The universal image of a snake is that of a limbless reptile moving by sinuous undulations across the ground. It may come as a surprise, then, that many species possess small, claw-like appendages called spurs located near their tail. These structures are external protrusions found flanking the cloaca, or vent, which is the common opening for the snake’s digestive, urinary, and reproductive tracts. The presence of these spurs offers an intriguing look into the evolutionary history of snakes and their current reproductive behaviors.
Anatomy and Location of Snake Spurs
Snake spurs, also known as pelvic spurs, are consistently found on either side of the cloacal opening, positioned on the underside of the snake near the tail’s base. The visible part is a small, hard, and often curved claw composed of keratin. This external claw is the terminus of an internal structure, giving the spur a degree of autonomous movement.
The internal support for the spur is composed of a reduced, bony remnant of the ancestral hind limb skeleton. This skeletal component is typically a reduced version of the femur, which floats freely within the muscle mass and is not connected to the snake’s spine. Well-developed musculature grants snakes the ability to move and erect the spurs independently.
These structures exhibit a noticeable sexual dimorphism, meaning they differ significantly between the sexes. In male snakes, the spurs are generally longer, thicker, and more sharply pointed than those found on females. This difference in size and shape is sometimes used by researchers to determine the sex of a snake. The larger, more robust spurs in males correlate directly with their function in reproductive behavior.
Spurs as Vestigial Structures
The underlying bony remnants within the spurs are physical evidence of the snake’s evolutionary lineage from four-legged ancestors. These reduced skeletal elements, which include a remnant of the pelvic girdle and the femur, are classified as vestigial structures. A vestigial structure is an anatomical feature that has lost its original primary function through evolution but is still retained by the organism.
The presence of this internal skeleton confirms that the ancestors of modern snakes possessed a fully functional pelvis and hind limbs, much like modern lizards. Fossil evidence, such as the extinct snake Najash rionegrina from the Cretaceous period, shows a transitional phase with small, external hind limbs, illustrating the progression toward leglessness. The complete loss of forelimbs and the reduction of hind limbs occurred over millions of years as snakes adapted to a limbless body plan.
While the original purpose of locomotion has been lost, the retention of the spurs suggests a genetic mechanism that did not fully suppress their development. The genes responsible for limb development are still present in snakes, but they are largely deactivated, resulting in these small relics of their past. The vestigial nature of the spurs links modern snakes to their limbed reptilian relatives.
The Role of Spurs in Snake Behavior
Despite their classification as vestigial, snake spurs have acquired a new, specific function centered on courtship, mating, and male-to-male competition. During the courtship ritual, the male snake uses his spurs to gently rake or scratch the female’s flanks and tail area.
This action serves as a form of tactile stimulation, encouraging the female to contract her muscles and maneuver her body into a position suitable for copulation. The repeated rubbing of the spurs helps the male align his cloaca with the female’s, a process that is challenging given the snakes’ long, cylindrical bodies. The spur acts as a tool to manipulate and stimulate the female, facilitating the final alignment necessary for the insertion of the hemipenis.
Males in some species also use their spurs during dominance displays or combat with other males. They attempt to poke, grip, or scratch their rival with the spurs, especially in wrestling bouts over access to a female. The ability to move the spurs independently makes them an effective anchor or scratching instrument in these behavioral contexts.
Snake Species Known for Prominent Spurs
Prominent spurs are restricted to the more ancient, non-venomous lineages of snakes. The two major snake families known for possessing the most easily observed spurs are the Boidae and the Pythonidae. These two groups, which include the world’s largest constrictors, are considered more primitive compared to advanced snakes like cobras or vipers, which lack external spur structures.
Within the Boidae family, the spurs are common in species like the Red-tailed Boa (Boa constrictor) and the Green Anaconda (Eunectes murinus). Similarly, the Pythonidae family features spurs on well-known species such as the Ball Python (Python regius) and the Burmese Python (Python bivittatus). In these large constrictors, the spurs are often easily seen and felt as small, curved hooks near the vent.
While the Boids and Pythons possess the most dramatic examples, small, less obvious spurs or internal pelvic remnants can be found in other basal snake groups. Some species of blind snakes, which are small, burrowing snakes, also retain vestigial pelvic structures. However, the large constrictors remain the prime examples where the function of the spur is most clearly tied to modern reproductive success.