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…
Kiwi and night vision January 29, 2008Posted by David Corney in Uncategorized.
Tags: nocturnal vision
A (fairly) new paper by Graham Martin et al. in the wonderful PLoS ONE discusses kiwi and their eyes. Coming from (nearly) the opposite side of the world, I am (was) embarrassingly ignorant about kiwi. I knew they were large and flightless birds from New Zealand, but that was about it. (I even thought the plural was “kiwis” .) I now know that they’re nocturnal, like quite a few birds, but they’ve evolved surprising eyes.
To see in the dark, many nocturnal animals have evolved relatively large eyes, such as owls, lemurs and some monkeys, to gather what little light there is. Against this however, eyes are heavy, being balls of mostly-water, and weight is always a concern if you’re flying. So at first glance, you might expect (as the authors mention) that a bird that stops flying might evolve to have larger and larger eyes, as weight becomes less of an issue. Especially if it’s nocturnal. However, kiwi have small eyes for their bodies, and what’s more, they have small optic nerves and small visual cortices. They’re not blind like cave fish, although given a few million years more, who knows?
Moving forward a few inches, all birds have nostrils, usually at the base of the bill or even inside the mouth. Kiwi, uniquely, have their nostrils at the tip of their bills, coupled with fine touch sensors all over the bill tips. They feed by pecking at surface-living insects or by probing the soil with their long bill and sensing underground insects, suggesting a convergent evolution to the same ecological niche filled by mammals in many parts of the world. And if you’re finding grubs underground, you don’t need vision, of course.
According to the “wiki-kiwi” page, in areas where people are absent, kiwi are active during the day. Which makes me wonder if they have ended up with reduced visual processing simply because they can’t see what they’re eating anyway, whether it’s day or night, so why waste the effort? It seems to me that there are two evolutionary stories that fit this data:
- In version one, kiwi evolved to find food in the topsoil with their beaks and so didn’t need good vision; in turn, they spent less energy growing and using eyes; and then finally they tended towards nocturnal behaviour because there was no extra cost to them.
- In version two, kiwi became nocturnal to avoid predators (not that there were any mammals to compete with until recently) or to find nocturnal insects perhaps; and then they developed poorer eyesight because good eyesight was no longer required.
It could of course be some mixture of the two, as evolutionary histories needn’t have a nice clear narrative. In either case, I guess they still need at least rudimentary vision for mate selection, not walking into trees, that kind of thing. Anyway, I’ve learned a lot about kiwi, for which I am grateful!
Martin, G.R., Wilson, K., Wild, J.M., Parsons, S., Kubke, M.F., Corfield, J., Iwaniuk, A. (2007). Kiwi Forego Vision in the Guidance of Their Nocturnal Activities. PLoS ONE, 2(2), e198. DOI: 10.1371/journal.pone.0000198
See also: “The allometry and scaling of the size of vertebrate eyes” doi:10.1016/j.visres.2004.03.023
“Some nocturnal animals rely on senses other than vision, which is reflected in their small eye size. Others take the strategy of increasing eye size as much as possible to compensate for the low light conditions.”
 I checked the OED to see what it said about the word “kiwi”. It gives the etymology as “Maori” which isn’t terribly informative, but does have a quote from a Walter Lawry Buller and his 1873 text, A history of the birds of New Zealand: “Last Sunday I dined on stewed Kiwi, at the hut of a lonely gold-digger.” So I’ve learned something already…
Night and day January 11, 2008Posted by David Corney in Uncategorized.
Tags: illumination, insect vision, nocturnal vision
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Although I’ve been researching vision and creating synthetic images for a while now, I’d never really thought about night-time illumination. Well, not beyond thinking, “It’s dark at night!” I suppose. But then I read a paper  by Javier Hernandez-Andres and his group in Granada about crepuscular and nocturnal vision, and learned that natural light at night is a lot more complex (and interesting) than I thought. During the day, all light come from the sun or from skylight, which is just scattered sunlight. But at twilight and through the night, there are many and varied light sources.
After the sun passes below the horizon, it still lights up the sky for a while so that’s one source of light. Then there’s moonlight, which is a direct reflection of the sun and has a very similar spectrum to daylight, at least for a high and full moon. And there’s starlight, which has a spectrum roughly like daylight but fainter and with four distinct spikes around the yellow / red region. Then illumination starts getting really exotic. There’s “airglow”, which was first (officially) noticed by Anders Ångström (he of the unit) in the mid-19th century. It consists of various light-emitting molecular processes in the upper atmosphere, which produces a faint blue-ish glow across the sky. Then there’s “zodiacal light”, which was noted by Cassini (he of the Saturn orbiter) in the 17th century. It consists of sunlight bouncing off scattered cosmic dust between the planets of our solar system, so again it has the same spectrum as sunlight, albeit fainter. And apparently, the very dark blue sky seen during late (“nautical”) twilight is that colour because of ozone absorption, and not (just) due to sunlight scattering effects. In other words, it’s not just “blue sky but a bit darker”, but is blue for a different reason.
The final nocturnal light source Hernandez-Andres et al. mention is anthropogenic light – light pollution. This varies enormously across space and time of course, with a strong yellow/red shifted spectrum suddenly appearing whenever a million streetlights click on at dusk, along with car headlights, office lights, advertising hoardings etc. etc. Scientists are now realising that many nocturnal animals, including some moths, rely on very subtle colour cues for foraging and mating, just as diurnal animals do. What effect light pollution is having on these creatures seems to be unknown, but presumably it forms a strong selection pressure, at least near built-up areas. Sounds like a ripe area of future study…
Johnsen, S. (2006). Crepuscular and nocturnal illumination and its effects on color perception by the nocturnal hawkmoth Deilephila elpenor. Journal of Experimental Biology, 209(5), 789-800. DOI: 10.1242/jeb.02053