Here it is:
|Quokka, photographed by Loetifuss, 9/23/2005|
I know, I know. So cute!
The important point is that like wallabies, quokkas are marsupials. This means that wallabies are much more closely related to quokkas than they are to placental mammals, like dogs. However most placental mammals (except for some primates) have only two types of cones and thus have limited color vision (they're dichromates). In contrast, many marsupials, including quokkas, are trichromatic, having three types of cones. Yet, the closely related wallaby is a dichromate.
How do we know? Well, by training the critters to indicate whether two light panels appeared to be the same color.
It’s pretty clear that the wallabies are dichromatic. What’s not clear is why they differ in this fundamental way from other, closely related marsupials.