Lying immediately east of the Rocky Mountains, the Great Plains consist of sprawling expanses of grassland up to 400 miles wide. Although the region is essentially grassland, copses of willow and cottonwood grow near water sources such as streams and rivers. The Great Plains extend from Alberta more than 2,000 miles south into central Texas. Portions of three provinces and 10 states, including Alberta, Saskatchewan, Manitoba, Montana, North Dakota, South Dakota, Minnesota, Wyoming, Nebraska, Colorado, Kansas, Oklahoma, New Mexico, and Texas, lie in this region. This culture region is adjacent to the Southwest, Far West and Subarctic cultures. These grasses made ideal grazing for the number one animal that sustained the lives of this region's peoples, the bison, better known as the buffalo. There were once literally millions of them, but the natives made the most of the relatively few they harvested, using virtually all of their parts. The Great Plains people were different from their counterparts elsewhere because their lifeways radically changed following encounters with Europeans. The horse was introduced into this region by the Spanish in the 1600s. Many tribes traded goods for this fleet creature, which freed them from the immediate area surrounding their villages when they searched for game — especially the bison. Now they were mounted hunters in long-distance pursuit of the thunderous, roving herds. In a word, the natives' way of life changed from agrarian to nomadic hunting and gathering. Later, access to the firearms, which added power, would further change their culture. With the encroachment of Europeans onto native lands to the East (as well as periods of drought), many agrarian tribes trekked west to the Plains seeking new farmlands, as well as the bison. Among these groups were the Arapaho, Cheyenne, Crow, Gros Ventre, Osage, Otoe Quapaw, and Sioux. With these new tribes on the Plains, a common and easily grasped form of communication among tribes of different language backgrounds became necessary. Sign language was the answer. From time to time, some 35 groups and sub-groups existed on the Plains. Gradually, relationships among members of different culture groups led to a melding of tribal customs. Many Great Plains tribes comprised related families, often numbering in the hundreds. These groups went their separate ways during most of the year, then gathered over the summer months to collaborate in bison hunts and spiritual ceremonies. Given the nomadic lifestyle of many Plains tribes, the movable teepee was the dwelling of choice. These structures consisted of a peaked pole armature covered with buffalo skin. The semi-nomadic Mandan used teepees during their hunting expeditions, but maintained permanent earth lodge villages situated along the rivers. The Wichita and Caddo continued to build houses of grass. Plains natives wore deerskin clothing. They used hefty buffalo hides for winter robes and moccasin soles. Men wore breechcloths, leggings, and simple shirts, just as their eastern counterparts did. They wore their hair in two long braids. The most courageous chiefs won the right to wear headgear of eagle feathers. They were like a general's stars or war decoration. Warfare among tribes consisted of hit-and-run raids staged by limited numbers of warriors. A ceremony always preceded the raid and the warriors always left assuming they would not return. A warrior would frequently carry spiritually charged items on his person for protection and strength. These were kept in a pouch called a "medicine bundle." The Plains people believed in Manitou, the Great Spirit. They also believed that the land had been created by the Great Spirit for all to use and everything on earth was sacred. Life was held to be a sacred circle from birth to death. The circle figures dominantly in their artwork and on their tools. They believed that each animal had a spirit that would give them luck and strength, including success in a war or a hunt. The bison was uppermost. Elders were venerated for their spiritual and historical knowledge. At the age of 14, a man-to-be would retreat to a quiet place to pray and fast for several days. He would petition for a vision from the Great Spirit or one of the Spirit’s guides, then take instruction for his coming life as a hunter, warrior, chief, or shaman (religious and ritual specialist). He also would be told how to gather and use the special stones, plants, and other objects believed to embody supernatural powers, to be kept in his medicine bundle. The most widely observed ritual was the elaborate and demanding Sun Dance, held to thank the spirits for the past year's successes and petition them for plenty in the coming year. It was typically held by each tribe, men, women, and children, during the Summer Solstice. The dance lasted from four to eight days. It also celebrated the regenerative continuity between life and death, and is still celebrated today.