Brazilian false rice rat, False oryzomys

Pseudoryzomys simplex

Pseudoryzomys simplex, also known as the Brazilian false rice rat or false oryzomys, is a species of rodent in the family Cricetidae from south-central South America. It is found in lowland palm savanna and thorn scrub habitats. It is a medium-sized species, weighing about 50 grams (1.8 oz), with gray–brown fur, long and narrow hindfeet, and a tail that is about as long as the head and body. The IUCN has assessed its conservation status as being of least concern, although almost nothing is known about its diet or reproduction.

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The only species in the genus Pseudoryzomys, its closest living relatives are the large rats Holochilus and Lundomys, which are semiaquatic, spending much of their time in the water. The three genera share several characters, including specializations towards a semiaquatic lifestyle, such as the presence of membranes between the digits (interdigital webbing), and a reduction in the complexity of the molar crowns, both of which are at incipient stages in Pseudoryzomys. Together, they form a unique assemblage within the oryzomyine tribe, a very diverse group including over one hundred species, mainly in South America. This tribe is part of the subfamily Sigmodontinae and family Cricetidae, which include many more species, mainly from Eurasia and the Americas. Pseudoryzomys simplex was independently described in 1888 on the basis of subfossil cave specimens from Brazil (as Hesperomys simplex ); and in 1921 on the basis of a live specimen from Paraguay (as Oryzomys wavrini ). This was confirmed in 1991 that both names pertained to the same species.

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Pseudoryzomys simplex is a nondescript, medium-sized rat with long, soft fur. The upperparts are gray–brown and the underparts are buff; the color changes gradually over the body. The small ears are covered with short hairs. The tail is as long as or slightly longer than the head and body, and is dark above and light below. Despite the presence of short hairs, the scales on the tail are clearly visible. The hairs on the feet are pale. The hindfeet are long and narrow and have five toes, the first and fifth of which are short. Webbing is present between the second, third, and fourth toes, but the membranes are not as large as in Lundomys or Holochilus. The tufts of hair on the toes and several of the pads are reduced, other common characteristics of semiaquatic oryzomyines. The head-body length is 94 to 140 millimeters (3.7 to 5.5 in), tail length 102 to 140 mm (4.0 to 5.5 in), hindfeet length 27 to 33 mm (1.1 to 1.3 in), ear length 13 to 19 mm (0.5 to 0.7 in) and body mass 45 to 55 g (1.6 to 1.9 oz).

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The female has four pairs of teats, including one on the chest and three on the belly, and the gall bladder is absent, both important characters of Oryzomyini. As is characteristic of Sigmodontinae, Pseudoryzomys has a complex penis, with the baculum (penis bone) displaying large protuberances at the sides. In the cartilaginous part of the baculum, the central digit is smaller than those at the sides.

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Biogeographical realms
Pseudoryzomys habitat map
Pseudoryzomys habitat map

Habits and Lifestyle

Pseudoryzomys simplex is known from northeastern Argentina, probably south to about 30°S, northward through western Paraguay to eastern Bolivia and from there eastward through Brazil in the states of Mato Grosso, Goiás, Tocantins, Minas Gerais, São Paulo, Bahia, and far in the northeast, Alagoas and Pernambuco. Paraguayan animals are somewhat smaller than those from Bolivia and Brazil and those from Bolivia have darker fur than Paraguayan specimens, but these differences are not considered significant enough to recognize subspecies. Certain bats show a similar pattern of variation: they are smaller and paler in the Chaco region, which includes much of Paraguay. Two specimens from Paraguay, collected 600 kilometres (400 mi) apart, differed by 1.4% in the sequence of the cytochrome b gene, but nothing is known about genetic variation in other parts of the range. The species has long been rare in collections; in 1991, Voss and Myers could use less than 50 specimens for their study of the species, including Lund's fragmentary material from Lagoa Santa.

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A fragmentary lower jaw of "Pseudoryzomys aff. P. simplex " (i.e., an unnamed species close to Pseudoryzomys simplex ) is known from a cave deposit in Cueva Tixi, Buenos Aires Province, Argentina, outside the current distribution of the species. It is dated from the first millennium CE. The jaw's morphology agrees with that of P. simplex, but the toothrow is relatively long (5.78 mm; 4.61 to 5.60 mm in three specimens of P. simplex ) and the first molar is relatively narrow (1.28 mm; 1.30 to 1.40 mm in five P. simplex ).

P. simplex inhabits open, usually humid tropical and subtropical lowlands. In Argentina, it is mainly a species of the eastern Chaco and in Brazil it is found in the Cerrado and Caatinga. Most specimens for which habitat data are known were caught on the ground in humid grassland, some in seasonally flooded areas; an Argentinean specimen was captured in dense swamp vegetation. It is terrestrial and semiaquatic, living on the ground but also spending time in the water.

Nothing is known about behavior or diet. P. simplex has frequently been found in pellets of the barn owl (Tyto alba ) and also in those of the great horned owl (Bubo virginianus ). It is a preferred prey of the maned wolf (Chrysocyon brachyurus ).

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Population number

The species is not known to be threatened and its conservation status is classified as least concern by the IUCN. It is a widely distributed species without substantial threats to its continued existence, but degradation of its habitat may endanger some populations. It was assessed as "potentially vulnerable" in Argentina.


1. Pseudoryzomys Wikipedia article -
2. Pseudoryzomys on The IUCN Red List site -

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