Cavia magna, commonly known as the greater guinea pig, is a species of rodent in the family Caviidae, native to South America.

It is found primarily in the coastal regions of southern Brazil and Uruguay, inhabiting grasslands, marshes, and other wetland environments.

[3] This species exhibits adaptations to its wetland habitats, including a preference for dense vegetation near water.

Greater guinea pigs are herbivorous, feeding on grasses, sedges, and other vegetation, and play a role in maintaining the ecological balance of their native habitats.

The Greater guinea pig is the larger out of all three of these species, with wider upper molars and longer digits.

[7] A variant diploid number of 2n = 62 and FN = 104 has been reported in this species, making it the first record of intra-specific chromosomal variation within Caviidae.

[9] This phenomenon is suspected to be due to pronounced peaks of androstenedione during the second trimester of pregnancy, a period associated with sexual differentiation of the genitalia.

Home-ranges have wide areas of overlap between individuals, which consist of complex networks of tunnels and runways through vegetation.

Regardless, the Greater guinea pig has a high mortality rate, highest during the dry summer months.

It is suspected that males use their larger home range sizes for roaming, and mate with every female in estrous that they are able to encounter.

[11] Females seem capable of breeding at any time of year, but births predominate in spring and the early part of summer (September to December).

[10] Newborns weigh approximately 18% of their mother's body weight in the wild and are considered an extreme example of precocial small mammals.

Litters are small, consisting of one to three pups, which are born with eyes open, fully furred and able to run within the first hour of birth.

Excitement is vocalized by high-pitched squeaking, stress by chirping, and teeth chattering to ward off perceived threats.