Behavioural evidence of dichromacy in a species of South American marsupial

Colour vision in marsupials is a controversial issue, especially among the genus Didelphis (Didelphidae, Didelphimorphia). While behavioural tests have diagnosed these animals as trichromats and electrophysiological studies have diagnosed them as monochromats, recent molecular genetics studies provide evidence for dichromatic colour vision, having found two classes of cone opsins in a species of this genus. This study examines the colour perception of a male and female white-eared opossum, Didelphis albiventris, through a series of tasks involving a behavioural paradigm of discrimination learning. Both opossums succeeded in discriminating pairs of stimuli consisting of Munsell colour cards presented in random brightness values that are assumed to be easily discriminated by dichromats and trichromats (e.g. blues versus oranges). However, both subjects failed to discriminate between colours that are expected to be easily discriminated only by trichromats (e.g. greens versus oranges). The opossums were also unsuccessful in distinguishing a colour against itself (e.g. oranges versus oranges), demonstrating that discrimination was based only on visual cues. These results are consistent with recent predictions based on molecular genetics suggesting that the genus Didelphis is routinely a dichromat.