Guinea pigs have a stubby body, with short limbs and no tail (although caudal vertebrae are present). The animal’s maximal development is reached at 15 months of age; the average weight of a male is 900-1200 g while that of a female is 700-900 g. The compact body retains heat well, but conversely cannot dissipate it easily; in fact, high temperatures rapidly lead to death (particularly in pregnant sows).
The body temperature varies from 37.2°C to 39.5°C. The ideal environmental temperature is in the range between 18°C and 26°C.
The average lifespan is considered to be between 3 and 4 years, although these animals can reach 6 or 7 years of age.
The anterior limbs have four toes each, while the posterior limbs have three; the inferior surface of the limbs is hairless with well-developed footpads and the toes have nails (Figs. 1a and 1b).
The eyes, located laterally, give the animal a visual field of 340 degrees. The third palpebra is rudimentary.
The ears are rounded and covered with fine hair; there is a hairless area behind the pinna.
The skin of the coccygeal and perianal region is rich in sebaceous glands used for territorial marking; the secretions from theses glands can dirty the fur giving it a greasy appearance.
The lips behind the incisors form a fold facing inwards to the interior of the mouth.
The teeth, both incisors and cheek teeth, grow continuously (elodent dentition); their correct length is maintained by the constant attrition provided by chewing fibrous material (grasses and other vegetables). The dental formula is: incisors 1/1, canines 0/0, premolars 1/1 and molars 3/3, for a total of 20 teeth. The incisors are white; the free part of the inferior incisors is about three times longer than that of the superior incisors (Fig. 2). There is a space, called the diastema, between the incisors and the cheek teeth. The premolars and molars are the same as each other and form a single surface for mastication. The inferior molars are inclined towards the tongue while the superior ones are inclined towards the cheeks. The plane of occlusion is oblique, angled at 30 degrees. When the mandible is in a resting position, the occlusion planes of the inferior and superior cheek teeth are almost in contact.
The tongue, which is fleshy, is poorly mobile. The posterior part of the tongue is connected to the soft palate, except for a small midline aperture called the palatal ostium, which is the only communication between the oral cavity (oropharynx) and the pharynx. This, together with the poor opening of the mouth and the fact that the oral cavity is long and narrow, makes orotracheal intubation of the guinea pig very difficult.
The hearing is very developed and the tympanic bullae are bulky.
The thoracic cavity is small compared to the abdominal one; the heart, which is relatively large, is located between the second and fourth intercostal spaces. The thymus extends from the mediastinum to the neck; most of this organ is replaced by adipose tissue in adults.
The gastrointestinal system is bulky. The small bowel occupies the left-hand side of the abdominal cavity, while the caecum occupies the right part (Fig. 3). The caecum is the most voluminous organ of the digestive tract and holds up to 65% of the intestinal contents. It is 15-20 cm long and sacculated as the result of three longitudinal ribbons of muscle fibres (dorsal, ventral and medial) called taenia coli, which run along its whole length forming pouches of the wall called haustra. The colon is about 70 cm long; from a functional point of view, the colon is divided into a proximal part (20 cm long) and a distal part (50 cm long). Ingested food is separated in the proximal part into faeces and caecotrophes (caecal pellets); reverse peristalsis of the proximal colon propels the pellets containing high quality proteins into the caecum where they are fermented through the effect of symbiotic micro-organisms, producing volatile fatty acids. The guinea pig can process and digest fiber more efficiently than the rabbit can.
The liver is a reddish-brown colour and smooth. It has six lobes (right, medial, left lateral, left medial, caudate and quadrate). The gallbladder is located at the level of the quadrate lobe and the descending duodenum; it is pinkish and has three lobes.
The digestive system of the guinea pig is very delicate and gastrointestinal disorders, which are often severe, constitute one of the main problems of this species. The causes are frequently related to dietary errors (e.g. fibre-poor diet, abrupt changes in diet).
The guinea pig is a monogastric herbivore; gastric emptying takes about 2 hours, while intestinal transit is completed in 8-30 hours, with an average of 22 hours.
The guinea pig, like the rabbit, practices coprophagy, eating the caecal pellets directly from the anus. However, the production of caecotrophes in the guinea pig does not take place at regular intervals as it does in the rabbit and chinchilla, but occurs about 150-200 times throughout the whole day. It is thought that the role of coprophagy in the guinea pig is the same as that in the rabbit, that is, production and use of vitamins and other valuable nutrients. Neonatal guinea pigs eat the caecotrophes of their mother, in this way acquiring a normal intestinal flora.
The inguinal canal remains patent throughout life and the testes can be withdrawn into the abdomen. There is a bulky adipose body associated with the testicle. The seminal vesicles are very developed and extend from the pubis to the abdomen (Figs. 4 and 5).
Both males and females have two nipples, although the mammary glands in the male are rudimentary. Despite the fact that the sow has only two mammary glands, she can provide milk for three or four whelps without problem, with these suckling in turn.
The male has a smallpenile bone. Scent glands are present laterally to the anus and are more developed in males. The anus is covered by a skin fold.
The distinction between the two sexes is easy in adults, but more difficult in very young subjects (Figs. 6 and 7). The anogenital distance is similar in both sexes, but in the male the opening of the prepuce is rounded and the penis can be visualised by exercising gentle compression at its base. In males that have not been castrated, the testicles are evident at the sides of the penis. In the female, the perineum forms a “Y” shape; the urethra has a separate opening from the vulva and its external aperture is on a papilla in the space between the two arms of the Y, while the vulva is found where the two arms meet. During anoestrus, the vulva is covered by a membrane that seals it (Fig. 8). The anus is found at the base of the Y. The uterus is bicornuate, with a short body and a single cervix.
The daily water intake is about 10 ml/100 g of body weight, but varies depending on the type of alimentation; if guinea pigs are provided a lot of fresh food, they drink very little. The urine is turbid and dense, with a pH of 9; it may contain crystals of ammonium phosphate and calcium carbonate. The average daily output of urine is 20-25 ml.
The main physiological characteristic of the guinea pig is that, like primates, it cannot synthesise vitamin C because of the lack of L-gluconolactone oxidase, an enzyme involved in the synthesis of this vitamin starting from glucose. For this reason the diet must contain adequate amounts of the vitamin in order to prevent vitamin C deficiency. The dose needed is 10-30 mg/kg of live weight, but should be increased during pregnancy and during disease states. Hypervitaminosis C does not occur because any excess of this vitamin is eliminated in the urine.