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THE GUINEA PIG OR CAVY
Taxonomic Relations
The most recent revision of the family Caviidae concludes
that the domestic guinea pig is a derivative of Cavia aperea and designates
all other forms including C. porcellus. C. rufescens. C. cobaya, and C.
cutleri, as invalid synonyms or subspecies of C. aperea. The Latin name most
frequently used today for the domestic guinea pig is C. porcellus,
but papers prior to 1940 frequently used C. cobaya. The confusion is due
to several factors including the total lack of records from the first
domestication and conflicting reports of genetic and zoological studies of
the large and widely distributed genus. It can be said with reasonable
certainty, thought, that a wild progenitor of the domestic cavy is still
available.
The four genera making up the family Caviidae are all found in South
America. The genus Cavia is widely distributed in almost all areas except
the Amazon rain forest. Southern Argentina is the home of the Patagonian
cavy, a hare=like animal frequently seen in zoos in this country. A
closely related family, Hydrochoeridae, contains the largest living rodent
the capybara. which may weigh up to 50 kg. This animal is also frequently
seen in zoos.
History
The Spanish conquistadors brought guinea pigs back to Europe with
them 400 years ago. The Incas of Peru had been breeding them in complete
domesticity for an unrecorded length of time. There has never yet been any
conclusive evidence of the exact geographic origin of their stocks. The animal
was kept as a table delicacy and both the wild and domestic forms are eaten
today in South America.
The name "guinea pig'' is of unknown origin: the only excuse for it
is the squeal that the animal emits when excited. Fanciers and the pet trade
prefer the name "cavy" and their disdain for "guinea pig" is absolute. A
veterinarian uses the two names for the -ame animal according to the owner's
sensitivity. The German name is meerschwein, which translates "sea pig).
Strains.
Guinea pigs are grouped into three 'breeds" by fanciers on the basis of
pelage. The English type has short, straight, coarse hair that follows the body
contours. The Abyssinian type has similar hair, but the coat is arranged in
rosettes or swirls over the body, giving the animal a somewhat appearance.
Crosses are as common as the pure types, so total confusion reigns. A third
type is called Peruvian; this type has hair (very much like the angora type in
rabbits) that is long and silky and totally unsuited to a laboratory animal.
The range of colors available is the same as for mice and rabbit. Albino,
English-type animals are often referred to as Dunkin-Hartley guinea pigs.
Dunkin and-Hartley were geneticists who developed a strain for their own work
that proved to be very productive, relatively docile, and bred true to color.
Genetics in guinea pigs has been very thoroughly and systematically in-
vestigated by the famous geneticist Sewall Wright. His inbred lines, some of
which were begun in 1906, are the most completely inbred strains. In 1958 two
of Wright's original 23 lines were still being maintained and were then tested
by skin grafting for homozygosity. At that time both strains (numbers 2 and 13
of the original series) were found to be isologous. They had been continuously
bred B x S for over fifty generations. Coat color Zenes have not been selected
for, so that these strains remain heterozygous for several color types, yet are
apparently homozygous for the histocompatability genes. 1, 2 These two strains,
therefore, provide the largest mammals available for tissue transplant work that
display complete histocompatability.
The domestic cavy will hybridize easily with C. aperea and
several of its subspecies, although in some crosses there is F1 male sterility.
See J. P. Rood and B. J. Weir, 1970, "Reproduction in female wild guinea pigs",
J. Reprod. Fertility, 23:393-409, for a full discussion.
The Special Anatomy of the Guinea Pig
General Description
A medium sized rodent of stocky, square profile with the appearance of
being laterally compressed. The face is blunt and the head about the same width
as the shoulders. The tail is externally absent; only a coccyx remains. The
feet show a reduction of toes with four in front and three on the hind feet.
All-toes have strong, straight claws. The soles of the feet are naked.
Although the wild coat color is agouti, most domestic cavies are white or white-
spotted. The body weights of adult animals average 850 gms and 1000: gms for
females and males, respectively.
Bauer, J.A., Jr., 1958. Histocompatability in inbred strains of guinea pigs.
Ann. N.Y. Acad. Sci., 73:663-672.
8
Bauer, J.A.,Jr., 1960. Genetics of skin transplantation and an estimate of the
number of histocompatable genes in inbred guinea pigs. Ann. X.Y. Acad. Sci.,
87:78=92.
The sexes are commonly referred to as sows and board and several other
terms from swine husbandry are transferred to guinea pigs. The animals are
docile, they very seldom bite but are always excitable, nervous, and constantly
chuckle, squeal, and chatter amongst themselves in colonies. They are commonly
maintained in harems for breeding purposes with one male for as many as 12
females. The young are born fully furred, their eyes open, and dentition ready
for solid food.
Their need for exogenous vitamin C is unique among rodents and must be
provided for in their husbandry.
External Features
The pelage is coarse and somewhat rough. Depending upon breed, their
growth patterns will vary from the normal smooth contours to a series of large
rosettes or whorls over the body. Color is variable in most strains although
the dark-pointed strains with white skin and fur generally breed true.
The short, fleshy ear is naked. Its veins are nor large enough for routine
injections although blood may conveniently be obtained from a nick in the ear.
The blunt nose is different enough from the more pointed nose of rats that
slotted feeder designed for rat should not be used in guinea pig cages. The
lips close behind the incisors as in other rodents. There are no cheek pouches.
Posture is like that in many other rodents; the anus is pressed to the
ground in the normal standing position. A large scent gland field in the
perineum is thus brought into contact and marks the surface where the animal
rests. These perineal glands open into deep clefts between the anus and genital
papilla of both sexes.
A single pair of inguinal nipples is seen in BOTH SEXES although mammary
gland development normally occurs only in females.
Skeleton
The vertebral formula ia 7 cervical, 13 thoracic. 6 lumbar. 2 sacral, and 6
coccygeal. The clavicle is vestigial. The symphysis of the mandibular rami is
ossified. The pelvic symphysis of females is destroyed at time of parturition.
This process begins more than two weeks prior to parturition and results in a
gap of 22 mm at the time of birth. A practiced technician can estimate the
stage of pregnancy by palpating this developing gap.
There are no differences in the times of closure of epiphyses in the sexes.
Guinea pigs have a similar growth pattern to rats and other rodents in that
sexual maturity fails to appreciably slow the rate of skeletal growth.
Dentition
Dental formula: I 1/1, C 0/0, P 1/1, M 3/3 = 20. The incisors are open
rooted chisel-shaped teeth. The molars are also rootless. in contrast to the
case in myomorphs.
The molariform teeth are arranged in convergent rows so that the premolars are
closer to each other than the last molars. There is also a tendency for the
teeth to wear at an acute angle rather than squarely. The lateral movements are
restricted;
grinding is accomplished by fore and aft movement. The angular process of the
dentary is very long to provide insertion for the horizontal component of the
masseter which drives the forward thrust of the lower jaw in both gnawing and
food grinding movements.
The pterygoid fossa of the guinea pig skull is open to the orbit allowing
the internal pterygoid muscle to spread its origin over the orbital wall. The
infraorbital canal is much enlarged for the pulley-like passage of a long head
to the masseter that has its origin on the maxilla.
Digestive System
The stomach is not divided into glandular and non-glandular regions as
described for all the other rodents studied in this sequence. The small intes-
tine of a guinea pig of 22 cm body length measures 120 cm (vs 100 cm for a rat
of the same body length. The cecum is relatively larger in the guinea pig.
measuring 14 cm (vs 5 cm in the rat). The colon of the guinea pig is roughly
60 percent the length of the small intestine: whereas that of the rat is only
about 16 percent. The guinea pig would appear to be more highly specialized as
a strict herbivore.
The guinea pig cecum is a source of volatile fatty acids derived from
bacterial degradation of cellulose and presumably also of several vitamins. The
cecum enlarges enormously in so=called germ-free animals, shortening their life
in many cases. The enlargement is not understood. The organ returns to normal
size if the animal is carefully infected with a normal flora of enteric
microorganisms. The "normal" anatomy would seem to be in reality as much a
function of the animals' internal ecology as of its genes.
The long colon is doubled and coiled on itself before reaching the direct
transverse and descending branches.
The liver is about 3:7 percent of the body weight in a 1 kg animal. It has
eight lobes. The bile duct and gallbladder are rather loosely attached to the
liver and therefore easily observed, cannulated, or tied. The formation of an
ampullary swelling in the common bile duct just at its entry to the duodenum is
presumably unique. This chamber fills, is cut off by sphincter action, then
empties itself into the gut.
The pancreas is not so diffuse as that of murid rodents, although well
defined head and tail regions can be identified.
Spleen and Thymus
The spleen of the guinea pig is somewhat broader in its proportions than
seen in rabbits and murid rodents. Its vascular pattern shows some variation
from other species also. There are no sheaths (of Schweigger-Seidel) and the
arterioles ramify in the red pulp as a well=defined system of capillaries before
entering the venous sinuses. The red pulp does not store large quantities of
red blood cells in its RE cells, the storage capacity of this spleen is largely
a function of the venous sinuses. According to T. Snook (Anat. Rec. 89:413-
428, 1944) the guinea pig spleen is thus more like the human than either the cat
or mouse.
Myeloid elements are absent from the normal guinea pig spleen. The red
pulp harbors large numbers of plasma cells amongst its RE cells.
The thymus differs in two important aspects from that of all other mammals
commonly used in the laboratory:
(1) It is located entirely in the neck where it occurs as a pair of distinctly
separate, ovoid masses.
Spleen and Thymus (cont.)
(2) It has a well developed system of efferent lymphatic that serve to
distribute the small lymphocytes (thymocytes) to other lymphoid organs.
Diapedesis is not observed in its blood vessels.
See Harris and Templeton, Acta Anat., 69:366-377, 1968, for details of this
lymphatic drainage and further discussions of comparative functional morphology.
Although: most species of mammals may be thymectomized experimentally, no
species offers the ready access to this organ that you will see in the guinea
pig where the organ can be exposed with relatively little trauma and observed
during experimental procedures.
Endocrine Organs
Pituitary: This gland is not encased by the sella turcica in this species.
Average weight for adult animals is 50 mg Per kg body weight.
Adrenals: Relatively very large with most of the unusual size due to
hypertrophy of the zona reticularis of the cortex. The functional significance
of this excessive development remains obscure (as is true for this region of the
cortex in all mammals).
Thyroid: Size and location conforms to-that of mammals generally.
Parathyroids: Found four sites embedded in the thyroid lobes.
Reproductive Systems
Male. The testis of the guinea pig matures rapidly. There are spermatozoa
in the ejaculate at about 50 days of age (average body weight of 611 gms). The
testes weigh between five and six gms. There is no seasonal variation in testis
weight or fertility. Males are retained in breeding colonies as long as three
years.
The testes are found in shallow scrotal sacs on either side of the penis.
The typical rodent pattern with broadly open inguinal canal and large epididymal
fat pad is present. The deferent duct unites with the vesicular gland duct
before entering the prostatic urethra. These is no separate ampullary gland.
The vesicular glands are very long (up to 10 cm), slender, tubular
structures; the coagulating glands are short and not so clearly differentiated
from the prostatic tissue as in myomorphs. There are separate dorsal and
lateral lobes to the prostate.
Bulbourethral glands are present. There are small preputial glands. The
prepuce and glans penis are adorned with rows of cornified papillae forming a
corona-like structure. An os penis is present.
Female. The ovary is not so completely encased by its bursa as was the
case for the myomorph rodents. The uterine tube is also less coiled, though
tortuous in its path. These structures are nearly always embedded in fat which
develops in the mesotubarium and mesovarium.
The guinea pig ovary products functional corpora lutea at each 16- to 17-day
cycle; they are grossly visible as pink structures in the cut ovary, the color
fading as the cycle passes the 13th day. Graafian follicles are also visible
to the naked eye.
The uterine horns are joined at the cervices and a single os cervicis opens
to the vagina. The vagina is a long structure passing the entire length of the
long pelvis. The mammalian cycle of vaginal cornification, sloughing, and repair
which follows the development of follicles; their rupture and leuteinization
were first studied in this animal (Papanicolaou, 1917).
The urethral meatus is sub-terminal to the clitoris. Perineal glands are
conspicuously present in both sexes on the walls of the cleft-like vestibule
which encloses anus, vaginal opening, and urethral meatus.
The vagina is closed by a membrane which ruptures just before estrus, stays
open for about four days, then reseals. If the animal is bred and conceives,
there will be a period of opening around the end of the fourth week of pregnancy
that coincides with the transition from ovarian (luteal) control of pregnancy
to placental control. After this time a pregnant female may be ovariectomized
without aborting.
The membrane opens again at term and a postpartum estrus is the normal time
for rebreeding. Lactation does not interfere with the subsequent implantation.
The young are weaned at about three weeks and the gestation period is about 68
days.
Growth
Guinea Pigs are born fully furred, with their eyes and ears open and are
able to begin taking solid food within two days. Birth weight is a function of
litter size and may range from 45 to 100 gms. The young are usually weaned at
160 gms rather than according to an age criterion; the age for this weight may
range from 14 to 28 days. Sexual maturity, as judged by rupture of the vaginal
membrane and willingness to copulate, may occur as early as 33 days of age,
which will mean that some females may be bred by their sires before weaning.
Such breedings are always unsuccessful, however, and the normal practice is to
keep the sexes separated until the females reach three and a half to four and
a half months of age or about 500 gms. There is a danger in not breeding them
at this age in that the pubic symphysis may ossify so completely as to prevent
its proper dissolution at parturition. Earlier breedings result in a high
percentage of abortions.
The males are normally not used as breeders until six months of age or
about 700 gms although spermatozoa are present after ten weeks and fertility is
high. The reason for the practice of using older males is that the usual
ratio of females per male in breeding harems is 12 to 1. The adage is "don't
send a boy out to do a man's job."
1 Ford, et al., Anat. Rec., 109:707-714, 1951.
Growth of Guinea Pigs. Weights in grams.
Age in Weeks Males Females
Birth 45 - 104 45 - 98
2 134 131
4 189 186
6 258 259
8 296 314
10 386 380
12 411 400
14 507 483
18 596 564
22 676 609
26 722 655
30 723 687
36 750 825
42 765 873
52 780 825
100 1100 900
Reference: H.H. Kibler, S. Brody, and D. Worstell, 1947. J. Nutrition 33:33.
GUINEA PIG (CAVIA PORCELLUS)
Age of puberty 45 to70 days
Minimum breeding age 12 weeks (female averages 450 grams and male 500 grams)
Breeding season Any time of year
Estrus cycle Polyestrous; all year
Duration of estrus cycle 16 to 19 days
Deration of period of heat (cstrus) 6 to 15 hours (for acceptance of the male)
Gestation period 58 to 75 days (average is 68)
Litter size 1 to 8 (average is 3)
Ovulation time 10 hours from onset of estrus; type spontaneous
Number of ova 2 to 4, both avaries involved
Copulation time At estrus
Sperm transit, vagina to tube 15 minutes
Ovum transit, tube to uterus 3-1/2 days
Fertilization time A few hours after ovulation
Cleavage of ovum to formation 5 to 6 days
of blastocoele
Implantation or attachment of ova 6 days
Return to estrus, postpartum 6 to 8 hours
Sperm deposition site Uterus
Fertilization site Fallopian tube
Chromosome number, diploid 64 in somatic cells
Birth weight 75 to 100 grams
Weaned 14 to 21 days
Eats solid food 5 days
Breeding life of female 3 years
Breeding life of male 4 years
Breeding habits 1 male to 10 or 12 females
Conception interval Variable (6 to 8 hours is usually satisfactory)
Sex ratio at birth 50 to 54% males average is 52
Lactation period 21 days
Fertilizable longevity of 22 hours
sperm in female tract
Fertilizable longevity 20 hours after ovulation
of egg in oviduct
