The Bactrian camel (Camelus bactrianus), also known as the Mongolian camel, domestic Bactrian camel or two-humped camel, is a large even-toed ungulate native to the steppes of Central Asia. It has two humps on its back. Domesticated Bactrian camels have served as pack animals in inner Asia since ancient times. With its tolerance for cold, drought, and high altitudes, it enabled the travel of caravans on the Silk Road. These camels are migratory, and their habitat ranges from rocky mountain massifs to flat steppe, arid desert, (mostly the Gobi Desert), stony plains and sand dunes. Conditions are extremely harsh – vegetation is sparse, water sources are limited and temperatures are extreme. The coat of the Bactrian camel can withstand cold as low as −22 °F in winter to 122 °F in summer. The camels' distribution is linked to the availability of water, with large groups congregating near rivers after rain or at the foot of the mountains, where water can be obtained from springs in the summer months, and in the form of snow during the winter. They have a remarkable ability to go without water for months at a time, but when water is available they may drink up to 30 gallons at once. Bactrian camels are diurnal, sleeping in the open at night and foraging for food during the day. They are primarily herbivorous. With tough mouths that can withstand sharp objects such as thorns, they are able to eat plants that are dry, prickly, salty or bitter, and can ingest virtually any kind of vegetation.