It's been years since I've been through the tunnels, but I never noticed any wildlife in them (which is fortunate, because otherwise you'd be walking through bat guano and other such artifacts).

DNR rule is to walk bikes through the tunnels. The tunnels are usually dripping wet, with a narrow walkway and two small ditches to channel the water out of the tunnel. When you're in the tunnel, the only light you have is the light at the end of the tunnel and whatever light you bring with you. If it's a humid day, you have fog in the tunnel and you often can't see the other end. Unless you have a big light, it's extremely difficult to keep your bike on the walkway, and if you have a big light, you're just blinding everybody else in the tunnel.