Elevational patterns of sexual size dimorphism in a Mediterranean lizard

Sexual size dimorphism (SSD), typically the result of different optimal body sizes for males and females, shows an enormous inter- and intraspecific variation. Lizards have become a model in the study of SSD, as they show either monomorphism, male- or female-biased SSD. Variations in the strength of intrasexual selection (assumed to favor larger body size in males) and fecundity selection (assumed to favor larger body size in females) are usually considered the main reasons for variation in SSD. Here, we take advantage of a 2,200 m elevational gradient for studying the elevational variation in SSD in the Mediterranean lizard Psammodromus algirus. Previous studies on this species suggest that the strength of sexual selection decreases with ascending elevation, while the strength of fecundity selection on females increases with ascending altitude. Accordingly, we predicted that SSD bias to males should increase at low elevations, while female-biased SSD should increase at high altitudes. We found that both males and females were larger with ascending elevation, but the magnitude of variation in body size with altitude differed between sexes. Females were slightly larger than males (1.1%), although SSD bias toward females increased with elevation. SSD was male-biased only in one locality, at the lowest elevation. Nevertheless, elevational differences in SSD resulted from a concomitant sexual variation in age structure, as females were older than males in every locality except at the lowest elevation, where males were older than females. Since this species shows post-maturational growth, sexual differences in age structure could explain the elevational pattern in SSD.