Pelvic spur

Modified reptilian appendages

Pelvic spurs (also known as vestigial legs) are external protrusions found around the cloaca in certain superfamilies of snakes belonging to the greater infraorder Alethinophidia. These spurs are made up of the remnants of the femur bone, which is then covered by a corneal spur, or claw like structure. Due to the fact that the spurs derive from the ancestral state of functional legs, but are no longer functional for locomotion specifically, these structures do meet the criteria for being considered vestigial. Nonetheless, uses for the structures have been thoroughly documented. Species that have external spurs have corresponding muscles, neurological structures, and vascularization to allow for independent movement. The spurs are specifically used in the clasping and stimulation of females by males during courtship and mating. In certain species, males will also use their spurs to engage in combat with one another. Nonetheless, multiple fossilized specimens document the progression of the development of leglessness within the suborder Serpentes. The species of extinct snake Najash rionegrina was first described in 2006, and has been proposed as the earliest branching taxa of the suborder Serpentes. The fossils were found in the Patagonia region of Argentina, and were dated to the Upper Cretaceous period. Najash rionegrina exhibited a sacrum, pelvic girdle, and robust hind limb structures outside of the ribcage, all of which led researchers to the conclusion that these hind limbs were functional for locomotion. These two superfamilies are sometimes referred to as primitive snakes, as they are considered to be the earliest diverging taxa of Macrostomata, the clade which includes the majority of described living snake species, and whose only other constituent beyond Booidea and Pythonoidea is Caenophidia. While species of Caenophidia lack spurs, the presence and use of spurs across Booidea and Pythonoidea is well documented. In these superfamilies, spurs can be observed as tools for courtship and competition between males, and are sexually dimorphic. The authors of this study noted that others in the field had made similar observations before, but had not formally researched the topic specifically. Spurs were found to be significantly larger in males, while in females spurs were much shorter, and were sometimes externally absent. Another study found the use of spurs in madagascan boas (Sanzinia madagascariensis), an arboreal species. In this species, researchers did not observe biting. In fact, the authors observed that the heads of combating males were frequently out of sight of one another. In its place, observed males would tightly grip on to each other using the posterior regions of their bodies, orienting their spurs to be perpendicular to their body while doing so.

Similar to the described observations of sexual dimorphism prior to their publication, William H. Stickel and Luccille F. Stickel also noted that observations had been made by others of pelvic spur use by males on females during mating. Furthermore, the males exhibited use of the spurs to better position or adjust the female’s cloaca for mating. The authors note that spur use for combat can also be observed in this species between males. In this species, multiple males were observed mating with the same female simultaneously, and males did not engage in combat. Instead, it was observed that spurs were used by males to manipulate the tail of the female. Corroborating these results in a New World species, a study in 2023 documented the use of spurs in mating by red tailed boas, Boa constrictor. It was observed that males repeatedly alternated the orientation of the spurs between horizontal and vertical positions, which the authors suggest stimulated muscle contractions in the female, which would allow for better alignment between the individuals for mating.