Red vision in animals is broadly associated with lighting environment but not types of visual task

Red sensitivity is the exception rather than the norm in most animal groups. Among species with a long wavelength sensitive (LWS) photoreceptor, peak wavelength sensitivity (λmax) varies substantially and it is unclear whether this variation can be explained by visual tuning to the light environment or to visual tasks such as signalling or foraging. Here, we examine long wavelength sensitivity across a broad range of taxa showing diversity in LWS photoreceptor λmax: insects, crustaceans, arachnids, amphibians, reptiles, fish, sharks and rays. We identified 161 species with a LWS photoreceptor (λmax ≥ 550 nm). We found evidence supporting visual tuning to the light environment: terrestrial species had longer λmax than aquatic species, and of these, species from turbid shallow waters had longer λmax than those from clear or deep waters. Of the terrestrial species, diurnal species had longer λmax than nocturnal species, but we did not detect any differences across terrestrial habitats (closed, intermediate or open). We found no association with proxies for visual tasks such as having red morphological features or utilising flowers or coral reefs. These results support the emerging consensus that, in general, visual systems are adapted to broad range of tasks, rather than tuned to certain tasks.

Methods
Full details of data collection and processing are in the manuscript.