ANATOMY AND PHYSIOLOGY
Guinea pigs have 7 cervical, 13-14 thoracic, 6
lumbar, 2-3 sacral and 4-6 coccygeal vertebrae.
Vertebral formula:
C7 T13-14 L6 S2-3 Cy4-6
Dental formula
Guinea pigs are monophyodont, that is, they get one
permanent set of teeth. Their dental formula is
unique for rodents, as most rodents do not have
premolars. All the teeth are hypsodont, or open
rooted.
Dental formula:
2(I 1/1 C 0/0 PM 1/1 M 3/3) = 20
Digits
Guinea pigs have four
digits on the forelimbs
and three digits on their
hindlimbs.
Ears
The ears of the guinea pig
are small and access to the
veins is difficult. Guinea
pigs have large tympanic
bullae and the internal
structures of the ear are
easily reached features that
facilitate auditory research.
TEATS
Both the male and
female have teats.
The female has a
single pair of
inguinal mammary
glands.
The thymus
The thymus of the
guinea pig surrounds
the trachea. This is in
contrast to rats, mice
and other murid
rodents, in which the
thymus is deep within
the thoracic cavity,
overlying the heart.
The adrenal glands
The adrenal glands are
bilobed and large compared
to most rodents. They are
situated cranial to each
kidney. In this image, in
which the animal's head is to
the left, the kidney is
obscured by renal fat, but
the left adrenal gland is
indicated by an arrow.
Gastrointestinal
Guinea pigs are
monogastric, but they have a
lower intestinal tract typical
of herbivores. The large
intestine occupies most of
the abdominal cavity, and
the cecum, identified by the
arrow, contains up to 65%
of the total gastrointestinal
contents.
Vaginal closure
The female guinea pig (sow)
can be distinguished from
the male by a shallow, Ushaped break in the ridge
between the urethral orifice
and the anus. This break is
covered with a vaginal
closure membrane which is
only open during estrus and
parturition.
Which one is the male?
Everted Penis
Sexing is more easily
accomplished by applying
gentle pressure to evert the
penis of the boar, or male.
The mature boar has extraabdominal paired testes,
which lie in the scrotal
pouches and open inguinal
canals.
Penile style
The os penis, or baculum, is
found within the dorsal
surface of the entire length
of the glans. The
intromittent sac, unique to
hystricomorph rodents, is in
the ventral aspect of the
glans. When the penis is
erect, the intromittent sac
everts, revealing two
keratinaceous, horn-like
styles attached to its caudal
end.
Female and male reproductive tracts
The male guinea pig, on the left, has large vesicular glands,
shown by the larger arrow, that are bilateral, smooth and
transparent. These may be mistaken for the uterine horns of
the female, which are identified by the smaller arrow on the
right.
Physiologic values
The average life span is four-five years.
Normal body temperature of the guinea pig is
99.0-103.1 F
Respiratory rate is 42-104 per minute
The heart rate is 230-380 beats per minute.
Adult body weight for the sow is 700-900
grams and, for the boar, 900-1200 grams.
Physiologic values cont..
Puberty 45-70 days
Breeding age 3-4 months
Gestation period 59-72 days
Litter size 2-5 pups
Weaning age 14-21 days
Unique future
Guinea pigs are the only rodents known to
require a source of vitamin C in their daily
diet. This is because they lack Lgulonolactone oxidase, an enzyme required
for the synthesis of vitamin C.
REPRODUCTION
Sow and Boar
The sow is a nonseasonal, continuously polyestrous
breeder, with an estrous cycle of 15-17 days. The
sow also exhibits a fertile estrus approximately 1215 hours postpartum.
Usually the sow is bred at two-three months of age,
which corresponds with a body weight of 350-450
grams. A sow should be bred for the first time
before 7 months of age.
For the boar, age at first breeding is typically threefour months (600-700 grams body weight)
Placentation
Placentation in the guinea pig is discoid and
hemomonochorial. Gestation is between 59
and 72 days, with an average of 68 days;
gestation length varies inversely with litter
size.
discoid placenta
a placenta in which the chorionic villi are arranged in a circular plate as in
human and rodent placentae.
hemochorial placenta
a type of placenta in which all maternal layers are lost so that fetal tissue is
in contact with frank maternal blood, as occurs in insectivores, rodents,
rabbits and most primates.
Handier w/sow
In late gestation, abdominal
distention is obvious and
body weight may nearly
double. The pubic
symphysis begins to
separate in the last half of
gestation due to increased
production of relaxin. The
diameter of the separation
48 hours prepartum is
approximately 15 mm and
may be as large as 22 mm at
parturition.
When are
you due?
Within 24 hrs!
Hey doc, can’t you at
least wear a glove?
Newborns
A litter of two to five piglets is
born within about 30 minutes, with
3 to 7 minutes between births.
Each newborn weighs between 60
and 100 grams, with individual
birth weights inversely
proportional to the litter size
Care of Young
precocial – born
furred, eyes open, teeth
erupted
walking within 2 hours
two nipples – inguinal
region
can care for four young
litters 3-4 (range 1-6)
Identification
Guinea pigs may be
identified individually.
Permanent methods
include color pattern
records, ear notching,
microchip, and tattoos.
An ear tag, as shown
here, is another
acceptable identification
method
Resources
LABORATORY ANIMAL MEDICINE AND SCIENCE SERIES II
GUINEA PIGS:
Biology and Use in Research
V-9023
L. J. Tambrallo, DVM, MS
R. E. Fish, DVM, PhD
Office of Laboratory Animal Medicine
University of Missouri
Columbia, Missouri