The interacting effects of total light intensity and chromatic contrast on visual signal visibility in an Anolis lizard

Abstract

The sensory drive hypothesis states that selection acts on signals to make them more detectable in the habitat conditions in which they occur, resulting in signal divergence for species occupying different habitats. For colour signals, visibility depends on the luminance contrast and the chromatic contrast between the signal and the viewing background. Sensory drive has been tested in studies of the colourful dewlaps of anolines occupying different habitats. These studies found that red or orange dewlaps were more visible than yellow or white dewlaps across all habitat types, counter to the predictions of sensory drive that a species’ signal should be more visible in its own habitat than in habitats of other species. However, in these, and other sensory drive studies, chromatic contrast was calculated with a visual perception model that assumed that total light intensity has little or no effect on chromatic contrast perception. We carried out behavioural experiments testing the probability of detection of green, yellow and red stimuli presented against luminance-matched green backgrounds, at low and high light intensity typical of shaded and unshaded habitats. We found that the red stimulus was most detectable in the high light condition, while in low light, yellow and red stimuli were equally detectable. We modelled the stimuli with a receptor noise model that takes total light intensity into account. The model predictions were consistent with the behaviour results. We conclude that there is an important interaction between total light intensity and chromatic contrast in determining the visibility of colour signals, which should be taken into account in visual ecology studies. For animals with small eyes, shade level, which strongly influences total light intensity, may be as important as, or more important than habitat spectral quality in the evolution of signal colour diversity through sensory drive. • Anolis lizards signal with red and yellow throat fans. • We tested effects of ambient light on signal visibility against a green background. • In high light, typical of low shade, red is most visible. • In low light, typical of shaded habitat, yellow and red are equally visible. • Habitat shade level may drive the evolution of Anolis signal colour differences.