Mice are normally light brown to dark grey on the body, with a light cream belly. Adults have a body length of about 75 mm and weigh up to 30 g. The tail is about as long as the body and is almost hairless.
In relation to their body, the ears are large and the eyes and feet are small. The long whiskers are very sensitive and are used as sensors when moving about in the dark. Mice have prominent incisor teeth that grow continuously: the length of the teeth is controlled by gnawing. Material may be gnawed yet not tasted or swallowed, so it is difficult to devise a repellent coating against mice.
The house mouse is distinguished from Australian native mice by:
The house mouse, Mus domesticus, originated near the present border of Iran and the former USSR, from where they spread to Europe and subsequently throughout the world. They were probably introduced into Australia by the first European settlers and quickly established throughout the continent. Within 100 years of introduction, mouse plagues were being reported which resulted in devastating damage to agricultural production.
The house mouse is not restricted to houses or buildings as its name might suggest. They are found throughout NSW in all habitats and have adapted to a wide range of environmental conditions. They are common on all agricultural lands, particularly cereal and summer cropping areas.
Mice are generally described as ‘commensal’ rodents – i.e., they live with humans, and are adapted to living in houses and buildings. Most farm buildings have a few mice. In the field, mice are always present but in most years are in relatively low numbers. Refuge areas such as channel banks and dense pastures are ideal habitat where detection is difficult. Poultry and pig sheds or grain storage facilities are also favoured, particularly if the ground can be excavated easily to establish burrows.
Mice consume a wide range of foods, eating 3–5 g daily. In a field situation, mice survive on the seeds of native grasses and thrive on introduced cereal grains. In food storage areas, their diet can include grains, vegetables, meat, fruit and dairy products. They are particularly attracted to high protein grains and aromatic vegetable oils. When selecting a bait type, it is important to know that mice will sample all foodstuffs within their range, but may not return to a particular feed type for many days. Mice can successfully live and breed without free water if the moisture content of the food is at least 15%. Where mice live in sheds and areas where the food supply has a low moisture content, they need 1–2 ml of water daily to survive. In these situations their activity can be limited by cutting off their access to water.
Mice are most active at night but can also be seen during the day, particularly around buildings or areas with adequate cover. Their home range is limited to an area of about 5 m2 in closed buildings, but in crop situations, with available food and water, the home range may be even less. Young mice are forced to seek new areas during periods of high breeding and this is one of the factors associated with the development of a plague. When mice move, they tend to follow the same path from refuge to feeding area. Paths are often confined to walls, pipes or natural barriers, so the tell-tale smear marks can be an indication of mouse activity. In the field, distinct tracks through the vegetation become obvious. Mice can swim and remain under water for lengthy periods. They can dig, jump upwards at least 30 cm, jump downwards at least 2.5 m without injury and squeeze through openings as small as 8 mm in width. In addition they can climb almost any rough surface, climb upside down and run down ropes and coated electric wires.
The social behaviour of the house mouse is not rigidly fixed and depends on the environmental context. In situations where water or food supply may be limiting and the mice are living in relatively low densities with large territories, they display territorial behaviour with both females and males displaying aggressive behaviour to other individuals of the same sex. Males mark their territory by scent marking with urine. In marked territories, intruders showed significantly lower aggression than the territory residents. When mice are living in dense populations with small home ranges, they switch from territorial behaviour to a hierarchy of individuals, and aggression is often mediated as the risk of injury becomes too great. The social unit of house mouse populations generally consists of one male and two or more females, usually related. These groups breed cooperatively, with the females communally nursing. This cooperative breeding and rearing by related females helps increase reproductive success. When no related females are present, breeding groups can form from non-related females. House mice show a male-biased dispersal; males generally leave their birth sites and migrate to form new territories whereas females generally stay and are opportunistic breeders rather than seasonal.
Mice can start breeding at 6–10 weeks of age and produce 10–12 litters per year. The gestation period is 19–21 days, with the female re-mating almost immediately after giving birth. Young mice begin eating solid food at 11 days and are weaned at 21 days of age. They have a life span of only one year in field situations. Litter size is generally 5–6 but can be up to 10. The young are born hairless and blind in a nest of collected materials such as grass, paper, hair, cloth remnants or anything soft that is available. If there is no infant mortality, one breeding pair of mice could theoretically produce 500 mice within 21 weeks. In Australia, mice living under field conditions have a seasonal pattern of breeding. This generally begins in early spring and continues until cold or wet conditions develop in late autumn (approximately 8-9 months duration). Mice living in unfavourable seasonal conditions may have a shorter breeding period, while those with nests in the warmth of buildings or haystacks are likely to have an extended breeding period.
Mouse plagues tend to occur when there is plenty of food and water available, environmental temperatures are not extreme, soil is moist and easy to dig, nesting conditions are favourable and diseases, parasites and predation are at a low level. Widespread mouse plagues in Australia were reported as early as 1900 and they have occurred on average once every 4 years since then. Mouse plagues can have dramatic agricultural, environmental and sociological impacts.
Predation may play a role in regulating mouse numbers until there is a rapid population build-up. Predators are unlikely to have any effect on numbers once plague proportions are reached. A noticeable increase in the number of predators in an area may indicate a large increase in the mouse population. In south-eastern Australia the main predators of mice are foxes, feral cats, snakes and all birds of prey. The presence of itinerant bird species such as the black-shouldered kite is a good indicator that mouse populations may be increasing. Domestic cats have no impact on localised mouse populations, contrary to popular myth. Although disease can cause a sudden decline in mouse numbers, marking the end of a mouse plague, declines in numbers occur mainly when mice are stressed from restricted food and shelter. It is more likely that overcrowding will allow parasite infestations to develop and contribute to the spread of disease.
Mice cause damage to almost all sown crops, no matter whether they are winter or summer crops or seeds of cereal, oilseed, maize or pasture. By digging into the loose soil immediately after sowing, mice are able to establish nests and feed on the seed or newly emerging seedlings. Most crops suffer damage prior to seedling emergence and when the grain or seed begins to mature. However, in cereal crops such as wheat, mice chew the growing nodes of the plant and can stop the development of the head or cause the stem to collapse. Mice can also damage horticultural crops like melons, pumpkins and tomatoes. Stored produce, buildings and machinery Mice will be active in most farm areas where produce is stored. Normally, there may be little pressure put on such storage until there are mice in plague numbers that will test security to the limit. Mice can find the smallest hole and gnaw on it until it is large enough to allow entry. During a plague it is difficult to maintain the mouse-free status of any facility unless there has been a mouse-proof component incorporated into the initial design and construction.