They typically live an average of four to five years, but may live as long as eight years. According to the 2006 Guinness Book of Records the longest living guinea pig survived 14 years, 10.5 months.
In the 1990s, a minority scientific opinion emerged proposing that caviomorphs, such as guinea pigs, chinchillas, and degus, are not rodents and should be reclassified as a separate order of mammals (similar to lagomorphs).
Subsequent research using wider sampling has restored consensus among mammalian biologists that the current classification of rodents as monophyletic is justified.
Domesticated guinea pigs thrive in groups of two or more. Guinea pigs learn to recognize and bond with other individual guinea pigs.
Domestic guinea pigs generally live in cages, although some owners of large numbers of guinea pigs will dedicate entire rooms to their pets. Cages with solid or wire mesh floors are used, although wire mesh floors can cause injury and may be associated with an infection commonly known as bumblefoot (ulcerative pododermatitis
After its cage has been cleaned, a guinea pig will typically urinate and drag the lower body across the floor of the cage to mark its territory. Male guinea pigs may also mark their territory in this way when they are taken out of their cages.
Guinea pigs do not generally thrive when housed with other species. Cohousing of guinea pigs with other rodents such as gerbils and hamsters may increase instances of respiratory and other infections, and such rodents may act aggressively toward the guinea pig.
The guinea pig is able to breed year-round, with birth peaks usually coming in the spring; as many as five litters can be produced per year.
Male and female guinea pigs do not differ in external appearance apart from general size.
Males reach sexual maturity at 3–5 weeks; females can be fertile as early as four weeks and can carry litters before they are adults.
Grass is the guinea pig's natural diet. Guinea pigs benefit from feeding on fresh grass hay, such as timothy hay, in addition to food pellets.
Common ailments in domestic guinea pigs include respiratory tract infections, diarrhea, scurvy (vitamin C deficiency, typically characterized by sluggishness), abscesses due to infection (often in the neck, due to hay embedded in the throat, or from external scratches), and infections by lice, mites, or fungus.
If handled correctly early in their life, guinea pigs become amenable to being picked up and carried, and seldom bite or scratch. They are timid explorers and often hesitate to attempt an escape from their cage even when an opportunity presents itself. Still, they show considerable curiosity when allowed to walk freely, especially in familiar and safe terrain. Guinea pigs that become familiar with their owner will whistle on the owner's approach; they will also learn to whistle in response to the rustling of plastic bags or the opening of refrigerator doors, where their food is most commonly stored.
Guinea pigs should be kept in pairs or, preferably groups, unless there is a specific medical condition that requires isolation. Lone guinea pigs are more likely to suffer from stress and depression. Domesticated guinea pigs come in many breeds, which have been developed since their introduction to Europe and North America.