An Arctic musk ox has four stomach compartments as a ruminant.
Like cattle, sheep, and deer, the musk ox is adapted to digest fibrous vegetation through a specialized four-part stomach. The compartments are the rumen, reticulum, omasum, and abomasum. Microorganisms in the rumen help break down cellulose in grasses, sedges, willows, and other tundra plants that ordinary digestion could not process efficiently.
Food is first swallowed and softened by fermentation. The musk ox later regurgitates partly digested material as cud, chews it again, and reswallows it. This repeated chewing reduces plant particles and improves the efficiency of microbial digestion before the material passes through the remaining compartments.
The adaptation is valuable in the High Arctic, where nutritious forage is seasonal and winter vegetation can be sparse. Musk oxen also conserve energy by forming dense defensive groups during severe weather and against predators. The four-compartment system is a ruminant trait, not a feature unique to musk oxen, so the animal is sometimes incorrectly described as having multiple separate stomachs rather than one stomach with four specialized chambers.