April 6, 2004 | David F. Coppedge

Underground Rodents Have Better Eyes Than Darwin Predicted

European scientists looked into the eyes of African mole-rats, expecting to find retinas that had deteriorated due to disuse in the underground, lightless environment.  What they found were several surprises that “call for a revision of our current views on the visual system of subterranean mammals,” reports a Max Planck Society press release.
    The eyes look smaller on the outside, but that belies their internal complexity.  The scientists “discovered that in contrast to previous assumptions, the eyes of subterranean African mole-rats have a rather well-structured retina with an unusually high proportion of cone photoreceptors,” says the report.  Since cones are the photoreceptors for daylight vision, “their usefulness in the lightless world of mole-rats is puzzling.”  Also puzzling was that 90% of the cones are sensitive to blue light, whereas in most mammals 90% are sensitive to green.  “The density of rods, the photoreceptors for low-light night vision,” furthermore, “is much lower in the mole-rats than in nocturnal surface-dwelling rodents.”  These findings were the opposite of what was expected:

The retinae were anatomically well-developed and showed no obvious deficits.  To the contrary, the researchers found an unusually high proportion of 10% cones among the photoreceptors.  Surface-dwelling nocturnal rodents like rat and mouse have only 1 – 3% cones, which is not surprising as cones do not operate in moonlight or starlight.  Even most diurnal mammals have no more than 5 – 20% cones.  Why should the mole-rats, living in constant darkness, invest so highly in the cones that only work in daylight?  The dominant majority of photoreceptors in all nocturnal and most diurnal mammals are the rods, which are used for vision at low light levels (night vision).  Here the mole-rats are less well equipped.  Their rod density is only one quarter of that of, for example, mice.  Why are the mole-rats so sparing with their light-sensitive rods?

The press release offers no new hypothesis to explain these observations.  It just admits the assumptions were wrong, and it’s back to the drawing board:

In summary, the photoreceptors of African mole-rats show stark deviations from the common mammalian pattern.  But none of these peculiarities fit the concept of a general regression of the retina in adaptation to a lightless living environment.  Evolutionary biology would predict that obsolete structures are removed because they are metabolically too expensive.  Hence these photoreceptor features should be interpreted as specializations for particular visual needs. 

It’s going to take more work to sort this out, they say.  “At present we know too little about the visual challenges and capabilities of these animals,” they admit.  But one thing they do know: “Certainly, the hypothesis of a general, convergent reduction of the eyes in subterranean mammals is up for re-examination.”

So another evolutionary assumption has been falsified by observations.  Nice work.  Since Charlie is taking a stoning as a false prophet, how about giving the intelligent design community a shot at making and testing hypotheses?

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Categories: Mammals

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