Wallabies lack other marsupials' capacity to distinguish colors, but this doesn't stop the members of the kangaroo family getting hooked on color-based computer games.
Reptiles and birds have four different color sensors in their eyes. Some invertebrates go even further, but most placental mammals make do with just two, probably because the ancestral species were nocturnal.
Humans can thank our primate ancestors for (most of us) being able to see the color range we can. Somewhere along the line a third cone type was added to the retina, making us trichromatic. “For fruit eaters the ability to spot a red apple against green foliage, rather than get stomach ache from eating an unripe fruit, is a big advantage,”says Curtin University's Dr Wiebke Ebeling.
So what about marsupials? Having branched off the evolutionary tree from placental mammals early in the piece scientists thought they might have retained extra color sensors.
Past research found that honey possums and fat-tailed dunnarts are trichormatic. Quokkas (tiny kangaroos too cute to be believed) also show signs of trichromacy. However, in 2010 Ebeling produced evidence tamar wallabies might have only two color sensors (dichromatic). Tammar wallabies are quite closely related to quokkas, while being larger and almost as cute. They also make excellent study animals, adapting well to captivity.
Ebeling, then based at the Australian National University, set out to conclusively test the wallaby's eyesight. She reports in PloS ONE showing them differing colored lights, and training them to push the button lit up to be the most similar color in return for a food pellet. Pressing the wrong button led to the wallaby being locked out of the game for a period.
“The most remarkable result was the determination of the ‘Neutral Point’ which describes a single color that to wallabies looks identical to white, where the animals cannot make up their mind which switch to choose,” Ebeling says. “In the case of wallabies, this was a shade of cyan (greenish blue).” Neutral Points are restricted to dichromatic species.
Ebeling also learned a lot about wallaby behavior. Although the buttons could be pressed with paws her subjects preferred to use their noses. The wallabies also got so competitive they often wouldn't stop to eat the food reward – researchers would arrive in the morning to find the nocturnal animals with a tray full of food pellets from playing all night.
However, one wallaby had to be booted out of the program when she began pressing the buttons at random. Investigations revealed she had recently become pregnant. “We didn't intend this,” Ebeling explained, but keeping the males and females apart was insufficient. Wallabies, like most kangaroos, can put their embryos in suspended animation, or embryonic diapause and restart the pregnancy when they are good and ready. Perhaps an abundance of food pellets inspired the wallaby it was time to get into the pregnancy proper.
Puzzles remain, however. The gene for the third photoreceptor in other marsupials has not been found, leading Ebeling to speculate that some other function has been doubled up to detect light at a third wavelength. Moreover, it is unclear how wallabies could have lost this receptor while quokkas kept it. There are rumors of dichromacy in certain possums and Ebeling is keen to get funding to study a variety of species. “I'd really like to test koalas,” she says, “But you can't find an incentive to get them to play.” Instead she would like to enroll some wombats in her program, but agrees she may need a more sturdy machine.