Australasian Science: Australia's authority on science since 1938

Pokies Machine Tests Wallaby Vision

By Stephen

Wallabies with an interest in computer games have revealed intriguing questions about the evolution of marsupial eyesight.

Humans and some other primates are trichromatic – we assemble the colours we see from three types of colour receptors in our eyes. Some species are far more exotic in their colour perception (AS, August 2008, p.10) but other placental mammals have just two sorts of colour receptors (dichromatic), giving them much poorer capacity in this regard.

“For fruit-eaters, the ability to spot a red apple against green foliage, rather than get stomach ache from eating unripe fruit, is a big advantage,” says Dr Wiebke Ebeling of Curtin University’s Department of Imaging and Applied Physics.

Ebeling has been testing colour vision in marsupials by sticking pellets of food to a machine with two buttons. When eating, test animals would accidentally push one of the buttons, sometimes leading to food rewards.

Further exploration teaches the animal that food is delivered when the button pushed is the one more closely resembling a colour displayed elsewhere. Pushing the wrong button locks the animal out of the game for a while.

“Our wallabies learned quickly and seemed to enjoy working on the machine, and often did not bother actually eating the food – they just played for the fun of it,” Ebeling says.

“The most remarkable result was the determination of the ‘neutral point’, which describes a single colour that to wallabies looks identical to white so they cannot make up their mind which switch to choose,” Ebeling says. “In the case of wallabies, this was a shade of cyan (greenish-blue).” The existence of a neutral point is a feature of dichromatic animals.

Dichromatism in wallabies is surprising because their close relative, the quokka, shows signs of trichromatism. Dunnarts are trichromatic despite the fact that as carnivores they need smell more than good eyesight. “It still remains a mystery what exactly the additional photoreceptor in other marsupials is and why the wallaby should be the only one to miss it,” Ebeling says.

While primates have a specific gene that codes for a green photoreceptor, no equivalent has been found in marsupials, leading Ebeling to wonder if some other gene has been conscripted to serve a dual purpose. She says that if this turns out to be the case it could lead to exciting new paths for vision research.

The study was reported in PLOS One.