The canary in the coal mine has long been a metaphor for early warnings, but for actual coal miners those birds were a matter of life and death. In Colorado coal mines, however, the canary wasn't an actual canary. Most of the time, Centennial State miners relied on mice to perform the functions normally carried out by coal mine canaries. Before we discuss the unusual relationship between mice and men in the subterranean world of the Colorado coal fields, it's worth looking at how animals…