Throwback: Happy Thought for 17 December 2021

Have a Happy Thought: 

 

Reindeer eyes, which in the summer are a beautiful gold, turn blue in the winter.

(Okay okay, if you’re a big Game of Thrones fan, learning that something’s eyes turn blue as Winter is Coming is maybe not a happy thought, but stick with me here)

 

Reindeer, and many other animals, have this layer in their eyes called the tapetum lucidum, which helps to reflect more light onto the retina. If you’ve ever seen a cat’s or owl’s eyes shining a bit green at you at nighttime, you’re actually seeing the reflection off of this layer, which sits right behind / under the retina. Humans do not have this layer in our eyes.

 

The other thing that animal eyes do, ours included this time, is expand or contract the size of the pupil, to let less or more light in. (Side note: pupils are not actually black. They’re actually clear. It’s just that the inside of our eyes are dark. This is why pupils can look red sometimes in photos, it’s actually that there’s enough light shining into our eyeball in the moment the photo is taken that you can see the inner reddish surface of our eyeball. Mind=blown.) Pupils change size when muscles in the iris (the coloured part of the eye) contract or expand. (Second side note: pupils also change size in response to other situations, like when your brain is really interested in something. Or someone. Just saying.)

 

Back to reindeer. Reindeer irises in the summer are this beautiful gold colour (Third side note: prey animals often have rectangular pupils. Predator animals often have oval pupils. Humans have round pupils. In case you were wondering.):

 


Image: Nejron Photo/ Shutterstock

 

But in the low light of long nights in the arctic winter, reindeer pupils expand to let as much light in as possible. And it works! With pupils wide open, and the help of the tapetum lucidum, their eyes are at least a thousand times more sensitive to light, than in the summer!

 

The thing is, though, that with the pupils wide open, it puts pressure on that tapetum lucidum, which changes the wavelength (colour) of light that is reflected off of that layer. More closely packed = shorter wavelength reflected = blue eyes.

 

 

As an unrelated bonus, check out this **incredible** short video, taken earlier this week by NASA’s Parker Solar Probe. This shows the Milky Way and several planets, as seen from **inside the sun’s atmosphere**!! This also marks the first time that any human-made object has intentionally touched the sun. (and it survived!!) Just incredible!

https://youtu.be/IQXNqhQzBLM

 

Explanation of the video, and more about the Parker Solar Probe, can be found on its website, here: http://parkersolarprobe.jhuapl.edu/News-Center/Show-Article.php?articleID=173

 

More information about reindeer eyes, here:

https://www.amusingplanet.com/2019/10/reindeers-eyes-change-color-with-seasons.html

 

More information about human eyeball anatomy, and how and why pupils change size, here:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6634360/

 

More information about the shape of an animal’s pupil, and what it says about them, here:

https://www.npr.org/sections/health-shots/2015/08/07/430149677/eye-shapes-of-the-animal-world-hint-at-differences-in-our-lifestyles

 


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