Accurate knowledge of species colour discrimination is fundamental to explain colour based behaviours and the evolution of colour patterns. We tested how the receptor noise limited model, widely used in behavioural ecology, matched actual colour discrimination thresholds obtained using behavioural tests. Guppies (Poecilia reticulata) were first trained to push a target coloured disk placed among eight grey disks of various luminances on a grey plate. Guppies were then tested to find target disks, which varied in colour contrast from the plate. The target disks followed a gradient going from high contrast to inconspicuous against the grey background. We plotted the percentage of correct choices of each colour in the gradient against the model prediction and determined the discrimination thresholds using the inflection point of the fitted sigmoid curve. We performed the experiment on six colour gradients: red, orange, yellow, green, blue and purple. Four colour gradients: red, orange, green and blue, showed a discrimination threshold that matched the model predictions. However, deviations of the model for the yellow and purple gradients suggest that ecological relevance of some colours could affect decision-making in behavioural tests and that we can no longer assume that the rules for colour discrimination are independent of colours.

Visual pigments can vary across the retina in many vertebrates, but the behavioural consequences of this retinal heterogeneity are unknown. Guppies (Poecilia reticulata) vary dorsoventrally in visual pigments and forage both on the ground and at the water surface, exposing different retinal regions to two very different visual environments. We tested guppy behaviour towards a moving stimulus presented below or above the guppy. We used 12 different narrow-band wavelength stimuli matching each of the opsin peak sensitivities presented either at the top or the bottom of our experimental apparatus. We analysed behaviours of 50 male and 50 female guppies over 4800 trials where a moving stimulus pattern was presented to each guppy. We found that wavelength, position and speed of the stimuli influenced male and female behaviour and seems to be mediated by the long wavelength sensitive photoreceptors. Males also had stronger behavioural responses than females whereas females performed more foraging-related pecking behaviour. Our results suggest that the spatial requirement of visual tasks and their ecological context are important and appear to be partly correlated with photoreceptor arrangement in the retina.