By Annie Hill
The original article in Russell Lights | Ngā Mārama o Kororāreka, August 2025 edition
It’s hard to envy the life of a mouse, low down on the food chain, living in perpetual terror, where humans are as big as a dinosaur and far too many creatures are convinced that its raison d’être is to provide a tasty snack. Their large, dark eyes, stare from their little heads as they move around in dashes and darts, trying to avoid open spaces and alert to anything that might mean danger.
It is the House Mouse who shares Aotearoa New Zealand with us, Mus musculus, a name that conjures the physique of a wrestler. Mus is the Latin word for mouse and musculus means small mouse, so ‘mouse small mouse’ in fact. Mice are renowned for being able to fit through minuscule spaces; indeed, their tiny bones allow them to squeeze through a hole the diameter of a pencil. Because they’re quite furry, it’s easy to overlook that the largest part is its head. It assesses dimensions using its sensitive whiskers: if it can get its narrow skull into a space, then the rest of it will follow. Most people have encountered house mice, appropriately enough, in a house; wooden houses are much easier for them to live in than those built of bricks or stones.
Like all rodents, their incisors never stop growing and they need constantly to gnaw things to file their teeth down. If they can sense food not far away, they will soon make a small hole through softwood. Once inside, as well as eating and spoiling food, they can damage wiring, pipes and textiles. Active and acrobatic climbers, they can jump 45cm (18 inches) vertically and I’ve seen them shin up a wire without difficulty. They also have a very low mass to surface area, which means that they have slow terminal velocity, so that even if they fall from a great height, they don’t hit the ground at force and can scamper off straight away. Apparently this is an issue in some of the fenced sanctuaries where mice have been eradicated: the odd mouse gets dropped by a careless morepork and, should she happen to be a pregnant female, the place can quickly become reinfested.
Generally speaking, mice live in communities, burrows, or rock piles or anywhere that gives shelter and safety from predators. They like to live close to their food sources, usually within about 50 metres, and are more or less territorial depending on food abundance and population density. Generally, one male will mate with several females who are frequently related. The female gives birth to a litter of 3 to 14 young after 19 to 21 days. Born blind and naked, they start getting fur after about 6 days, their eyes open after a fortnight, and they’re weaned a week later. Females look after and feed each other’s babies which become sexually mature at around 6 weeks. Favourable conditions such as availability of food and shelter, are more important than the season and as a female can produce up to 10 litters a year, the population can expand very rapidly. Densities of up to 1,000 mice per hectare have been found in Australia. Once they’re weaned, young males generally leave their birth sites and migrate to form new territories, whereas females generally stay in the area.
House Mice are so called, because they’ve lived around humans for about 8,000 years. You’ll occasionally read that they rely on humans to survive; however, while house mice are generally poor competitors, in Aotearoa there are no other small animals to compete with them and so they can survive most places where there is food and shelter. Possibly too, the fact that they breed very quickly, enables them to evolve the necessary traits to adapt very quickly. Mice are generally afraid of rats which often kill and eat them, but populations of rats and mice do exist together in forest areas in Aotearoa New Zealand.
You could be forgiven for thinking that of all the introduced pests, mice are the least of our worries, but in fact they predate on birds’ eggs and have been recorded eating the rumps and heads of defenceless seabird chicks while they were still alive, leading to a lingering death. This has reached epidemic proportions on Gough Island, where in the absence of rats, mice evolved to grow much larger than their usual (nose to base of tail) length of 7.5–10 centimetres (tail length of 5–10 cm), and heavier than their typical 11–30 grams, and started eating the unfledged albatross chicks on their nests.
In all, the island was losing around 2,000,000 seabirds a year to these introduced predators, including Tristan albatross, MacGillivray prions and grey petrels. (Sadly, an attempt to eradicate the mice proved unsuccessful, although it gave the birds a temporary reprieve.) As well as their attacks on birds, mice also predate lizards and invertebrates and do a lot of damage to young plants. In the few places where they have been completely exterminated, it becomes obvious just how much damage they inflict.
Unfortunately, they’re very difficult to eradicate at scale and are not part of the Predator Free 2050 programme. Conservationists are concerned that once the dominant predators are removed, mouse populations will explode because they are prey for rats and mustelids. Indeed, there is a school of thought that we should remove mice first, then get rid of rats, weasels, stoats, ferrets, and feral cats in that order.
Hopefully, we will develop new and better ways to deal with mice populations and not end up with our motu overrun with mus musculus.
Mouse. Author: JAMIE HALL / The-Nelson-Mail
