  The last mentioned difficulties are perhaps most famously illustrated by the 10-year cycle of snowshoe hares, muskrats and their predators in the North American boreal forests and, especially, the 4-year cycle of lemmings and, possibly, other microtines in the arctic tundra of Eurasia and North America. In spite of almost seventy-five years of continuous research on these well-documented cycles the mechanisms driving them remain unresolved. Models producing such cycles abound, but the structural uncertainty of most of these models, coupled with partial observability of many of the parameters in the field have precluded resolution of the  debate.[15]