Evolutionary Shifts in Rhodopsin Function Illuminate Dim-Light Adaptations in Birds

Rhodopsin evolution suggests the ancestral bird lineage underwent a shift from dim-light environments to niches characterized by brighter conditions. Despite this, little is known about functional variation of rhodopsin or its consequences for sensory ecology in crown birds. Here, we performed in vitro expression and determined the retinal release rate and spectral tuning of rhodopsin pigments in 16 extant bird species, including seven members of two clades adapted to nocturnal niches, namely owls and nightbirds (nightjars and allies). We found that rhodopsin retinal release was conserved in owls but accelerated in nightbirds, suggesting adaptation for higher temporal resolution in dim light. The oilbird (Steatornis caripensis), which inhabits caves and uses echolocation, showed the fastest release rate, possibly representing convergent evolution with echolocating bats that exhibit similar kinetics. Unexpectedly, the flightless cassowary (Casuarius casuarius) exhibited a blue-shifted spectral tuning of rhodopsin relative to its close relative, the emu (Dromaius novaehollandiae). Blue-shifted rhodopsins were also identified in another bird, the white-bellied erpornis (Erpornis zantholeuca), and in a terrestrial mammal, the okapi (Okapia johnstoni), but not in its close relative, the giraffe (Giraffa camelopardalis). Thus, the blue-shifted rhodopsin, normally associated with deep-sea or fossorial amniotes, appears to have undergone convergent evolution in these three lineages, suggesting possible visual adaptation to dim-light conditions in forest habitats.