Although the phenomenon of tail autotomy has traditionally been viewed in a purely adaptive light, functional constraints imposed by the locomotor system appear to have influenced the presence and extent of autotomy in lizards. Them. caudifemoralis longus is an unsegmented hind limb retractor that originates from the caudal vertebrae. It does not participate in autotomy and thus limits the proximal position of autotomic septa. Variation in the extent of the m. caudifemoralis is correlated with locomotor type. The muscle is large and originates from a long series of caudal vertebrae in fast moving lizards with powerful limb retraction, as exemplified by taxa capable of bipedal running. In slower lizards with sprawling postures, such as geckos, the m. caudifemoralis is small and restricted to the first few postsacral vertebrae. Autotomy is typically restricted or absent in the former lizards, while in the latter only the most proximal vertebrae are incapable of autotomy. In the evolution of existing patterns of caudal autotomy, functional demands intrinsic to the tail may be subservient to locomotor constraints imposed on the tail base by the m. caudifemoralis longus.