Reindeer herding is when reindeer are herded by people in a limited area. Currently, reindeer are the only semi-domesticated animal which naturally belongs to the North. Reindeer herding is conducted in nine countries: Norway, Finland, Sweden, Russia, Greenland, Alaska (the United States), Mongolia, China and Canada. A small herd is also maintained in Scotland. Reindeer herding is conducted by individuals within some kind of cooperation, in forms such as families, districts, Sámi and Yakut villages and sovkhozy (collective farms). A person who conducts reindeer herding is called a reindeer herder and approximately 100,000 people are engaged in reindeer herding today around the circumpolar North. The domestication of the reindeer does not lend itself to a simple explanation. There is no doubt that when the glaciers retreated at the end of the last Ice Age, people followed reindeer to the North, using traps during the reindeer hunt. Modern archaeological data (rock art) suggest that domestication may have taken place for the first time in the Sayan Mountains between Russia and Mongolia, possibly 2-3 thousand years ago. According to another theory, the Tungus (the ancestors of the present Evenks and Evens) independently domesticated reindeer to the east of Lake Baikal, and that reindeer herding originated in several places simultaneously. Reindeer herders have their own stories about how reindeer were domesticated, and about the relationship between wild and domestic reindeer. Whatever the debate, the very fact of domination of a reindeer led to a reindeer revolution that spread to the North, East, and West. Sleds pulled by reindeer appeared later than dog sleds. The reindeer sleds made accessible areas of the tundra and mountains, which can only be accessed by helicopter. Reindeer became the preferred vehicle on the expanses of Eurasia. The Sámi people lived and worked in so-called siiddat (reindeer herding groups) and reindeer were used for transport, milk and meat production.