Oligoryzomys flavescens
Yellow Pygmy Rice Rat
Description:
Oligoryzomys flavescens is a small rodent with a pointed muzzle, oval-shaped
ears and a long, slender, naked tail. Its tail, which is longer than its body, ranges from
85-155mm in length. The body length ranges from 70-110 mm and it generally weighs
anything from 9 to 40 grams (Nowak 1999).
Redford and Eisenberg (1992) describes O. flavescens’ body coloration in some
detail. The dorsal part of its head and body is brown with yellow and black tipped hairs
mixed into the fur. Continuing down the sides and rump, the brownish fur is washed with
an orange color. Finally, its venter is a whitish-yellow color that occasionally contains
hints of gray fur mixed in. Also, the fur of O. flavescens is very soft compared to other
rodents.
O. flavescens also has a very delicate skull which lacks supraoccipital ridges
(Dickerman and Yates Feb. 1995).
Distribution:
O. flavescens is found from southern Brazil through Uruguay into parts of
northern Argentina at least through to the Buenos Aires Province (Redford and Eisenberg
1992).
Ontogeny and Reproduction:
Very little is known about the reproduction of O. flavescens reproduction.
However, studies have shown that their reproductive peak is during the months of April
and May with average litters containing about 5 young, with as little as three and as much
as 7 offspring produced.
Ecology and Behavior:
The genus Oligoryzomys and Oryzomys are very closely related in phylogeny.
One thing that is common with both genuses is that they tend to occupy three types of
habitats. They can arboreal, terrestrial or live near the water and be semi-aquatic.
Because of their versatility, sharing of territory among species exists (MacDonald 2001).
O. flavescens can utilize all three of these habitats. This mammal typically
occupies pasture fields surrounded by weedy margins found in crop land regions
throughout its range (Busch et. al. 2001). Sometimes these fields border rivers, streams
and wetlands. O. flavescens can also be found in the tall marsh grasses near these bodies
of water (Redford and Eisenberg 1992). Also, the yellow pygmy rice rat has the ability to
climb trees in the area. This could be a possible adaptation to escape predators (Nowak
1999).
O. flavescens typically is a solitary, nocturnal rodent that does not leave its home
range and habitat much. Trapping studies have shown that the species tends to be
residential in its habitat (Busch and Kravetz 1992).
O. flavescens is a very generalized herbivore. It’s a very opportunistic species,
feeding on the grasses and green plants that surround it in its habitat (Ellis et. al. 1998).
Some cases of omnivory have been found in the wild. Some individuals that lived near
aquatic habitats had stomachs that contained invertebrate material (Redford and
Eisenberg 1992).
Today, the numbers of O. flavescens being found is decreasing. This is a general
trend found both in Oligoryzomys and Oryzomys. Human impacts and the introduction of
domestic cats and murid rats and mice have been keeping the population numbers very
low (MacDonald 2001). This could be one of the reasons why information about these
rodents is hard to find.
Remarks:
One of the most common things that O. flavescens is associated with is
Hantavirus Pulmonary Syndrome (HPS). HPS is an infectious disease typically
characterized by fever, myalgia and headache followed by hypotension or shock. These
viruses can be fatal. They are mainly transmitted to humans through the inhalation of
infectious rodent feces, urine, or saliva, including O. flavescens (Bayard et. al. 2004).
Uruguay is one country that has had a growing number of hantavirus cases in
recent years. 38 cases have been reported since April 1997 with a fatality rate of 21%
(Delfraro et. al. 2003).
Surveys have been conducted to understand the origin of HPS in Uruguay.
Rodent traps were set out in places humans occupy and where HPS has been found
before, including peridomestic areas, cropland fields, and road borders. The survey
found that 85.4% of all rodents captured where found in area disturbed by humans, which
include the three areas listed above. Yellow Pygmy Rice Rats were among the most
frequently captured species in the survey. It also was the only species that showed
evidence of HPS antibodies with 2.6% prevalence (Delfraro et. al. 2003).
This study was only conducted in the southern part of Uruguay where HPS has
been found in humans. Studies are being conducted to determine whether morphological
differences between O. flavescens species in Uruguay and Argentina exist and that HPS
antibodies are present in those identified species.
Literature Cited:
Bayard, Vincent et. al. Sept. 2004 Outbreak of Hantavirus Pulmonary Syndrome,
Los Santos, Panama, 1999-2000. Emerging Infectious Diseases. Vol. 10.
No. 10. Pg. 1635-1642.
Busch, M. et. al. 2001. Habitat Selection by Akodon azarae and Calomys laucha in
Pampean Argoecosystems. Mammalia. Vol. 65. Issue 1. Pg.
29.
Busch, M. and Kravetz, F.O. 1992. Competitive Interactions Among Rodents in a Two
Habitat System: The Effect of Species Removal. Mammalia. Vol. 56. Issue 4. Pg. 541.
Delfraro, Adriana et. al. July 2003 Yellow Pygmy Rice Rat (Oligoryzomys flavescens)
and Hantavirus Pulmonary Syndrome in Uruguay. Emerging Infectious Diseases. Vol. 9.
No. 7. Pg. 846-852.
Dickerson, A.W. and Yates, T.L. Feb. 1995. Journal of Mammalogy. Vol. 76. Issue 1.
Pg. 172.
Ellis, Barbara A. et. al. 1998. Dietary Habits of the Common Rodents in an
Argoecosystem in Argentina. Journal of Mammalogy. Vol. 79. Issue 4. Pg.
1203.
MacDonald, David. 2001. The Encyclopedia of Mammals. Andromeda Oxford
Limited. Pg. 623.
Nowak, Ronald M. 1999. Walker’s Mammals of the World. 6th edition. Vol. 2. The
John’s Hopkins University Press- Baltimore and London.
Redford, Kent H. and Eisenberg, John F. 1992. Mammals of the Neotropics: The
Southern Cone. The University of Chicago Press-Chicago and
London. Vol. 2. Pg.
266.
Reference written by Matt Koslowski, Biology 378 (Mammalogy), University of
Wisconsin – Stevens Point. Edited by Christopher Yahnke. Page last updated August 15,
2005.
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