Moles and their eyes May 12, 2008Posted by David Corney in Uncategorized.
Tags: eyes, moles, nocturnal vision, photoreceptors
I always thought that moles, being subterranean, were virtually blind. Turns out I was right, but their eyes are much more interesting than I would have thought. A new paper by Glösmann et al. in the Journal of Vision taught me lots. Briefly, moles have at least two colour photoreceptor cells (i.e. cones), potentially giving them colour vision in line with most mammals. However, their short-wavelength cone is down-shifted relative to humans, meaning that they can see ultraviolet (UV) light. The lens and cornea of the human eye scatters most blue/violet/UV light, to protect the sensitive retina from potentially damaging UV light. Presumably, if you’re subterranean, then such damage isn’t an issue, so moles have lenses that transmit blue/UV light much better than ours do.
Also, it seems that many / most of their cones co-express both medium- and short-wavelength sensitive opsins (light sensitive proteins). I’d always thought that each cone type only had a single photopigment, so ‘S’ cones just had ‘S’ opsins, and ‘M’ cones just had ‘M’ opsins. Turns out that many mammals, including moles and humans show co-expression of S and M opsins, during at least some stage of their development. Co-expression means that the sensitivity functions are broader than would otherwise be expected, so a co-expressing ‘blue’ cone will be more sensitive to green/yellow light that before, and a co-expressing ‘green’ cone more sensitive to blue light. I suppose that in theory, given three opsins one could have three single-expression cones, plus 6 3 (see comments) dual-expression cones plus 1 triple-expression cone type. Could the rest of the visual system make sense of this? Yes! (I think.) Having more cone types may reduce the spatial acuity, as it reduces the density that any single cone type could have, but increases the colour sensitivity. And if the response functions largely overlapped, then I don’t think the loss of spatial sensitivity would be too great anyway. It might require a few new post-receptoral channels, but as long as each cone gave an essentially unchanging response to any given stimulus, then the rest of the visual system should be able to interpret things correctly.
The Final Fascinating Fact I learnt from this paper is why moles can see at all: the main reason seems to be so they can detect breaks in their tunnels. If something is burrowing in to eat them, or if a passing heavy cow accidentally causes a mini-collapse, the mole has to know so that it can run away or repair the damage. Which makes we wonder: if the soil above part of a tunnel becomes progressively weakened, e.g. by air or water erosion, would UV light get through before visible light? Might UV sensitivity allow a mole to go and fix an otherwise invisible weakness and prevent tunnel collapse? Or is their UV sensitivity merely a left-over from some other evolutionary branch? Or does it somehow help them to simply mess about in boats?
Reference: Glösmann, M., Steiner, M., Peichl, L., Peter , A. (2008). Cone photoreceptors and potential UV vision in a subterranean insectivore, the European mole. Journal of Vision, 8(4), 1-12.
PS Don’t forget, of course, that every mole contains 6.02214×10^23 molecules…