Marking dogs - what are they; a bit about training; effectiveness and their place in the pack. First off, what is a marking dog? - a marking dog is a dog that uses its nose to locate rats where they cannot be seen. This could be underground, hidden behind something, in piles of straw or rubbish. Picture this, you have been asked to clear the rats from a pig field, in the field are 100 feeders and 100 drinkers all with holes round them and holes in the banks surrounding the fields. A marking dog