Sexual size dimorphism (SSD) is common in lizards, with most reported trends being towards larger trunks in females and wider heads in males. Such trends are often explained in terms of fecundity selection favoring larger females capable of producing more offspring, and sexual selection favoring large-headed males able to win duals against rivals for accessing females, respectively. SSD may also be a side effect of a possible sexual difference in age structure, thus highlighting the need for explicitly considering individual age in morphometric comparisons between sexes. In this study, we used morphometric and skeletochronological data on a sample of Spotted fringe-fingered lizards from an arid area in southern Tunisia to investigate SSD, while accounting for individual age. Our major aim was to assess whether this understudied species fits the general SSD trends recognized in lizards. As expected, trunk size showed a significant female-biased dimorphism. However, there was no sexual dimorphism in head size. Moreover, using behavioral observations, we found that females engaged in fighting to the same extent as males, and that fighting success increased with increasing head size in both sexes. These results suggest that aggressive conflicts did not exert selective pressure solely on males. We hypothesize that in poorly productive arid environments, aggressive competition among females for resources may exert selective pressure on female head size comparable to that exerted by mate competition on males. We also call for future research to address possible links between environmental aridity, resource competition, and lizard morphology.