Reindeer eyes change hues with the seasons

Sunday, December 28th, 2025

In 2013, scientists discovered that reindeer eyes change hues with the seasons:

If you look into the eyes of an Arctic reindeer (Rangifer tarandus) in the summer, when the days are long and the Sun is bright, you will see shining back a gold and turquoise glow, similar to the emerald reflection of cats’ eyes in the night.

In wintertime, however, when darkness reigns, a reindeer’s eye does something unique. It turns a stunning, deep blue.

[…]

Reindeer feed at twilight, and during the Arctic winter, twilight can last for more than a third of the day, casting an extremely blue light over the icy landscape.

[…]

To aid in the reindeer’s ability to see lurking wolves and yummy lichen in the dimness, scientists think that the animal’s eyes may have evolved to reflect more blue light in winter. This gives the low light another pass through the retina, allowing more information to be gleaned by the eye’s photoreceptors.

As such, the reindeer gets a brighter view of the twilit landscape (up to a thousand times brighter), but the trade-off is an image with significantly less resolution, like looking through misted glass.

[…]

In 2022, Fosbury and colleagues studied the difference between the eyes of reindeer that had died in summer and those that had died in winter.

Their findings support the idea that constant dilation of the pupils in low light affects the eyes’ fluid balance, possibly causing structural changes in the tapetum.

Posted in Animals, Science | 13 Comments »

Comments

Jim says:

December 28, 2025 at 5:17 pm

Does this suggest that blue eyes in humans are in substantial part an adaptation for twilight hunting?
Jim says:

December 28, 2025 at 5:18 pm

…Or twilight warfare?
Bruce says:

December 28, 2025 at 5:53 pm

Twilight warfare adaptation, spread by sex slave trade?
Jim says:

January 1, 2026 at 12:36 pm

That’s a grim thought.
Jacob G. says:

January 5, 2026 at 1:39 pm

Blue eyes in humans is probably an adaptation to foggy nothern locations, such as the baltic states. See Morgan Worthy.
Isegoria says:

January 5, 2026 at 2:06 pm

I might have to read Morgan Worthy’s memoir, I Have Known the Eyes Already:

I am a retired psychologist. More than 40 years ago I started studying the relationship between eye color and behavior in humans and animals. In 1973, Newsweek had an article, The Eyes Have It, in their “Science” section, which was devoted to my research. That led to a lot of similar articles in other publications and interviews on radio and television. I was a guest on the show, To Tell the Truth. “Will the real Morgan Worthy please stand up?”

This memoir is an attempt to let the real Morgan Worthy stand up. The beginning theme will deal with my role as a researcher trying to study a topic, eye color and behavior, outside the mainstream of current psychological research. Prior to 1974, I studied humans and animals; after 1974, all my research was of animals. Here, I want to give a final overview of those research findings and tell what happens to a fellow who wanders off the beaten path of current research.

Then I want to tell, for the first time, about a personal research project that I undertook to understand why I kept injuring my right thumb. It took me 15 years to sort it out, but since 1991, I have not injured that thumb again. It all had to do with the curious fact that I am one of the few people who has been involved in three gun accidents. The first, and worst, happened more than 70 years ago; in fact, it happened Tuesday noon, the week before the Sunday morning attack on Pearl Harbor. Many lives were changed that week, and my life was no exception. I was five years old at the time…
T. Beholder says:

January 5, 2026 at 10:56 pm

Jim says:

Does this suggest that blue eyes in humans are in substantial part an adaptation for twilight hunting?

There are humans with shiny blue pupils? Details, please, details!
Jim says:

January 6, 2026 at 1:11 am

Isegoria, I don’t understand that excerpt. It seems utterly incoherent.

Jacob G., why did the Neanderthals have blue eyes? And why are blue eyes so ludicrously overrepresented among sailors?
Isegoria says:

January 6, 2026 at 10:27 am

I don’t think the opening to his memoir is aiming for coherence.
Isegoria says:

January 6, 2026 at 10:40 am

Grok lists these associations with light eyes:

Increased sensitivity to light (photophobia): People with light eyes have less pigment to absorb bright light or UV rays, making them more prone to squinting in sunlight, glare discomfort, or needing sunglasses indoors/outdoors. This is due to reduced melanin acting as a natural filter.

Higher risk for certain eye conditions: Lighter eyes are linked to elevated risks of age-related macular degeneration (ARMD), uveal melanoma, and UV-related damage, as less melanin provides weaker protection against sunlight.

Other associations: Some studies suggest light-eyed individuals may tolerate alcohol less (due to melanin links) or perform better in self-paced tasks (e.g., golf or bowling), though evidence is mixed and requires more research.
Phileas Frogg says:

January 6, 2026 at 2:52 pm

Jim,

With regard to the sailing overrepresentation, I wonder how much of it is blue eyedness, vs the fact that it’s simply other Northern European traits, which have significant overlap with blue eyedness, blue eyedness being a common Northern European trait.

Bone density levels, body fat percentages, certain behavioral and cognitive habits, etc.

It strikes me as being more correlative than causal.

Incidentally, I didn’t realize there WAS such an association. Thanks.
Bruce says:

January 11, 2026 at 1:20 pm

I just read Worthy’s memoir; charming but disjointed. But his Animal Eye Colors: Yellow-Eyed Stalkers, Red-Eyed Skulkers, and Black-Eyed Speedsters looks really good.

If he’s right, and he is learned, then domestication in general looks like a branch of pouncer hunting, and more wild and feral adaptations look like a branch of chaser hunting.

Blue eyes, or light eyes in general, being a pouncer hunter thing; darker eyes being chaser thing. All cats with yellow eyes, almost all dogs with black eyes, except some dogs running cat software.

And yes, this would mean something about human race and sex.
Jacob G. says:

January 12, 2026 at 12:40 pm

Worthy’s larger hypothesis is that light eyes equals better timing to act like ambush predators and that darker eyes better reaction like see and chase predators, or their prey.

But lighter eyes are more sensitive to light in general, particularly the blue light which trains the circadian rhythm.

His theory is humans adapted to low-light conditions of northern foggy areas, particularly the Baltic the epicenter of blue-eyed humans. Presumably it would be the same for Neanderthals? But they were exclusively ambush predators weren’t they, so perhaps it had to do with that. Our species seems to be generalists, and never specialized as much as the Neanderthals, that’s probably why he didn’t use the reaction v. timing aspect to explain why blue eyes have been selected for. He does pretty clearly show the reaction v. timing holds true for us.

He guesses that dark eyed peoples would prefer garish colors, reds and blues, while light eyed peoples would prefer more muted greys, blacks, and browns. Again, relating to light sensitivity.

I haven’t read his memoir, just the book Eye Color, but it sounds like it has similar information.

Felix: “I don’t think our public schools have been working hard to assimilate everyone into our shared culture in a long, long time.” The key word is “our”. Schools, world-wide, certainly teach a shared culture. Out in the sticks there may be a few unenlightened, deplorables of the lower orders who have issues with that shared culture. But, pfft.
Gaikokumaniakku: “In 2003, the United States Department of Education gave about 18,000 randomly selected Americans the National Assessment of Adult Literacy (NAAL).” Of course, these must have been 18,000 randomly selected white Americans, to make a reasonable comparison with the historically white population of the majority of America, right? … right? The ghost of William Shockley is hovering silently over me and shaking his head from side to side in a manner I can only interpret as ominous.
Phileas Frogg: Every educational concern or observation in the modern world comes down to this: “Unless and until we finally accept that people are no more equal in their educability and intelligence than they are in their ability to sprint 100m, dunk a basketball or stand 7 feet tall, we will continue to waste limited time, energy, and resources on solving a problem without a solution: men are not equal.” God needs all sorts. And yet, every class of person projects their narrow measure of...
Jim: The labor market pays you for what you know now… The “labor market” is a centrally planned, authoritarianly organized corporate economy, a free market for employers as against employees, not a free market for employees as against employers. Fortunately, the Singularity is poised to render mutational load a thing of the past.
Isegoria: Caplan’s response would be that almost no one uses any of those skills, and those skills don’t generalize or transfer, at least not for most people most of the time.
Gaikokumaniakku: Geometry can be taught in many ways. Knowledge of proofs might be more useful than knowing the volume of a sphere. A bigger problem is that school is intended to reinforce the social order. If schools taught students how to resist cops and lawyers, government would find maintaining power to be more difficult. Another angle is skills versus facts. I probably don’t need to know the year in which Shakespeare wrote (a fact) but I certainly do benefit from understanding how Shakespeare...
Isegoria: Excellent example, Bob. I’m reminded, oddly enough, of how Musk calculated that on a good day he made a hundred command decisions as he walked the floor of his Tesla factory: “At least twenty percent are going to be wrong, and we’re going to alter them later,” he said. “But if I don’t make decisions, we die.”
Bob Sykes: I once had a family doctor who commented that doctors knew when to to add new prescriptions but not when to stop old ones.
Isegoria: Bryan Caplan is definitely allergic to shared culture. On the other hand, I don’t think our public schools have been working hard to assimilate everyone into our shared culture in a long, long time. I was just reading Charles Murray’s Coming Apart, and that is one of his main points: The main vehicle for nineteenth-century socialization was the reading textbook used in elementary school, the variants of which were modeled on the overwhelmingly most popular series, the McGuffey Readers. They...
Phileas Frogg: Caplan is one of the most hit/miss popular academics today. He goes from cogently and insightfully detailing the flaws and issues with a complex system or behavior, to proposing the most obviously asinine and unworkable solutions imaginable to anyone with practical experience in that field, to slipping a totally original and unlooked for consideration into the mix that gives you pause, all within the span of a small handful of pages, only to do it again a few pages later. Reading Caplan...
Bob Sykes: One argument in favor of Emily et al is social cohesion and group self-identification, national-building. Caplan seems to think humans are identical, interchangeable atoms, without history or culture. Or is he just furthering the usual sabotage of the surrounding, somewhat alien, culture in which he is embeddded
David Foster: “The great 20th-century composer Igor Stravinsky wrote, in The Poetics of Music, “You cannot create against a yielding medium.” Stravinsky’s innovations were nothing if not revolutionary, but he knew that he could not have produced them if he had not been constrained by the traditions of music and the mathematical strictures of tone. “Let me have something finite, definite,” he wrote. “My freedom will be so much the greater and more meaningful the...
Jim: “They were free to imagine and play and invent and write,” he said. “They were inventing one thing after another, after another, after another and for an engineer, what more can you ask for?” That’s the worst imaginable environment for engineers, who invariably need to be held down and whipped with soapful socks like Private Pyle at boot camp.
Jim: Yet maintaining the universalist, human-rights based legal infrastructure constructed after the Second World War takes priority over addressing these issues. The fact that this infrastructure was created for an entirely different world, where there was much less international migration, and where “asylum seeker” meant a political dissident from the Eastern Bloc, does not matter. If the post-WWII occupational court infrastructure were universalist, it would be possible for Englishmen to immigrate to...
Isegoria: That’s exactly the kind of thing he’s talking about, Phileas. I don’t want to share too many lengthy excerpts from a brand new book, but I’m happy to pique everyone’s interest.
Phileas Frogg: Looking forward to reading more excerpts, I thoroughly enjoyed, “The Sports Gene,” so much so that I went and bought a copy a few years ago. I expect some similarly provocative and interesting fodder from this one. On the topic of the book, whenever I was struggling to write a drum part for the band I played for in college, I would begin systematically taking pieces of my kit away until I developed something that worked, and then slowly add pieces back in. Limiting myself...
Jim: Phileas Frogg: Then we are substantially in agreement, our only meaningful divergence being that I reflexively view utility from the perspective of the world-controllers, as opposed to from the citizen-subjects. The major difference, then, is not moral righteousness or justice or other such thing, but the habitual understanding that the perpetuation of a system, any system, is owing, ultimately, to the perspective of its managers, formal or informal, that the purpose of a system is what it does…and...
Jim: Free men do not live subject to video surveillance or electronically locked buildings.
Bob Sykes: If you want a career in education, engineering, medicine, law, business, or the sciences, an accredited degree is mandatory. The days when you could just sit in the back of the room and listen ended long ago, even for state schools. At most schools, private and public, key cards of some sort are needed to get into most buildings, and surveillance cameras are everywhere.
Isegoria: Wow, times have changed! The recent Brown University shooting footage revealed that security cameras are common now, but the idea of locking a college down is really foreign to me.

Isegoria

Reindeer eyes change hues with the seasons

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