My cat Squimmy is heavy (13 lbs) and as far as I know she's never killed anything bigger than a small moth. When we were infested with mice, she loved to stare at the wall behind which they were scritchity-scratching, but that was as helpful as she got. We had to get old-fashioned mouse-traps, the cartoon Tom&Jerry type -- which worked well, baited with pnut butter crackers.