Arctic reindeer can do something no other mammal can, though to be fair not many mammals can survive on the North Pole.

Twelve years ago, scientists discovered the fabled sled-pullers changed their eye color depending on the season, Live Science reported. They have brownish golden eyes in the summer (to counteract 24 hours of sunlight) and switch to blue in the winter to help see in the darkness.

Only now are scientists beginning to understand how reindeers make this adaptive switch, according to Live Science. Neuroscientist Glen Jeffery, who studies vision at University College London, recalled the initial discovery and mused on the time its taken researchers to understand its mechanisms.

"I opened the eyes up on my lab bench and almost fell off my chair," Jeffery said. "The differences were so dramatic, and I knew no one had ever seen anything like that before. However, it has taken us 12 years to slowly find out what is going on and why.

"No one has ever seen anything like this in a mammal before, let alone such a large shift," he said.

Part of the reason it took so long is that researchers had to make yearly trips to the arctic, where "the environment can be brutal," according to Jeffery. He's had some interesting adventures with the environment and the native Sami people during his trips.

"Thousands of animals would suddenly turn up, and they were all turned into meat in a couple of days, but this would go on miles from anywhere," Jeffery said. "Once, we drove deep inland for about four hours in a 4X4 [truck] close to the winter solstice - had to take a 30-kilometer (18 miles) diversion into Finland because wine is much cheaper there than anywhere else in Scandinavia. Magical trip - hardly saw anyone on the journey."

Jeffery attributes the color change to the movement of collagen fibers in the retina, Live Science reported. When pressure increases and they compress together, the reduced spacing reflects blue light (and less light overall). When spacing is increased, brown light is reflected. During the blue period, Jeffery believes light reflects from a greater number of cells, meaning the reindeer stands a better chance at seeing in the dark though the overall acuity of its vision will actual decrease.

Jeffery and researchers wish to continue their work on the vision of arctic animals, in particular the polar bear.

The bears have "the most diminutive tapetum lucidum (the reflective area where the collagen cells compress and decompress) I've ever seen and unlike any other bear," Jeffery said. "It's milky with no color. There has to be a reason for that."