Sexual dimorphism of tail length in lacertid lizards: test of a morphological constraint hypothesis

Abstract

Males of many lizard species have longer tails than similarly‐sized females. We hypothesized that this dimorphism is induced by a longer non‐autotomous tail part in males, which is associated with the presence of the copulatory organs at the tail base, and presumably reduces the males' ability to escape predation by tail shedding. A compensatory mechanism would be an increase of total tail length in males, to achieve equal lengths of the autotomous tail part in both sexes. A critical prediction of this ‘morphological constraint’ hypothesis is that the extent of dimorphism in total tail length increases with the magnitude of sexual differences in length of the non‐autotomous tail base. We tested this prediction through a comparative study in a small clade of lacertid lizards. Within each of nine species, sexual differences in length of the non‐autotomous tail base and in total tail length do not change with body size. All species, except one, exhibit a clear male‐biased dimorphism in length of the non‐breakable tail base. In all species studied, males have longer tails than females. We used the method of phylogenetically independent contrasts to explore the interspecific relation between dimorphism in length of the tail base and sexual differences in total tail length. Contrary to our prediction, we found no evidence for a positive correlation between the extent of dimorphism in both traits. Thus, constraints imposed by the male copulatory organs on tail autotomy do not seem to be a significant factor in the evolution of dimorphism in tail length in this clade of lacertid lizards.