The size of rats has long captured human curiosity, leading to fascination with both living species and the colossal rodents known from the fossil record.
Record-Breaking Living Rats
Among living rat species, some stand out for their substantial size. The Sumatran bamboo rat (Rhizomys sumatrensis) can reach nearly 50 centimeters (20 inches) in length with a 20-centimeter (7.9-inch) tail, and weigh up to 4 kilograms (8.8 pounds). These bulky, slow-moving rodents inhabit extensive burrow systems in Southeast Asia, where they feed on bamboo roots and other underground plant parts.
Another large living rat is the recently discovered Uromys vika from the Solomon Islands. This tree-dwelling species can weigh up to 1 kilogram (2.2 pounds) and measures about 45.7 centimeters (1.5 feet) from nose to tail tip. Local islanders have long spoken of this “vika,” known to crack open coconuts with its teeth. The Uromys vika is considerably larger than common black rats, which typically weigh around 200 grams (0.44 pounds).
The African giant pouched rat (Cricetomys gambianus) is recognized for its impressive length. It can have a body length of up to 43 centimeters (17 inches) and a tail that adds another 45 centimeters (17.7 inches), bringing its total length to 88 centimeters (34.6 inches). On average, these rats weigh about 1.3 kilograms (2.9 pounds).
Giants of the Past
The fossil record reveals rodents that dwarfed any living species. The largest rodent known to have ever existed is Josephoartigasia monesi, an extinct species that lived in South America 2 to 4 million years ago. This massive rodent, related to modern-day pacaranas, is estimated to have weighed nearly 1,000 kilograms (about 2,200 pounds), with some estimates reaching up to 2,586 kilograms (5,701 pounds). Its skull alone measured up to 53 centimeters (1.7 feet) in length, and its body length could have been around 3 meters (9.8 feet) with a height of 1.5 meters (4.9 feet).
While Josephoartigasia monesi was a rodent, it was not a “true rat” in the narrow taxonomic sense of the Muridae family. However, other extinct giant murids, or true rats, have also been discovered. For instance, the Timor giant rat (Coryphomys musseri) is an extinct species known from sub-fossil remains found on Timor Island, Indonesia. Archaeological research indicates these rats could have weighed up to 6 kilograms (13.2 pounds) as adults.
Island gigantism often explains the large size of extinct rodents found on islands. This evolutionary trend occurs when small species colonize islands lacking large predators and competition, allowing them to grow to larger sizes due to relaxed selective pressures and potentially greater resource availability. The Flores giant rat, another example from an Indonesian island, was twice the size of an average brown rat, illustrating this principle.
Distinguishing Large Rodents
The term “rat” is often broadly applied to many large rodents, leading to confusion about what constitutes a true rat. True rats belong to the family Muridae, which includes the Rattus genus. Many large rodents that might be mistaken for giant rats are actually part of different rodent families.
The capybara (Hydrochoerus hydrochaeris) is the largest living rodent in the world, averaging 1.2 to 1.5 meters (3.9 to 4.9 feet) in length and weighing between 35 to 66 kilograms (77 to 146 pounds). While a rodent, it is not a rat. Capybaras are semi-aquatic mammals with robust bodies and short, almost invisible tails, distinctly different from the slender body and long tail of a true rat.
The nutria (Myocastor coypus), also known as coypu, is a large semi-aquatic rodent native to South America. Nutria can weigh up to 10 kilograms (22 pounds) and measure 40 to 60 centimeters (16 to 24 inches) in body length, with a 30 to 45 centimeter (12 to 18 inch) tail. Despite sometimes being called “swamp rats” due to their rat-like tail, they are not true rats and belong to a different family, Echimyidae.
Pacas (Cuniculus paca) are large, ground-dwelling rodents found in Central and South America. These animals can weigh up to 14 kilograms (31 pounds) and measure 50 to 77 centimeters (20 to 30 inches) in length, excluding their short tails. With their distinctive spotted coats and stocky build, pacas are often colloquially referred to as “royal rats” or “jungle rats,” but they are rodents of a separate family, Cuniculidae.
Beavers (Castor canadensis), known for their dam-building, are the largest rodents in North America, weighing between 16 to 45 kilograms (35 to 100 pounds). They are easily distinguished by their broad, flat tails.