December 10, 2010

Wallabies Have Poor Color Vision

Contrary to the recently established theory that marsupials have excellent color vision, research has shown that the wallaby is a rare exception.

The research 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 colors because they are missing one type of visual pigment cell.

"Dr Ebeling has demonstrated that wallabies do not have the complete set of three visual pigments in their cones "“ the vision cells that provide us with color vision," says Dr Hemmi. "Wallabies are dichromats "“ they have only two visual pigments out of the three "“ this means that they get less color input to their brain and see things similar to other mammals, or a color-blind human."

"Given that most marsupials are trichromats "“ having three visual pigments, this poses the million dollar question: why do wallabies differ in color vision when they come from the same family, as the quokka for instance?"

Dr Hemmi explains 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 color discrimination. However, upon the discovery that marsupials may be able to discern colors just as well as humans, it was 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," he says. "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 "“ color vision in bright lights. This could have contributed to the loss of diversity in their vision cells."

"Up to now we still do not have a clear idea of why marsupials are exceptional in their color vision compared to other mammals. An example of this is the trichromatic dunnart which has a third cone that has not yet been identified."

"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 color vision."

The research team is currently investigating the vision of the brushtail possum, which is a nocturnal marsupial. Their current finding, "ËœDiversity of color vision: not all Australian marsupials are trichromatic' has been published in the journal PLoS ONE.

The Vision Centre is funded by the Australian Research Council as the ARC Centre of Excellence in Vision Science.


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