VSC/443/543
RESEARCH ANIMAL METHODS 2004
Lecture notes for 11/10/2004

Students should know the general characteristics of the Order Rodentia, the taxonomic classification of common laboratory rodents, the various genetic and microbial categorizations of laboratory rodents, general concepts of rodent husbandry, and the unique anatomic, physiologic, and behavioral features of each of the common laboratory rodent species.

I. GENERAL CHARACTERISTICS OF RODENTS

Rodents comprise the largest and most diverse group of mammals with over 1700 different species.  They occupy a wide range of habitats from subterrestrial to terrestrial to arboreal, and vary greatly in size and appearance.  The smallest rodent is the harvest mouse which weighs in at 6 grams and the largest is the capybara which can weigh over 50 kilograms.  Specialized anatomical features emphasize the diversity of the group and include the beaver tail, porcupine quills, and the patagium of the flying squirrel.

The major feature of rodents is their dentition.  In fact, the term “rodent” is derived from the Latin verb “rodere” which means “to gnaw”.  The incisors are always “open-rooted” or hypsodont (grow continuously throughout life) and have enamel only on the anterior surface.  These two features result in a continual sharpening of the incisor teeth as the upper and lower incisors grind against each other when the rodent gnaws.  The cheek teeth of rodents may be hypsodont or brachydont (do not grow continuously throughout life).  The dental formula of rodents is never greater than (I 1/1, C 0/0, Pm 2/1, M 3/3), and the gap between the incisors and cheek teeth (created by the absence of the second incisors, the canines, and the anterior premolar teeth) is called the diastema.

II. WHY ARE RODENTS USED IN BIOMEDICAL RESEARCH?

Estimates are that 95% of all animals used in biomedical research are rodents (90 %  mice and rats, 2% hamsters, 2% guinea pigs, 1% other rodents).  There are numerous reasons why rodents are used. 

1. Easy to care for and handle, small and inexpensive to maintain.

2. High reproductive capacity.

3. Short generation time and lifespan.

4. A wealth of baseline information exists for many rodent species.

5. Large numbers of well-defined strains and stocks for traditional laboratory rodents.

6. Some species amenable to germfree and pathogen-free production techniques.

7. Diversity of species-specific characteristics result in many models for ailments which affect man.

III. TAXONOMY

Outbred Stock- A closed population (for at least 4 generations) of genetically-variable animals that is bred to maintain maximum heterozygosity.  The goal is to limit inbreeding to less than 1% of the genome per generation.  In general, the larger the population the lower the rate of inbreeding, so large numbers of breeders  (greater than 100 pairs) can maintain a stock by total random mating, while small numbers of breeding pairs require strict adherence to a systematic mating system that maximizes heterozygosity.  The standardized nomenclature for outbred stocks consists of a laboratory designation, a colon, and a stock designation, and a dash and mutation designation (if present), e.g. Hsd:SD or Crl:ZU-fa.  

Inbred Strain- A closed population of genetically-uniform animals that results from 20 or more consecutive generations of brother “ sister (or parent “ offspring) matings.  Inbreeding reduces genetic variability by increasing the probability that animals homozygous at a particular locus will be mated with each other.  Approximately 98% of gene loci are homozygous after 20 generations (F20) and 99.5% are homozygous after 40 generations (F40).  Inbred animals are less vigorous than non-inbred animals because of chance fixation of deleterious recessive genes in the homozygous condition.  This is called inbreeding depression and many attempts to establish inbred lines fail because of this (less than 50% of attempts make it past 20 generations).  If a subpopulation of a strain branches off between F20 and F40 or is maintained separately for more than 100 generations it is designated as a substrain.  The standardized nomenclature for inbred strains consists of a strain designation, a diagonal line, and then a substrain designation which usually represents the laboratory that maintains the substrain, e.g. C57BL/6J.  One to three letter strain abbreviations can also be utilized, e.g. B6. 

F1 Hybrid- A genetically uniform population of animals with numerous heterozygous gene loci produced and maintained by mating two different inbred strains.  Standardized nomenclature consists of the female parent strain designation (long hand or abbreviated), the male parent strain designation, and F1, e.g. C57BL/6 “ DBA/2 F1 or B6D2F1.

Coisogenic and congenic- An inbred strain that differs from the parent inbred strain at only one gene locus.  Coisogenic implies the mutation arose spontaneously within the parent inbred strain, while congenic implies the mutation arose in another strain or stock and was backcrossed onto the parent inbred strain.  Nomenclature for coisogenic and congenic strains consists of the inbred strain designation, a dash, and the alleles at the mutant gene locus, e.g. BALB/c- nu/nu.

Knock out-  Basically the same as a coisogenic or congenic strain, except that the mutation was specifically targeted to a gene of interest using current molecular technologies, i.e. the mutation did not arise spontaneously but was induced by genetic manipulation.  This is performed by transfecting a mutated gene into a culture of embryonic stem (ES) cells and then inoculating some of these ES cells into a 32 cell embryo (called a blastocyst).  The blastocyst is transplanted into a pseudopregnant female and the animal that develops from that embryo has some cells (including germ line cells- sperm or ova) that contain the mutation.  The “founder” knockout is then bred and offspring are screened for the mutation of interest.  The nomenclature is the same as for coisogenic and congenic strains except for the mutant gene designation has a laboratory designation added as a superscript, e.g. BALB/c- Mbp^m1Dn/Mbp^m1Dn.

Transgenic- An animal which has a foreign segment of DNA in every cell of its body.  These are produced by injection of the foreign DNA into the pronucleus of a one-cell embryo.  Subsequent homologous recombination results in incorporation of the foreign DNA into the host genome, the embryo is transplanted into a pseudopregnant female animal, and the animal that develops from that embryo is transgenic.  This “founder” transgenic animal can then be selectively bred to place the transgene on a genetically-defined background.  Currently, even though the transgene is present in all cells of the transgenic animal, transgene expression can sometimes be manipulated so that it occurs only in certain cell populations or can be turned on and off.  Nomenclature for a transgenic strain consists of the rodent strain/stock designation, a dash, Tg, the transgene designation in parentheses, and the laboratory designation, e.g. C57BL/6J-Tg(CD8Ge)23Jwg.

Axenic- These are animals that are germ-free.  They have no microbial flora in the gastrointestinal tract or on the skin surface, and must be maintained in a totally sterile environment.  They can be produced by (1) ceaserean rederivation (hysterectomy with removal of near-term fetus) followed by hand-rearing or cross-fostering onto an axenic lactating female or (2) embryo transfer into a pseudopregnant axenic female.  These animals in general have extremely large cecums since there is no breakdown of plant material by microbial fermentation, and this has been a limiting factor in the development of certain species of axenic animals.

Gnotobiotic- These animals have a very-defined microbial flora.  They are produced initially by administering a microbial “cocktail” of bacteria to an axenic animal.  These microbes allow fermentation and other physiologic processes that require bacteria to proceed.  Thereafter the gnotobiotic population is self-perpetuated as the offspring are exposed to the microbial “cocktails” as it is shed in the feces of parents. 

Specific Pathogen Free (SPF)- These animals are free of specified pathogens, but otherwise their microbial flora is not well defined.  Virus antibody free (VAF) rodents would be included in this classification.

Conventional- These animals have an undefined microbial flora, and pathogens may or may-not be present.

Caging- A wide variety of caging is available for rodents and in general the choice depends upon the type of study and the microbial status of the rodent.  The size and height of the cages used for each rodent species varies in accordance with the NIH Guide for the Care and Use of Laboratory Animals, and the cages are generally constructed of durable materials like stainless steel or polycarbonate plastic which can withstand repeated sanitization.  Shoe box cages are used in most situations, especially in breeding colonies.  Hanging racks are often used for individually housed rats, while larger hanging drawers are used to house guinea pigs.  Gnotobiotic (and often SPF) rodents are maintained in some type of sterile caging or housing system.  This can range from microisolator caging to ventilated rack systems to flexible film isolators to barrier facilities (sterile animal rooms into which everything is autoclaved and personnel shower in and wear sterile clothing).  Cubicles can be used to maintain small populations of SPF rodents or to quarantine/maintain rodents which have an infectious disease.  Metabolic cages are often used for toxicology and pharmacology studies and allow collection of urine and feces and monitoring of food, water, and even gaseous exchange.

Food, water, and bedding- The nutritional requirements for the traditional laboratory rodent species have been worked out and a variety of commercial pelleted diets are available.  Most are certified to be free of chemical and microbial contaminants.  Nutritional supplements (vegetables, fruits, etc.) are generally not supplied to laboratory rodents since they are unnecessary and may introduce an unwanted experimental variable.  Food is supplied generally in hoppers which are suspended above the cage floor to prevent soiling by urine or feces, although food is sometimes placed on the cage floor if pre-weanling rodents are in the cage.  Chemically defined diets sometimes are in a powder form and require the use of a special feeder to prevent spillage.  Water is distilled or purified by a reverse osmosis process to remove any potential chemical contaminants.  Water is supplied by sipper tube bottles or by automatic watering systems.  Aspen shavings, crushed corn cob, hardwood chip, and shredded paper bedding are used in solid-bottom caging.  Cedar chips are avoided since the aromatics they contain can alter liver metabolism and may affect experiments.  Nestlets (square pads of a fiber-like material) are placed in cages that contain pre-parturient females.

Environmental conditions- Temperature and humidity is maintained in accordance with the NIH Guide and for rodents these parameters are 64-79°F and 30-70% humidity.  Room air is changed between 10-15 times per hour, and light cycles are generally 12 hours light:12 hours dark, although some rodents breed better at 14 hours light: 10 hours dark.

Genetics-    Of the over 200 inbred strains of mice that have been established, only about 25 are used extensively.  A wide range of characteristics have become fixed in these strains including a high incidence of specific types of tumors.  Although mice within an inbred strain are nearly identical genetically, they do not necessarily respond identically because the effect of non-genetic (environmental) variables may be accentuated in inbred animals. 

Anatomy-  External features of the mouse include a pointed snout with long vibrissae (tactile hairs), erect hairless round ears, five digits on each foot, five pairs of mammary glands (nipples absent on males), and a long hairless tail.  Mice have horseshoe-shaped glands deep within the orbit of the eye called Harderian glands.  When a mouse is stressed these glands excrete a reddish-brown substance called porphyrin, resulting in a periocular discharge.   The dental formula is 2 × (I 1/1, C  0/0, Pm 0/0, M 3/3) = 16 teeth, and the stomach is divided into nonglandular and glandular regions.  The male has an os penis, testicles which can be retracted into the abdominal cavity, and numerous accessory sex organs (vesicular, coagulating, prostate, bulbo-urethral, and ampullary glands) which aid in copulation and form the post-copulatory vaginal plug.  The female has a duplex uterus with uterine horns communicating just prior to entering the cervix. 

Physiology-  Average body weight for adults is 20-40 grams with males only slightly larger than females and average lifespan is 1.5-3 years.  These parameters are largely strain-dependent.  Additional information is available in Appendix I. 

Behavior-  Mice are nocturnal, social, and their behavior is often influenced by pheromones (substances produced by the preputial glands of the male mouse and excreted in the urine).  They groom themselves constantly which helps them maintain a smooth, glossy haircoat.  A behaviorally dominant mouse sometimes chews the hair from a subordinate mouse in distinctive patterns, often around the muzzle; this behavior is called barbering.  Group housed females usually do not fight, but group housed males often inflict severe wounds to each other, often in the genital region.

Reproduction-  Male mice have a long anogenital distance, a large genital papilla, testicles, and no nipples, whereas female mice have a short anogenital distance, a small genital papilla (vaginal orifice), a separate urethral orifice, and five pairs of nipples.  Mice are continually polyestrous with an average estrous cycle of four to five days.  Monogamous and polygamous (1 male to 2-6 females) mating schemes are commonly used for breeding.  Females often have a post-copulatory vaginal plug which remains in place for approximately 24 hours.  Females can be removed prior to parturition, or can be left in the cage with the male to take advantage of the fertile post-partum estrus which occurs 14-28 hours after parturition.  Gestation averages 19-21 days (although this can be delayed 4-10 days if the female is also lactating) and pups are weaned at 21-28 days.  Females should be left alone for about 48 hours post-parturition or they may cannibalize their young.

Olfactory stimuli play an important role in mouse reproduction. The Lee-Boot effect is when the estrous cycle of females caged together is suppressed, especially if the females are crowded.  Exposing these anestrous females to male urine results in estrus approximately 72 hours following exposure and is called the Whitten effect.  These two effects are frequently utilized together to time mate large numbers of mice as is required for cesearean rederivation.  Odors originating from a strange male mouse, particularly a male of a different strain, may prevent implantation or produce pseudopregnancy in recently bred females and is known as the Bruce effect. 

History-  Rattus norvegicus, the common laboratory rat, is thought to have originated in temperate Asia.  Rats were the first mammalian species domesticated for use in experiments.  Philipeaux studied adrenalectomized rats in France in 1856 and reports on nutritional and breeding research quickly followed.  Neuroanatomical studies by Hatai at the University of Chicago in the early 1890's represented the first known experimental use of rats in the USA.  The albino rats used in these early studies probably originated from European stocks in Switzerland.  Beginning in 1906 there was an effort to standardize the albino color variant by the Wistar Institute in Philadelphia, PA.  The Wistar bloodline has contributed to the development of more new strains of rats than any other. 

Genetics-  There are many outbred and inbred strains of rats.  A few of the common outbred strains include the Sprague Dawley (SD) , Wistar (WI) and Long Evans (LE); the latter is often called the hooded (or piebald) rat.  The  most frequently used  inbred strains include the Fisher 344 (F344), Brown Norway (BN), Lewis (LEW),  and Wistar-Furth (WF).    

Anatomy-  Exterior features of rats include a pointed snout with long vibrissae, erect hairless ears, five digits on all four feet, six pairs of mammary glands (nipples absent on males), and long hairless tails which may account for 85% of the total body length.  Rats have horseshoe-shaped glands deep within the orbit of the eye called Harderian glands.  When the rat is stressed, these glands excrete porphyrin (a reddish-brown substance) which produces a periocular and nasal discharge.  Rats have an extracoronary blood supply to the heart from branches of the subclavian and internal mammary arteries, and there are usually two anterior vena cava present (most mammals have one). The dental formula is 2 × (I 1/1, C 0/0, Pm 0/0, M 3/3) = 16 teeth, and the stomach is divided into nonglandular and glandular regions that are separated by a large ridge that also prevents the rat from being able to vomit.  Rats lack a gall bladder and are therefore unable to concentrate bile.  The male has an os penis, testicles which can be retracted into the abdominal cavity, and numerous accessory sex organs which aid in copulation and form the post-copulatory vaginal plug.  The female has a duplex uterus with uterine horns that enter two distinct cervical canals.

Physiology- The rat tail plays a key role in thermoregulation.  Blood circulation leading to the tail is increased when the rat needs to eliminate body heat and decreased when it needs to conserve body heat. Male rats exhibit a prolonged period of growth, with ossification of the long bones incomplete until well into the second year of life.  Average body weight for adults is 250-520 grams, with males larger than females, and the average lifespan is 2.5-3.5 years.  These parameters are largely strain-dependent.  Additional information is available in Appendix I.(appendix handed out in class). 

Behavior-  Rats are nocturnal, curious, and readily learn tasks.  Healthy rats keep themselves groomed and clean, but their haircoat is generally less smooth and glossy than in mice.  Groups of males or females rarely fight, although problems may occur when mixing two or more, older males who have been breeding animals.  Rats are able to close their nares and can dive and swim underwater. 

Reproduction-   Male rats have a long anogenital distance, a large genital papilla, testicles with hairless scrotal sacs, and no nipples, whereas female rats have a short anogenital distance, a small genital papilla (vaginal orifice), a separate urethral orifice, and six pairs of nipples.  Rats are continually polyestrous with an average estrous cycle of 4-5 days.  Estrus begins in the evening hours and lasts about twelve hours.  Monogamous and polygamous (1 male to 2-6 females) mating schemes are commonly used for breeding.  Females often have a post-copulatory vaginal plug, which forms 3-8 hours after mating and remains in place for approximately 24 hours.  Females can be removed prior to parturition, or can be left in the cage with the male to take advantage of the fertile post-partum estrus that occurs within 48 hours after parturition.  Gestation averages 21-23 days and pups are weaned at 21-28 days.  Noise, movement and other alterations in the environment can increase cannibalization of the young.

History- The major gerbil species used in the United States is Meriones unguiculatus, the Mongolian gerbil or clawed jird.  This species was first used for research in 1933 when researchers found that it was susceptible to a blood parasite of humans (schistosomiasis).  In 1935 Kasuga captured twenty pairs of M. unguiculatus from the region of the Amur River basin in eastern Mongolia and Manchuria.  The animals were taken to Japan and maintained as a closed, random-bred colony in the Kitasato Institute.  A sub-colony of this stock was started by Nomura in 1949 at the Central Laboratories for Experimental Animals, and it was from there that Schwentker imported four breeding pairs into the USA in 1954. Schwentker's random-bred, closed colony at the West Foundation, Brant Lake, New York, USA., then became the center from which animals were distributed throughout the United States, to the United Kingdom and thence to Europe.    

Anatomy- The Mongolian gerbil has a short broad head, hairless erect ears, four pairs of mammary glands, a long furred tail, and larger rear legs than forelegs.  Most gerbils are agouti in color (hair shafts have white base, brown middle, black tip) with a gray ventrum and a black tail-tip, although solid black, albino, and other multicolor variants exist.  An orangish-brown ventral sebaceous marking gland lies on the midline of the body near the umbilicus.  It is used to scent-mark the environment and is much larger in males than in females.  The dental formula is 2 × (I 1/1, C  0/0, Pm 0/0, M 3/3) = 16 teeth.  Gerbils have an incomplete Circle of Willis, an arterial network adjacent to the ventral surface of the brain, which connects (anastomoses) with the common carotid arteries.  Ligation of one of the carotid arteries results in cerebral ischemia and therefore the gerbil is commonly utilized as a model of stroke.  Adrenal glands are proportionally larger than many other rodents (on a body weight basis, the adrenal glands are about four times that of the rat), and are thought to contribute to the gerbil’s ability to conserve bodily water.  

Physiology- Since it is a desert animal, the gerbil has a very low water requirement and a high heat tolerance.  The Mongolian gerbil can develop high levels of plasma lipid, especially when fed certain diets, e.g. those with a high proportion of sunflower seeds.  Adult gerbils weigh 55-100 grams with males slightly larger than females, and the lifespan varies from 3-4 years.  Additional information is available in Appendix I. 

Behavior-  Gerbils are burrowers and should always be housed in solid bottom caging with bedding.  They are diurnal, but tend to be more active at night.  Gerbils are docile, inquisitive, clean, and rarely bite.   Foot stomping is a common sign of aggression and excitement.  They are able to jump great distances and heights and cagelids should be well-secured. 

Reproduction- Sexing can be carried out from birth onwards by comparing the anogenital distances; the distance in the male is about twice that of the female.  The dark-colored scrotum of the young male is also obvious in recently weaned stock, and the female has separate vaginal, urinary and anal openings.  The gerbil is continually polyestrous with an estrus cycle of 4-6 days.  The estrus period begins at night and lasts less than twenty-four hours.  A post-copulatory vaginal plug is formed during mating, but it is small and lies deep in the vagina and cannot be readily detected.  The gestation period is 24-26 days, but gestation can be prolonged up to 16 days when the female is suckling more than two young.  Most females show a fertile post-partum estrus.  Abandonment or cannibalism may be seen with small litters or with excessive handling of the offspring.  The male often assists in caring for the young.  The young are usually weaned at 21-28 days.

Gerbils are monogamous and a mature animal who has lost its mate will usually not accept another.  It is best to assemble breeding pairs before they reach sexual maturity.  Once a pair is established the animals will live in harmony and breed con­tinuously.

History- The most common species of hamster used in research is Mesocricetus auratus, the Syrian or golden hamster.  In 1930, a professor at the Hebrew University in Jerusalem captured a mother golden hamster with eleven pups while on a zoological expedition in Syria.  He quickly realized that these animals were useful for research and could be readily bred in captivity.  He distributed offspring to researchers throughout the world.  Many of the Syrian hamsters purchased today as pets or research animals originated from this original litter captured in Syria in 1930; therefore, although laboratory suppliers describe their stock as being outbred, these animals are genetically quite homozygous. 

Anatomy-  The Syrian hamster has abundant and loose skin, short legs with four digits on the front feet and five on the rear feet, and a short furred tail.  They are most commonly gold-reddish brown with a white chest and ventrum, but can also be cinnamon, piebald, longhaired “teddy bear”, or albino.  The dental formula is 2 × (I 1/1, C 0/0, Pm 0/0, M 3/3) = 16 teeth, but unlike other rodents the incisors are present at birth.  Hamsters have large internal cheek pouches that are well developed, highly distensible invaginations of the lateral buccal walls.   When full, they extend to well back over the shoulder. The cheek pouches are considered to be “immunologically-privileged sites” and can be everted; in the past researchers used these characteristics to study a variety of immune processes including experimental tumor implantation.  The stomach is divided into nonglandular forestomach and glandular stomach by a constriction.  As a desert species, the Syrian hamster has water-conserving kidneys with extremely long papillae which extend into the ureters.  The sexual dimorphism in size and weight of the adrenal glands is reverse to that found in other rodents, with male adrenal glands larger than those in females.   This is due to a much thicker (3X) zona reticularis in male adrenal glands which is related to the breeding season and sexual maturity (androgen production).  Hamster urine is normally extremely turbid and milky because it contains many small crystals.  Although both sexes possess flank glands that are sebaceous scent glands, they are more darkly pigmented and larger in the male.  Hamsters possess extremely large testicles for their size which are retractable, an os penis, and numerous accessory sex glands.  The female reproductive tract consists of a duplex uterus with two cervical canals that merge into a single external cervical opening.  Females usually have twelve or fourteen mammary glands, but as many as twenty-two have been reported. 

Physiology- Hamsters will enter a pseudohibernation if the ambient temperature drops below 50°F or light decreases to less than 12 hours a day.  During this time the hamster’s metabolic activity slows down (body temperature drops to ambient temperature, respiratory and heart rate decrease), however they do not fatten up prior to entering dormancy, must awake periodically to eat, and can also be aroused by stimulation (thus this is not a true hibernation). Average body weight ranges from 85-150 grams with females larger than males, and life span varies from 1.5-2 years.  Additional information is available in Appendix I.

Nutrition- Hamsters should be fed on the floor since adults have difficulty using slot feeders (because of their blunt snouts) and because the young begin to eat solid food at 7-10 days of age.   

Behavior- Hamsters are solitary, nocturnal, burrowers, and the most escape-prone of the rodents.  In general, they are not docile and commonly feign biting or sometimes will bite when disturbed.  Hamsters use their cheek pouches to store food and also will hide their young in the pouches if they feel threatened.  Females are dominant and will fight other females and males; males will also fight but less commonly.  A common site for fight wounds are the flank glands and the genital region. 

Reproduction- The male has a greater anogenital distance, one urogenital opening, and the testes cause the posterior end to appear rounded.  The female has a short anogenital distance, separate vaginal and urinary openings, and the posterior end is pointed.  Hamsters undergo a winter breeding quiescence induced by decreased photoperiod that manifests as poor reproductive performance and infertility.  During this time the male’s testicles will regress in size.  The female has a four day estrus cycle, with estrus during the evening hours and ovulation between midnight and 1:00 a.m.  At the end of estrus, there is a copious thick, white post-ovulatory discharge.  The female can be successfully mated in the evening of the third day after this post-ovulatory discharge.  Hand-mating is preferred to prevent injury to the males, although monogamous and polygamous breeding systems can work if groups are established at a young age.  After breeding females are housed individually until the time of parturition.  Gestation averages 15-16 days, and the post-partum estrus is infertile.  Hamsters are notorious for cannibalizing their young at the least disturbance, and should be left undisturbed from 2 days prior to parturition to one week post-parturition.  Pups are usually weaned at 21-28 days of age.

History- The guinea pig or cavy, Cavia porcellus, is a member of the suborder Hystrichomorpha, and therefore differs from the other traditional laboratory rodents in many ways.  The origin of the common name “guinea pig” is vague.  Possibly the name is due to the  resemblance to a suckling pig and because the carcass is scalded and then scraped free of hair before cooking, similar to a pig.  Although not closely related to swine, certain terms are similar: adult females are called “sows”, adult males are called “boars” and parturition may be referred to as “farrowing”.  Guinea pigs were first used as a research animal in 1780 when Lavoiser used guinea pigs to measure metabolic heat production. 

Genetics- Guinea pigs are popular as pets world-wide and hobby breeders have selected animals for various coat characteristics including smooth short hair (English shorthair), medium length hair which forms whorls and rosettes (Abyssinian), and long-haired varieties (Peruvian, Angora).  The outbred Dunkin-Hartley breed (stock) is the most common type used in research. This breed has short, smooth hair and may be albino, white, brown, red, or black as a monocolor or as bands and patches in bicolored or tricolored individuals.  Inbred strains include Strain 2 and Strain 13 that differ in their major histocompatability II gene complex and are used in studies of immune function.  A hairless strain has been developed for research in transdermal drug penetration and other studies in which hair may interfere with research results. 

Anatomy-  Guinea pigs have stocky bodies, short legs with four digits on the front feet and three on the rear feet, and no tails.  The dental formula is 2 × (I 1/1, C 0/0, Pm 1/1, M 3/3) = 20 teeth.  All teeth, including the flat molars and premolars, are hypsodont.  Because the molars are used to grind food, the muscles of mastication (cheek and jaw) are well developed.  The mouth does not open very wide. The thymus is located in the cervical region and present only in immature animals.  Before athymic animals were developed, the guinea pig was used extensively for immunologic studies because the cervical thymus could easily be surgically removed.  The stomach is entirely glandular.  Like many herbivores most digestion occurs in the cecum, which may contain up to 65% of the gastrointestinal contents.  The cecum occupies the left side of the abdominal cavity and is a thin walled sac (15-20 cm long) which has three parallel bands (taenia) and small out-pouches (haustra).  Like the hamster, the adrenal glands are larger in the males.  The male has an os penis, retractable testicles, and numerous accessory sex glands.  The female reproductive tract consists of two uterine horns that open into the cervix and there is a single external cervical opening into the vagina.  Both males and females have a single inguinal pair of mammary glands with nipples that are surrounded by small hairless areas.   

Physiology- Average body weight ranges from 700-1200 grams with boars larger than sows, and life span varies from four to five years.  Additional information is available in Appendix I. 

Behavior- Guinea pigs are most active in the morning and evening.  Groups of guinea pigs form social hierarchies usually centered around one alpha male.  High ranking males are intolerant of sexual activity of lower ranking males and can be very aggressive.  Hair chewing and barbering can occur.  Communication is vocal/auditory (with chatters, whines, whistles, screams and purrs) and olfactory (through scent marking with anal glands).  Guinea pigs are easily handled and rarely bite, however they will stampede (circle) and may scratch when excited.  Their response to danger is either immobility or sudden explosive scattering of the group. 

Reproduction- The anogenital distance is virtually the same in young animals and adults.   The male has only one urogenital opening, and the penis is normally retracted but may be extruded by gently retracting the preputial skin.  The female has separate vaginal, urinary and anal openings, and has a vaginal closure membrane that lies in a shallow U-shaped fold between the anus and urethral opening.  This membrane is perforated only during estrus and parturition.  The female has a 16 day estrous cycle, with a 6-11 hour estrus that usually occurs during the evening. Monogamous and polygamous (1 male per 4-10 females) breeding systems are successful. After mating, a vaginal plug forms and is retained for one to two days.  Females should be mated between two and three months of age since the pubic symphysis ossifies if females are not bred before six months of age which can result in dystocia.  In females bred before six months of age, the ligamentous portion of the pelvic symphysis softens during pregnancy and degenerates about two weeks prior to parturition to allow passage of the young through the birth canal.  Palpation of this separation can be used to estimate the date of parturition.  The symphysis returns to normal within 24 hours post-partum.  Gestation is 59-72 days (average of 68 days); this relatively long gestation allows more in utero development and the young are born precocious with hair, teeth, and open eyes.  The young may begin to eat solid food and drink water within several hours of birth.  They will continue to nurse for 14-24 days, despite the inclusion of solid foods. Newborn guinea pigs usually are incapable of voluntary micturition for several days post-partum and maternal licking is necessary to stimulate defecation and urination. 

Fox JG, BJ Cohen, and FM Loew.  Laboratory Animal Medicine. Academic Press, Inc., Orlando, FL (1984).

Harkness JE and JE Wagner.  The Biology and Medicine of Rabbits and Rodents.  4th ed., Lea and Febiger, Philadelphia, PA (1995).