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The best-known rat species are the black rat (Rattus rattus) and the brown rat (Rattus norvegicus). This group, generally known as the Old World rats or true rats, originated in Asia. Rats are bigger than most Old World mice, which are their relatives, but seldom weigh over 500 grams (17+1⁄2 oz) in the wild.[1]

The term rat is also used in the names of other small mammals that are not true rats. Examples include the North American pack rats (aka wood rats[2]) and a number of species loosely called kangaroo rats.[2] Rats such as the bandicoot rat (Bandicota bengalensis) are murine rodents related to true rats but are not members of the genus Rattus.

Male rats are called bucks; unmated females, does, pregnant or parent females, dams; and infants, kittens or pups. A group of rats is referred to as a mischief.[3]

The common species are opportunistic survivors and often live with and near humans; therefore, they are known as commensals. They may cause substantial food losses, especially in developing countries.[4] However, the widely distributed and problematic commensal species of rats are a minority in this diverse genus. Many species of rats are island endemics, some of which have become endangered due to habitat loss or competition with the brown, black, or Polynesian rat.[5]

Wild rodents, including rats, can carry many different zoonotic pathogens, such as Leptospira, Toxoplasma gondii, and Campylobacter.[6] The Black Death is traditionally believed to have been caused by the microorganism Yersinia pestis, carried by the tropical rat flea (Xenopsylla cheopis), which preyed on black rats living in European cities during the epidemic outbreaks of the Middle Ages; these rats were used as transport hosts. Another zoonotic disease linked to the rat is foot-and-mouth disease.[7]

Rats become sexually mature at age 6 weeks, but reach social maturity at about 5 to 6 months of age. The average lifespan of rats varies by species, but many only live about a year due to predation.[8]

The black and brown rats diverged from other Old World rats in the forests of Asia during the beginning of the Pleistocene.[9]

Rat tails

A rat in a city street The characteristic long tail of most rodents is a feature that has been extensively studied in various rat species models, which suggest three primary functions of this structure: thermoregulation,[10] minor proprioception, and a nocifensive-mediated degloving response.[11] Rodent tails—particularly in rat models—have been implicated with a thermoregulation function that follows from its anatomical construction. This particular tail morphology is evident across the family Muridae, in contrast to the bushier tails of Sciuridae, the squirrel family. The tail is hairless and thin skinned but highly vascularized, thus allowing for efficient countercurrent heat exchange with the environment. The high muscular and connective tissue densities of the tail, along with ample muscle attachment sites along its plentiful caudal vertebrae, facilitate specific proprioceptive senses to help orient the rodent in a three-dimensional environment.[citation needed] Murids have evolved a unique defense mechanism termed degloving that allows for escape from predation through the loss of the outermost integumentary layer on the tail. However, this mechanism is associated with multiple pathologies that have been the subject of investigation.[citation needed]

Multiple studies have explored the thermoregulatory capacity of rodent tails by subjecting test organisms to varying levels of physical activity and quantifying heat conduction via the animals' tails. One study demonstrated a significant disparity in heat dissipation from a rat's tail relative to its abdomen.[12] This observation was attributed to the higher proportion of vascularity in the tail, as well as its higher surface-area-to-volume ratio, which directly relates to heat's ability to dissipate via the skin. These findings were confirmed in a separate study analyzing the relationships of heat storage and mechanical efficiency in rodents that exercise in warm environments. In this study, the tail was a focal point in measuring heat accumulation and modulation.[citation needed]

On the other hand, the tail's ability to function as a proprioceptive sensor and modulator has also been investigated. As aforementioned, the tail demonstrates a high degree of muscularization and subsequent innervation that ostensibly collaborate in orienting the organism.[13] Specifically, this is accomplished by coordinated flexion and extension of tail muscles to produce slight shifts in the organism's center of mass, orientation, etc., which ultimately assists it with achieving a state of proprioceptive balance in its environment. Further mechanobiological investigations of the constituent tendons in the tail of the rat have identified multiple factors that influence how the organism navigates its environment with this structure. A particular example is that of a study in which the morphology of these tendons is explicated in detail.[14] Namely, cell viability tests of tendons of the rat's tail demonstrate a higher proportion of living fibroblasts that produce the collagen for these fibers. As in humans, these tendons contain a high density of golgi tendon organs that help the animal assess stretching of muscle in situ and adjust accordingly by relaying the information to higher cortical areas associated with balance, proprioception, and movement.[citation needed]

The characteristic tail of murids also displays a unique defense mechanism known as degloving in which the outer layer of the integument can be detached in order to facilitate the animal's escape from a predator. This evolutionary selective pressure has persisted despite a multitude of pathologies that can manifest upon shedding part of the tail and exposing more interior elements to the environment.[15] Paramount among these are bacterial and viral infection, as the high density of vascular tissue within the tail becomes exposed upon avulsion or similar injury to the structure. The degloving response is a nocifensive response, meaning that it occurs when the animal is subjected to acute pain, such as when a predator snatches the organism by the tail.[citation needed]

As pets

Main article: Fancy rat

A domesticated rat Specially bred rats have been kept as pets at least since the late 19th century. Pet rats are typically variants of the species brown rat - but black rats and giant pouched rats are also sometimes kept. Pet rats behave differently from their wild counterparts depending on how many generations they have been kept as pets.[16] Pet rats do not pose any more of a health risk than pets such as cats or dogs.[17] Tamed rats are generally friendly and can be taught to perform selected behaviors.

Selective breeding has brought about different color and marking varieties in rats. Genetic mutations have also created different fur types, such as rex and hairless. Congenital malformation in selective breeding have created the dumbo rat, a popular pet choice due to their low, saucer-shaped ears.[18] A breeding standard exists for rat fanciers wishing to breed and show their rat at a rat show.[19]

As subjects for scientific research

Main article: Laboratory rat

A laboratory rat strain, known as a Zucker rat, bred to be genetically prone to diabetes, a metabolic disorder also found among humans. In 1895, Clark University in Worcester, Massachusetts, established a population of domestic albino brown rats to study the effects of diet and for other physiological studies.[citation needed] Over the years, rats have been used in many experimental studies, adding to our understanding of genetics, diseases, the effects of drugs, and other topics that have provided a great benefit for the health and wellbeing of humankind.[citation needed]

The aortic arches of the rat are among the most commonly studied in murine models due to marked anatomical homology to the human cardiovascular system.[20] Both rat and human aortic arches exhibit subsequent branching of the brachiocephalic trunk, left common carotid artery, and left subclavian artery, as well as geometrically similar, nonplanar curvature in the aortic branches.[20] Aortic arches studied in rats exhibit abnormalities similar to those of humans, including altered pulmonary arteries and double or absent aortic arches.[21] Despite existing anatomical analogy in the inthrathoracic position of the heart itself, the murine model of the heart and its structures remains a valuable tool for studies of human cardiovascular conditions.[22]

The rat's larynx has been used in experimentations that involve inhalation toxicity, allograft rejection, and irradiation responses. One experiment described four features of the rat's larynx. The first being the location and attachments of the thyroarytenoid muscle, the alar cricoarytenoid muscle, and the superior cricoarytenoid muscle, the other of the newly named muscle that ran from the arytenoid to a midline tubercle on the cricoid. The newly named muscles were not seen in the human larynx. In addition, the location and configuration of the laryngeal alar cartilage was described. The second feature was that the way the newly named muscles appear to be familiar to those in the human larynx. The third feature was that a clear understanding of how MEPs are distributed in each of the laryngeal muscles was helpful in understanding the effects of botulinum toxin injection. The MEPs in the posterior cricoarytenoid muscle, lateral cricoarytenoid muscle, cricothyroid muscle, and superior cricoarytenoid muscle were focused mostly at the midbelly. In addition, the medial thyroarytenoid muscle were focused at the midbelly while the lateral thyroarytenoid muscle MEPs were focused at the anterior third of the belly. The fourth and final feature that was cleared up was how the MEPs were distributed in the thyroarytenoid muscle.[23]

Laboratory rats have also proved valuable in psychological studies of learning and other mental processes (Barnett 2002), as well as to understand group behavior and overcrowding (with the work of John B. Calhoun on behavioral sink).[citation needed] A 2007 study found rats to possess metacognition, a mental ability previously only documented in humans and some primates.[24][25]

Domestic rats differ from wild rats in many ways. They are calmer and less likely to bite; they can tolerate greater crowding; they breed earlier and produce more offspring; and their brains, livers, kidneys, adrenal glands, and hearts are smaller (Barnett 2002).

Brown rats are often used as model organisms for scientific research. Since the publication of the rat genome sequence,[26] and other advances, such as the creation of a rat SNP chip, and the production of knockout rats, the laboratory rat has become a useful genetic tool, although not as popular as mice. When it comes to conducting tests related to intelligence, learning, and drug abuse, rats are a popular choice due to their high intelligence, ingenuity, aggressiveness, and adaptability. Their psychology seems in many ways similar to that of humans.[citation needed]

Entirely new breeds or "lines" of brown rats, such as the Wistar rat, have been bred for use in laboratories. Much of the genome of Rattus norvegicus has been sequenced.[27]

General intelligence Early studies found evidence both for and against measurable intelligence using the "g factor" in rats.[28][29] Part of the difficulty of understanding animal cognition generally, is determining what to measure.[30] One aspect of intelligence is the ability to learn, which can be measured using a maze like the T-maze.[30] Experiments done in the 1920s showed that some rats performed better than others in maze tests, and if these rats were selectively bred, their offspring also performed better, suggesting that in rats an ability to learn was heritable in some way.[30]

As food Main article: Rat meat Rat meat is a food that, while taboo in some cultures, is a dietary staple in others.[31]

Working rats Main article: Working rat Rats have been used as working animals. Tasks for working rats include the sniffing of gunpowder residue, demining, acting and animal-assisted therapy.

For odor detection Rats have a keen sense of smell and are easy to train. These characteristics have been employed, for example, by the Belgian non-governmental organization APOPO, which trains rats (specifically African giant pouched rats) to detect landmines and diagnose tuberculosis through smell.[32]

In the spread of disease Rats can serve as zoonotic vectors for certain pathogens and thus spread disease, such as bubonic plague, Lassa fever, leptospirosis, and Hantavirus infection.[33]

They are also associated with human dermatitis because they are frequently infested with blood feeding rodent mites such as the tropical rat mite (Ornithonyssus bacoti) and spiny rat mite (Laelaps echidnina), which will opportunistically bite and feed on humans,[34] where the condition is known as rat mite dermatitis.[35]

As pests

Rodent Bait Station, Chennai, India Rats have long been considered deadly pests. Once considered a modern myth, the rat flood in India occurs every fifty years, as armies of bamboo rats descend upon rural areas and devour everything in their path.[36] Rats have long been held up as the chief villain in the spread of the Bubonic Plague;[37] however, recent studies show that rats alone could not account for the rapid spread of the disease through Europe in the Middle Ages.[38] Still, the Centers for Disease Control does list nearly a dozen diseases[39] directly linked to rats.

Most urban areas battle rat infestations. A 2015 study by the American Housing Survey (AHS) found that eighteen percent of homes in Philadelphia showed evidence of rodents. Boston, New York City, and Washington, D.C., also demonstrated significant rodent infestations.[40] Indeed, rats in New York City are famous for their size and prevalence. The urban legend that the rat population in Manhattan equals that of its human population was definitively refuted by Robert Sullivan in his book Rats but illustrates New Yorkers' awareness of the presence, and on occasion boldness and cleverness, of the rodents.[41] New York has specific regulations for eradicating rats; multifamily residences and commercial businesses must use a specially trained and licensed rat catcher.[42]

Chicago was declared the "rattiest city" in the US by the pest control company Orkin in 2020, for the sixth consecutive time. It's followed by Los Angeles, New York, Washington, DC, and San Francisco.[43] To help combat the problem, a Chicago animal shelter has placed more than 1000 feral cats (sterilized and vaccinated) outside of homes and businesses since 2012, where they hunt and catch rats while also providing a deterrent simply by their presence.[44]

Rats have the ability to swim up sewer pipes into toilets.[45][46] Rats will infest any area that provides shelter and easy access to sources of food and water, including under sinks, near garbage, and inside walls or cabinets.[47]

As invasive species

Rat-catching, 1823, by Edwin Landseer, engraving, published by Hurst, Robinson & Co. When introduced into locations where rats previously did not exist, they can wreak an enormous degree of environmental degradation. Rattus rattus, the black rat, is considered to be one of the world's worst invasive species.[48] Also known as the ship rat, it has been carried worldwide as a stowaway on seagoing vessels for millennia and has usually accompanied men to any new area visited or settled by human beings by sea. The similar species Rattus norvegicus, the brown rat or wharf rat, has also been carried worldwide by ships in recent centuries.[citation needed]

The ship or wharf rat has contributed to the extinction of many species of wildlife, including birds, small mammals, reptiles, invertebrates, and plants, especially on islands. True rats are omnivorous, capable of eating a wide range of plant and animal foods, and have a very high birth rate. When introduced to a new area, they quickly reproduce to take advantage of the new food supply. In particular, they prey on the eggs and young of forest birds, which on isolated is