A yak's coat is so well-insulated that snow resting on its back will not melt.

The domestic yak (Bos grunniens) and its wild counterpart (Bos mutus) are uniquely adapted to the high-altitude environments of the Tibetan Plateau and the Himalayas. Their survival depends on a specialized coat consisting of three distinct layers. The outer layer is made of long, coarse guard hairs that can reach lengths of 12 inches (30 centimeters) and serve to repel moisture and wind.Beneath the guard hairs lies a mid-layer of shorter hair and a dense, woolly undercoat known as 'khullu.' This undercoat is composed of fine fibers that are typically 15 to 20 microns in diameter, which is comparable to high-quality cashmere. These fibers trap air close to the skin, creating a thermal barrier that prevents convective heat loss even in extreme conditions.Research published in journals like 'Nature Communications' highlights how yaks maintain their core temperature in environments that drop to -40°F (-40°C). Because the insulation is so effective at preventing heat from reaching the surface of the fur, the outer temperature of the coat remains near the ambient air temperature. This thermal efficiency is what allows snow to accumulate on their backs without melting, as there is no escaping body heat to provide the energy for a phase change from solid to liquid.In addition to their fur, yaks possess larger lungs and a higher red blood cell count than cattle found at sea level. These physiological traits allow them to process oxygen more efficiently at altitudes of 13,000 to 20,000 feet (4,000 to 6,000 meters). Their sweat glands are also largely non-functional, which further prevents heat loss through evaporation during the brutal Himalayan winters.