Here’s a season-themed science post.
It’s not really the color of the reindeer iris that changes, but the color of the tapetum lucidum (TL), a reflective layer at the back of some animals’ eyes that helps them see better. The TL does this by bouncing photons that escape the retina back to the retina again, increasing what they can see. It’s the reflectance from the tapetum lucidum that makes your cat’s eyes glow when you shine a light on them in the dark or take a flash photo, like this:
Noctural animals tend to have TLs, as they need that extra vision in the dark.
The two papers below, the first in i-Perception and the second in Proc. Roy. Soc. B, explain a phenomenon appropriate to the season: the change in the color of the TL in reindeer (Rangifer tarandus) when winter comes on. Click the title each paper to read it, and there’s also a summary of the two articles in National Geographic with some useful photos. First here’s a reindeer from Norway (there’s only one species worldwide, also called “caribou”):
The two papers displaying and giving explanations for the color change:
So what is the phenomenon we’re discussing? Actually there are two.
First, reindeer live in the far north, and in that environment the light regime changes drastically during the seasons, from being daylight all 24 hours in summer to twilight between 12 and 24 hours a day in winter, with only 60 days of the sun even peeking over the horizon. This causes a drastic change in the color of the environment, making it and the sky turn dark blue during the winter (b and d) below, as opposed to the daylight sky with which we’re familiar.
The TL of arctic mammals helps deal with the low light—after all, if they’re not hibernating they need to escape predators and find food, but the reindeer have two extra tricks. The normal gold color of the TL (similar in other mammals), shown in (a) below, itself turns deep blue (c). Reindeer are the only mammals known in which the TL shifts its color, though not a lot of mammals (especially in the Arctic or Antarctic) have been examined,
The other trick is that reindeer have corneas and lenses that transmit up to 60% of UV light, which is usually blocked in mammals active during the day. (This tends to give captive reindeer cataracts, but in the wild the diet of reindeer may protect them from this malady.) All of this together means that, in the darkness of winter, reindeer have a better view of UV-absorbing and -reflecting objects than do other mammals.
But why these traits? It turns out that reindeer feed in the winter largely on lichens, particularly the species Claudia rangiferina, or “reindeer lichen”, which looks like this (picture from Wikipedia):
And, as it happens, many edible lichens are light-colored, like the one above. As humans we’d be hard pressed to find them, but since they absorb UV, like the one in the first row below, they appear dark against the snow to any animal sensitive to UV light. This makes it easier for them to find food. (The lichen in the second row is not so visible since it, like snow, reflects ultraviolet light):
The other hypothesis, suggested in the earlier Dominey et al. paper, is that the TL change as well as increased sensitivity to UV light of reindeer eyes can help them detect predators like white wolves. But Dominey et al. also note that the eyes of other Arctic prey like musk oxen and roe deer have not been studied. If prey that don’t eat lichen show similar temporal changes in TL color and also similar UV-permissiveness of the cornea and lens, that would support the predator-detection rather than the lichen-detection hypothesis. Looking at the eyes of a few other species could discriminate between these hypotheses (of course, both could be true at once). The eyes studied in the second paper, by the way, were supplied by Sámi people, many of whom herd reindeer for a living.
Two other questions: how do reindeer protect their eyes from UV light (which of course is high in the summer) given that their corneas and lenses still transmit lots of UV in the summer? Why don’t wild reindeer get cataracts? Well, high levels of vitamin C (ascorbic acid) protect against cataracts, and Dominy et al. say this:
Linking UV visual sensitivity to feeding ecology raises tantalizing questions of how Cladonia might simultaneously protect reindeer eyes from UV damage. Cladonia has impressive antioxidant properties (Kosanić et al., 2014), and its combination with other favored foods––namely, the buds and leaves of Arctic willow (Salix arctica) and dwarf birch (Betula nana)––both of which have exceedingly high levels of Vitamin C (Rodahl, 1944), could provide a diversified suite of protective measures.
They follow this with some science humor, which you can see throughout the paper, including its cute title about “Enlichenment”:
Whatever the case, our essay has far-reaching practical applications by suggesting that orange juice and carrots are ideal treats for supplementing reindeer on Christmas Eve.
And here’s a bit more humor from the paper:
Taken together, our limited study of lichens suggests chromatic conspicuousness to reindeer eyes under twilight conditions. They also cast new light on the benefits of a luminescent nose––it may light the way for Santa to see by, but it is Rudolph’s blue-eyes that allow him to find dinner after a long Christmas season.
Second, what is the mechanism of color change in the TL? This is the subject of the second paper, and the mechanism is fascinating—a testament to the power of natural selection. Fosbury and Jeffery did modeling studies on the internal structure of the reindeer eye under various postulated conditions, using data taken from reindeer eyes obtained from slaughterhouses. I’ll quote the National Geographic article to give the postulated mechanism of TL color change, as I’m lazy and it’s Christmas:
Fosbury puzzled through the optical conditions during Arctic twilight and found that the tapetum would “tune” itself to that frequency of light.
He and Jeffery went into the lab to dissect and experiment on a vast supply of reindeer eyes—bags and jars of them, collected over years from reindeer herds owned by the Sami, a Scandinavian Indigenous people.
Reindeer tapeta are made of little fibers of collagen suspended in fluid, forming a changeable reflective crystal. The collagen fibers in summer eyes floated loosely in the fluid, creating a crystalline mirror that best reflected reddish light. But in winter-collected eyes, the collagen fibers were packed much tighter, changing the crystal shape and making it reflect primarily blue light, with a smear into the nearby UV.
In the darkness, reindeer likely dilate their pupils, which in turn blocks a little drainage hole for eye fluid, causing the eye’s internal pressure to build up, compressing the tapetum collagen and changing the crystals’ shape. In summer, the animals’ pupils return to normal.
“You add these things up and the sensitivity of their eyes is at least a thousand times higher in winter than in summer,” says Tyler.
Fantastic! But there’s a downside to this increased sensitivity:
But their unique adaptation may hurt them. Today, high-voltage power lines crisscross traditional Sami herding territory leaking bursts of UV light, which to reindeer looks “like fireworks; they won’t go near it,” says Jeffery—leading the Sami to engage in contentious court battles to protect their herds.
I’d be surprised if this suite of adaptations was limited to reindeer, but nobody has looked at the eyes of other Arctic mammals. That would give a clue to the selective pressures that have likely molded these adaptations.
Merry Christmas!
h/t: Athayde
I was going to say that this article is very enlightening, but they beat me to it with Enlichenment.
Well, UV’e still had a good idea there!
+1
Fascinating! I wonder if Vitamin C is protective against cataracts in humans. (An obvious question, given the report above, and probably well studied.)
I remember learning about how insects can see UV in Bob Silberglied’s (z”l) Biology of Arthropods course in 1978. A most unexpected and interesting finding, and the experiments that demonstrated the phenomenon were paragons of creativity. I served as Bob’s teaching fellow in the course the next year.
I wonder how manny mammals can respond to UV—other than by getting cataracts, of course.
It would be interesting to find out (and relatively simple, experimentally) how much vitamin C reindeer pee away if given a diet rich in free vitamin C.
I take it that everyone here knows that if a human takes, say 5 grammes (not the milligrammes normal for vitamin RDAs) a day of vitamin C, they’ll pee 4 grammes of it away within hours of ingesting the dose. As a putative treatment for everything from flat feet to VD, the actions of our kidneys undermine … I’ve forgotten who the “vit.C Guru” was.
It’s interesting that it’s the UV component that is effective. The Inuit, and mountain dwellers, have known for millennia that snow and ice are relatively effective at reflecting UV, leading to relatively high irradiations on the snow fields. Recent generations of skiers have rediscovered this fact – for which the archaeological record has shown protective devices against back into at least Viking times.
This is really interesting stuff! I also appreciate the touches of levity. One wonders what else we don’t know about common animals, simply because we haven’t thought to ask before?
Thanks for posting this one! I have sometimes been party to questions around spectral properties of biological material. Light and biology — surprises and fascination.
Re what to leave out for the reindeer: In Spanish-speaking countries, the main magical gift-givers are the Three Kings, and you’re supposed to leave out forage for their camels!
GCM
What a great article. Incredible stuff to learn about. Thanks so much!
Great post!
Very interesting! Thank you for sharing.
Amazing. Your articles and brilliant explanations of these marvelous animal adaptations have opened my eyes to much that I didn’t understand or notice in life. The Caribou’s adaptation I think is one of the most incredible things yet.
Another interesting science post – thanks!
And best wishes for a Happy Coynezaa.
Given that caribou live on average about 15 years, I presume their eyesight would probably deteriorate at about the same rate as ours, which seem to be good for about 25-30 years before going downhill. But then I started having to wear glasses by age 11. I would have been a casualty of natural selection if I had to survive in the wild, although I also happen to have very good night vision.
Really interesting. I wonder if they can also detect lichen fluorescence induced by blue or UV light? I do a lot of fluorescence work with lichens and they often fluoresce in brilliant red and yellow, whereas pure snow has little or no fluorescence. This fluorescence would however be many times weaker than the incident light that causes it. So maybe that’s how they find lichens in the summer, against gray and white rocks????
Ah, and there is an error probably made by Autocorrect. The lichen genus is Cladonia, not Claudia.
Isn’t it fascinating that they give themselves angle-closure glaucoma to improve their winter sight? The same process in humans gives rise to acute glaucoma as we cannot re-open the angle and allow aqueous humor to drain from the eye once it is under high pressure. I wonder how they do it?
Anyway, Jerry, this is the kind of article that illustrates well why you would please all of us by continuing your science posts!