Intraspecific variation in life cycle impacts the morphology of Alpine newts.

Facultative paedomorphosis refers to the ability of individuals within a species to reach sexual maturity either by undergoing metamorphosis (metamorph) or by retaining larval traits into adulthood (paedomorph). This developmental flexibility provides a natural framework to investigate how facultative ontogenetic pathways shape morphological variation. Here, we investigate how life cycle variation (metamorphs, paedomorphs, and paedomorphs undergoing metamorphosis) affects the size and morphology of the body and feeding apparatus (cranium, mandible, and hyoid) in the Alpine newt (Ichthyosaura alpestris), using both linear and geometric morphometric approaches. Our results reveal clear differentiation in size, allometric trajectories, and morphology of both the body and the feeding apparatus. Paedomorphs are smaller and exhibit morphologies consistent with aquatic functional demands and the consequences of incomplete metamorphosis, particularly in the feeding apparatus, which normally undergoes extensive remodeling during metamorphosis. Individuals undergoing metamorphosis are morphologically distinct from paedomorphs but overlap with metamorphs in body size, suggesting that metamorphosis involves rapid skeletal reorganization before post-metamorphic growth occurs. Moreover, patterns of morphological integration and modularity do not differ between life cycle categories, suggesting that the developmental interdependence of phenotypic traits does not promote the morphological changes observed in species that experience facultative paedomorphosis. Paedomorphic populations nevertheless show higher morphological disparity in the feeding apparatus, likely reflecting variation in the degree of ossification and remodeling resulting from incomplete metamorphosis. Overall, these results show that facultative life cycles strongly structure body and feeding morphology in Alpine newts, while leaving trait covariation largely unchanged. These findings emphasize the potential of developmental plasticity to generate substantial morphological diversity within a same species, providing a basis for understanding how paedomorphic lineages maintain morphological variation despite persistent developmental constraints.