Mantled Howler Monkey

Currently, these monkeys are small and skinny. They are perfect for the rainforest. If there was an ice age, change happens. Their first adaptation would turn slightly bigger as it cools, its fur gets thicker, and it starts crawling on logs instead of swinging in trees. Inside, its heart gets used to beating faster for more blood warmth, and the arm muscles get replaced with a sort of thick blubber, to improve warmth and not need unnecessary muscle mass if its not climbing trees anymore. Once the age fully develops, the monkey must move into a cave and fully live on the ground. It gets even larger and develops a bigger head. Inside all of the previous changes just intensify. It changes back to brown to try to look like a scarier animal, like a bear.