What Are Spurs in Snakes and What Do They Do?

Spurs in snakes are small, claw-like structures that protrude on either side of the cloaca (the single opening used for reproduction and waste), and they are the visible remnants of hindlimbs that snakes’ ancestors possessed millions of years ago. They are most familiar to anyone who has handled a boa or python, where they appear as tiny, hard hooks near the base of the tail. Far from being useless leftovers, research shows that spurs play an active role in courtship and, in some species, in combat between rival males.

What Spurs Actually Are, Physically

Each spur is a keratinized claw that pokes through the skin, connected internally to a small bone called the femur, and in many species to remnants of a pelvic girdle as well. The internal anatomy varies by species, but in a number of basal snakes the skeleton behind each spur includes a recognizable pubis bone and sometimes other pelvic elements. Evolutionary analysis indicates that the most recent common ancestor of all living snakes likely retained all three pelvic bones and rudimentary hindlimbs in the form of femoral spurs.¹ So what you see as a tiny claw on a ball python is really the outermost tip of a miniature leg skeleton buried inside the snake’s body.

In embryonic development, these structures form relatively late. Studies on the pipe snake Anilius scytale show that the pelvic bones (ilium, pubis, ischium) and femur become visible as cartilage during mid-development and begin to ossify toward the end of embryonic life, while the cloacal spurs themselves do not appear until after hatching.² This means the bony scaffold is built first and the external claw forms later, a developmental sequence that mirrors the layered evolutionary history of the structures.

Which Snakes Have Them

Spurs are most prominent in the boid snakes, the family that includes boas and pythons. In large species like Burmese pythons and green anacondas, the spurs can be several millimeters long and are plainly visible to the naked eye. Males typically have larger spurs than females, a difference that is useful to breeders and field researchers for sexing animals.

But boas and pythons are not the only snakes with pelvic vestiges. Pipe snakes, shield-tailed snakes, and some other groups classified as “basal” or “intermediate” snakes retain varying degrees of internal pelvic skeleton and, in some cases, external spurs.³ Among the blind snakes (scolecophidians), the pelvic skeleton has been reduced along a different route altogether. In some of those tiny burrowing species, only a single rod-like bone remains, identified as the ischium rather than the pubis that persists in boas and pythons.¹ These snakes generally do not have visible external spurs.

Advanced snakes, the colubrids, elapids, and vipers that make up the vast majority of living species, are completely limbless with no visible spurs and no significant internal pelvic remnant. If you own a corn snake, a king cobra, or a rattlesnake, you will not find spurs on the animal.

What Spurs Do During Courtship

The most well-documented function of spurs is their use during mating. Male boas and pythons use them to stimulate the female’s body during courtship. The male positions himself alongside the female and rhythmically strokes or scratches her sides and cloacal region with his spurs, a behavior that can last for hours. This tactile stimulation appears to help elicit the female’s receptivity and cloacal gaping, facilitating copulation.

Phylogenetic analysis of courtship behavior across snake families supports the idea that rubbing with spurs was an ancestral courtship behavior in the boid lineage.⁴ In other words, it is not a quirk that evolved independently in a few species; it has been part of boid mating for a very long time. The fact that males tend to have larger spurs than females fits this picture, because sexual selection would favor males whose spurs were more effective at stimulating females.

The broader pattern across basal snakes, where many species share a configuration of spur, femur, and sizeable pubis, is thought to be functionally related to this widespread use of hindlimb remnants in mating rather than being a mere evolutionary leftover with no purpose.¹ That distinction matters: calling spurs “non-functional vestiges” undersells their current biological role.

Spurs in Male-Male Combat

Courtship is not the only context where spurs come into play. In several boid species, males that compete for access to a female engage in ritualized combat, and spurs can be part of the arsenal. Rival males coil around each other, raise their bodies, and attempt to push or topple the opponent. In boids specifically, poking with spurs was added to this ancestral combat repertoire.⁴

The spurs are not weapons in the way that fangs are. They are too small to inflict serious injury. Instead, spur-poking during combat seems to function more as a form of physical assertion, a way to push, prod, and unsettle a rival. Think of it as grappling rather than striking. The behavior is well documented in captive breeding colonies, where males housed together during the breeding season will sometimes engage in spur-assisted wrestling bouts.

Colubrid and viperid males, which lack spurs, have their own forms of ritualized combat (the iconic raised-body “dance” of male rattlesnakes, for example). Those species manage combat perfectly well without spurs, which reinforces the point that spurs are not essential for combat in general but are a tool available to the lineages that still have them.

How Snakes Lost Their Legs in the First Place

Understanding spurs also means understanding the evolutionary story of leg loss. Snakes descended from limbed lizard-like ancestors, and the transition to a limbless body plan happened gradually over tens of millions of years. The fossil record captures intermediate stages. A 95-million-year-old fossil snake from the Middle East preserves the most extreme hindlimb development known in any snake, retaining a tibia, fibula, ankle bones, foot bones, and toes.⁵ These were small but recognizable legs attached to an unmistakably snake-like body.

Between that ancient legged snake and a modern boa with its tiny spurs, a long process of skeletal reduction took place. Bones were progressively lost or reduced, from toes to ankle to shin to thigh, until what remained in many lineages was just a nub of femur tipped with a keratinized claw. The reduction did not follow the same path everywhere. As noted earlier, the two main groups of living snakes (scolecophidians and alethinophidians) reduced their pelvic skeletons by retaining different bones, showing that limb loss played out independently multiple times across snake evolution.¹

Perhaps the most surprising finding is that the process was not strictly one-directional. The evolutionary data suggest that some lineages lost ossified pelvic elements and then regained them. And the snake fossil record indicates that complete legs were re-acquired at least once, meaning the developmental potential for limb formation was not entirely erased even in species that had already reduced their limbs significantly.³

The Genetics Behind Limb Reduction

The genetic machinery behind snake limb loss has come into sharper focus in recent years, and it centers on a gene called Sonic hedgehog (Shh), which is a key driver of limb outgrowth in vertebrates. Snakes still carry the Shh gene itself, but the regulatory switch that turns it on specifically in developing limbs has been badly degraded.

Researchers identified snake-specific mutations within this long-range limb enhancer. When they tested the enhancer from various vertebrates in transgenic mice, versions from fish, from other mammals, even from animals separated by hundreds of millions of years of evolution, all drove normal limb development. But when the snake version of the enhancer was swapped in, the result was severe limb reduction. Strikingly, restoring just a single lost binding site in the snake enhancer was enough to bring back full function.⁶ The gene is there; the switch is broken.

Work specifically on python embryos tells a complementary story. In pythons, the limb-specific Shh enhancer has accumulated mutations that destroy essential binding sites, so Shh activity in the developing hindlimb bud is weak and short-lived. This causes the genetic circuit that normally drives outgrowth to shut down early.⁷ But other genetic programs are better preserved. The HOXD digit enhancers, which control toe formation, are still intact in pythons, and HOXD genes are actually expressed in the hindlimb buds. Python embryos temporarily develop cartilage precursors of a tibia, fibula, and footplate before the structures regress. This means the blueprint for a more complete leg has not been entirely erased, it just stalls partway through construction.

This finding explains how re-emergence of hindlimbs could have happened in certain fossil lineages without requiring the from-scratch reinvention of legs. The developmental toolkit was sitting there, partially functional, waiting for the right regulatory mutation to let it run a little further.

Are Spurs Really “Vestigial”?

The word “vestigial” gets thrown around a lot in discussions of snake spurs, and it deserves some pushback. In the strict evolutionary sense, spurs are vestigial: they are reduced remnants of a structure (a full leg) that was present in the ancestral form. Nobody disputes that. But “vestigial” in casual conversation often implies “useless,” and that is where the label misleads.

Structures can be vestigial in origin and functional in their current state. The human appendix is a commonly cited parallel: it is a remnant of a larger cecum found in herbivorous ancestors, yet it appears to play a role in immune function and gut flora maintenance. Snake spurs are similar. They are anatomically reduced, yes, but they remain under selection pressure because they contribute to reproductive success through courtship stimulation and rival intimidation. Males with larger, more effective spurs may gain a mating advantage, which means the structures are not drifting neutrally but are actively maintained by natural selection in the lineages that retain them.

When researchers describe the shared spur-femur-pubis configuration of many boid snakes as “likely to be related functionally to the widespread use of the hindlimbs in mating and courtship, rather than the usual representation of the bones as non-functional vestiges,” they are making exactly this point.¹ The bones are reduced, but calling them non-functional misrepresents the biology.

Sexing Snakes by Their Spurs

For snake keepers and breeders, one of the most practical aspects of spurs is their usefulness in determining an animal’s sex. In boas and pythons, males generally have noticeably larger and more prominent spurs than females. An experienced keeper can often make a good guess about sex just by comparing spur size, though it is not foolproof. Juvenile animals may have spurs too small to differentiate, and individual variation can occasionally produce a female with larger-than-typical spurs or a male with unusually small ones.

The more reliable method of sexing snakes is probing, which involves inserting a small, lubricated probe into the cloaca to check for the inverted hemipenes of a male. But spur size provides a quick visual check that is non-invasive and can narrow things down before more definitive methods are used. In breeding colonies of ball pythons or boa constrictors, keepers routinely note spur size as part of their records.

The sexual dimorphism in spur size also reinforces the functional interpretation. If spurs were truly non-functional remnants drifting toward disappearance, you would not expect consistent size differences between the sexes. The fact that males, who use the spurs actively during courtship and combat, tend to have larger ones is evidence that the structures are under sex-specific selection.

Tactile Sensory Structures in Spurless Snakes

Advanced snakes that lack spurs entirely have evolved other tactile tools for courtship. Sea snakes in the genus Emydocephalus provide a striking example. Males of the turtle-headed sea snake have enlarged scale structures called genial knobs (on the chin) and anal knobs (near the cloaca) that contain dense arrays of mechanoreceptors. Research indicates that these sensory structures help amorous males orient to the direction of female movement and give feedback for cloacal alignment during mating.⁸

These structures are not homologous to spurs. They evolved independently in a lineage that lost its hindlimb remnants long ago. But the parallel is instructive: the need for tactile feedback during courtship is apparently strong enough that when one set of structures was lost, another was cobbled together from different raw materials. The problem that spurs solve in boas, providing physical contact cues during mating, is solved by different anatomy in snakes that no longer have spurs. Courtship across snakes is a deeply tactile affair, and the body finds ways to support that regardless of which structures happen to be available.

Spur Health and Problems in Captivity

Keepers of boas and pythons occasionally encounter spur-related health issues. Because spurs are essentially modified claws, they can become ingrown, break, or develop infections at the base. Retained shed skin around the spurs is a common minor problem: when a snake sheds, the thin skin covering each spur sometimes fails to come off cleanly, and over successive shed cycles, layers of retained skin can build up and restrict blood flow or trap bacteria.

If a spur breaks at the base or becomes infected, it can cause localized swelling and discomfort. In most cases, warm soaks and gentle cleaning resolve the issue. More serious infections may require a veterinary visit and possibly removal of the affected spur, which does not appear to cause the snake any lasting harm beyond the loss of that particular courtship tool. Captive snakes that lose a spur through injury or surgery generally behave normally, though breeders may notice reduced courtship effectiveness in males.

Proper humidity and clean substrate go a long way toward preventing spur problems. If a keeper notices a spur looking swollen, discolored, or crusty with retained shed, addressing it early prevents it from becoming a bigger issue. It is a small part of the animal, but one worth keeping an eye on during routine health checks.