Do Lizards Have Two Penises? The Role of Hemipenes

Lizards do have two penises, though the proper term is hemipenes (singular: hemipenis). These paired reproductive organs sit at the base of the tail, tucked inside the body when not in use, and a male lizard only inserts one at a time during mating. The same anatomy is shared by snakes, and the group name for lizards and snakes together, Squamata, is effectively the club of animals with hemipenes. The story behind why these organs come in pairs, how they work, and what they reveal about lizard biology is considerably stranger than the basic fact of their existence.

How Hemipenes Actually Work

Unlike the mammalian penis, a hemipenis is not a tube that simply becomes erect. Each hemipenis is stored inverted inside the tail, like a sock turned inside out. When a male is ready to mate, one hemipenis is everted outward through the cloaca, the single opening lizards and snakes use for waste, reproduction, and everything else. The process is hydraulic: the organ fills with blood to achieve its functional shape, then retracts by muscle contraction once mating is complete.

Research on rattlesnake hemipenes provides a detailed picture of the internal mechanics. Each organ consists of two concentric cylinders of tissue riddled with irregular spaces that fill with blood during arousal. Radial bundles of smooth muscle act like struts, controlling how much the outer cylinder expands. When those muscles relax, the organ swells outward; when rings of smooth muscle contract, the whole structure shortens and can be pulled back inside.

¹

The surface of the hemipenis often carries a groove called the sulcus spermaticus, which channels sperm from the cloaca to the tip of the organ. In many species, this groove forks at the end, with each branch ending near an opening that aligns with the female’s paired oviducts. In the green anole lizard, the hemipenis does not actually penetrate deep into the female’s reproductive tract. Instead, the two apical openings of the forked groove press against the openings of the oviducts from within the cloaca, delivering sperm directly to where it needs to go.

²

Why Use One at a Time and Then Switch

If a lizard has two hemipenes, you might expect it to use both simultaneously, but that is not how it works. Only one hemipenis is inserted during a single mating. What researchers have found is that many male lizards alternate sides between successive copulations, and this switching appears to serve a real reproductive purpose.

In the brown anole, males that mated twice within a 24-hour window showed a strong tendency to switch from one hemipenis to the other for the second mating. Males that were experimentally prevented from alternating transferred significantly fewer sperm in the second copulation compared to males allowed to switch. The advantage was clear when copulations happened within a day of each other but disappeared at longer intervals of around 72 hours, suggesting the sperm supply in each hemipenis needs recovery time.

³

This alternation pattern has been confirmed across follow-up studies. The behavior depends on actual intromission rather than just mounting; males that physically insert one hemipenis seem to “remember” which side they used and preferentially choose the opposite side next time.

⁴

Think of it like having two loaded water guns. Fire one, and while it refills, the other is ready to go. A male lizard that mates with multiple females in quick succession can deliver a full sperm load each time by switching sides. In species where males compete intensely for mating opportunities and a male might encounter several receptive females in a single day, this built-in redundancy could be a meaningful reproductive advantage.

Why Lizards Developed Paired Organs in the First Place

The developmental explanation is surprisingly straightforward. In all amniotes (reptiles, birds, and mammals), the earliest embryonic stage of external genital development involves paired swellings that appear near the base of the developing hind limbs. In mammals, turtles, and crocodilians, those two swellings merge into a single genital tubercle that eventually becomes a penis or clitoris. In lizards and snakes, the paired buds simply never fuse. Each one develops independently, producing two separate organs.

⁵

This is not a case of lizards evolving something extra. It is a case of mammals and other groups evolving something merged. The paired state may actually be the more ancient starting point. Research on tuatara embryos supports this view. Tuataras are the closest living relatives of lizards and snakes, yet adult tuataras lack any intromittent organ entirely and mate by simply pressing their cloacas together. When researchers examined preserved Victorian-era tuatara embryos in 3D, they found that even tuatara embryos develop the beginnings of external genitalia, which then regress before hatching. This conserved developmental stage across all amniotes suggests a single evolutionary origin for the external genitalia program, with different lineages either merging, retaining, or losing the paired structures.

⁶

The Wild Diversity of Hemipenis Shapes

If you imagine a lizard hemipenis as a smooth, simple tube, you are wildly off base. Hemipenes are among the most varied structures in the entire animal kingdom. They come covered in spines, hooks, ridges, cups, and fringes. Some look like medieval weapons. Others resemble fleshy flowers. The diversity is so extreme that herpetologists have long used hemipenis shape as a primary tool for telling species apart.

In South American anoles alone, researchers cataloging hemipenis morphology across 13 species found that the main differences between species involved the organ’s overall shape and surface ornamentation, while variation within a single species was low. A fleshy projection on the surface away from the sperm groove, called the asulcate process, turned out to vary dramatically between species and may serve as one of the most useful identifying features for this group.

⁷

A similar pattern appears in ablepharine skinks (the “fused eyelid” skinks of the genus Ablepharus). All species in the group share the same basic deeply forked hemipenis shape without large spines or hard structures, but subtle differences in the proportions of the forked lobes, whether long and slender or short and robust, reliably separate species and align well with molecular genetic data. The hemipenis essentially serves as an anatomical fingerprint for species classification.

⁸

What the Spines and Hooks Are For

The spines that stud the hemipenes of many species are not just decorative. A recent study that tested the puncture mechanics of hemipenial spines in 3D found that spine shape and sharpness vary enormously. Some spines are needle-like and can pierce tissue across a wide range of angles with very little force. Others are deeply hooked and blunter, more suited to gripping than piercing. Tip sharpness and structural curvature were the best predictors of how easily a spine could puncture a surface.

⁹

These features likely serve multiple purposes during mating. Sharp spines may help anchor the hemipenis inside the female during copulation, preventing premature separation. Hooked spines could serve a similar anchoring function by catching on tissue folds. There is also the possibility that spine morphology is shaped by sexual conflict: males benefit from secure attachment and prolonged copulation, while females might benefit from the ability to dislodge a mating partner. This arms race between male and female interests could drive the rapid diversification of hemipenial ornamentation.

Evidence from Tropidurus lizards supports the idea that sexual selection plays a role in shaping hemipenis traits. Researchers studying this genus found allometric patterns in the hemipenis lobes consistent with directional selection, potentially driven by female cryptic choice (where females internally bias paternity toward preferred males) and genital coevolution between the sexes.

¹⁰

Females Have Them Too

One of the more surprising recent findings in reptile biology is that female squamates possess their own paired structures called hemiclitores, the anatomical counterparts of the male hemipenes. Because the paired genital buds in the embryo are the same in both sexes before hormonal differentiation kicks in, females retain a version of the same paired organs.

¹¹

In snakes, researchers identified hemiclitores as two separate structures in the tails of females, sitting behind the cloaca. Unlike hemipenes, the snake hemiclitores lacked spines, had no sperm groove, could not be everted, and were considerably smaller. But they contained extensive erectile tissue filled with blood cells and were threaded with nerve bundles, suggesting they can engorge with blood and provide sensory feedback during mating.

¹²

In at least one lizard species, the Mexican alligator lizard Barisia imbricata, the overlap between male and female anatomy is even more dramatic. All newborns in this species have hemipenes regardless of sex, and the only way to determine sex in neonates is to examine the internal reproductive ducts and gonads directly. The hemipenes in females begin to regress at about seven months of age and are fully gone by around 15 months.

¹³

The hemiclitores have historically been overlooked in reptile biology, partly because they are internal and partly because researchers simply did not look for them. Their discovery has opened up new questions about whether female genital anatomy plays an active role in mating dynamics, mate choice, or stimulation that affects sperm storage.

Hemipenes as a Communication Tool

Mating is the primary function of hemipenes, but some lizards have found a second use: intimidation. In at least two species of Liolaemus lizards from South America, males have been observed everting their hemipenes outside of any mating context. When researchers filmed male lizards in different social scenarios, hemipenis eversion occurred only during encounters with rival males, never when a female was present and never in isolation. The behavior appears to function as an aggressive display, a way of signaling dominance or willingness to fight without actually making physical contact.

¹⁴

This is an unusual example of a reproductive organ being co-opted for social signaling. It is not entirely clear what information the display conveys to rival males, whether it advertises body condition, testosterone levels, or simply serves as a generic threat. The fact that it appeared in two different species in the same genus raises the question of whether the behavior is more widespread than currently documented, since most field studies of lizard behavior have not been looking for it.

Veterinary Concerns for Pet Lizards

For anyone who keeps lizards as pets, hemipenis problems are one of the more common reproductive health issues. Hemipenile prolapse, where one hemipenis everts and cannot retract back into the tail, is a recognized veterinary emergency in captive reptiles. A case study of a leopard gecko that required surgical removal of a prolapsed hemipenis highlights the range of causes: infections, forced separation during mating, and inflammation caused by probing (a common technique used to determine sex in reptiles). Other contributing factors include calcium deficiency and kidney stones, though pinpointing the exact cause in any individual case is often difficult.

¹⁵

The silver lining of having paired organs is that losing one hemipenis to surgery does not make a male infertile. Males that have undergone hemipenectomy (removal of one hemipenis) can still mate successfully using the remaining one. This is a practical advantage of the paired anatomy that has nothing to do with evolutionary pressures and everything to do with surgical outcomes in a veterinary clinic.

The Bigger Reproductive Plumbing

Hemipenes do not work in isolation. The full male reproductive system in a lizard includes paired testes, a network of ducts that transport sperm, a sexual segment of the kidney that contributes seminal fluid, and the cloaca where all these outputs converge before reaching the hemipenes. In the ameiva lizard, detailed anatomical study shows that sperm travels from the testes through a series of increasingly larger ducts, receiving secretions along the way, before arriving at the cloaca. From there, the sperm flows along the sulcus spermaticus groove during copulation.

¹⁶

One detail worth noting is the sexual segment of the kidney. In many male squamates, a portion of the kidney produces secretions that mix with sperm and may serve as a mating plug, blocking the female’s cloaca after copulation to prevent rival males from inseminating her. This is yet another layer in the reproductive strategy, working alongside hemipenis morphology and alternation behavior to maximize a male’s chances of fathering offspring. The entire system, from testis to hemipenis tip, is tuned by millions of years of sexual selection acting on every component independently.