Zapus hudsonicus

[Ken Kinman]

The short answer is probably enzyme suppression.  You might be interested in reading Molecular Mechanisms of Metabolic Arrest (K.B. Storey, ed., 2001).  Storey has researched the hibernation of Zapus hudsonius, and his introductory chapter of the above-mentioned book ("Turning Down the Fires of Life") is conveniently available online at:

http://www.carleton.ca/~kbstorey/01-kbs5.pdf

  ---Cheers,
          Ken Kinman
P.S.  Given the decrease of the permafrost in Alaska (due to global warming), the range of Zapus hudsonius may well expand even further northward.