Trichromatic vision in toads: evidence from preference for colour objects during mate choice

Input of different spectral types of photoreceptors in amphibian colour vision has been assessed by studying the behaviour of male toads Bufo bufo and B. gargarizans in a laboratory. This method is based on the males’ innate reaction of approaching and clasping any visual object during the breeding season. A pair of colour stimuli made of paper or displayed on a LCD monitor has been selected to match the toad’s RGB colour space so that they differ by excitation of only a single spectral class of retinal photoreceptors. With pairwise presentation, the males are found to prefer a more ‘bluish’, or a less ‘reddish’, or a less ‘greenish’ stimulus from each pair. A microspectrophotometry of isolated rod and cone photoreceptors and an observation of behavioural responses in toads have revealed a trichromatic mesopic visual system based on a positive input from blue-sensitive photoreceptors and negative inputs from red-sensitive cones and green-sensitive rods. It is supposed that the observed colour preference and involvement of different chromatic mechanisms determine social interactions of anurans during mate choice.