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So the yak. It is a long-haired, herd bovid native to the Himalayas and Central Asia, even as far north as Russia and Mongolia. There are two species - the domesticated one of which there are a large number and the wild population, which is small and listed as vulnerable. By all accounts, yaks are friendly and easily trainable.
Domesticated yaks are used as beasts of burden and for their milk, meat and fibres. Their dried droppings are used as fuel, vital in treeless mountain regions. The milk is used to make cheese and butter. Milk is used in a popular milk tea and the butter is used in cooking, in lamps and as an ingredient in yak butter tea, which is drunk in large quantities throughout Nepal and Tibet. Meat is eaten fresh after the annual before winter slaughter, dried and after being naturally frozen. Meat and blood are also used in sausages. Hides are made into leather which is used to make children's coats, boots and gloves, for rope and bags and, in some areas, for making coracles.
Yaks produce several different types of fibre. The long "skirt" hair is used for tent fabric and ropes while down mixed with the skirt fibre is woven and used in a variety of ways in different areas. These include for tents, blankets, bags and clothing. It can also be made into felt. Down hair alone is used for clothes, suiting, knitwear, blankets and carpets. Yak down is marketed as yak cashmere which fetches high prices with its feel and lustre being thought to be better than that of wool.
It was interesting doing this research because I realised that I actually knew almost all of this. I just hadn't put it all together in my mind. If you want to find out more about yaks this wikipedia article is quite useful but you might also have a look at these websites.
10 Products From Yak and Their Utilisation