Great question, cologne, I love it. I've been reading all sorts of papers about bat evolution to avoid starvation by being more tolerant to hypoxia! Fantastic. Nothing too specific yet. I could guess at a mole, that's as close as I am at the mo' - but I bet I'm a miles away.
Don't know the form for asking for clues, so if it's not the done thing please ignore. But... Is it a hibernating animal?
You're not miles away Andy - it is a mole ! It is apparently able to reuse oxygen which enables it to spend so much time in cramped/gassy.. nest conditions underground. Not to be confused with the mole rat which is also a fascinating animal - apparently the only mammal immune to cancer. Over to you.

