Testing the morphological constraint hypothesis of tail length in the sexually dimorphic Cerastes vipera and new perspectives

The morphological constraint hypothesis (MCH) states that, in snakes, males typically have relatively longer tails than females to accommodate the hemipenes and retractor muscles. To date, most studies testing the MCH have been interspecific and results have been equivocal. We tested the MCH intraspecifically on Cerastes vipera, a species with a relatively short tail and suitable for testing the MCH. The relative tail length and length of the hemipenes pocket in Cerastes vipera were measured in preserved museum-maintained males (n = 35) and in free-ranging males and females (n = 277). Males exhibited relatively longer tails than females, which was explained fully by the length of the hemipenes pocket. The relatively short tail of C. vipera presents a constraint to the reproductive structures in males, as the length of the hemipenes pocket occupies a greater proportion in shorter- than longer-tailed individuals. This is the first report presenting these intraspecific findings in support of the MCH. Whether these relations are widespread among snake families, within Viperidae, or specifically within C. vipera warrants further studies.