Oligoryzomys microtis
Small-eared pygmy rice rat
Description:
Oligorhyzomys microtis is the smallest of 15 species in the genus. Its weakly
bicolored tail is long, slender and naked, and equals roughly 128% of the head and body
length. It has relatively long hind feet. The average head and body length for O. microtis
is 146mm, tail length is 188mm, hind feet are 24mm, and the average ear length is 14mm.
On average each weighs about 18g (Redford and Eisenberg 1992). The four longer hind
toes have tufts of silvery grey bristles that expand beyond the claws. The pelage is coarse
but not spiny or bristly. Its back is yellow-brown or gray mixed with black and it is white
or gray underneath with less black. Its throat is variably white to gray. The cheeks are
orangish brown or gray, the muzzle is pointed and the ears are oval and densely furred
inside (Nowak 1999). The whiskers are short, sometimes reaching the ear tip but never
the shoulder (Emmons 1990). The skull is small but stout and the rostrum is relatively
broad and stocky. However, these identifying features are common to the genus and may
not always be used to properly identify the species in the field.
Distribution:
O. microtis is found in the lowlands of eastern Peru and Bolivia extending east to
southern Brazil and south to northern Argentina into the province of Corrientes. It also
occurs throughout eastern Paraguay and into the Chaco (Redford and Eisenberg 1992).
Small-eared pygmy rice rats are found in many different habitats depending on
geographic location. Overall this species seems to prefer dense vegetation. They also
occupy houses and camps. In Paraguay and Argentina they inhabit marshes and wet
grasslands. In Brazil they occupy the edges of gallery forests and also secondary growth
habitats. In the Chaco they prefer wet and dry marshy habitats (Redford and Eisenberg
1992).
Ontogeny and Reproduction:
Females have four pairs of mammae. The average litter size is 5 but ranges from
2 -11 young per litter. Breeding occurs year round and mature females produce about 6
litters per year. Nests are made of woven grasses and placed in a slight depression in the
ground (Nowak 1999).
Ecology and Behavior
Oligoryzomys microtis are nocturnal, solitary, and terrestrial. While they are not
characterized as arboreal, they are good climbers. These animals feed on seeds, fruits
and insects. They can be found feeding in rice fields and storage barns (Nowak 1999).
Remarks:
O. microtis has some very important implications for humans. It is an agricultural
pest and can be found feeding in rice fields and storage barns. It is also a household pest
that may take up residence in homes and camps (Emmons 1990). While humans across
the world share their homes and barns with many rodent species, O. microtis could have a
more negative effect on humans than many other rodent species. Its most important role
in relation to humans is that it may be a vector of many diseases. Studies done on this
species have resulted in the first successful isolation of Rio Mamore virus and the first
evidence for the existence of hantavirus in Peru (Powers 1999). Other areas that have
reported cases of hantavirus from Oligoryzomys species include Paraguay, the Chaco,
Argentina, Chile, Bolivia, Brazil, Venezuela and Panama (Chu 2003). In most cases, the
viruses associated with human infections in these areas are closely related to
Oligoryzomys-borne viruses (Padula 2000). These diseases apparently do not harm the
rodent host but can be fatal to humans. Transmission may occur by rodent bites or
contact, but occurs most frequently from breathing virus-contaminated aerosols of rodent
excreta. Because the disease is apparently so easy to obtain from infected rodents,
knowledge and control of O. microtis is very important for human health (Schmaljohn
1997).
While the Small-eared pygmy rice rat is referred to as Oligoryzomys microtis
throughout this paper, the actual species name is under some discrepancy among
scientists. Allen gave it the name O. microtis in 1916 but it has since been referred to as
O. delicates, O. longicaudatus, O. fornesi, O. mattogrosssae, and O. chaparensis
(Carleton 1989).
Literature Cited:
Carleton, Michael D., and G.G. Musser. 1989. Systematic Studies of Oryzomyine
Rodents (Muridae, Sigmodontince): A Synopsis of Microryzomys. Bulletin of the
American Museum of Natural History. 191:71.
Chu, Yong-Kyu, R.D. Owen, L.M. Gonzalez, and C.B. Jonsson. 2003. The Complex
Ecology of Hantavirus in Paraguay. The American Journal of Tropical Medicine
and Hygiene. 69(3):263-268.
Emmons, Louise H. Neotropical Rainforest Mammal: A Field Guide. 1990. University
of Chicago Press. 180.
Nowak, Ronald M. Walker’s Mammals of the World. 6th Edition, Volume 2. 1999. John
Hopkins University Press. 1366-1369.
Padula, P.J., S.B Colavecchia, V.P. Marinez, M.O. Gonzalez Della Valle, A. Edelstein,
S.D.L. Miguel, J. Russi, J. Mora Riquelme, N. Colucci, M. Almiron, and R.D.
Rabinovich. 2000. Genetic Diversity, Distribution, and Serological Features of
Hantavirus Infection in Five Countries in South America. Journal of Clinical
Microbiology. 38(8):3029-3035.
Powers, A.M., D.R. Mercer, D.M. Watts, H. Guzman, C.F. Fulhorst, V.L. Popov, and R.
B. Tesh. 1999. Isolation and genetic characterization of a hantavirus
(Bunyaviridae: Hantavirus) from a rodent, Oligoryzomys microtis (Muridae),
collected in northeastern Peru. The American Journal of Tropical Medicine and
Hygiene. 61(1):92-98.
Redford, K.H., and J.F. Eisenberg. Mammals of the Neotropics: The Southern Cone.
Volume 2. 1992. University of Chicago Press. 266-267.
Schmaljohn, Connie and B. Hjelle. 1997. Hantaviruses: A Global Disease Problem.
Emerging Infectious Diseases. 3(2).
Reference written by Amy Schumacher, Biology 378 (Mammalogy), University of
Wisconsin – Stevens Point. Edited by Christopher Yahnke. Page last updated August 15,
2005.