Guinea pigs (Cavia porcellus), or cavies, are common household pets and the best-known species in the genus Cavia, subfamily Caviinae. They are hystricomorph rodents (having a specific jaw and facial muscle structure defined by a large infraorbital foramen through which the masseter muscle passes) native to the Andes region of South America. Guinea pigs were domesticated as livestock and considered a meat source in certain regions.
Among the several known breeds of guinea pigs, the most popular are the smooth-haired American shorthair; the Abyssinian, with hair swirls called rosettes in a well-defined pattern; and the long-haired Peruvian. Clubs, such as the American Cavy Breeders Association, exist in different countries.
Guinea pigs have stocky bodies with short limbs and no tail. They have rounded, hairless pinnae with physiological alopecia caudal to the ears. Hairless areas are also present on the medial antebrachia. Guinea pigs have androgen-dependent sebaceous glands along the dorsum and around the anus that are used for marking, such as the caudal gland that is dorsal to the anus at the base of the spine (see ).
Note the oily appearance created by secretions from the caudal (sebaceous) gland.
Courtesy of Dr. Miranda Sadar.
The forefeet of guinea pigs have four digits, with three on the pelvic feet, each with a claw that requires trimming. Excess keratin, called spurs or horns, can form on the forefeet and can also require routine trimming. A guinea pig's clavicle attaches at the coracoid process of the scapula and the manubrium. The pelvic symphysis is fibrocartilaginous, and in pregnant females, a gap can be palpated with parturition. Both female and male guinea pigs have one pair of inguinal nipples.
Guinea pigs are obligate nasal breathers and have four lung lobes (cranial, middle, caudal, and accessory) on the right and three (cranial, middle, and caudal) on the left. They possess a palatal ostium, where the soft palate is continuous with the base of the tongue, creating a small opening that connects the oropharynx with the rest of the pharynx. This arrangement makes endotracheal intubation challenging in this species.
Their teeth are elodont (continuously growing), hypsodont (high-crowned), and aradicular (open-rooted). The incisors are white, which is a unique feature; most rodent incisors have yellow enamel. The premolars and molars grow at an acute angle with the occlusal surface at approximately 30–35°.
Guinea pigs are obligate herbivores and hindgut fermenters. The stomach is positioned in the left cranial quadrant of the abdomen. The small intestines are positioned on the right side, because the large cecum occupies the left and central abdomen. The cecum is large and contains approximately 65% of the GI contents. Guinea pigs' microbiota is mostly Gram-positive and anaerobic, with Lactobacillus spp predominating. They exhibit coprophagy and do not produce cecotropes. How coprophagy contributes to the nutritional needs of guinea pigs is unknown. If this action is prevented, however, guinea pigs lose weight, excrete more minerals in the feces, and digest less fiber. Their liver has six lobes (right, medial, left lateral, left medial, caudate, and quadrate), and they have a well-developed gallbladder.
The reproductive system of male guinea pigs includes four accessory sex glands (vesicular, prostate, coagulating, and bulbourethral). The most prominent are the vesicular glands, which are long, coiled, blind ended, and commonly fluid-filled. Because of their appearance, the vesicular glands can be mistaken for uterine horns on ultrasonography or necropsy. Guinea pigs have open inguinal canals, and the testes lie within these and the scrotum, which is not well developed. Caudoventral to the urethral opening of the penis is an intromittent sac that is everted during an erection, and two horny styles project externally. This sac can be confused with the external urethral opening during urinary catheterization (see ). Guinea pigs possess an os penis. Males have a rectal pouch, which is located caudal to the anus and between the scrotal outpouchings. The rectal pouch can become impacted with feces, which commonly occurs in older, intact males.
Urethral catheterization in a male guinea pig. Note the paired horny styles projecting from the intromittent sac, which is often mistaken for the urethral opening. Paired blind pouches are also adjacent to the urethral opening.
Courtesy of Dr. Miranda Sadar.
Female guinea pigs have a bicornuate uterus with a short uterine body and paired uterine horns. They have a single cervix. The urethra exists outside of the vaginaon the urinary papilla, which is cranial to the vaginal opening. The urinary papilla is a good landmark during urinary catheterization, which is straightforward in this species and sex (see ). Guinea pigs are precocial—born with eyes and ears open, fully haired, and eating solid food.
Urethral catheterization in a female guinea pig. The urinary papilla is external in females, making urinary catheterization easier than in other species.
Courtesy of Dr. Miranda Sadar.
Guinea pigs are typically most active at dawn and dusk but will often adjust to their owners’ schedules. They often do best when housed as pairs or trios; however, caution must be exercised when housing intact males together or when housing intact males and females because of the risks of fighting and breeding, respectively. Social problems and breeding can be minimized or eliminated with surgical sterilization.
Normal guinea pig behaviors include chewing and hiding; often, guinea pigs will not explore their surroundings if they feel unsafe. They vocalize in a variety of ways, including wheeking, chutting, bubbling, rumbling, chattering, whining, chirping, and shrieking. Bouncing repeatedly, also called popcorning, is a sign of excitement. Guinea pigs do not climb readily and rarely jump.
Guinea pigs can be neophobic and might refuse to eat or drink if there are substantial changes in their husbandry or diet. Guinea pigs develop food preferences early in life but can be open to changes if they are made slowly and consistently.
For More Information
Also see pet health content regarding physical characteristics of guinea pigs.
