Of Mice and Humans

Est. Reading Time: 5 minutes
By George Shuklin (talk) - Own work, CC BY-SA 1.0, https://commons.wikimedia.org/w/index.php?curid=5521043

As the weather gets cooler, animals start looking for a nice comfy place to spend the winter. One of these unwelcome house guests is the house mouse. I thought now might be a good time to learn a little about this species and maybe get some advice on what you can do to prevent them from getting into your house in the first place.

House mice weren’t always here in the United States. They are indigenous to an area from the Mediterranean to China. But mice are nothing, if not smart, and they hitched rides on ships. Now they are found worldwide. Because of their close association with humans, house mice are even able to live in deserts where they might not otherwise survive.

House mice are 2.5-3.5 inches in length, not including their tails. Their tails add an additional 2-4 inches to overall length, and are scaly with no fur. House mice only weigh 0.5-1 ounce. Their fur can range in color from brown to black with a white or buff-colored belly. Scientists have found that when house mice live in close association with humans they tend to have longer tails and darker fur color. House mice have ears that are relatively large and black eyes. They have excellent hearing, vision, and sense of smell. Their whiskers are very sensitive and are used to transmit information about the surrounding environment.

(By 4028mdk09 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11516327)

In the wild, mice eat plant materials and insects. They eat the seeds, roots, stems, and leaves of plants. When available, house mice may also eat beetle larvae, caterpillars, cockroaches, and dead animals. When living with humans, mice eat human food as well. They have even been known to eat soap and glue. Mice either store their food in their burrows or live where humans store food.

House mice are preyed on by a wide variety of species such as house cats, foxes, snakes, and raptors. Their main defense against predation is to avoid being seen and avoid being caught. House mice are very agile animals. They are capable of running up to eight miles per hour, and they climb and jump quite well. Surprisingly, house mice are good swimmers too.

In the wild, house mice may make underground burrows that have a series of tunnels and three or four exits. There are several chambers in the system that are used for nesting and storage. Wild mice may also live in the walls of buildings. In or near human dwellings, house mice will nest in any hidden spot that is close to food, such as woodpiles or storage areas. Usually nocturnal, house mice may sometimes be active during the day when living in a human dwelling or if food is scarce.

(By OnkelJohn – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=7200962)

Male mice mate with more than one female at a time. Recent studies show that males may produce ultrasonic “songs” when they are exposed to female pheromones (scents). Breeding can occur throughout the year, but wild house mice generally breed from April through September. Estrus, the time when the female is receptive to the male, usually only lasts for a single day. The estrous cycle (fertility cycle) itself lasts four to six days. Females can have another estrous cycle as soon as 12 hours after giving birth.

Nests are made out of soft materials that are lined with more finely shredded materials. Females can have five to ten litters per year, with the average litter size being five or six offspring. Litters can have as few as three offspring or as many as 12. The gestation period lasts from 19-21 days, but may last longer if the female is nursing young while she is pregnant.

The young are naked and blind when they are born. After 10 days, the young will be completely covered in fur and their eyes open after 14 days. Females care for the young for about three weeks, at which time most of the young will leave the mother’s territory. Once the young are weaned, the adults become aggressive towards them which forces them to disperse. The only exception to this is that young female mice may sometimes stay close by. After five to seven weeks, the young will be fully mature and able to reproduce. The life span of a house mouse in the wild is only about 12-18 months, although they can live up to two years in captivity.

(CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=352524)

Paradoxically, house mice are both territorial and colonial when they live with humans. Dominant males maintain territories that consist of several females and their young. Subordinate males may share a territory with other males or may have a territory of their own if the conditions permit. Wild mice tend to be less territorial than mice that live with humans. Females have a dominance hierarchy within a territory, but they are less aggressive than males. All mice within a territory will defend against outsiders that intrude on their territory. Most mice do not travel more than 50 feet from their territory. When food is overly abundant, house mice tend to be commensal (colonial). However, even in these situations, the males will still maintain a small territory. In the wild, it is rarer to find house mice living commensally.

There are several methods that can be used to keep mice out of your house. The first method is to keep things properly cleaned up, such as cleaning up food spills immediately. Eliminating places around your house that can be used as cover or shelter can also help reduce the number of mice that get into your house. If possible, keep materials at least eight inches off the ground and about a foot away from the wall. These methods alone won’t keep mice out of your house, however. Exclusion methods must also be used. Find any openings that are a quarter inch or greater and either screen them off or find another way to remove the opening. Put sealant or mortar around gaps in pipes that are less than a half inch. For larger gaps, you can use copper or stainless-steel mesh. Make sure that windows, doors, and screens fit securely and cover the edges of doors and windows with metal to discourage gnawing. Using these methods make it more likely that you can live in harmony with house mice.

The Longmont Weekly Wild is a weekly column about all things wildlife and wild in Longmont. You can find previous articles under the Lifestyle section.