Impact of Life-Cycle Variation on Feeding System Musculature in Caudata

Synopsis Caudata (salamanders and newts) exhibit considerable diversity in terms of ecology, life cycle, morphology, and behavior, ranging from species with complete metamorphosis to those with facultative or complete loss of metamorphosis. These developmental differences are often tightly linked to ecological transitions and morphological transformations, influencing how salamanders exploit habitats, access resources, and feed during their lifetime. While skeletal transformations have been widely studied, the impact on cranial musculature remains poorly understood. This study explores how life-cycle variation and associated ecological transitions and morphological transformations affect the architecture of feeding muscles in salamanders. We conducted dissections of the feeding system in 25 salamander species representing different life cycles, ecological transitions, and morphological transformations. We quantified muscle volume and physiological cross-sectional area (PCSA) functional muscle group of the jaw and hyoid muscles. Our results revealed a differentiation in cranial musculature based on different ecological strategies, and that other factors, such as head size, play a prominent role in shaping muscle architecture. We identified consistent patterns associated with whether individuals undergo an ecological transition, experience a morphological transformation, and with their adult habitat use, suggesting that ecological context imposes functional constraints on the muscular organization of the feeding system. These findings suggest that life history, ecological, and developmental strategies impose constraints that influence the muscular organization of the jaw and the hyoid apparatus. Future work should broaden taxonomic sampling, integrate bone and muscular traits together, and examine the evolutionary pathways by which life-cycle variation interacts with functional morphology in Caudata.