The question that comes to mind is do they *really* bury themselves
in mud in nature?
I found an article where scientists actually dove into water in the
winter and found the frogs on top of the bottom surface. They
were in depressions in the bottom substrate.

I've had frogs show up in years we cleaned the pond in the
spring and years we haven't. Limited amt. of muck vs. more muck.
I always wondered if they hung out in the 'condos' in the builder's
brick that holds up the island.

I do know they need a limited amount of oxygen in the winter
which they manage to get thru their skins.
I think there are many factors in amphibian deaths over the winter.

If I had unlimited funds.... I'd set up an elaborate camera system and
lighting system underwater and *watch* them over the winter.....

k :-)