Rodents: Friend, Foe, and Laboratory Subject?

wild mouse

I. Rodents Throughout History

         Rodents are inconspicuous, nocturnal animals that are small in size and have nonspecialized dentition. Small rodents (rats, mice, etc.) have long lived side by side in sympatry with humans since ancient times, 12,000 B.P (Boursot, Auffray, Britton-Davidson, Bonhomme, 1993, p. 130).  Some small rodent populations even began living commensally in the homes of humans. The commensal habitat, the human home is constantly protected and food is unintentionally provided for rodents there.  As a result there is a significant reduction of interspecific competition, predation, and climatic pressures that would of normally affected survival in the wild (Boursot et. al., 1993, p.137). Rodentia commensalism with humans has made this mammalian order the second most successful mammalian clade on Earth.

Rodentia is the most abundant and diversified order of living mammals, representing about 43% of the total number of mammalian. Its species are distributed on every continent except Antarctica and include many of the most abundant and taxonomically diverse mammals. (Meerburg, Singleton, Kijlstra, p.221)

Diagram 1 (Appendix), an incomplete Phylogram of Mus (the genus name for mouse) shows the diversity of species present within the genus, this is only a small portion of Rodentia Order. Boursot et. al. (1993) mention how the commensalism of small rodents (rats and mice) with humans “…has modified the population structure mainly through fragmentation of populations and high local densities…[and] has led to …domestication in the form of laboratory strains.” (Boursot et. al., 1993, p. 120).

In high-density populations, the behavioral organization has lead to a hierarchical structuring of the population (Maly, Knuth, and Barrett, 1985).  Though territoriality remains the common structure in outdoor environments where food is more scattered (Mikesic and Drickamer, 1992).  A commensal population coexisting in a single dwelling/settlement is usually subdivided into several social units/demes (Crowcroft and Rowe, 1963).  A deme is usually never more that 12 individuals it consists of a dominant breeding male, a few subordinate males, and several females (Singleton, 1983).

Hippocrates, among others, wrote extensively about the human-rodent interaction and the common effects of infectious agents both in livestock and humans.  Furthermore, rodent-related diseases can be traced as far back as the Ten Plagues of Egypt.

“‘It will become fine dust over the whole land of Egypt, and festering boils will break out on men and animals throughout the land’, says Exodus 9, describing an air-borne infection causing disease in livestock and humans. The Old Testament also carries the description, in the First Book of Samuel, of a lethal outbreak of ‘groin tumors’ in Philistines, in conjunction with the presence of rats, a possible early description of bubonic plague or tularemia and its association with rodents …[I]n 1796, Edward Jenner commented that man ‘has familiarized himself with a great number of animals which may not, originally, have been intended for his associates’, and thus this deviation ‘seems to have proven to him a prolific source of diseases’ .” (Pappas, 2010, p.322)

Small-rodents (specifically Black rats for this example) are responsible for the causation of three human plagues: the Justinian Plague (540s-700s A.D.), the Black (Great) Plague (1300s), and the Modern Plague (1860s-early 1900s) (Khan, 2004, p. 270; Andrianaiyaoarimanana, 2012, p.1). During the Victoria era, in the UK rats were sold by the hundreds to dog owners who would use them to conduct rat sporting, the placing of rats into a pit with dogs where they (the rats) were mauled to death (Edelman, 2002, p.4).  Rat-sporting was quite lucrative at the peak of its popularity rat pit owners were able to make a large profit from charging audience members an entrance fee to watch the rats and to bet on how long it would take the dog to kill them all.  By the 1870s rat-sporting was completely prohibited by English law (Edelman, 2002, p.4).

Rodents (specifically mice and rats) have been documented as pets as early as 1100 B.C. in China.  However, rats did not truly become popular globally popular as pets until Jimmy Shaw and Jack Black began breeding and selling different colored fancy rats in England around the same time rat-sporting was dying out (Edelman, 2009, p.5). “The first known experiments performed on rats date back to 1856 in France, and in the following decades there were experiments involving rats in England as well as in Germany” (Edelman, 2009, p.5). However, the first modern lab rat is widely credited as white albino rats used in 1895 experiments at the Wistar Institute in Philadelphia (Edelman, 2009, p.5).

By the end of the 19th century the rat had established a threefold identity as: a detestable pest, a lovable pet, and a scientifically neutral laboratory subject. For us it’s all about the context we encounter the rat in. The way a rodent is treated throughout its life is largely determined by its’ birthplace. Rats born in a rat farm are treated as snake food, rats born in the sewers are treated as pests, rats born in laboratory are treated as a research subject, and rats purchased at the local pet store are treated as a pet.

II. Rodents as Pests

         Let us first examine rodents in their role as detestable pests.   Rodents that are considered detestable pests are commonly referred to as vermin. Rodents are opportunistic animals like with baboons of South Africa careless handling and storage of produce, along with improper removal of waste have contributed widely to the public’s opinion of rodents as pests.  Moreover, rodents are considered pests for the following reasons: they destroy farmers’ crops before they are able to harvest them, they spread zoonotic diseases, they have a gnawing tendency which tends to ruin building infrastructure, and lastly they can act as feral invaders to the natural habitats of other species. (Alpin et al., 2011,p.1).  All of the aforementioned reasons contribute to the demonizing of rodents, which shall be discussed a little later.

The first reason rodents are considered detestable pests are they compete with humans for food.  Rodents do this chiefly through the pre-harvest crop damage. In Asia, pre-harvest rice losses range from 5% in Malaysia to 17% in Indonesia (Singleton, 2003). To put this into perspective, a loss of 6% in Asia amounts to enough rice to feed 220 million people, roughly the population of Indonesia, for 1 year (Singleton, 2003).  Meerburg, Singleton, and Kijlstra (2009) mention how,

Rat damage is often patchy and family rice plots small, so it is not uncommon for farmers or villagers to lose half of their entire rice crop to rats. [Furthermore] On a global scale, it was recently estimated that almost 280 million undernourished could additionally benefit if more attention were paid to reducing pre- and post-harvest losses by rodents. (p. 221)

Obviously this issue is in dire need of addressing.  More compelling evidence of rodents as pests, is exemplified by the destructive behavior of the Black rat, which is perhaps the most widely distributed of all commensal animals and also the most destructive of all animal pests (Alpin, 2011, p.1). In rural areas the Black rat accounts for more than 85 % of small mammals captured alone (Andrianaivoarimanana, 2012).  The other 15% is composed largely of other small rodents.

The second reason rodents are considered detestable pests are for their ability to spread zoonotic diseases.  Pappas (2010) defines a zoonotic disease as, “ a disease that can be transmitted by animals to humans” (p.321). Rodents possess the ability to amplify pathogens from the environment and form reservoirs of (zoonotic) disease (Meerburg et. al., 2009, p. 222). Rodents are essentially the nexus between wildlife and human populations (Meerburg et. al., 2009, p. 221). Rodent-borne diseases can be spread via two different pathways (Diagram 4, Appendix). The first pathway is a direct route from rodents to human. Rodents can spread pathogens to humans by biting them or by human consumption of food products or water that is contaminated with rodent feces (Meerburg et. al., 2009,p. 222). Rodent-borne pathogens can also be spread indirectly to humans via transmission to ectoparisitc arthropods (ticks, mites, fleas) vectors, which can then infect humans (Meerburg et. al., 2009,p. 222). Also, “Rodents that are accidentally or on purpose ingested by livestock can transfer pathogens, which can result in death if these food products are not-thoroughly cooked. “ (Meerburg et. al., 2009,p. 222).

As mentioned earlier, the most famous zoonotic disease, the Black Plague, spread from Black rats to humans. A very intriguing example of rodents serving as a reservoir for zoonotic disease can be observed in Vietnam where rodent are being infected by Fishborne Zoonotic Trematodes (FZTs) via their drinking water, which has potential to increase the rate of transmittance of FZTs to humans (Thi et. al., 2012, p.1045).  The possibility of transfer of zoonotic diseases between humans and rodents has severe public and veterinary implications. There are over 100 diseases that can be acquired from wild rodents. Diagrams 2 and 3 (Appendix) lists some of these diseases for a comprehensive list check Meerburg et. al. (2009, p.223-251).  Also, rodents are sometimes involved in the horizontal transmission of pathogens that cause animal diseases, thus causing huge economic damages and image losses for animal husbandry that we depend on for our sustenance and livelihood  (Meerburg et. al., 2009, p.222).

The third and fourth reasons rats are considered detestable pests their gnawing habit and role as feral invaders to the natural habitats of other species will not be further elaborated upon.  So let us redirect our attention to the underlying feelings reason we all despise rodents. We have a tendency to demonize rodents. In the horror genre, films, books and cartoons rodents are often included to enhance disgust (Diagram 5 &6; Appendix).  Essentially in media rodents have become a symbol of evil, much as the dove is a symbol of peace, and the owl a symbol of wisdom.  “The identity of the rat as a symbol for dirt, disease and evil aggression…” (Edelman, 2002, p. 6).  Diagram 7 (Appendix) is a perfect example of the demonization of rodents; it is an advertisement for the rat exterminator. Two pictures are juxtaposed next to each other, each with its own accompanying message. One picture depicts a cute “cartoon” rat holding a bunch of flowers, and the other one a “real” dark rat, which is shown facing the viewer as if it is ready for attack.  The legend under the first picture says: ‘”The rats we are concerned with are very, very different [i.e. from the cute cartoon character]…’ The legend of the second picture announces: ‘…They are frightening. They steal, pollute, soil, infect us, and. sometimes they kill us!’ Rats are not only depicted as vermin, they are criminals.” (Edelman, 2002, p.6-7).

III.  Rodents as Pets

Next let us discuss rodents in their role as lovable pets. The first written reference of a rodent being kept as a pet is found in Ehya, the oldest extant Chinese dictionary, which dates back to 1100 B.C. (Royer).

“In Europe the breeding of fancy mice became popular through the introduction of Japanese stock in the early 17th century. By 1895 Walter Maxey founded the National Mouse Club in Victorian England, with its first official show held…that year. Since that time, mouse clubs have formed worldwide. Shows are held so competitive breeders can display their mice, where they are judged on color, body shape and behavior.” (Royer).

Also people like Jimmy Shaw, Europeans aristocrats of the Victorian Era and Mary Douglas have popularized breeding fancy rodents (rats and mice) both for pet keeping and for competition. There are many breeds of fancy rats and mice, not to mention other rodents such as chinchillas, hamsters, guinea pigs, hamsters, gerbils, mole rats, and chipmunks are kept as pets.

Mice are kept as pets in many countries for because they are relatively small, inexpensive, never need bathing, and can learn to enjoy regular handling (RMCA Mouse FAQ).   Female mice are popular with owners since they tend to cohabitate with other mice better than males. Additionally, the urine of female fancy mice does not contain as strong an odor as that of the male mice.   Rodents kept as pets are treated with affection and may be pampered with miniature clothing and ornate ribbons.  This is especially the case for fancy mice and rats.  Dressing the rodent in ornate ribbons and miniature clothing is aesthetically pleasing and also very amusing for the owner.

IV. Rodents as Laboratory Subjects

Last of all let us validate rodents in their role as neutral laboratory animals.  Laboratory mice are bred for smaller size, easier handling, quick reproduction and maturity, and shorter life spans in comparisons to their wild cousins (Robinson, 1979). In 1895, the Wistar Institute had a epiphany, they realized the following: clean, healthy, albino rats were essential for accurate research and that their production was a serious, difficult, and worthwhile task; secondly it is advantageous to directly and frequently handle the investigator’s stock instead of using “‘tongs or other rough devices’” (Edelman, 2002, pg.5).  Also, handlers are advised to only perform their jobs sufficiently enough to get the job done, so that the results of the experiments being conducted on the mice/rats who are supposed to neutral subjects are accurate.  Neutral here refers to the rodent’s role as neither a loved or a hated animal.  Despite the underlying stigma encircling rodents, specifically the demonization that was mentioned earlier there has been a discernible shift towards acknowledging the social character of rodents (Edelman, 2002, p.8).

Since Wistar’s introduction of albino rats to the laboratory setting laboratory rodents have been pivotal to advancements made both in science and the quality of human life. Rodent research is absolutely necessary and anyone who argues otherwise should consider the number of human lives rodent research has saved through the successful creation of new vaccines, medication, and surgical methods. Rodents make up the absolute majority of all lab animals.  Moreover, their numbers continue to grow, while the use of ‘higher’ animals in labs rapidly declines.  Rats do benefit from the laws and regulations that try to limit cruel experiments on animals, and medical papers often use euphemisms such as ‘put down’, ‘put to sleep’ or ‘sacrifice’ in order to avoid the starker ‘kill’ (Edelman, 2002, p.7).  Although this may be true, the Animal Welfare Act excludes rodents and birds from its’ definition of animals and by no coincidence most scientists chose rats as their research subjects.            “The [Animal Welfare] Act’s definition of the term animals…’Animal means             any live or dead …warm-blooded animal, which is being used, or is intended        for use for research, teaching, testing, experimentation, or exhibition purposes, or as a pet. This term [animals]             excludes: birds, rats of the genus        Rattus and mice of the genus Mus bred for use in research…” (Herzog,            2010,p.237).

The mouse/rat/bird exclusion means that we truthfully have no idea how many animals in total are used in research each year in the United States.  The responsibility for ensuring the ethical treatment of lab animals is placed solely on the institutions where the research is conducted.  Each institution is responsible for establishing a local Institutional Animal Care and Use Committee (IACUC). (Herzog, 2010, p. 238-9).

V. Conclusion

Historically as an inhabitant of the vile underworld, the sewers, rodents (referring mainly to rat and mice) have been considered vermin/pests and have been exterminated and expelled as result. It is clear that masses of rodents provoke disgust, while a single rodent can be accepted as a pet, provided that it is cleaned, presented well ‘dressed up’ preferably in a fancy fur color.   Human-rodent relations have changed dramatically since commensalism first occurred. Human-rodent relations has changed now a new threefold role of rodents has emerged, given context and a rodent’s place of birth it will be regarded as either a despised pest, a lovable pet, or neutral laboratory subject.


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Demelio U.

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