Back to the water II: Variation in eye and cranial morphology in Telmatobius frogs with different lifestyles.

Several tetrapod lineages have independently evolved an aquatic lifestyle, driving structural and functional modifications. Changes in eye size and position are key trends in this transition. While most frogs and toads have aquatic larvae, the majority of adults are terrestrial. In contrast, Telmatobius frogs exhibit a unique spectrum from semiaquatic to strictly aquatic lifestyles, linked to the colonization of the Andean highlands. This study examines variation in eye size, position, and cranial proportions, in two species: the semi-aquatic Telmatobius oxycephalus and the strictly aquatic T. rubigo. Measurements across post-metamorphic stages were used to compare groups and assess allometric patterns. Telmatobius rubigo, which inhabits high altitudes, has smaller eyes and a more dorsal/frontal eye position than T. oxycephalus. Among T. oxycephalus, the forest population at lower elevations has larger eyes compared to valley counterparts at intermediate altitudes. Cranial analysis reveals that T. rubigo has smaller orbits and larger otic capsules, while T. oxycephalus shows the opposite pattern. These findings suggest that smaller eye size and more frontal/dorsal positioning evolved in response to the aquatic lifestyle at high elevations. This pattern parallels that of highly aquatic pipid frogs, which adapted to aquatic habitats during the Cretaceous. In contrast, Telmatobius specialization for aquatic environments is relatively recent and associated with Andean colonization, providing a unique opportunity to study an adaptive radiation within a modern evolutionary context.