Fig. 8. Predicted relative threshold spectral sensitivity for the potentially tetrachromatic colour vision system of the peafowl Pavo cristatus. The model used is that given by Vorobyev and Osorio (1998) and assumes that thresholds are determined by photoreceptor noise, that noise in a given colour channel is proportional to the reciprocal of the square root of the relative proportion of that cone type in the retina, and that the level of noise in a cone is independent of spectral sensitivity. VS, SWS, MWS and LWS single cones were assumed to be present in the ratio 1:1.9:2.2:2.1, respectively (Hart, 2001a). Spectral sensitivities for the different cone types were those presented in Fig. 7.