Guinea pig
Biology

Guinea pigs are relatively large for rodents. In pet breeds, adults typically weigh between 700 and 1,200 g (1.5 and 2.6 lb) and measure between 20 and 25 cm (8 and 10 in) in length. Some livestock breeds weigh 3 kg (6.6 lb) when full grown. Pet breeds live an average of four to five years but may live as long as eight years. According to Guinness World Records, as of 2006, the longest-lived guinea pig was 14 years, 10 months, and 2 weeks old. Most guinea pigs have fur, but one laboratory breed adopted by some pet owners, the skinny pig, is mostly furless. In contrast, several breeds have long fur, such as the Peruvian, the Silkie, and the Texel. They have four front teeth and small back teeth. Their front teeth grow continuously, so guinea pigs chew on materials such as wood to wear them down to prevent them from becoming too long. In the 1990s, a minority scientific opinion emerged proposing that caviomorphs such as guinea pigs, chinchillas, and degus are not actually rodents, and should be reclassified as a separate order of mammals (similar to the rodent-like lagomorphs which includes rabbits and hares). Subsequent research using wider sampling restored the consensus among mammalian biologists regarding the current classification of rodents, including guinea pigs, as monophyletic.

Wild cavies are found on grassy plains and occupy an ecological niche similar to that of cattle. They are social animals, living in the wild in small groups (" herds ") that consist of several females ("sows"), a male ("boar"), and their young ("pups" not "piglets," a break with the preceding porcine nomenclature). Herds of animals move together, eating grass or other vegetation, yet do not store food. While they do not burrow themselves or build nests, they frequently seek shelter in the burrows of other animals, as well as in crevices and tunnels formed by vegetation. They are crepuscular and tend to be most active during dawn and dusk when it is harder for predators to spot them.

Male and female guinea pigs do not significantly differ in appearance apart from general size. The position of the anus is very close to the genitals in both sexes. Sexing animals at a young age must be done by someone trained in the differences. Female genitals are distinguished by a Y-shaped configuration formed from a vulvar flap. While male genitals may look similar, with the penis and anus forming a similar shape, the penis will protrude if pressure is applied to the surrounding hair anterior to the genital region. The male's testes may also be visible externally from scrotal swelling.
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