House Mice

Brief:

The house mouse, Mus musculus, ranges in colour from brown, grey to black and measures from 7 - 9cm’s combined head and body. They have large ears, a pointed nose, small eyes, small feet, sharp, flat teeth and a long tail that is darker in colour than the body.

The faecal droppings of mice are typically 6mm long and pointed. An adult mouse will weigh on average 15-18g.

Young males and females are not easily distinguished: females have a significantly smaller distance between their anus and genital opening. Females have five pairs of mammary glands and nipples; males have no nipples. When sexually mature, the most striking and obvious difference is the presence of testicles on the males. These are large compared to the rest of the body and can be retracted into the body. In addition to the regular pea-size thymus organ in the chest, house mice have a second functional pinhead-size thymus organ in the neck next to the trachea.

Female house mice have an estrous cycle about four to six days long, with estrous itself lasting less than a day. If several females are held together under crowded conditions, they will often not have an estrous at all. If they are then exposed to male urine, they will come into estrous after 72 hours.

Male house mice court females by emitting characteristic ultrasonic calls. The calls are most frequent during courtship when the male is sniffing and following the female; however, the calls continue after mating has begun, at which time the calls are coincident with mounting behaviour. Males can be induced to emit these calls by female pheromones. The vocalizations appear to differ between individuals and have been compared to bird songs because of their complexity. While females have the capability to produce ultrasonic calls, they typically do not do so during mating behaviour.

Following copulation, female mice will normally develop a copulation plug, which prevents further copulation. This plug stays in place for some 24 hours. The gestation period is about 19–21 days, and they give birth to a litter of 3–14 young (average six to eight). One female can have 5 to 10 litters per year, so the mice population can increase very quickly, with breeding occurring throughout the year. The newborn are blind and without fur. Fur begins growing about three days after birth, and the eyes open one to two weeks after birth. Females reach sexual maturity at about six weeks of age and males at about eight weeks, but both can copulate as early as five weeks.

Habits

The house mouse is omnivorous. Seeds are the preferred food but mice also like foods high in fat and protein such as butter, bacon, meat and sweets. Mice feed at multiple sites, often 20-30 different sites each day, taking a small amount of food at each. A typical mouse will consume about 3-4g of food each day, about 20% of its body weight.

As primarily nocturnal animals, house mice have little or no colour vision. Their visual apparatus is basically similar to humans, but differs markedly in at least one respect. The ventral area of the mouse retina has a much greater density of ultraviolet-sensitive cones than other areas of the retina, although the biological significance of this structure is unknown.

House mice also rely on pheromones for social communication, some of which are produced by the preputial glands of both sexes. The tear fluid and urine of male mice also contains pheromones, such as major urinary proteins.

Mice move with their tails lifted up off the floor, this distinguishes their tracks from those of the rat. 

Perhaps one of the clearest signs of rodents is the damage they cause by their perpetual need to gnaw.  Mice can be distinguished by the grooves caused by the incisors being roughly 1mm apart.

Habitat

The House Mouse will avoid wet or damp conditions, but is very tolerant of cold conditions.  Usually found to infest houses but during the latter end of the grain seasons in Australia they will feed on the loose grain in the fields and will breed extremely quickly.

There is a definite social ranking among mice that is linked to protection of individual territories that are scent marked with urine.

Subordinate mice tend to feed and be sexually active when the dominant males are inactive, generally during the day. Unrelated males are highly aggressive towards each other.

Males tend to have larger territories than females and each mouse will travel its entire territory daily to investigate any changes that may have occurred.

Threats

House mice can transmit diseases, and can damage food and food packaging. Some of the diseases the house mouse carries can be deadly: for example, leptospirosis, murine typhus, rickettsialpox, tularemia, lymphocytic choriomeningitis and potentially bubonic plague. Many of these diseases are harmful to both humans and livestock.

As well as disease mice can pose a threat to stored goods like food, with an estimated 12-14% of global food production lost to rodent activity.  Damage to buildings is also of major concern, with mice being documented chewing through concrete and metal shuttering in some extreme cases. This gnawing behaviour also brings an increased risk of fire as a result of damage to electrical cabling.

Prevention

A combined approach of different control methods, including chemical, baiting, trapping, barrier fencing, habitat modification, ultrasonic devices, repellents and biological control, integrated with land management practices is most effective.

In grain production areas, monitoring and early intervention are important. Rodent-proofing facilities, habitat modification and good farm hygiene assist prevent mouse numbers increasing.

Monitoring growing crops and using early in-crop baiting once mouse activity is seen is also important.

Baits containing rodenticides should always be used in accordance with label instructions or permits issued by the Australian Pesticides and Veterinary Medicines Authority.

Many rodenticides are only approved for use around buildings, grain storage area and animal housing.

A limited number of rodenticides can be used in grain, legume, canola, safflower, nut, sugarcane, macadamia, pineapple, sweet potato crops, pasture, and banana plantations. There are strict label instructions or permit conditions on how the rodenticides can be used in these situations to minimise risks of food product contamination and adverse environmental impacts.

Others

Photos and information are provided by PelGar.

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