Head form variation in mountain newts of the genus Calotriton (Amphibia, Salamandridae) in a phylogeographic context

Relief has a profound impact on the genetic structure of species. While the geographic ordination of genetic variation is assessed in many mountain species, the factors involved in phenotypic differentiation and its congruence with phylogeographic history have been largely underexplored. This is the case of the genus Calotriton. Here we use a protocol that allows the study, through geometric morphometrics, of the shape of the head of live animals in situ with the aim of obtaining information on the factors involved in morphological differentiation. To this end, we assess the general pattern of head shape variation in the two species of the genus Calotriton and evaluate its congruence with genetic and geographic structures. The results indicate that differences in head size and shape occur both between and within species. The degree of differentiation between species is congruent with the absence of hybridization and the precise pattern of change suggests that differences in head form could be, at least in part, adaptive. Sexual dimorphism in head size and shape was similar in both species, with males having bigger and more robust heads than females. Geographic variation in head shape and size was congruent with the phylogeographic pattern, but not with divergence times, especially within C. asper. This, together with the high divergence detected between some close populations and the convergence of distant populations, suggests that adaptation to local environments, as well as plastic and/or stochastic factors, are occurring. Our results let us to conclude that the procedure here employed is suitable for detecting head form variation at inter and intraspecific levels.