I don't think that the prey instinct is too far below the surface and since Dobes are used as scent dogs to find humans, I don't think it's a stretch to get them to track anything else. A month ago I saw Java, nose to the ground, following the track of 'something' from the mud room into the family room. I had heard a cricket chirp, figured it was just outside of the french doors. She tracked it down, and was pacing, barking and pawing the ground trying to reach whatever she had cornered like crazy. I was sure it was a mouse (thought it might have snuck in during the new construction) and wasn't too keen to investigate. Sure enough it was a large cricket. The way Java was carrying on you'd think it was a much bigger animal!
When I was growing up, we had a cottage two hrs N of Toronto. The chipmunks were always tormenting our Dobe - one would hide on top of one woodpile, the other on a pile of lumber on the other side of the lot, chirping and teasing the poor dog as she would race from one pile to the other. By four pm she was flaked out by the fireplace from hours of non-stop running. She always managed to find any frogs or water snakes that were hiding along the shoreline as well. Nothing much got past her. Thankfully she was well trained and would not venture out into the bush (we were ten miles from the nearest town) or chase after a bear. Truth be told, I've seen more deer since we've moved to NJ than I ever saw in the close to 20 years of vacationing in Northern Ontario!