Source: Hong Kong Food and Environmental Hygiene Department

Authors: Yuen Chi Keung, Leung Chi Kin, Tsang Wing Sze

Biological characteristics and characteristics of commensal mice

Among the rodents (Rodenta) in the world, there are the most species of mice (Muridae); at the same time, rodents are also the class with the most species among mammals (Mammalia).

A distinguishing feature of rats and other hyperodonts is the presence of a pair of sharp and continuously growing incisors in both upper and lower jaws, and the absence of canines. This pair of front teeth must pass constant drying to keep them sharp and prevent them from growing too long and affecting feeding. Although lagomorphs (lagomorpha) also have ever-growing incisors, rabies have two pairs of incisors in the upper jaw and have other features that differ from rodents; therefore are too taxonomically distinct to be assigned to rodents class (but also mammals).

Mice themselves and their external parasites are various vectors or hosts that endanger human health. Rodents, which live with humans, eat crops and pollute our food, causing direct economic losses and spreading diseases. Their gnawing habit can also bring us a lot of economic and property losses; when the biting objects are wires or cables, it can even cause a fire.

Common commensal mice in Hong Kong

Although most rats reproduce and grow in the natural environment, some rat species have adapted to the living environment and habits of humans, live together with humans, and pose a lot of nuisance to humans.

There are three common commensal mice in Hong Kong:

1. Ditch rat (Rattus norvegicus), also known as brown rat and Norway rat

2. Black house mouse (Rattus rattus), also known as boat mouse, roof mouse, house mouse

3. Mus musculus (Mus musculus), also known as mouse, house mole

1. Rat (Rattus norvegicus)

The ditch mouse is the largest of the three common commensal mice. Adult ditch rats have a stocky body and weigh between 200 and 450 grams, but may also weigh up to 600 grams: the total length from head to tail is about 32 to 47 cm.

Compared with other common rat species, the proportion of the head of the ditch mouse is smaller, the snout is round and flat, and the eyes are smaller; the ears are also smaller, and the forward folding will not cover the eyes. The tail of the ditch rat is thicker, but shorter than the total length of the body plus head; the top and bottom of the tail are not uniform in color (the bottom is noticeably lighter). Ditch rats are more common in places such as sewage drains, flower beds and burrows.

2. Black house mouse (Rattus rattus)

Although both the house mouse and the ditch mouse belong to the genus Rattus in the family Muridae, if you observe carefully, you can see that there are significant differences in their appearance. An adult house mouse has a slender body, weighing about 120 to 225 grams, and rarely more than 300 grams; the total length from head to tail is about 27 to 43 centimeters.

Compared with the ditch mouse, the proportion of the head of the house mouse is larger, the eyes are also larger, and the snout is sharper. In addition, the large ears of the house mouse fold forward to cover the eyes. The tail of the house mouse is usually longer than the combined length of the body and head; and the entire tail is dark and uniform in color. The house mouse is good at climbing, and is more common in false ceilings, building top floors, roof spaces and beams, etc.

3. Mus musculus (Mus musculus)

Mus musculus belongs to the genus Mus (Mus) in the family Muridae. Mus musculus is much smaller than the other two common mice. Its body is thinner and slender, usually weighing between 15 and 30 grams; the total length from head to tail is about 7 to 20 cm.

The head of Mus musculus is relatively large in proportion to the body, with large eyes and a pointed nose. Its ears are also large enough to cover the eyes when folded forward. The tail of Mus musculus is uniform in color, brownish-gray; but it can be slightly shorter or longer than the total length of the head and body. In addition, the size of the front and rear feet of Mus musculus is similar, and the proportion of the feet to the body is the smallest among the three common mice. Mus musculus is usually found indoors, especially near areas where food is stored.

Finally, it must be noted that the hair color of mice is controlled by genetics and environment, and the color of mice of the same species can also vary greatly. Although Rattus rattus is often referred to as the "black house mouse", the mouse is not necessarily black, and a black mouse is not necessarily a "black house mouse"; nor is the brown house mouse. . Most of the white mice commonly used in scientific experiments are white Rattus norvegicus and Mus musculus that have been specially bred.

sensory abilities of mice

1. Vision

Rats are nocturnal animals, so their eyes have evolved to provide side-by-side night vision, allowing them to see objects in very dim light. However, in order for the eyes to fully develop the ability to detect weak light in a dark environment, the acuity of vision must be sacrificed. Therefore, the vision of mice is very blurred, and they must rely on other senses to find the surrounding environment at the same time.

In addition, although there are two types of cone cells for sensing light and ultraviolet light in addition to the rod cells that sense light, the number of these two types of cells is very small; and experiments have shown that Only at ultraviolet levels far beyond the environment in which the mice live, the mice barely have some color recognition ability. Therefore, although mice are not completely color-blind scientifically, the above data show that mice can be regarded as color-blind under normal circumstances.

2. Tactile

To compensate for their lack of vision, mice have developed a keen sense of touch. Not only do they have long tentacles, but they also have needle hairs on their bodies, which help them sense the surrounding environment. In a dark environment, these tactile stimuli help mice distinguish the distance, roughness and texture of objects. Coupled with excellent memory, they can know the environment within the range of activities like the back of their hands.

3. Hearing

Rats also have very sensitive hearing. Human ears can only distinguish sounds up to about 20 kilohertz (20kHz), but mice can hear high-frequency ultrasound up to 100 kilohertz (100kHz). They can communicate with ultrasonic waves, and they respond very well to louder volumes or high-frequency calls for help from their companions.

4. Taste

Rats also have a good sense of taste. They have the ability to recognize very small amounts of rodenticides or other bitter and odorous substances in rat bait, and can remember the relevant smell. Therefore, if a mouse eats a dose of rodenticide that is not lethal, it will not eat food containing the rodenticide after the symptoms pass and it gradually recovers; and she will also warn her companions . In this case, re-applying the rodenticide has no effect on the feeding-resistant rat or even other rats in the same group.

5. Smell

Rats also have excellent vision, which helps them detect the location of food and identify animals of the same species and different species. They use smell to identify different rats, as well as the sex and reproductive status of the rats; rats with different status in the group also have different smells. Rats will also leave scents on the areas where they walk and live to facilitate road identification and delineate their own territories.

Mobility and living habits of mice

All urban rats (and most wild rats) are nocturnal and highly adaptable. But based on subtle differences in body shape and other physical characteristics, as well as sense of touch, different species of mice play to their own strengths in different ecological environments, so their living habits are not exactly the same.

1. Climbing ability

The ditch rat is the largest of the three commensal rat species. His climbing ability is relatively weak. The body length of the black house mouse is about the same as that of the ditch mouse, but its weight is only about half of that of the ditch mouse. Its climbing ability is relatively strong, and it can easily climb up the false ceiling or beam along the vertical pipes on the wall, wooden door frames and even wires, etc. Occupy nearby voids for shelter. Mus musculus usually haunts places where food is stored, using furniture or false ceilings as nests.

2. Digging ability

Ditch rats often dig burrows, the depth of which can reach 50 cm, and each burrow usually has two openings. Mice and Mus musculus seldom dig burrows. But the black house mouse uses the burrows left by the ditch mouse as shelter.

3. Jumping ability

In addition to climbing, mice are also very good at jumping. Even the small Mus musculus can jump to a height of 25 centimeters, while the larger and stronger musculus and mus musculus can jump to a height of one meter when they have an approach run.

4. Eating habits

All kinds of mice in the city are omnivorous animals, and basically all human food, no matter whether it has rotted or not, can become food for mice, but different types of mice have their own preferences. The preference of the gully mouse was sweet potatoes, bread and fresh or dried fish and meat, the preference of the house mouse was nuts, meat, apples, carrots and bread, and the preference of the mus musculus was grains and seeds. In terms of feeding habits, both the ditch mouse and the black house mouse will eat a lot of food at one time, and will take the food to a safe place to eat; the mus musculus is used to eating small amounts of food from many different places. But none of them had the habit of storing food for the winter.

In addition to feeding, rats also cause a lot of damage in the process of gnawing on different objects. The incisors of mice will continue to grow throughout their life, so as not to lose their function due to constant wear and tear during the feeding process. But it is precisely because the front teeth will continue to grow, so mice often keep the front teeth sharp and prevent the teeth from being too long to hinder feeding through continuous biting.

5. Domain Characteristics

The range of activity of the mouse is related to the size of the mouse itself. The activity range of the black house mouse and the ditch mouse is about 30 to 50 meters, while the house mouse usually only moves within the range of 5 to 10 meters. Commensal mice usually use urine and feces to demarcate their territory, while male mice, even late-pregnant or lactating female mice, are very hostile and aggressive to intruders.

6. Social organizations

Rats live in groups. Each group of rats is led by a male rat, and this male rat can enjoy the best food and habitat and the greatest chance of mating. Rats with lower status may therefore be excluded from the Edge or strange place.

7. Probe Features

Ditch mice, house mice and Mus musculus all have a strong desire to explore and curiosity, which helps them obtain more food and develop more living areas, but at the same time, city mice usually walk against walls, In this way, the guard hair on the body can sense the surrounding environment, so as to obtain a sense of security in the process of walking and exploring in the case of insufficient vision. In order to gain a sense of security, most of the time they will only walk on the usual walking path, thus leaving a barren road on the habitual grass, or leaving a smear of grease dirt (scratch) at the foot of the wall.

They also have a reaction of resistance to unknown objects (novelty resistance). During the long evolutionary process, avoiding contact with unknown objects can avoid danger, thereby greatly increasing the survival chances of the entire group and even species. The novel resistance response of Mus musculus is relatively weak.

8. Fertility

Rats are surprisingly fertile and can breed year-round. These commensal mice can produce five to eight litters per year, and each litter can produce 5 to 14 pups. If it is not controlled in time, a pregnant mouse can produce tens of thousands of offspring within a year, which shows the importance of timely eradication of rodents.

Other mice in Hong Kong

In addition to the ditch mouse, the black house mouse and the mus musculus, the mice that can be seen in the wild in Hong Kong include the black class dog (Rattus sikkdimensis), the pin hair mouse (Niviventer fulvescens) and the field mouse (commonly known as voles) (Bandicota indica), etc. But these mice are not commensal rodents. Most of them feed on plants and are almost extinct in urban areas.

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