of one hemipenis, pushing it outward through the cloaca in a process similar to turning a sock inside out. The everted organ is often surprisingly elaborate. In the puff adder, for example, each hemipenis is a bilobed structure covered in backward-pointing spines and hooks, with a groove called the sulcus spermaticus running along the outer surface to channel sperm from the cloaca to the tip of the organ.1Journal of King Abdulaziz University-Science. Macroscopic and Microscopic Anatomy of the Hemipenes of the Snake Bitis arietans arietans That groove typically forks near the end, splitting to reach both lobes.2Microscopy and Microanalysis. Comparative Aspects of the Hemipenial Architecture of the Arabian Horned Viper Cerastes gasperettii: Macroscopic and Microscopic Analysis With Evolutionary Perspectives
Internally, the hemipenis of a rattlesnake consists of two concentric cylinders of erectile tissue. The outer cylinder holds a mesh of blood-filled spaces that expand during erection, controlled by rings and struts of smooth muscle that regulate how much the organ inflates. A large retractor muscle, the retractor penis magnus, originates on one of the tail vertebrae and threads through the inner cylinder, allowing the snake to pull the hemipenis back inside after mating.3PLOS ONE. The Evolutionary Implications of Hemipenial Morphology of Rattlesnake Crotalus durissus terrificus (Laurent, 1768) (Serpentes: Viperidae: Crotalinae) The whole system is hydraulic: muscle relaxation lets blood fill the spaces and push the organ outward, and muscle contraction pulls it back in and squeezes the blood out.
Why Two Instead of One
In most mammals, birds, and turtles, the paired genital buds that appear early in embryonic development fuse along the midline to produce a single penis or clitoris. In snakes and lizards, the two buds never fuse. They stay separate, each developing into its own fully independent organ on either side of the tail base.4Current Biology. Evolution: One Penis After All Research on ball python embryos has shown that these paired outgrowths arise from different tissue than the genital tubercles of other amniotes. In mammals, a signaling molecule called Sonic hedgehog (a real name, borrowed from the video game character) plays a central role in driving phallus growth from the inner lining of the cloaca. In snakes, the hemipenes lack that endodermal contribution entirely, and the sulcus does not express Sonic hedgehog at all.5PubMed. Development of hemipenes in the ball python snake Python regius
This difference is deep enough that some researchers have raised the possibility that hemipenes are not direct evolutionary equivalents of the mammalian penis at all. They may represent an independently derived solution to the same reproductive problem, built from a different developmental program. The genetics of limb development appear to be entangled in this story: a set of genes called HOXD that help pattern both digits and external genitalia were retained in snakes even after they lost their limbs, likely because those genes remained essential for building hemipenes.6Current Biology. Degenerative Evolution of a Shh Limb Enhancer Underlies Limblessness in Snakes The evolutionary link between limb loss and genital development is still being untangled, but the overlap in genetic control is striking.
One at a Time, and Sometimes They Take Turns
Despite having two hemipenes, a male snake inserts only one during any single mating. Which one he uses is not always random. In garter snakes studied in the wild, the overall split between left and right hemipenis usage averaged close to 50/50 across a population, but individual males showed a clear tendency to alternate sides in successive matings. The likely explanation is that each hemipenis has its own sperm supply and its own capacity to produce a copulatory plug (more on that below), so switching sides lets a male replenish between encounters.7Behavioral Ecology. Are snakes right-handed ? Asymmetry in hemipenis size and usage in gartersnakes (Thamnophis sirtalis)
Similar alternation patterns have been documented in lizards. In brown anoles, males significantly favored the opposite hemipenis when matings happened within a day of each other, but the preference disappeared when matings were separated by three days, suggesting the driver is depletion of sperm or seminal fluid rather than a fixed behavioral rule.8Animal Behaviour. Alternation of hemipenis use as a behavioural means of increasing sperm transfer in the lizard Anolis sagrei Having two organs effectively doubles a male’s short-term mating capacity, a meaningful advantage in species where breeding seasons are short and competition is intense.
Spines, Hooks, and an Arms Race
One of the most visually startling features of hemipenes is the ornamentation. Many species have surfaces covered in spines, hooks, ridges, cups, or chalice-shaped structures, and the variation across species is enormous.9Integrative and Comparative Biology. The Squamate Clitoris: A Review and Directions for Future Research Some spines are needle-sharp and capable of piercing tissue across a wide range of angles, while others are blunt hooks that cannot puncture at all. A study testing spine mechanics in three dimensions found that sharpness and curvature were the best predictors of whether a spine could actually penetrate tissue.10PubMed Central. Evolutionary morphology of genital spines informed by puncture mechanics
What are these spines for? Some clearly serve as anchoring devices. In red-sided garter snakes, a large basal spine on the hemipenis inserts into the female’s cloaca during mating. When researchers experimentally removed this spine, males could still mate but copulation was significantly shorter and the copulatory plugs they produced were smaller.11PubMed Central. Sexual conflict over mating in red-sided garter snakes (Thamnophis sirtalis) as indicated by experimental manipulation of genitalia Some of the more elaborate ornamentations may serve functions beyond anchoring. A broad survey of Old World snakes found morphologies consistent with locking mechanisms during copulation, but also structures that seemed to correspond to the variety of mating behaviors observed across species. The researchers proposed that sexual selection is the primary force shaping hemipenial ornamentation, with different mating systems driving different ornamental strategies.12PubMed. Does Sexual Selection Influence Ornamentation of Hemipenes in Old World Snakes?
The diversity is taxonomically useful, too. Hemipenial shape varies so reliably between species that herpetologists have long used it as a diagnostic character for identifying and classifying snakes. Two species that look nearly identical on the outside may have wildly different hemipenes.
Copulatory Plugs and What Happens After Mating
In many snake species, the male deposits a gelatinous plug inside the female’s cloaca immediately after insemination. These copulatory plugs were first described decades ago as a kind of enforced chastity device, a physical barrier meant to prevent rival males from mating with the same female.13PubMed. Copulatory plugs in snakes: enforced chastity That framing turns out to be incomplete. Surgical experiments on garter snakes showed that the plug is produced by secretions from the kidney’s sexual segment and delivered through the ureter. Males whose ureters were surgically tied off could not produce a plug, and their sperm simply leaked out of the female immediately after mating. Critically, sperm were found distributed throughout the plug material itself, suggesting the plug functions as a kind of spermatophore: a protein matrix that holds sperm in place and releases them gradually as it dissolves.14Biological Journal of the Linnean Society. Not just a chastity belt: the functional significance of mating plugs in garter snakes, revisited
The ability to produce a plug also appears to be limited. In the same study, the likelihood of a male depositing a plug dropped significantly after his second mating. This aligns with the alternation behavior described earlier: if each hemipenis draws on its own supply chain of plug material, a male who switches sides can deliver a full plug with each mating. Run out on both sides and his reproductive success likely drops even if he can still physically copulate.
The Female Side of the Story
For most of the history of herpetology, attention has been laser-focused on hemipenes. Female genital anatomy was either ignored or actively misidentified. A 2022 paper finally confirmed what had been occasionally speculated but never rigorously demonstrated: female snakes have hemiclitores, paired structures analogous in position to the male hemipenes.15PubMed Central. First evidence of hemiclitores in snakes Dissection and imaging across nine species and four families showed that the hemiclitores are small, non-eversible structures separated by connective tissue and forming a triangular shape that extends toward the tail. In death adders, histology revealed erectile tissue and nerve bundles but no spines. The researchers concluded that these are genuine functional organs, not underdeveloped hemipenes or scent glands as some earlier studies had claimed.
Radiographic comparison in golden lancehead vipers showed that hemipenes appear as herringbone-shaped structures with bone-like density due to heavily ossified spines, while hemiclitores appear as softer triangular structures, suggesting the spines in hemiclitores fail to develop the same level of hardening seen in males.16PubMed. Morphological and structural differences between the hemipenes and hemiclitores of golden lancehead snakes, Bothrops insularis (Amaral, 1922), revealed by radiography The under-investigation of female anatomy was not subtle. A review described the squamate hemiclitoris as “conspicuously under-investigated,” with existing studies riddled with inconsistencies, while the male hemipenis has been extensively studied since the 1800s.9Integrative and Comparative Biology. The Squamate Clitoris: A Review and Directions for Future Research
Genitalia That Evolve Together
Male and female genital shape in snakes do not evolve independently. A study of three closely related watersnake species found that when hemipenis shape differed between species, vaginal shape tracked those changes in parallel. In two of the species, both the hemipenes and the female reproductive tracts were bilobed; in the third, neither sex showed bilobation.17Integrative and Comparative Biology. Divergent Genital Morphologies and Female–Male Covariation in Watersnakes Geometric morphometric analysis of another species showed that genitalia in both sexes became wider and more bifurcated as individuals grew larger and reached reproductive maturity, a synchronized change that researchers interpreted as evidence of co-evolution to improve copulatory fit.18Biological Journal of the Linnean Society. Examining the shape and size of female and male genitalia in snakes using three-dimensional geometric morphometrics
This kind of correlated evolution is consistent with what biologists see in many animal groups: sexual selection, and sometimes outright sexual conflict, shapes both partners’ anatomy simultaneously. The spines and hooks on hemipenes may benefit the male by anchoring him during mating, but they also impose costs on the female, creating pressure for the female tract to change in ways that either accommodate or resist those structures. The result is an ongoing evolutionary dialogue between male and female anatomy that drives the extravagant diversity herpetologists see across snake species.
Sexing Snakes in Practice
Because hemipenes are normally retracted and invisible, determining a snake’s sex from the outside is not straightforward. Breeders and veterinarians rely on two main techniques, both of which take advantage of the hemipenial pockets in the tail base. Probing involves gently inserting a smooth, blunt-tipped rod into the pocket on each side of the cloaca. In males, the probe slides deep because the pocket extends along the length of the inverted hemipenis. In females, the probe stops almost immediately because there is no deep pocket. Reference values for probe depth vary by species.19Veterinary Clinics: Exotic Animal Practice. Reproductive Disorders of Snakes
The other method, colloquially called “popping,” involves applying gentle pressure at the tail base to manually evert the hemipenes in young snakes. It works because the organs are small and pliable in juveniles. This technique carries some risk: performed roughly or on very young animals, it can cause spinal injury.19Veterinary Clinics: Exotic Animal Practice. Reproductive Disorders of Snakes
When Things Go Wrong
The most common clinical issue involving hemipenes is prolapse, where one or both organs evert and fail to retract. Causes range from infection and trauma to constipation and forced breeding attempts. A chronically prolapsed hemipenis loses blood supply and becomes necrotic, at which point the standard treatment is amputation. Surgery involves fully everting the affected hemipenis, tying it off with sutures, and transecting it. Because hemipenes are entirely separate from the urinary tract, removing one or both does not interfere with the snake’s ability to urinate or defecate.20Veterinary Clinics: Exotic Animal Practice. Reproductive biology and disorders of captive reptiles A snake that loses one hemipenis to surgery can still mate using the other, though naturally he loses the alternation advantage. A snake that loses both can live a normal life but will be unable to reproduce.
Prolapse is more common in captive snakes than in wild populations, partly because captive conditions can lead to chronic dehydration, infections, and handling-related trauma. Reptile keepers who notice tissue protruding from the vent area should seek veterinary care quickly, since the window for saving the organ closes as swelling and tissue death progress.