Guinea pigs

The guinea pig (Cavia porcellus), also commonly called the Cavy, is a species of rodent belonging to the family Caviidae and the genus Cavia. Despite their common name, these animals are not pigs, nor do they come from Guinea. They originated in the Andes, and studies based on biochemistry and hybridization suggest they are domesticated descendants of a closely related species of cavy such as Cavia aperea, C. fulgida, or C. tschudii, and therefore do not exist naturally in the wild. The guinea pig plays an important role in the folk culture of many Indigenous South American groups, especially as a food source, but also in folk medicine and in community religious ceremonies.

In Western societies, the guinea pig has enjoyed widespread popularity as a household pet since its introduction by European traders in the 16th century. Their docile nature, their responsiveness to handling and feeding, and the relative ease of caring for them, continue to make the guinea pig a popular pet. Organizations devoted to competitive breeding of guinea pigs have been formed worldwide, and many specialized breeds of guinea pig, with varying coat colors and compositions, are cultivated by breeders.

Biological experimentation on guinea pigs has been carried out since the 17th century. The animals were frequently used as a model organism in the 19th and 20th centuries, giving way to the metaphor "guinea pig" for a test subject, but have since been largely replaced by other rodents such as mice and rats. They are still used in research, primarily as models for human medical conditions such as juvenile diabetes, tuberculosis, scurvy, and pregnancy complications.

History
The common guinea pig was first domesticated as early as 5000 BC for food by tribes in the Andean region of South America (present-day Ecuador, Peru, and Bolivia). Statues dating from ca. 500 BC to 500 AD that depict guinea pigs have been unearthed in archaeological digs in Peru and Ecuador. The Moche people of ancient Peru worshipped animals and often depicted the guinea pig in their art. From ca. 1200 AD to the Spanish conquest in 1532, selective breeding resulted in many varieties of domestic guinea pigs, which form the basis for some of the modern domestic breeds. They continue to be a food source in the region; most households in the Andean highlands raise the animal, which subsists off the family's vegetable scraps. Folklore traditions involving guinea pigs are numerous; they are exchanged as gifts, used in customary social and religious ceremonies, and frequently referenced in spoken metaphors. They also play a role in traditional healing rituals by folk doctors, or curanderos, who use the animals to diagnose diseases such as jaundice, rheumatism, arthritis, and typhus. They are rubbed against the bodies of the sick, and are seen as a supernatural medium. Black guinea pigs are considered especially useful for diagnoses. The animal also may be cut open and its entrails examined to determine whether the cure was effective. These methods are widely accepted in many parts of the Andes, where Western medicine is either unavailable or distrusted.

Spanish, Dutch, and English traders brought guinea pigs to Europe, where they quickly became popular as exotic pets among the upper classes and royalty, including Queen Elizabeth I. The earliest known written account of the guinea pig dates from 1547, in a description of the animal from Santo Domingo; because cavies are not native to Hispaniola, the animal was likely introduced there by Spanish travelers. The guinea pig was first described in the West in 1554 by the Swiss naturalist Conrad Gessner. Its binomial scientific name was first used by Erxleben in 1777; it is an amalgam of Pallas' generic designation (1766) and Linnaeus' specific conferral (1758).

Name
The scientific name of the common species is Cavia porcellus, with porcellus being Latin for "little pig". Cavia is New Latin; it is derived from cabiai, the animal's name in the language of the Galibi tribes once native to French Guiana. Cabiai may be an adaptation of the Portuguese çavia (now savia), which is itself derived from the Tupi word saujá, meaning rat. Guinea pigs are called quwi or jaca in Quechua and cuy or cuyo (pl. cuyes, cuyos) in the Spanish of Ecuador, Peru, and Bolivia. Paradoxically, breeders tend to use the more formal "cavy" to describe the animal, while in scientific and laboratory contexts it is far more commonly referred to by the more colloquial "guinea pig".

How the animals came to be thought of as "pigs" is not clear. They are built somewhat like pigs, with large heads relative to their bodies, stout necks, and rounded rumps with no tail of any consequence; some of the sounds they emit are very similar to those made by pigs, and they also spend a large amount of time eating. They can survive for long periods in small quarters, like a 'pig pen', and were thus easily transported on ships to Europe.

The animal's name carries porcine connotations in many European languages. The German word for them is Meerschweinchen, literally "little sea pig", which has been translated into Polish as świnka morska and into Russian as морская свинка. This derives from nautical history: sailing ships stopping to reprovision in the New World would pick up stores of guinea pigs, which provided an easily transportable source of fresh meat; Schweinswal (pig-whale) is German for porpoise, which was another food source for sailors. The French term is Cochon d'Inde (Indian pig); the Dutch used to call it guinees biggetje (Guinean piglet) or Spaanse rat (Spanish rat) in some dialects, and in Portuguese the guinea pig is sometimes referred to as porquinho da Índia (little Indian pig). This is not universal; for example, the common word in Spanish is conejillo de Indias (little rabbit of India / the Indies). Equally peculiar, The Chinese refer to them as Holland pig (荷蘭豬).

The origin of "guinea" in "guinea pig" is harder to explain. One theory is that the animals were brought to Europe by way of Guinea, leading people to think they had originated there. "Guinea" was also frequently used in English to refer generally to any far-off, unknown country, and so the name may simply be a colorful reference to the animal's foreignness. Another theory suggests the "guinea" in the name is a corruption of "Guiana", an area in South America, though the animals are not native to that region. A common misconception is that they were so named because they were sold for the price of a guinea coin; this theory is untenable, because the guinea was first struck in England in 1663, and William Harvey used the term "Ginny-pig" as early as 1653. Others believe "guinea" may be an alteration of the word coney (rabbit); guinea pigs were referred to as "pig coneys" in Edward Topsell's 1607 treatise on quadrupeds.

Traits and environment


Guinea pigs are large for rodents, weighing between 700 and 1200g (1.5-2.5 pounds), and measuring between 20 and 25 cm (8–10 inches) in length. They typically live an average of four to five years, but may live as long as eight years. According to the 2006 Guinness Book of Records the longest living guinea pig survived 14 years, 10.5 months.

In the 1990s, a minority scientific opinion emerged proposing that caviomorphs, such as guinea pigs, chinchillas, and degus, are not rodents and should be reclassified as a separate order of mammals (similar to lagomorphs). Subsequent research using wider sampling has restored consensus among mammalian biologists that the current classification of rodents as monophyletic is justified.

Natural habitat
Cavia porcellus is not found naturally in the wild; it is likely descendant from some closely related species of cavies, such as Cavia aperea, Cavia fulgida, and Cavia tschudii, which are still commonly found in various regions of South America. Some species of cavy identified in the 20th century, such as Cavia anolaimae and Cavia guianae, may be domestic guinea pigs that have become feral by reintroduction into the wild. Wild cavies are found on grassy plains and occupy an ecological niche similar to that of the cow. They are social, living in the wild in small groups which consist of several females (sows), a male (boar), and the young (which in a break with the preceding porcine nomenclature are called pups). They move together in groups (herds) eating grass or other vegetation, and do not store food. While they do not burrow or build nests, they frequently seek shelter in the burrows of other animals, as well as in crevices and tunnels formed by vegetation. They are crepuscular, tending to be most active during dawn and dusk, when it is harder for predators to spot them.

Domestic habitat
Domesticated guinea pigs thrive in groups of two or more; groups of sows, or groups of one or more sows and a neutered boar are common combinations. Guinea pigs learn to recognize and bond with other individual guinea pigs, and testing of boars shows that their neuroendocrine stress response is significantly lowered in the presence of a bonded female when compared to the presence of unfamiliar females. Groups of boars may also get along, provided that their cage has enough space, they are introduced at an early age, and no females are present. Domestic guinea pigs have developed a different biological rhythm from their wild counterparts, and have longer periods of activity followed by short periods of sleep in between. Activity is scattered randomly over the 24 hours of the day; aside from avoidance of intense light, no regular circadian patterns are apparent.



Domestic guinea pigs generally live in cages, although some owners of large numbers of guinea pigs will dedicate entire rooms to their pets. Cages with solid or wire mesh floors are used, although wire mesh floors can cause injury and may be associated with an infection commonly known as bumblefoot (ulcerative pododermatitis). "Cubes and Coroplast" (or C&C) style cages are now a common choice. Cages are often lined with wood shavings or a similar material. Bedding made from Red Cedar and pine, both softwoods, was commonly used in past decades, but these materials are now believed to contain harmful phenols (aromatic hydrocarbons) and oils. Safer beddings include those made from hardwoods (such as aspen), paper products, and corn cob materials are other alternatives. Guinea pigs tend to be messy within their cages; they often jump into their food bowls or kick bedding and feces into them, and their urine crystallizes on cage surfaces and can be difficult to remove. After its cage has been cleaned, a guinea pig will typically urinate and drag the lower body across the floor of the cage to mark its territory. Male guinea pigs may also mark their territory in this way when they are taken out of their cages.

Guinea pigs do not generally thrive when housed with other species. Cohousing of guinea pigs with other rodents such as gerbils and hamsters may increase instances of respiratory and other infections, and such rodents may act aggressively toward the guinea pig. Larger animals may regard guinea pigs as prey, though some (such as dogs) can be trained to accept them. Guinea pigs can be safely housed with degu as they share the same dietary needs and have similar behavioural traits. Opinion is divided over the cohousing of guinea pigs and domestic rabbits. Some published sources say that guinea pigs and rabbits complement each other well when sharing a cage. However, as lagomorphs, rabbits have different nutritional requirements, and so the two species cannot be fed the same food. Rabbits may also harbor diseases (such as the respiratory infections Bordetella and Pasteurella), to which guinea pigs are susceptible. Even the dwarf rabbit is much stronger than the guinea pig and may cause intentional or inadvertent injury.

Behaviour
Guinea pigs can learn complex paths to food, and can accurately remember a learned path for months. Their strongest problem solving strategy is motion. While guinea pigs can jump small obstacles, they cannot climb, and are not particularly agile. They startle extremely easily, and will either freeze in place for long periods or run for cover with rapid, darting motions when they sense danger. Larger groups of startled guinea pigs will "stampede", running in haphazard directions as a means of confusing predators. When excited, guinea pigs may repeatedly perform little hops in the air (known as "popcorning"), a movement analogous to the ferret's war dance. They are also exceedingly good swimmers. Like many rodents, guinea pigs sometimes participate in social grooming, and they regularly self-groom. A milky-white substance is secreted from their eyes and rubbed into the hair during the grooming process. Groups of boars will often chew each other's hair, but this is a method of establishing hierarchy within a group, rather than a social gesture. Dominance is also established through biting (especially of the ears), piloerection, aggressive noises, head thrusts, and leaping attacks. Non-sexual simulated mounting for dominance is also common among same-sex groups.

Guinea pigs have poor sight, but well-developed senses of hearing, smell, and touch. Vocalization is the primary means of communication between members of the species. Some sounds are:


 * Wheek - A loud noise, the name of which is onomatopoeic, also known as a Whistle. An expression of general excitement, it may occur in response to the presence of its owner or to feeding. It is sometimes used to find other guinea pigs if they are running. If a guinea pig is lost, it may wheek for assistance.
 * Bubbling or Purring - This sound is made when the guinea pig is enjoying itself, such as when being petted or held. They may also make this sound when grooming, crawling around to investigate a new place, or when given food.
 * Rumbling - This sound is normally related to dominance within a group, though it can also come as a response to comfort or contentment. In these cases the rumble often sounds higher and the body vibrates shortly. While courting, a male usually purrs deeply, swaying and circling the female in a behavior called "rumblestrutting". A low rumble while walking away reluctantly shows passive resistance.
 * Chutting and Whining - These are sounds made in pursuit situations, by the pursuer and pursuee, respectively.
 * Chattering - This sound is made by rapidly gnashing the teeth, and is generally a sign of warning. Guinea pigs tend to raise their heads when making this sound. A more relaxed type of gnashing often means the guinea pig wants a treat that is somewhere nearby but out of reach.
 * Squealing or Shrieking - A high-pitched sound of discontent, in response to pain or danger.
 * Chirping - This less-common sound, likened to bird song, seems to be related to stress, or when a baby guinea pig wants to be fed. Very rarely, the chirping will last for several minutes.

Breeding


The guinea pig is able to breed year-round, with birth peaks usually coming in the spring; as many as five litters can be produced per year. The gestation period lasts from 59–72 days, with an average of 63–68 days. Because of the long gestation period and the large size of the pups, pregnant females may become large and aubergine-shaped, although the change in size and shape varies. Newborn pups are well-developed with hair, teeth, claws, and partial eyesight; they are immediately mobile, and begin eating solid food immediately, though they continue to suckle. Litters yield 1–6 pups, with an average of three; the largest recorded litter size is 17.

In smaller litters, difficulties may occur during labour due to over-sized pups. Large litters result in higher incidences of stillbirth, but because the pups are delivered at an advanced stage of development, lack of access to the mother's milk has little effect on the mortality rate of newborns. Cohabitating females assist in mothering duties if lactating.

Male and female guinea pigs do not differ in external appearance apart from general size. The position of the anus is very close to the genitals in both sexes. Female genitals are distinguished by a Y-shaped configuration formed from a vulvar flap; while the male genitals may look similar with the penis and anus forming a like shape, the penis will protrude if pressure is applied to the surrounding hair. The male's testes may also be visible externally from scrotal swelling.

Males reach sexual maturity at 3–5 weeks; females can be fertile as early as four weeks and can carry litters before they are adults. Females that have never given birth commonly develop irreversible fusing of the pubic symphysis, a joint in the pelvis, after six months of age. If they become pregnant after this has happened, the birth canal will not widen sufficiently; this may lead to dystocia and death as they attempt to give birth. Females can become pregnant 6–48 hours after giving birth, but it is not healthy for a female to be thus constantly pregnant.

Toxemia of pregnancy is common and kills many pregnant females. Signs of toxemia include anorexia, lack of energy, excessive salivation, a sweet or fruity breath odor due to ketones, and seizures in advanced cases. Pregnancy toxemia appears to be most common in hot climates. Other serious complications of pregnancy can include a prolapsed uterus, hypocalcaemia, and mastitis.

Diet


Grass is the guinea pig's natural diet. Their molars are particularly suited for grinding plant matter, and grow continuously throughout the animal's life. Most grass-eating mammals are quite large and have a long digestive tract; while guinea pigs have much longer colons than most rodents, they must also supplement their diet by coprophagy, the eating of their own feces. However, they do not consume all their feces indiscriminately, but produce special soft pellets, called cecotropes, which recycle B vitamins, fiber, and bacteria required for proper digestion. The cecotropes (or caecal pellets) are eaten directly from the anus, unless the guinea pig is pregnant or obese. They share this behaviour with rabbits. In older boars (the condition is rarer in young ones), the muscles which allow the softer pellets to be expelled from the anus for consumption can become weak. This creates a condition known as anal impaction, which prevents the boar from redigesting cecotropes, though harder pellets may pass through the impacted mass. The condition may be temporarily alleviated by carefully expelling the impacted feces.

Guinea pigs benefit from feeding on fresh grass hay, such as timothy hay, in addition to food pellets which are often based from timothy. Alfalfa is also a popular food choice; most guinea pigs will eat large amounts of alfalfa when offered it, though there exists some controversy over the feeding of alfalfa to adult guinea pigs. Some pet owners and veterinary organizations have advised that, as a legume rather than a grass hay, alfalfa consumed in large amounts may lead to obesity, as well as bladder stones due to excess calcium, in any but pregnant and very young guinea pigs. However, published scientific sources mention alfalfa as a source for replenishment of protein, amino acids, and fiber.

Like humans, but unlike most other mammals, guinea pigs cannot synthesize their own vitamin C and must obtain this vital nutrient from food. If guinea pigs do not ingest enough vitamin C, they can suffer from potentially fatal scurvy. Guinea pigs require about 10 mg of vitamin C daily (20 mg if pregnant), which can be obtained through fresh, raw fruits and vegetables (such as broccoli, apple, cabbage, carrot, celery, and spinach) or through dietary supplements. Healthy diets for guinea pigs require a complex balance of calcium, magnesium, phosphorus, potassium, and hydrogen ions; adequate amounts of vitamins E, A, and D are also necessary. Imbalanced diets have been associated with muscular dystrophy, metastatic calcification, difficulties with pregnancy, vitamin deficiencies, and teeth problems. Guinea pigs tend to be fickle eaters when it comes to fresh fruits and vegetables, having learned early in life what is and is not appropriate to consume, and their habits are difficult to change after maturity. They do not respond well to sudden changes in diet; they may stop eating and starve rather than accepting new food types. A constant supply of hay or other food is generally recommended, as guinea pigs feed continuously and may develop habits such as chewing on their own hair if food is not present. Because guinea pigs' teeth grow constantly, they routinely gnaw, lest their teeth become too large for their mouth, a common problem in rodents. Guinea pigs will also chew on cloth, paper, plastic, and rubber.

A number of plants are poisonous to guinea pigs, including bracken, bryony, buttercup, charlock, deadly nightshade, foxglove, hellebore, hemlock, Lily of the Valley, mayweed, monkshood, privet, ragwort, rhubarb, speedwell, toadflax, and wild celery. Additionally, any plant which grows from a bulb (e.g., tulip and onion) is normally considered poisonous.

Health


Common ailments in domestic guinea pigs include respiratory tract infections, diarrhea, scurvy (vitamin C deficiency, typically characterized by sluggishness), abscesses due to infection (often in the neck, due to hay embedded in the throat, or from external scratches), and infections by lice, mites, or fungus.

Mange mites (Trixacarus caviae) are a common cause of hair loss, and other symptoms may also include excessive scratching, unusually aggressive behavior when touched (due to pain), and, in some instances, seizures. Guinea pigs may also suffer from "running lice" (Gliricola porcelli), a small white insect which can be seen moving through the hair; the eggs of these lice, which appear as black or white specks attached to the hair, are sometimes referred to as "static lice". Giving a bath with neem oil soap is a gentle and effective way of ridding the pig of lice. Other causes of hair loss can be due to hormonal upsets caused by underlying medical conditions such as ovarian cysts.

Foreign bodies, especially small pieces of hay or straw, can become lodged in the eyes of guinea pigs, resulting in excessive blinking, tearing, and in some cases an opaque film over the eye due to corneal ulcer. Hay or straw dust will also cause sneezing. While it is normal for guinea pigs to sneeze periodically, frequent sneezing may be a symptom of pneumonia, especially in response to atmospheric changes. Pneumonia may also be accompanied by torticollis and can be fatal.

Because the guinea pig has a stout, compact body, the animal more easily tolerates excessive cold than excessive heat. Its normal body temperature is 101–104 °F (38.5–40 °C), and so its ideal ambient air temperature range is similar to the human's, about 65–75 °F (18–24 °C). Consistent ambient temperatures in excess of 90 °F (32 °C) have been linked to hyperthermia and death, especially among pregnant sows. Guinea pigs are not well suited to environments that feature wind or frequent drafts, and respond poorly to extremes of humidity outside of the range of 30–70%.

Guinea pigs are prey animals whose survival instinct is to mask pain and signs of illness, and many times health problems may not be apparent until a condition is severe or in its advanced stages. Treatment of disease is made more difficult by the extreme sensitivity guinea pigs have to most antibiotics, including penicillin, which kill off the intestinal flora and quickly bring on episodes of diarrhea and in some cases, death.

Similar to the inherited genetic diseases of other breeds of animal (such as hip dysplasia in canines), a number of genetic abnormalities of guinea pigs have been reported. Most commonly, the roan coloration of Abyssinian guinea pigs is associated with congenital eye disorders and problems with the digestive system. Other genetic disorders include "waltzing disease" (deafness coupled with a tendency to run in circles), palsy, and tremor conditions.

Pets
If handled correctly early in their life, guinea pigs become amenable to being picked up and carried, and seldom bite or scratch. They are timid explorers, and rarely attempt to escape from their cages, even when an opportunity presents itself. Guinea pigs who become familiar with their owner will whistle on the owner's approach; they will also learn to whistle in response to the rustling of plastic bags or the opening of refrigerator doors, where their food is most commonly stored.

Domesticated guinea pigs come in many breeds, which have been developed since their introduction to Europe and North America. These varieties vary in hair and color composition. The most common varieties found in pet stores are the English shorthair (also known as the American), which have a short, smooth coat, and the Abyssinian, whose coat is ruffled with cowlicks, or rosettes. Also popular among breeders are the Peruvian and the Sheltie (or Silkie), both straight longhair breeds, and the Texel, a curly longhair.

Cavy Clubs and Associations dedicated to the showing and breeding of guinea pigs have been established worldwide. The American Cavy Breeders Association, an adjunct to the American Rabbit Breeders' Association, is the governing body in the United States and Canada. The British Cavy Council governs cavy clubs in the United Kingdom. Similar organizations exist in Australia (Australian National Cavy Council) and New Zealand (New Zealand Cavy Club). Each club publishes its own Standard of Perfection and determines which breeds are eligible for showing.

Cultural and media influence
As a result of their widespread popularity in human domestic life, and especially because of their popularity in households with children, guinea pigs have shown a presence in culture and media. Some noted appearances of the animal in literature are The Fairy Caravan, a novel by Beatrix Potter, and Michael Bond's Olga da Polga series for children, both of which feature guinea pigs as the central protagonist. Another appearance is in The Magician's Nephew by C. S. Lewis: in the first (chronologically) of his The Chronicles of Narnia series, a guinea pig is the first creature to travel to the Wood between the Worlds. The short story Pigs Is Pigs by Ellis Parker Butler is a tale of bureaucratic incompetence; two guinea pigs held at a train station breed unchecked while humans argue as to whether they are "pigs" for the purpose of determining freight charges. In the Golden Hamster Saga books, there are two guinea pigs named Enrico and Caruso who are modern-day thespians (named after Enrico Caruso) who serve as secondary characters, and often irritate the main character, Freddy Auratus who strongly dislikes their acting antics.

Guinea pigs have also been featured in film and television. In the TV movie Shredderman Rules, the main character and the main character's crush both have guinea pigs which both play a minor part in the plot. A guinea pig named Rodney, voiced by Chris Rock, was a prominent character in the 1998 film Dr. Dolittle and Linny the Guinea pig is a co-star on Nick Jr.'s Wonder Pets. Guinea pigs were used in some major advertising campaigns in the 1990s and 2000s, notably for Egg Banking plc, Snapple, and Blockbuster Video. The Blockbuster campaign is considered by some guinea pig advocates to have been a factor in the rise of cohousing guinea pigs and rabbits. In the South Park season 12 episode "Pandemic 2: The Startling", giant guinea pigs, and giant guinea pigs dressed in costumes, are portrayed as rampaging over the entire Earth. The Walt Disney Pictures movie G-Force, scheduled to be released in the United States in July 2009, features a group of highly intelligent guinea pigs trained as operatives of the U.S. government.

Scientific research
The use of guinea pigs in scientific experimentation dates back at least to the 17th century, when the Italian biologists Marcello Malpighi and Carlo Fracassati conducted vivisections of guinea pigs in their examinations of anatomic structures. In 1780, Antoine Lavoisier used a guinea pig in his experiments with the calorimeter, a device used to measure heat production. The heat from the guinea pig's respiration melted snow surrounding the calorimeter, showing that respiratory gas exchange is a combustion, similar to a candle burning. Guinea pigs played a major role in the establishment of germ theory in the late 19th century, through the experiments of Louis Pasteur, Émile Roux, and Robert Koch. Guinea pigs have been launched into orbital space-flight several times, first by the USSR on the Sputnik 9 biosatellite of March 9, 1961 - with a successful recovery. China also launched and recovered a biosatellite in 1990 which included guinea pigs as passengers.

In English, the term guinea pig is commonly used as a metaphor for a subject of scientific experimentation. This dates back to the early 20th century; the Oxford English Dictionary notes its first usage in this capacity in 1913. In 1933, Consumers Research founders F. J. Schlink and Arthur Kallet wrote a book entitled 100,000,000 Guinea Pigs, extending the metaphor to consumer society. The book became a national bestseller in the United States, thus further popularizing the term, and spurred the growth of the consumer protection movement. The negative connotation of the term was later employed in the novel The Guinea Pigs by Czech author Ludvík Vaculík as an allegory for Soviet totalitarianism.

Guinea pigs were popular laboratory animals until the later 20th century; about 2.5 million guinea pigs were used annually in the U.S. for research in the 1960s, but that total decreased to about 375,000 by the mid-1990s. As of 2007, they constitute approximately 2% of the current total of laboratory animals. In the past they were widely used to standardize vaccines and antiviral agents; they were also often employed in studies on the production of antibodies in response to extreme allergic reactions, or anaphylaxis. Less common uses included research in pharmacology and irradiation. Since the middle 20th century, they have been replaced in laboratory contexts primarily by mice and rats. This is in part because research into the genetics of guinea pigs has lagged behind that of other rodents, although geneticists W. E. Castle and Sewall Wright made a number of contributions to this area of study, especially regarding coat color. In 2004, the U.S.'s National Human Genome Research Institute announced plans to sequence the genome of the domestic guinea pig.

The guinea pig was most extensively implemented in research and diagnosis of infectious diseases. Common uses included identification of brucellosis, Chagas disease, cholera, diphtheria, foot-and-mouth disease, glanders, Q fever, Rocky Mountain spotted fever, and various strains of typhus. They are still frequently used to diagnose tuberculosis, since they are easily infected by human tuberculosis bacteria. Because guinea pigs are one of the few animals which, like humans, cannot synthesize vitamin C but must obtain it from their diet, they are ideal for researching scurvy. Complement, an important component for serology, was first isolated from the blood of the guinea pig. Guinea pigs have an unusual insulin mutation, and are a suitable species for the generation of anti-insulin antibodies. Present at a level 10 times that found in other mammals, the insulin in guinea pigs may be important in growth regulation, a role usually played by growth hormone. Additionally, guinea pigs have been identified as model organisms for the study of juvenile diabetes and, because of the frequency of pregnancy toxemia, of pre-eclampsia in human females.

Guinea pig strains used in scientific research are primarily outbred strains. Aside from the common American or English stock, the two main outbred strains in laboratory use are the Hartley and Dunkin-Hartley; these English strains are albino, although pigmented strains are also available. Inbred strains are less common and are usually used for very specific research, such as immune system molecular biology. Of the inbred strains that have been created, the two that are still used with any frequency are, following Sewall Wright's designations, "Strain 2" and "Strain 13".

Hairless breeds of guinea pigs have been used in scientific research since the 1980s, particularly for dermatological studies. A hairless and immunodeficient breed was the result of a spontaneous genetic mutation in inbred laboratory strains from the Hartley stock at the Eastman Kodak Company in 1979. An immunocompetent hairless breed was also identified by the Institute Armand Frappier in 1978, and Charles River Laboratories has reproduced this breed for research since 1982. Cavy fanciers then began acquiring hairless breeds, and the pet hairless varieties are referred to as "skinny pigs".