Colour Vision in the Virginia Opossum

Abstract

COLOUR vision in mammals has long been considered to be restricted to primates, possibly as a result of independent evolution of the ability after it was lost during a nocturnal stage of mammalian evolution¹. A variety of investigations, however, have indicated some ability to discriminate on the basis of colour in the European red squirrel², the pigmy goat, red deer, and Nilgai antelope³, the Masai giraffe⁴, the common cat⁵, and the antelope ground squirrel⁶. Because the Virginia opossum (Didelphis virginiana) is very similar to the earliest known mammals⁷, the presence or absence of colour vision in this species would be of particular relevance to theories of the phylogeny of colour vision in both marsupial and placental mammals. The marsupial eye is essentially like that of a placental mammal in structure, while the retina is basically a reptilian one containing single and double cones bearing oil-droplets and a large number of filament rods. In particular, the opossum has a clearly nocturnal eye and some of the single cones lack oil-droplets and are similar to the cones of placental mammals⁸. A further consideration is that the marsupials are the only class of vertebrates having double cones in the retina (fish, amphibians, reptiles, and birds being the others) which have not yet been shown to have colour vision⁹.