I found this explanation at Smithsonian:
National Geographic‘s Ed Yong reports on the finding:The bit that actually changes colour is the tapetum lucidum or “cat’s eye”—a mirrored layer that sits behind the retina. It helps animals to see in dim conditions by reflecting any light that passes through the retina back onto it, allowing its light-detecting cells a second chance to intercept the stray photons. The tapetum is the reason why mammal eyes often glow yellow if you photograph them at night—you’re seeing the camera’s flash reflecting back at you.Reindeer eyes, by default, are gold. But during the long winter, their pupils dilate for months on end, Yong explains. All of this effort takes a toll on the reindeers’ eyes, which begin to swell and in turn exert pressure on tapetum.This layer is mostly made up a collagen, a protein whose long fibres are arranged in orderly rows. As the pressure inside the eye builds up, the fluid between the collagen fibres gets squeezed out, and they become more tightly packed. The spacing of these fibres affects the type of light they reflect. With the usual gaps between them, they reflect yellow wavelengths. When squeezed together, they reflect… blue wavelengths.The wintery blue, Yong writes, is about 1,000 times more sensitive to light than the summery gold. The latter color, on the other hand, helps in the summer by bouncing the majority of light off of the animals’ eyes, effectively acting like a pair of natural sunglasses.
You learn something every day.
None of the explanation of why seems to make sense. simply refelecting back light that was scattered by the retina would not improve vision, it would generally make it worse since the light would be reflected to a different focal point making a double image, and probably out of focus. The exception would be an extremely tight coupling of the curvature of the outerlens to the altered lens density to make the reflector and lens work in a miraculous cancelation--not only unlikely, but even less likely to work as the eye was refocused or re-aimed. On top of all that anything that made the lens layer more reflective of scattered light from the retina would have the even stronger effect of reducing the amount of light entering the eye in the first place! So again it would work less well. I suspect that possible better hypotheses might be that there is some chemical change in the retina that increases its sensitivity in winter (articel says 1000x !) but for some reason the pigment that provides that isn't chemically stable to blue light, so a blue light filter is added to the front the lens. Another possible hypothesis is that since it is true that a larger iris also will increase any chromatic abberations in any imperfect lens, the visually accuity is improved by removing the blue light from the spectrum. A final hypothesis is that since blue light is more easily scattered, and winter has snow which will reflect more blue light there is increased blue light glare in winter. So they reduces that. This by the way is the same reason they sell "blue blocker" sun glasses. While those are orange looking, that's because they work by absorption so they absorb blue. The lenses of the deer are working by reflection and so reflect blue. My money would be on this last effect.
ReplyDeleteMy understanding is that the tapetum lucidum, being behind (interior to) the retinal photoreceptors, reflects back any light rays that traveled between and missed the rods/cones on the inward journey, thus giving those cells a second chance to receive that light, thus improving sensitivity.
ReplyDeleteI don't understand the lens to be more reflective - just the tapetum.
Hmmm good point. The photo has me puzzled. It looks almost like there is an outer iris (slit) and an outer iris (round). Perhaps the inner one is a trick of the light passing through the lens but maybe not?
Deleteif it is there then I'd say the lens itself seems to have blue cast and I would assume that is what is being discussed. But if not then perhaps you are right that there is an ambient blue scatter from somewhere deep in the eye. I have read owl retina also have reflectors, but night time flight is more demanding than twilight darkness of the northern regions. The slit pupil is intriguing. My knee jerk guess is that like a telescope aperature this would trade enhanced horizontal resolution for vertical resolution.
I don't understand, wouldn't it be detrimental to have their eyes being 1000x more sensitive to light? I would think snow would reflect a lot of bright, white light so in a snowy environment I imagine their vision would be whitewashed and it'd be like when we look into the bright headlights of a vehicle and cannot see what's on either side of the vehicle behind the headlights.
ReplyDeleteIn the winter, in the Arctic, sunlight is a rare commodity.
DeleteWow! My mistake, I was thinking of regular deer and somehow completely missed that this change in eye color happens with REINDEER.
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