Macroevolutionary changes such as variation in habitat use or diet are often associated with convergent, adaptive changes in morphology. However, it is still unclear how small-scale morphological variation at the population level can drive shifts in ecology as observed at a macroevolutionary scale. Here, we address this question by investigating how variation in cranial form and feeding mechanics relate to rapid changes in diet in an insular lizard (Podarcis siculus) after experimental introduction into a new environment. We first quantified differences in the skull and jaw muscle architecture between the source and introduced population using 3D geometric morphometrics and dissections. Next, we tested the impact of the observed variation in morphology on the mechanical performance of the masticatory system using computer-based biomechanical simulation techniques. Our results show that the small differences in shape, combined with variation in muscle architecture, can result in significant differences in performance allowing access to novel trophic resources. The confrontation of these data with the already described macroevolutionary relationships between cranial form and function in these insular lizards provides insights into how selection can, over relatively short time scales, drive major changes in ecology through its impact on mechanical performance.

The muscle architecture dataset is composed of 18 specimens from Pod Kopište and 19 specimens from Pod Mrčaru. Each muscle bundle was extracted by dissecting the left side of the head. Muscle bundles were blotted dry and weighed with a digital balance (Mettler AE100; ± 0.1 mg). The connective tissue surrounding the muscles was digested by submerging the muscles in a 30% aqueous nitric acid solution for 24h. Next, the nitric acid was removed, and a 50% glycerol solution was added to arrest the muscle digestion. Muscle fibers were drawn under a binocular scope (Leica) with camera lucida (see Taverne et al. 2021) and measured using Image J (Rasband 1997). Muscle volume was calculated as the ratio between muscle mass and muscle density (1.06 g.cm^-3; see Mendez & Keys 1960). The physiological cross-sectional area (PCSA) of each muscle bundle was subsequently calculated by dividing muscle volume by the mean fiber length. The muscles were grouped into four functional groups: the external adductors, the pseudotemporalis group, the adductor posterior, and the pterygoid group (Table S2). The jaw depressors and the constrictor dorsalis muscles were not considered since they are not involved in jaw closing.

The dataset is an .xcl file that can be opened with any table sheet editor.