There is speculation as to what caused this variation within the three chronospecies. Variations in environment, climate change, and migration surely played roles in the evolutionary process of the mammoths. Take "M. primigenius" for example: Woolly mammoths lived in opened grassland biomes. The cool steppe-tundra of the Northern Hemisphere was the ideal place for mammoths to thrive because of the resources it supplied. With occasional warmings during the ice age, climate would change the landscape, and resources available to the mammoths altered accordingly.