Cave-animal eyes astonish researchers

Riotous photon cell elevation in subterranean rodents

Cave living Graumull © MPG
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The eyes of underground African Mulle (sand digger) have, contrary to previous assumptions, a normally developed retina with a high proportion of a certain form of Lichtsinneszellen, the pin. These are actually responsible for seeing in daylight, and their use in the lightless world of Mulle is therefore extremely puzzling. The new findings challenge common beliefs about the visual organs of subterranean mammals.

While in other mammals, as well as humans, a green-sensitive cone pigment dominates, most cones of Mulle contain a blue-sensitive visual pigment. The density of light sensory cells responsible for twilight and night vision, the rods, is much lower in the Mullen than in above-ground rodents. The research team from the Max Planck Institute for Brain Research in Frankfurt, the University of Duisburg-Essen and the Charles University in Prague published these findings in the European Journal of Neuroscience.

Underground life

Remarkably many mammals have become completely or partially subterranean during evolution - nearly 300 species of rodents, insectivores and marsupials. Presumably as an evolutionary adaptation to the lightless habitat, most of these subterranean species have regressed small eyes; many are considered blind. The African Mulle (sand digger, Bathyergidae) are among those rodents who spend their lives completely underground in the current state of knowledge. The animals feed on roots and tubers, to which they dig searches, and also raise their cubs underground.

The eyes are small, depending on the type 1.5 to 2.5 millimeters in diameter. Leo Peichl from the Max Planck Institute for Brain Research in Frankfurt / Main, Pavel Nemec from the Charles University in Prague and Hynek Burda from the University of Duisburg-Essen have the eyes of three species, the Graumull Cryptomys anselli, the giant gastric C. mechowi and the Nacktmull Heterocephalus glaber, examined more closely and found amazing.

Enigmatic cone pile

The retina of the eyes was anatomically normal and showed no abnormal regression. On the contrary, the researchers found an unusually high proportion of ten percent cones among the light-sensory cells. Nocturnal rodents such as the rat and the mouse have a cone proportion of only one to three percent - which is not surprising since in the case of lunar or starlight these sensory cells do not yet respond. Even the most diurnal mammals get by with five to twenty percent cones. display

Why should the living in the dark Mulle invest so high in the cones, which are active only in daylight? The majority of the photoreceptor cells are the little streaks of all nocturnal and most diurnal mammals, which are responsible for seeing at low magnitudes. Here, the Mulle prove to be less well equipped: Their St bchendichte is only a quarter as high as that of the mouse. Why save the mulle just on the light-sensitive St bchen?

Reversal of the normal

Another surprise came when they examined the visual pigments of the cones. Usually, the retina of most suckers contains two spectral cone types: about 90 percent are green-sensitive cones, only 10 percent are blue-sensitive. This allows a passable, so-called dichromatic color vision. In the Mullen, however, about 90 percent of the cones contain the blue-sensitive visual pigment, the remaining 10 percent are pure green cones. The Mulle are the first mammals to observe such a radical reversal of the mixing ratio of green and blue cone pigment.

None of these special features found in the Mullen (high proportion of blue cones, small number of stabs) fits the thesis of the general retraction of the retina in adaptation to the lightless underground habitat. Evolutionary biologists generally assume that unused structures are degraded because they cost unnecessary metabolic energy - nature does not provide any superfluous benefits. Rather, one would interpret these peculiarities as specializations for special visual tasks.

What these visuals might be like must show future behavioral and field studies. Maybe the animals sometimes leave their subterranean habitats? There is still too little knowledge about the visual challenges and achievements of Mulle. The hypothesis of the general, convergent reduction of the organ of vision in subterranean habitants is now definitely on the test.

(MPG, 07.04.2004 - NPO)