Key Identification Features
House mice have brown to grey fur with their under parts being slightly paler, there is considerable variation in colour between different populations depending on the habitat in which they are found. House mice have a pointed snout with long whiskers, they have large ears in relation to their heads with small eyes that allow them to see best in low light conditions and in near darkness. Females are larger than males with their size also varying between different populations. Generally males measure between 16 and 18cm and females between 15 and 18cm. Adult males weigh up to 25 grams with females weighing up to 30 grams. The head and body can measure up to 9cm long with a scaly ringed tail also reaching to 9cm in length. House mice are similar in appearance to wood mice with the main distinguishing feature being a scalar tail resembling that of a rat on a smaller body size. Tracks are smaller than those of the wood mouse with the four-toed front feet reaching 1.3cm long while the five-toed hind feet can be up to 1.8cm in length. Vocalistions are composed of a range of high audible squeaks with a squeal sound used if threatened. Ultra sound signals are primarily used for communication between mother and infants.
House mice are common in a wide range of urban habitats including houses, factories, warehouses and shops, in the countryside they will occupy farm buildings, refuse tips and hedgerows. Urban populations of house mice living in and around buildings usually do not dig burrows but shelter in crevices in walls and below floors. These populations will build nests of paper and cloth for extra warmth, individuals in such locations will generally have small home ranges which can be as little as 10m2. Rural populations avoid open habitats, digging shallow burrows which are lined with grass, nearby they will construct store chambers which resemble molehills and contain grains, grass and corn ears. A home range of such an individual will be much larger than those found within buildings. House mice populations can even be found on islands where they prefer more open field conditions then their urban dwelling cousins.
Food and Feeding Habits
House mice will feed on practically any food source but prefer cereals, they can be described as opportunistic omnivores. Urban populations will feed on whatever is available and have even been known to eat soap, glue and plaster. Rural populations consume insects and their larvae, worms, fruit and vegetables which they store in food cashes near their burrows, these can hold up to 10kg of seeds, grains and cereals. They are nocturnal feeders with the night divided into two active spells for foraging, the first soon after dusk and the second just before dawn. When a range of different food is available they will consume a little of each making up to two hundred visits to the food source and picking up 20mg each trip. On average they will consume 3.5 grams of food in a day. House mice can go for long periods without water if their food source contains a high moisture content, food is picked up using the front paws and turned regularly while nibbling. When foraging house mice are curious but suspicious of any new objects they find, they will scurry about in short fast bursts often pausing to stand on their hind legs to sniff the air. They avoid moving across open areas preferring to use regular covered trails or along the edges of walls.
Reproduction and Life Cycle
Breeding in house mice populations can vary depending on the structure of the group and the population density of an area, scent information is important for initiating the breeding cycle. Unlike the wood mice, males of this species do not use ultra sound when fighting for mates. Urban populations of house mice may breed throughout the year if their food supply is sufficient producing between five and ten litters in a year with each litter containing four to nine pups. Pregnancy lasts for about twenty days with the young being born blind and naked weighing only 1.5 grams. The pups will be weaned by their mother for two weeks until they reach a weight of 7 grams after which they will begin to leave the nest and consume solid food. Female house mice can become sexually mature after only six weeks with the males taking up to eighteen months before they are able to reproduce. For rural populations the breeding season generally runs from spring to autumn with the females producing a litter every five weeks. After birth the mother mouse gives parental care to pups by providing milk and food with the male only occasional involved in grooming. The lifespan of rural house mice is usually shorter than those found in more built up areas with two years being the longest they live for in Ireland.
The house mouse is originally a native of the warm grassland areas of Asia and central Europe and would most likely have lived in rock crevices feeding on seeds, grasses and other plants. Over time it developed a close association with human settlements and increased its range along with human movement. The species is now widespread throughout the whole of the European continent including most islands, they can also be found in areas of colder climates due to their adaptation to human settlements. It is not known how or when the house mouse arrived in Ireland but they were probably inadvertently brought with Iron Age settlers. Now they can be found throughout Ireland even on some islands if human settlements are present. The population density of house mice can vary greatly depending on such factors as location, the time of year and the availability of food sources. In some urban areas such as warehouses or food stores there may be the equivalent of several thousand individuals per hectare with rural areas containing up to fifty per hectare in the peak breeding period of summer and early autumn.
The house mouse is largely dependent on human settlement and the food sources that can be found there, they are generally out competed by other small mammals for food and habitats in rural areas and are absent from open countryside. They are an important prey species for a number of large Irish predators including foxes, stoats, owls, kestrels and hawks so their continued presence in Ireland is important to Irish eco-systems. The main cause of mortality in Ireland is from the cold but the ever-growing populations of domestic and feral cats can account for large numbers of house mice deaths each year. They can be seen as a pest species when their numbers grow due to their ability for rapid reproduction when extra food sources are located, this and the fact that they can carry a number of diseases leads to widespread poisonings and trappings in some urban locations.