Golden snub-nosed monkeys live higher and colder than almost any other non-human primate outside Japan, in forest that is snow-covered for months. Their diet tracks the season: young leaves and buds in spring, fruit and seeds in summer and autumn, and lichen through winter, when it can make up the greater part of what they eat.
Lichen is a poor food, low in energy and hard to break down, but it is present on trunks and branches when the forest is otherwise bare. Like other colobines they carry a chambered fermenting stomach, which is what makes such marginal material usable, and they slow down and reduce daily travel through the cold months.
The flattened face has an untested folk explanation, that the absent nose bridge prevents frostbite. No evidence supports it, and the trait is shared with snub-nosed relatives living in warmer forest, so the cold-adaptation story is best treated as a guess rather than a finding.