I've had people ask me what is the difference between reindeer and caribou, so reindeer are a domesticated animal and a caribou is a wild animal. Reindeer tend to be much small and are a slightly different color than their wild caribou cousins.
Caribou live in very extreme climates of Alaska, Scandinavia, and Russia where there is 19-20 hours of sunlight in the summer and 5-7 hours of daylight in the winter. Caribou have an very cool adaption which helps them to deal with these light extremes. They actually change the way the tapetum in the back of their eyes work. Click here for a cool picture of the color of their eyes in the different seasons. Like many animals which come out a night or are most active during the early morning and late evenings they have a tapetum, a reflective layer of cells in the back of their eyes, which allows their eye to collect the most amount of light making it easier for them to see. Caribou take this one step further and through creating more or less pressure in their eyes, depending on the season, pack the tapetum cells closer together or spread them out. Spreading them out during the brighter summer season allows them to reflect more light back out through their eyes, but in the winter caribou increase the pressure in their eyes causing the tapetum cells to pack closer together. These closely packed cells allow caribou to catch more light making it much easier for them to see in the darkness. Adjustable night vision goggles!
So have no fear whatever the reindeer the caribou will be able to see their way to your house to drop off those presents.
More fun stuff about caribou.
http://animals.nationalgeographic.com/animals/mammals/caribou/
http://www.livescience.com/17621-surprising-facts-reindeer-caribou.html
http://www.fws.gov/alaska/nwr/arctic/carcon.htm
http://www.mnh.si.edu/arctic/html/caribou_reindeer.html
