Two types of trichromatic squirrel monkey share a pigment in the red-green spectral region.
Microspectrophotometric measurements have been obtained for individual photoreceptors from four female squirrel monkeys (Saimiri sciureus) that had been shown behaviourally to be trichromatic. Relative to a normal human observer, two of the monkeys required more red light for a Rayleigh match; the other two required more green light than a normal human observer. In the red-green spectral region, the first type of monkey was found to have two cone pigments with peak sensitivities at approximately 536 and 549 nm, whereas the second type was found to have pigments with peak sensitivities at approximately 549 and 564 nm. By maximum likelihood estimation it was shown that the microspectrophotometric data could be described by a model that assumed only three underlying distributions, two of which were present in each type of monkey. The fit of this model was as good as one in which a "double normal" distribution was fitted individually to the data for each animal. This result is consistent with a genetic theory that postulates in Saimiri three possible alleles for a single locus on the X-chromosome; the heterozygous female enjoys trichromacy because Lyonisation ensures that only one photopigment is manufactured in any given cone.