Why do yaks have long hair on their bodies?

Yaks have long hair because it is essential for survival: it provides superior insulation against the extreme cold of their native high-altitude environments, originally stretching from the Himalayas and Tibetan Plateau in the south to Lake Baikal in the north. The dense, double-layered coat, consisting of coarse outer hair and soft undercoat, effectively traps body heat and protects against chilling winds and snow, allowing yaks to thrive in temperatures as low as −40 °C (−40 °F).