There are four species of pest rodent in New Zealand. The Mouse (Mus musculus), the Roof Rat (Rattus rattus) also known as the Ship Rat and Black Rat, the Norway Rat (Rattus norvegicus) also known as the Water Rat, and the Kiore (Rattus exulans) also known as the Pacific Rat.
How to tell them apart? You may think that the mouse would be easily distinguished because it is so much smaller. But consider a young rat, it will also be small. There are some ways to tell the rodents apart and they are found in the table below. However, the table describes adult rodents and the way to tell a mouse from a young rat is the relative size of the head. A young rat will have a much larger head in proportion to its body than a mouse.
You are unlikely to be able to examine the rodent closely and it is often only the evidence of droppings that you have to identify your infestation. The size and shape of these dropping can be a good indication of what rodent you have.
How to tell rats and mice apart:
|Ship Rat |
|Norway Rat |
|Other common names||Maori rat, Polynesian rat, Native rat, Pacific rat||Black rat, Blue rat, Bush rat, House rat, Roof rat, Matapo||Brown rat, Water rat, Sewer rat, Pouhawaiki||Field mouse|
|Adult weight (g)||Typically 60-80; (up to 180)||Typically 120-160; (up to 225)||Typically 200-300; (up to 450)||Typically 15-20; (up to 30)|
|Max head+body length ("HBL")||180mm||225mm||250mm||115mm|
|Tail||Slightly shorter or longer than HBL; Thin and uniformly dark all over.||Much longer than HBL; Uniformly dark all over.||Clearly shorter than HBL; Thick, with pale underside.||Slightly shorter or longer than HBL; Uniformly grey-brown.|
|Ears||15.5-20.5 mm; Cover eyes when pulled forward; Fine hairs do not extend beyond edge of ear.||19.0-26.0 mm; Cover eyes when pulled forward; Fine hairs do not extend beyond edge of ear.||14.0-22.0 mm; Do not cover eyes when pulled forward; Obvious hairs extend beyond edge of ear.||12.0-15.0 mm|
|Adult hind-foot length||24.5-31.0 mm||28.0-38.0 mm||30.0-41.5 mm||15.0-21.0 mm|
|Colour of upper-side of hind-foot||Outer edge dark near ankle; Rest of foot and toes pale.||Uniform colouring over whole foot, usually dark.||Always completely pale.||Uniformly grey.|
|Fur on back||Brown.||Three colour variations: rattus: uniformly black; alexandrinus: brown with long black guard hairs; frugivorous: brown with long black guard hairs.||Brown.||Dull grey-brown.|
|Fur on belly||White-tipped grey giving irregular colour.||Three colour variations: rattus: uniformly grey; alexandrinus: uniformly grey; frugivorous: uniformly white or creamy-white.||White-tipped grey giving irregular colour.||Uniformly grey.|
|Length of droppings||6.4 - 9.0mm||6.8 - 13.8mm||13.4 - 19.1mm||3.9 - 7.6mm|
|Number of nipples||8||10-12 (usually 10)||12||10-12|
|Habits||Agile climber; Digs small holes; Nests mainly on the ground; Feeds both on the ground and in trees; Infrequent swimmer.||Very agile and frequent climber; Rarely burrows; Nests mainly in trees and shrubs; Infrequent swimmer.||Burrows extensively; Climbs much less frequently than the other rats; Strong swimmer; Nests underground; Very wary.||Mainly ground-dwelling, though capable climber; Nests in small holes.|
Sourced from: The Handbook of New Zealand Mammals (1990), King (ed) Guide to the Identification and Collection of N.Z. Rodents (1996), Cunningham and Moors