Both gnaw. Both leave droppings. But a house mouse explores a 5-metre radius while a Norway rat patrols 30 to 50 metres, and that single fact decides where you put traps, how many you need and whether exclusion is even possible.
The droppings tell you which
House mouse droppings are 3–6 mm, pointed at both ends, and usually scattered. Norway rat droppings are 12–18 mm, capsule-shaped with blunt ends, and tend to sit in the same few places along a route. Roof rat droppings are similar to Norway but often found on beams, in loft insulation or on top of wall plates.
Fresh droppings are dark and soft. Old ones are grey and brittle. Wear gloves and a mask; rodent waste can carry hantavirus and leptospirosis, and dry sweeping aerosolises it.
How fast they breed
A house mouse gestates in 19–21 days and produces 5–6 pups per litter, five to ten litters a year. Pups are weaned at three weeks and sexually mature at six to eight weeks. A pair can produce 30–60 descendants in a year.
Norway rats gestate in 21–23 days with 8–12 pups per litter, three to six litters a year. Roof rats are similar. Rats need more food and more space, so their numbers grow more slowly — but a rat colony is harder to starve out because it forages so widely.
Other signs worth reading
- Grease marks. Dark smears along skirting boards where bodies brush the same path nightly.
- Gnaw marks. Mice leave fine parallel grooves; rats leave broader, coarser ones. Both gnaw wiring and pipe insulation.
- Urine pillars. Small grease-and-urine deposits at corners, visible under UV light.
- Runs. Flattened grass or dust-free channels along walls and behind appliances.
What works
Snap traps and secure bait stations, placed against walls with the trigger end facing the wall. Rodents travel with their whiskers touching a vertical surface; a trap in the open is walked around. Set traps for three to five nights before judging them.
Glue boards catch insects and small lizards as often as they catch mice, and they are banned in several countries. Snap traps are faster and more selective.
Exclusion is the real fix
A mouse needs a gap of about 6 mm — the width of a pencil. A rat needs about 20 mm. Seal pipe penetrations with steel wool and sealant, fit brush strips to doors, repair vent screens, and check the gap behind kitchen plinths. Trapping without exclusion just clears a room for the next animal.