Washington: Contrary to the theory that marsupials have excellent colour vision, a new study has shown the wallaby is a rare exception.
An international team, led by Dr Jan Hemmi from the ARC Centre of Excellence in Vision Science and The Australian National University, has shown that Tammar wallabies are much weaker in discriminating different colours because they are
missing one type of visual pigment cell.
"Wallabies do not have the complete set of three visual pigments in their cones - the vision cells that provide us with colour vision.
"Wallabies are dichromats - they have only two visual pigments out of the three - this means that they get less colour input to their brain and see things similar to other mammals, or a colour-blind human," Dr Hemmi said.
He added: "Given that most marsupials are trichromats- having three visual pigments, this poses the million dollar question: why do wallabies differ in colour vision when they
come from the same family, as the quokka for instance?"
The scientists explain that it has long been thought that all mammals have dichromatic vision and that only the primate eye later evolved to have a finer sense of colour
discrimination.
However, upon the discovery that marsupials may be able to discern colours just as well as humans, it`s suggested that the ancestors of the pouched-mammals were always
trichromatic whereas placental mammals were dichromatic.
"It is speculated that mammals lost all but two of their ancestors` cone pigments during evolution. Mammals were proposed to be nocturnal animals early in their evolutionary history, which means they mainly used their rods - vision in low light - to see, instead of their cones - colour vision in bright lights. This could have contributed to the loss of diversity in their vision cells.``
"Knowing that not all marsupials are trichromats allows us to make comparisons and leads us one step closer to locating the X factor that drives the evolution of good colour
vision," Dr Hemmi said.
The findings have been published in the latest edition of the `PLoS ONE` journal.
PTI
