A reindeer has four stomach compartments.
Reindeer are ruminants, a group of hoofed mammals that digest tough plant material through fermentation. Their four-part stomach consists of the rumen, reticulum, omasum, and abomasum. Food is first fermented by microbes, then later regurgitated and chewed again as cud.
This system lets reindeer extract nutrients from lichens, grasses, sedges, leaves, and other vegetation that would be difficult to digest directly. In winter, ground and arboreal lichens can become especially important when snow covers other plants.
The four compartments are not four separate stomachs in the ordinary sense; they are connected sections of one specialised digestive system. Reindeer are also unusual among deer because both males and females commonly grow antlers, although their antler timing differs.