Euryoryzomys russatus, also known as the russet oryzomys, russet rice rat, or big-headed rice rat, is a species of rodent in the family Cricetidae. It is a member of the genus Euryoryzomys, which was split off from Oryzomys in 2006. It was first described by Johann Andreas Wagner in 1848. It is found in southern Brazil, eastern Paraguay and northeastern Argentina. It is considered a large species in its genus, with a reddish-brown coat, long tail length, and large skull. It is a terrestrial rodent, spending its time foraging for seeds, fruits, and insects. It is listed by the IUCN as least concern, although studies have shown it to be influenced by anthropogenic disturbances. Predators consist of small members of the order Carnivora.
Etymology
The prefix eury- comes from the Greek word 'eurys' meaning "wide" or "broad". The specific epithet russatus comes from the Latin word 'russatus', meaning "clothed in red".
Taxonomy
Euryoryzomys russatus is the currently accepted name for the russet rice rat. It is a member of the order Rodentia and family Cricetidae with the genus Euryoryzomys comprising six valid species.
Distribution and habitat
Euryoryzomys russatus is found in continental Brazil and outlying islands. This species is also found in Bolivia, Argentina, and Paraguay. It is found along altitudinal gradients consisting of lowland and mountainous areas. It is found in the Atlantic Forest as well as some areas of the Amazon rainforest. Specific locations where E. russatus has been collected include: Desterro Environmental Conservation Unit central Santa Catarina Island, Brazil ; Alem Paraiba, Minas Gerais, Brazil ; Guaricana National Park, Paraná, Brazil ; Ilha do Cardoso, São Paulo, Brazil ; Parana, Southern Brazil ; Picinguaba, São Paulo, Brazil ; Rio de Janeiro, Brazil ; Morro Grande Forest Reserve, São Paulo, Brazil. This species is considered a forest specialist, dwelling only in habitats that have extensive forest canopy cover. Abundance and immigration rates have been shown to increase as forest coverage increases.
Life history
Morphology
The genus Euryoryzomys is described as having pelage that varies from yellow in color to a red-brown dorsally, while having a lighter ventral color. The ears are typically medium to large. Vibrissae do not extend past the ears. Most species possess a jugal. Euryoryzomys russatus is described as having a large skull and relatively large body length; with a body length range of. Weights have been recorded in some studies, and averaged. The tail averages between in length. Pelage of the rice rat is a reddish-brown on the dorsal portion of the body and white on the ventral portion. The pinnae and tail are both grey in color. The fore and hindlimbs are a pale pink, along with the nose. The facial vibrissae are black in color. Parietals in E. russatus possess no lateral expansions.
Ecology
Euryoryzomys russatus is a nocturnal, terrestrial rodent that moves primarily over leaf litter found on the forest floor. A seasonal microhabitat selection study found variation in microhabitat choice in warm-wet and cool-dry seasons. Euryoryzomys russatus was shown to have greater abundance in areas with woody debris, low leaf litter height, and high arthropod biomass during the warm-wet season and during the cool-dry season the greatest abundance was seen in areas with high leaf litter humidity. It is an opportunistic eater, consuming seeds, fruit, and insects when possible. A study of seed predation in the Brazilian Atlantic forest found E. russatus to be an efficient seed predator, eating a majority of seeds offered. A study of the population dynamics of a population of E. russatus on Santa Catarina Island in southern Brazil showed them to have nearly equal sex ratios. Populations of E. russatus have been shown to have a monogamous mating system. Females show reproductive activity throughout the year, and correlate with availability of food resources. The nests of E. russatus are cup-shaped, and built with fibers from bamboo and other grasses from the family Poaceae. Offspring are born altricial, lacking hair, and with eyes and ears closed. A study tracking individuals of E. russatus calculated the average number of offspring per pregnancy to be 3.6, with three to six nestlings being typical.
Genetics
Genetic analyses have grouped different populations of E. russatus into three clades using mitochondrial and nuclear gene regions, however no subspecies have been identified. A study focused on the genetic structuring of populations in the Atlantic Rain Forest of southern Brazil found no genetic structuring throughout the species distribution. Through karyotyping of E. russatus individuals from Parque Estadual da Serra do Mar it was found that they possess a chromosome number of 2n=80. This number is shared with E. emmonsae and E. nitidus.
Conservation
Euryoryzomys russatus is listed as a species of least concern by the IUCN as of September 2016. However, studies have shown this species to be susceptible to anthropogenic disturbances such as habitat degradation or destruction.
Predators
Studies have found the E. russatus to be a prey item of several neotropical feline species including jaguarundi, oncilla and ocelot. Domestic cats have also been shown to prey on E. russatus''.
Parasites
Research into the gastrointestinal parasites of E. russatus found eight different endoparasites in island and continental populations. A new species of nematode, Hassalstrongylus luquei, was found in the small intestine of E. russatus. A single individual was found to possess antibodies from a systemic fungal infection with Paracoccidioides brasiliensis. A study on Rickettsia rickettsii, Brazilian Spotted-fever has shown E. russatus to be a host for the tick species Amblyomma ovale, which is a known vector for the zoonotic disease.
