Yaks use their stomachs as internal heaters to survive freezing temperatures.

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 primary survival mechanism is a highly efficient ruminant digestive system. Like cows, yaks have a four-compartment stomach consisting of the rumen, reticulum, omasum, and abomasum.The rumen acts as a massive fermentation vat where anaerobic bacteria and protozoa break down fibrous plants and grasses. This chemical reaction is exothermic, meaning it releases a significant amount of heat as a byproduct of microbial fermentation. Research indicates that this metabolic heat production is so effective it allows yaks to maintain a stable core body temperature even when external temperatures drop to -40°C or -40°F.To retain this heat, yaks possess a thick, double-layered coat consisting of a long, shaggy outer hair layer and a dense, woolly undercoat. Their skin is also thick and lacks the sweat glands found in most other bovines, which prevents heat loss through evaporation. Furthermore, yaks have a larger lung capacity and higher red blood cell count than cattle at lower altitudes, allowing them to process oxygen more efficiently in thin air.Studies by the International Yak Information Center highlight that yaks can survive at altitudes up to 20,000 feet (6,000 meters). Their specialized digestive furnace is so powerful that yaks often struggle with heat exhaustion if temperatures rise above 15°C (59°F). This combination of internal fermentation and external insulation makes them one of the most cold-hardy mammals on Earth.