The culture of the plains indians was changed by

The culture of the Plains Indians changed by the introduction of horses by the Spanish.

Explanation:

One of the mai things that changed the Indians' every day lives were horses. Horses were brought by the Spanish conquerors because these animals were not in America. So when the Spanished conquered territories in the Americas, they started using horses in that territory as they did in Europe. This really benefitted the Indians because they transported things faster and carried more thing than with any other known method by the Indians in the Plains.

the social contract theory

yes i think there would be volunteers