Most animals have a particular color of eye shine which depends on the composition and structure of their tapetun lucidum but the Eurasian mountain reindeer is different. It's the only one we know of that switches colors, changing from a summery golden yellow to a deep blue in winter. (18:14)
And scientists think that's a side effect of how their eyes have adapted to long periods of darkness. The arctic summers where they live have 24 hour days and the winters include long stretches of night. The animals compensate for this in part by making their retinas more responsive to low light in winter. (18:30) 
And it's believed they can see some wavelengths of UV light but that's not the most obvious adaptation. 
To see better when it's so dang dark. a reindeer's pupil open wide to let more light in, kind of like yours do if you go into a dark room, but when a pupil stretches wider it also flattens the front of the eye ever so slightly, which increases the pressure inside the eye. (18:50)
Since their pupils are constantly wide open during winter, that pressure builds up, squeezing the tiny collagen fibers from the deer's tapetum lucidum closer together. And this compression causes the color change. (19:02)
The tightly-packed fibers strongly reflect shorter, blue wavelengths of light, while the more spaced out fibers reflect longer, yellow ones instead - a phenomenon known as Bragg's Law. (19:12)
Researchers can actually compress a dissected piece of the reindeers eye with a tiny weight and induce the same yellow-blue color change but the color isn't what's really important to the animal. 
The compressed fibers also scatter light sideways through the retina instead of reflecting it out of the eye, increasing the amount of light the retina can absorb. While summer eyes reflect more than 95 percent of the light shone into them, winter eyes reflect only 40 percent. And that likely means the reindeer can see in what seems like total blackness. The only downside to this amazing feed is because light rays inside the eye are more scattered, everything also looks a little fuzzier but that seems a small prize to pay. I wish I could trade a little blurriness for night vision. (19:54)

But siamese cats also have those super-cute boots and that little brown mask and that's because tyrosinase is functional in their extremities, like their tales, legs, noses and ears. Compared to their volume these parts of the body have a larger surface area for heat to escape from. So these small and slender body parts loose heat more quickly than the cats central core. (22:23)
That's just like how your fingers, and feet, and nose are the first things to get cold when you step outside or how a narrow icicle is gonna melt before a compact icecube made of the same volume of water. (22:35)
In a cat that means the face, limbs and tale are just a few degrees cooler compared to the rest of the animals body and that difference is enough to preserve delicate tyrosinases shape and function, allowing it to color the fur. (22:50)
If you ever get to see a newborn siamese kitten you can actually watch this process happen life. When the cats are born they're totally white since the inside of the womb is a uniform and toasty 38 degrees celsius or so. Out in the world, the temperature around the kitten is cooler than its body so it loses heat to the environment especially from its tiny toes and tail. (23:13)
Within a few weeks the characteristic siamese color pattern starts to emerge as new fur grows and tyrosinase gets to work in the cats extremities. So unless you keep mittens on your kittens siamese cats will develop their trademark coloration, all thanks to this delightful mutation. (23:31)
Cats are unsurprisingly just so great. Thank you for watching this episode of SciShow. Compilations like this make me realize how much content we have put out over the years and whe couldn't have done it without the support of our patrons on Patreon and our channel members. If you support SciShow you helping put more free educational videos out into the world to be watched by people wjo love them, to be used in classrooms all over the place and we are really grateful for that. (24:00)