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All About Mice

The Secret Life of Field Mice:

Most of the mice used in laboratories are white albino house mice (Mus musculus). The house mouse, a member of the rodent family, originated in ancient Asia and later spread throughout Europe. In the 1500s, they traveled to North and South America on English, French and Spanish ships. Today house mice appear both in the wild or commensally (nesting within human dwellings) in nearly all parts of the United States.

The mouse has been used in biomedical research since the early 20th century. Today, over 3,000 genetically defined strains of lab mice are used for research purposes. The primary supply source of laboratory mice is commercial breeders. Relatively few are captured from the wild or bred in research laboratories, although the latter practice is becoming more common with some genetically engineered mice.

Several characteristics have made the mouse an appealing research subject. These include the mouse's genetic similarity to humans (at least 80% of DNA in mice is identical to that of humans), small size, short lifespan and reproductive cycle, low maintenance in captivity, and mild manner. For these reasons, house mice constitute the majority of mammals used in research, testing, and education. Over ten million mice are used each year in U.S. laboratories alone, in tests of new procedures and drugs as well as in research involved in the production of biological products such as vaccines.

Mouse Biology

The average lifespan of a wild mouse is typically one to two years, while mice kept as pets usually live about three years and have survived up to six years. Mice reach sexual maturity at between five and six weeks of age and have tremendous reproductive potential. They breed throughout the year and may produce as many as eight litters in a single year, with the average litter consisting of four to seven pups. Female mice care for young pups who are fully furred at ten days old and weaned at about three weeks. After this time, young mice venture out of the nest and leave their mother's territory, although female mice frequently remain nearby.

The mouse's sense of smell is perhaps the most important of the five senses. A wide range of pheromones, or olfactory social signals, and other smells are key in communication regarding social dominance and family composition. Through territorial marking, mice are able to speed up or retard sexual maturation in juvenile females, synchronize reproductive cycles in mature females, and terminate pregnancies. Smell is also critical for survival, alerting mice to food and predators.

Hearing is also an important sense in mice and helps compensate for their poor eyesight, which is particularly true of the albino mice used in research. Mice are highly sensitive to sound, detecting frequencies from 10-70 kHz and possibly up to 100 kHz. (The human range is from 200 Hz to 16 kHz, with the most important range below 6 kHz.) Sounds with frequencies over 20 kHz are considered ultrasonic or beyond the range of human hearing. Mice are able to hear ultrasonic sound and even use it for communication in a variety of circumstances, including parent-pup relations, sexual encounters and stressful situations.

The sense of taste is highly developed in mice, evident through the varied diet of wild mice. An omnivorous species, mice eat a variety of plants, seeds, grains, insects and occasionally carrion. Varied diets are important in distinguishing territory, as food is a factor in the smell of urine which is used by mice to mark territory. Mice consume about one-tenth their body weight in food each day. They have 16 teeth that continue to grow throughout their lifetime, so apart from eating, mice have to gnaw to prevent tooth overgrowth.

Vision is relatively unimportant, as mice are mainly nocturnal mammals and rely more heavily on their other senses to travel. Mice use their whiskers to help navigate by sensing air movements and surfaces. When traveling long distances they prefer to keep one side of their body pressed against a wall or other surface. This also helps them stay out of sight.

There is no question that mice feel pain. In fact, mice are used in pain research studies to help determine the role genes play in pain sensitivity. Pain may not be readily noticeable in mice, as they instinctively attempt to hide illness and injury because weak or sick animals are targeted by predators in the wild. However, mice in significant pain or distress, or that are seriously sick, typically display a lack of activity, sunken eyes, ruffled fur, and an arched back or huddled position.

Mouse Behavior

The natural habitat of mice is forests and grasslands, although nests may be found in nearly any undisturbed place with a nearby food source. In the wild, mice live in single, male-dominated groups called colonies. A typical colony consists of a dominant male, several females, and their young. Occasionally, males may share territories. Females are less aggressive than males but may establish their own hierarchy within a territory.

Mice kept in a confined environment, such as a lab setting, are usually more territorial than wild mice. There is rarely fighting among female mice groups but grouped male mice can inflict serious injuries on each other. To show their dominance over other mice in their territory, males may exhibit a behavior termed "barbering" in which they chew distinct areas of fur, usually around the muzzle or whiskers, from subordinate mice, leaving a bald patch.

Mice construct nests, but may also live in group dwellings that consist of complex networks of tunnels, with separate common areas for nesting, storage, feeding, urinating, and defecating. They spend a great deal of time burrowing into and manipulating nesting material, exploring, and foraging for food. Grooming is also important to mice; their shiny, smooth fur is the result of constant grooming. In fact, mice will also help each other groom areas of the head and back that are difficult to reach.

Of Mice and Humans

Despite their bad reputation, not much about the average mouse's appearance could be considered imposing or threatening. Adult mice generally weigh less than one ounce and range in length from two-and-a-half to three-and-a-half inches, minus the tail. A mouse's tail is usually only slightly shorter than the animal's body.

While the mild-mannered mouse is ill-suited for confrontation and nearly always prefers flight to fighting, a single mouse can incite a near panic among people simply by scurrying across a room. Their lightning speed most likely puts people off guard. Mice can run up to eight miles per hour and are adept at jumping, climbing, and swimming. But mice have good reasons to be fast runners as their potential predators include hawks, snakes, falcons, owls, ferrets, mongeese, weasels and foxes, as well as domestic cats, dogs and, of course, people. So mice are constantly on guard, spending a good part of their life running and hiding.

Because human habitations are appealing to mice, they can cause considerable trouble for humans if proper precautions are not taken to exclude them from dwellings and food storage areas. The biggest problem that mice create for humans is contamination of food, of grain and seed in particular, with urine and feces. Killing problem mice is not the best answer; as long as mice have access to a building there will always be more of them to deal with. The best solution to mouse problems is to prevent them from entering buildings and homes.

Despite the problems mice can cause for humans, there can be no doubt of the fascination humans have with this animal, and that people appreciate the mouse's charming characteristics. For many decades the mouse has been a favorite in cartoons, children's literature, and movies portraying fictional characters such as Mickey Mouse, Mighty Mouse, Mrs. Frisby, Stuart Little, The Country Mouse, and recently, the well known cartoon characters, Pinky and the Brain, two laboratory mice plotting world domination from their cage.

Mice are the smallest domesticated mammals and many people keep them as pets. In fact, the popularity of pet mice has grown in the past 15 years. This may be due in part to the growing ownership of reptile pets, some of whom require live mice as food. Some reptile owners who purchase mice as food for their reptiles discover that they are entertaining and easy to care for, not to mention cute. So many mice who might have been lunch for a snake end up as pets instead. Albino house mice, the same kind used in labs, are the most commonly-kept pet mice. "Fancy mice," originally bred for show purposes, are also kept as pets. Fancy mice are larger than other mice, have longer tails and larger ears, and occur with a wide variety of colors, markings, and coats.

Research and Testing Conducted on Mice

Mice are the work-horses for medical research on many human illnesses and conditions. They are the primary research model used to study disorders such as cancer, AIDS, seizures, Alzheimer's disease, multiple sclerosis, and transplant rejection. Medical research on mice has produced advances in treatments for depression, cancer prevention and treatment, and malaria vaccines. Mice are also widely used in genetic engineering research. Genetically engineered or "transgenic" mice have selected human genes artificially introduced into their genomes that make them more susceptible to certain human diseases and conditions, such as certain types of tumors, diabetes, cancer, leprosy, tuberculosis, and even obesity. These transgenic mice are then used by researchers to study a particular ailment. Mice who fail to successfully incorporate genes that researchers inject into their embryos are killed. The number of lab mice that meet this fate is quite high, as only between 1 and 10 percent of genetic engineering in mice is successful.

Many of the tests and experiments conducted on mice cause them a great deal of pain and distress. One of the most inhumane tests mice are subjected to is the "classical" LD50 test. The aim of the test is to estimate the dose that kills 50 percent of the animals to which it is administered (hence the name: "Lethal Dose 50%"). The LD50 involves injecting several groups of mice with varying doses of the product to be tested and seeing if they die within three to four days. During BOTOX® testing, mice endure different levels of muscular paralysis. Those mice injected with a high enough dose die from suffocation after their diaphragm becomes paralyzed. Others may languish with varying degrees of paralysis before being euthanized at the end of several days.

When testing or medical experiments are completed, virtually all test mice subjects are euthanized, typically by carbon dioxide inhalation. However, various studies suggest that CO2 inhalation may cause significant pain and distress to animals.

What protections apply to mice in laboratories?

House mice bred for research (virtually all mice used in research) are excluded from the definition of "animal" in the Animal Welfare Act, and are therefore excluded from protection under the primary federal law giving some protection to research animals. Mice receive some level of protection under federal guidelines applicable to research institutions that receive funding from the U.S. Public Health Service. These institutions must have Institutional Animal Care and Use Committees to see that these recommendations are followed.

In other situations, however, it is routine for research to be conducted on mice with no federal oversight of their humane treatment. Attempts to secure greater oversight through litigation have been thwarted by opposition from the research community.

A Better Life for Laboratory Mice?

Provisions for mice used in laboratories is a currently a topic of intense debate in the scientific community. Researchers have argued for generations that the laboratory environment for animals must be highly standardized to reduce the variability of results. But the trend is now under scrutiny, with many researchers acknowledging the complex nature of the tiny mouse. Small, barren and dull housing for lab mice is now being blamed for exactly what it was said to eliminate: compromised data. Researchers are realizing that an enhanced quality of life in the lab lowers stress levels in mice and avoids possibly compromising test results. Studies of mouse populations reveal that they are a highly-evolved and social animal. Mice, like many other animals, need to be able to exert some control over their environment in order to feel secure, and in many cases laboratory life gives them limited opportunities to do so.

Laboratory housing designs for mice are dictated by human convenience, and the behavioral needs of mice are given little consideration. The majority of laboratory mice are housed in cages not much larger than shoe boxes, which rarely contain any kind of environmental enrichment. Mouse cages sometimes have wire mesh or grid floors for sanitation purposes. However, these surfaces are uncomfortable for mice and may result in pressure sores. Grid flooring also does not allow for the use of substrate, which mice need to build nests and tunnels, one of their favorite activities. The mouse's food and water sources are usually suspended from the cage top to prevent spillage and make cage cleaning easier. But mice prefer to forage for their food and prefer to search for food (as they do in the wild) instead of eating from a container.

Mice kept in conditions far removed from what they would experience in the wild can suffer depression, anxiety, and frustration, and may display abnormal behaviors such as constant jumping or repeatedly moving in stereotyped patterns around their cages.

Some provisions that help mice adapt to lab life are housing in compatible groups (as opposed to solitary housing), nesting materials for burrowing and hiding, opportunities to explore and forage for food, regulation of light and dark cycles, noise reduction, and limiting transport within or between labs. Transport of lab mice under even the best conditions is stressful for them, and they may require several days to recover from this event.

No other animal has been subjected to more testing or medical research than the mouse. Yet progress towards making laboratory life more humane for these animals has been slow. Animal protectionists and researchers concerned with lab animal welfare advocate for reducing the number of mice necessary for experiments, refining procedures to make experiments and lab life more humane for mice and replacing live mouse experiments with modern and improved techniques, wherever possible.

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