Marsupials are an infraclass of mammals, characterized by a distinctive pouch (called the marsupium), in which females carry their young through early infancy. Most living species occur in Australia, New Guinea, and nearby islands, but there are also many species in the Americas. 
Like humans, many marsupials have trichromatic colour vision. (That is, they perceive colour using three distinct colour-sensitive pigments: blue sensitive, yellow sensitive, and red sensitive.) Among humans the gene for the third red sensitive pigment is X-linked and polymorphic. As a result a large minority of human males perceive red and green slightly differently from the majority. Although many marsupials have three colour sensitive pigments, there is no genetic evidence that the red sensitive X-linked pigment is present in marsupials. Instead, the red sensitive pigment is replaced with a pigment that is antigenically distinct from the human homolog. In addition, the third colour sensitive pigment appears to be missing altogether in tammar wallabys. Unlike humans who perceive three primary colours (blue, yellow, and red) wallabys may perceive only two (blue and yellow). As a result, wallabys may not be able to distinguish between the colour red and the colour green.