Groundhogs are actually also our largest ground squirrels. The bone projection above its eye socket (postorbital process) help place it in the squirrel family, Sciuridae. They are not only champion diggers (their name Monax comes from Carolina Algonquian Indian for "digger"), but can be good climbers like other squirrels, despite their large sizes and fat. They may not do it often, but they will climb, especially in search of fruit. I've twice surprised them (and myself) when I came upon them in trees.
Groundhogs are also the only rodents found locally that I am aware of that have white incisors. All other rodents that I know of around here have hardened enamel that is often yellow, orange, or otherwise discolored. Finding any rodent skull with white incisors is usually a good clue that you found a woodchuck.
Whistlers (so-called due to their whistle-like calls) are also one of the few local mammals that are true hibernators. Most other so-called hibernators wake up periodically and do not undergo the metabolic processes of real hibernation. So this Groundhog's Day, all self respecting woodchucks are in deep slumber. So have a Happy Groundhog Day, but don't expect to see any real wild woodchucks for some 6 more weeks or so.