Their biological name is “Japanese macaque” and they are called “snow monkey” as well.
They are living in “Monkey Mountain” in the zoo, where their community is just like that in the wild. It is so interesting to see their habits and “monkey relations,” so Monkey Mountain is one of the most popular sections in Ueno Zoo.
I remember my biology teacher at high school. He loved animals and fish and… any living creatures so much. He told us that he visited Ueno Zoo in his holiday and watched these monkeys for 3 hours. For 3 hours.
… My legs will be numbed if I stand in front of them for 3 hours.
But it is true that they are interesting animals.
The Quickest Guide to Ueno: http://tokyo-direct-guide.com/mustsees/useno.html