Scapteromys is a genus of South America

South America

South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

n rodent

Rodent

Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

s in the tribe Akodontini

Akodontini

Akodontini is the second most speciose tribe of the subfamily Sigmodontinae. It includes at least 106 living species in 19 genera and is distributed mainly in the southern half of South America, with only two genera extending into Guyana and Venezuela . It also includes genera previously placed in...

 of family Cricetidae

Cricetidae

The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, and New World rats and mice...

. Two species are known, found in northern Argentina

Argentina

Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

, southern Brazil

Brazil

Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

, Paraguay

Paraguay

Paraguay , officially the Republic of Paraguay , is a landlocked country in South America. It is bordered by Argentina to the south and southwest, Brazil to the east and northeast, and Bolivia to the northwest. Paraguay lies on both banks of the Paraguay River, which runs through the center of the...

 and Uruguay

Uruguay

Uruguay ,officially the Oriental Republic of Uruguay,sometimes the Eastern Republic of Uruguay; ) is a country in the southeastern part of South America. It is home to some 3.5 million people, of whom 1.8 million live in the capital Montevideo and its metropolitan area...

. They are as follows:

• Argentine Swamp Rat
  Argentine swamp rat
  The Argentine swamp rat, Scapteromys aquaticus, is a semiaquatic rodent species from South America. It is found in northeastern Argentina and Paraguay, where it lives in freshwater marshes and along the southern coast of the Río de la Plata estuary, as well as in forests.-References:*...
  
   (Scapteromys aquaticus)
• Waterhouse's Swamp Rat
  Waterhouse's Swamp Rat
  Waterhouse's Swamp Rat, Scapteromys tumidus, is a semiaquatic rodent species from South America. It is found in southern Brazil, Uruguay and northern Argentina, where it lives in freshwater and salt marshes, as well as open grassland of the pampas....
  
   (Scapteromys tumidus)

Both species are semiaquatic, living in and near marshes and other bodies of water. They reach a body length of 15 to 20 cm and a tail length of 13-17 cm, and weigh 110-200 g. Fur color is dark gray on top and light gray on the underside. They are primarily crepuscular

Crepuscular

Crepuscular animals are those that are active primarily during twilight, that is during dawn and dusk. The word is derived from the Latin word crepusculum, meaning "twilight." Crepuscular is, thus, in contrast with diurnal and nocturnal behavior. Crepuscular animals may also be active on a bright...

 and nocturnal. Their diet consists mainly of insects

Insectivore

An insectivore is a type of carnivore with a diet that consists chiefly of insects and similar small creatures. An alternate term is entomophage, which also refers to the human practice of eating insects....

; they also consume other invertebrates and plant material.

The two species differ in karyotype

Karyotype

A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...

, with aquaticus having 2n = 32 and tumidus 2n = 24.