TIL that reindeer eyeballs turn blue in winter to help them see at lower light levels. (They're golden-colored in summer.) No other mammals are known to have this ability.

https://royalsocietypublishing.org/doi/10.1098/rspb.2022.1002
#til #todayilearned
https://www.reddit.com/r/todayilearned/comments/1g9ces0/til_that_reindeer_eyeballs_turn_blue_in_winter_to/