Answer: How infuriating! Clematis in general are very brittle, so accidents can happen and it takes very little to sever the stems. While I am not aware of any animals with a particular taste for clematis, that doesn't rule out any of your suspects. Until you can verify the culprit, I would suggest a variety of barriers: a fence around the base to keep out small animals and pets, some thorny clippings (from say a rose bush) stuck in around the base to deter little critters, and a stern warning to two footed folks to be more careful. I would also perhaps add one of the commercial sprays aimed at keeping an assortment of animals off (available at garden centers and farm supply stores) to be applied according to label instructions.
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