Loss of Privilege
Blue, green and red - our eyes put together the colorful world of colors from three different photoreceptor cells. This sensory-physiological trinity has hitherto been considered unique among mammals, which is only realized in apes including humans. But a small marsupial is challenging this primate privilege.
In 1801, the British doctor and physicist Thomas Young put forward a hypothesis whose physiological foundations were still completely unknown: he assumed that any color that the human eye is able to perceive consists of light of three different wavelengths can be combined.
Today, this three-color theory – further developed by the physicists Hermann von Helmholtz and James Clerk Maxwell – has been confirmed by the discovery of the corresponding visual cells: in addition to the highly light-sensitive but color-blind rods that ensure twilight vision, our retina has three different types of cones that perceive light of different wavelengths and therefore different colors. From the spectral ranges blue (430 nanometers), green (535 nanometers) and red (565 nanometers) our eye assembles the colorful world of colors.
This trichromatic color vision system has probably evolved several times independently of each other. It is well known that many insects, such as bees, are excellent at distinguishing between colors because they also have three types of photoreceptor cells with different levels of sensitivity. And also a number of fish, amphibians, reptiles and birds rely on the alliance of three.
Among the mammals, however, humans are one of the color-physiological exceptions. Although most mammals can perceive colors, they are content with two types of cones: one in short-wave blue light and another for long-wave light. This dichroic system is thought to have evolved as the nocturnal ancestors of mammals gradually transitioned to diurnal lifestyles about 65 million years ago. It wasn't until about 35 million years ago, when Old World and New World monkeys went their separate ways, that primates-which includes humans-evolved a third color receptor. So much for the theory.
The biologists were all the more surprised when the researchers led by Catherine Arrese from the University of Western Australia in Crawley discovered a third color receptor in an inconspicuous marsupial in 2002: the fat-tailed narrow-footed marsupial mouse (Sminthopsis crassicaudata) has three types of cones. Two absorb in the mid-wavelength range of 509 and 535 nanometers, respectively, while a third specializes in ultraviolet with a wavelength of 363 nanometers.
Now the existence of three types of cones is an important indication for trichromatic color vision, but the final proof was still pending. Together with her Australian colleague Lyn Beazley and Christa Neumeyer from the University of Mainz, Arrese has now provided this proof. The scientists taught three marsupial mice that choosing a certain color would give them a coveted treat. By mixing light of different wavelengths, the researchers were then able to test the extent to which their test animals could distinguish colors. And it actually turned out that the color perception of marsupial mice – like that of primates – is based on three different photoreceptor cells.
This means that the primates lose a privilege: Apparently not only they but also marsupials – a very original group of mammals – have a fully developed trichromatic color vision system. Unlike us, they can also see ultraviolet light; the color red, on the other hand, leaves them rather cold. This should make the colorful world of a bag mouse a little different from ours.
