Recurrent evolution of ultraviolet vision in frogs and toads is linked to transitions in locomotor mode

The evolution of shortwave vision in frogs and toads (anurans) has remained unclear, because molecular and behavioural evidence suggest a contrasting picture regarding their UV sensitivity. Although all anurans studied to date express violet-peaking SWS1 pigments, many species possess UV-transmissive lenses and show behaviours consistent with UV detection, raising questions about how SWS1 spectral tuning has evolved. Here, we combined sequence analyses with functional characterization of visual pigments from ancestral and modern species and behavioural assays to resolve this uncertainty. We show that the ancestral anuran possessed a violet-sensitive pigment (393 nm) and that early diversification involved further shifts toward longer wavelengths (404-414 nm). Despite this overall trend, UV-peaking SWS1 pigments (approx. 360 nm) have re-evolved independently in at least five lineages. These repeated tuning shifts are associated with locomotor ecology, such that violet sensitivity is maintained in jumping species, whereas UV sensitivity recurs in lineages that use burrowing. Behavioural experiments in the Asian toad, which shows violet-peaking SWS1, confirm UV perception. We propose that repeated transitions in locomotor mode have shaped shortwave spectral tuning in anurans, linking visual ecology to changes in movement.