From Stephen Mikesell:

Re: What? Where? When? Some animals may know (Aug. 12): In the article it was said that there is one minor quibble, which is that voles have a very keen sense of smell and might have thus been able to detect which cage had the receptive female by smell even though it had been cleaned. It seems that this could be overcome by using a different, but exactly the same kind of, cage in the same location.