Coevolution and allometric scaling of cranial soft tissues in geckos (Gekkota)

Miniaturization is a widespread evolutionary phenomenon that can impose strong constraints on the organization of the vertebrate head, yet the structural consequences of body-size reduction for cranial soft tissues remain partially unexplored. Geckos (Gekkota) are an especially informative clade for investigating these patterns because they include a broad range of adult body sizes, including a high prevalence of miniaturized species. Furthermore, geckos provide a useful system in which the brain, eyes, and feeding musculature must be accommodated within a finite cranial space. Here, we combine three-dimensional volumetric data derived from DiceCT and μCT scans with phylogenetic comparative analyses to examine how major cranial soft tissues scale across geckos of varying adult body sizes. We evaluate allometric scaling using phylogenetic RMA regression against two complementary size proxies, snout-vent length and a cranial size index, and further assess whether scaling within the feeding musculature is uniform by partitioning the jaw muscles into temporal, palatal, and protractor functional groups. Our results show that cranial soft tissues do not scale uniformly with size: head volume is approximately isometric, brain volume shows negative allometry, eye volume is proxy dependent, and jaw adductor muscle volume shows positive allometry. Functional partitioning further reveals that the temporal and palatal groups drive positive allometry in the feeding musculature, whereas the protractor muscles remain approximately isometric. These findings provide a quantitative framework for understanding how neurosensory and musculoskeletal systems are co-accommodated within the gekkotan skull and offer new insight into the structural consequences of body-size evolution and miniaturization in vertebrates.