Sexual Dimorphism in Lacertid Lizards: Male Head Increase vs Female Abdomen Increase?

Abstract

Lacertid lizards exhibit sexual dimorphism in size corrected values for abdomen (always larger in females) and head (larger in males) lengths. Relative abdomen length increased with SVL in females but did not in males. The mean abdomen/head ratio for juvenile lizards (sexes pooled) was lower than that of females but did not differ from that of males in any of the studied species. Therefore, the ontogenetic development of the main body segments (abdomen and head) was isometric in male lizards, whereas female abdomen exhibited positive allometric growth. Standardized independent contrasts (Felsenstein's method) of female abdomen to head ratio and of the slope of the regression of clutch size on SVL explained a significant amount of variation in sexual size dimorphism in a stepwise multiple regression model. The fact that sexual size dimorphism was best explained by variables related to female reproductive investment, together with the ontogenetic trajectories of body segments suggest that sexual size dimorphism results mainly from variation in female size. Despite the suggested prominent role of selection on female body size in determining the outcome of size dimorphism, there was also evidence of selection for increased body size in males, which were the largest sex in species with low selective pressure towards increased female size (constant clutch size or low fecundity slope over size). Evidence for intersexual food (prey size) partitioning was weaker than expected from the widespread dimorphism in body size or relative head size found among lacertid lizards. Furthermore, the development of body segments was in some instances inconsistent with the competitive hypothesis, the largest sex having relatively smaller trophic structures.