Thylamys pusilluscommon fat-tailed mouse opossum(Also: small fat-tailed opossum)

Geographic Range

This lowland species is distributed in south-eastern Bolivia, western Paraguay, and north-eastern and central Argentina. (Giarla, et al., 2010)


Thylamys pusillus inhabits the arid and semi-arid lowlands of central South America, including the Chaco, Monte, and Pampas ecoregions. (Giarla, et al., 2010)

  • Range elevation
    0 to 1000 m
    0.00 to 3280.84 ft

Physical Description

Like other members of its genus, Thylamys pusillus is notable for its incrassate (fattened) tail. The size of the tail varies by season in accordance with food availability. Although this species is a marsupial, females do not have a pouch. This species is tricolored, with darker dorsal fur, paler lateral fur, and a white ventral region. This species is broadly similar morphologically to Thylamys macrurus but much smaller. It is also quite similar to Thylamys pallidior, a species that occurs in some of the same areas as Thylamys pusillus. Dorsal hair length and ventral hair color are useful characters for differentiating between these two species (Giarla et al., 2010). Giarla et al. (2010) report head and body lengths that range from 88 to 116 mm (average 98 mm) and tail lengths that range from 98 to 134 mm long (average 109 mm). (Giarla, et al., 2010; Voss, et al., 2009)

  • Range length
    186 to 250 mm
    7.32 to 9.84 in
  • Average length
    207 mm
    8.15 in


No published studies have examined mating systems in Thylamys pusillus.

Little is known about the general reproductive behavior of Thylamys pusillus.

  • Key Reproductive Features
  • gonochoric/gonochoristic/dioecious (sexes separate)
  • sexual
  • viviparous

Little is known about parental investment in Thylamys pusillus. Like all marsupials, females nurse their highly altricial young. However, because members of the genus Thylamys lack a pouch (marsupium), the young must cling to their mother's venter. (Giarla, et al., 2010)

  • Parental Investment
  • altricial
  • female parental care
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female


No records of this species' lifespan are available.


Little is known about the behavior of Thylamys pusillus. This species is likely solitary, as most small, insectivorous mammals are. As is the case for other members of this genus, Thylamys pusillus is likely nocturnal and probably enters torpor during the day. (Palma, 1997)

Home Range

No studies have examined the home range of Thylamys pusillus.

Communication and Perception

Because this species is small and nocturnal, communication between individuals is likely primarily olfactory in nature. Palma (1997) reports that the olfactory and visual regions of another Thylamys species' brain are especially well developed. (Palma, 1997)

Food Habits

Little is known about the food habits of this species. Like other Thylamys species, Thylamys pusillus likely consumes insects and perhaps occasionally eats small vertebrates, leaves, fruit, seeds, and carrion (Palma 1997). (Palma, 1997)


Like other small mammals, Thylamys pusillus is likely well adapted to avoiding predators by being nocturnal and inconspicuous. No records of known predators are available. (Giarla, et al., 2010)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Thylamys pusillus likely acts as an important predator to many arthropod species and perhaps some small vertebrates. It is likely prey to both bird and medium-sized mammals, such as owls and foxes. It is also likely host to many ecto- and endoparasites. More specific information about the ecosystem role of Thylamys pusillus is not presently available. (Palma, 1997)

Economic Importance for Humans: Positive

There are no known positive impacts of Thylamys pusillus on humans.

Economic Importance for Humans: Negative

There are no known negative effects of Thylamys pusillus.

Conservation Status

This species is listed as "Least Concern" by the IUCN.


Tom Giarla (author), University of Minnesota, Sharon Jansa (editor), American Museum of Natural History, Robert Voss (editor), American Museum of Natural History, Tanya Dewey (editor), University of Michigan-Ann Arbor.



living in the southern part of the New World. In other words, Central and South America.

World Map


uses sound to communicate


young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.


Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat


uses smells or other chemicals to communicate


having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.


animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females


An animal that eats mainly insects or spiders.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


uses touch to communicate


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.


A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate


reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.


Giarla, T., R. Voss, S. Jansa. 2010. Species Limits and Phylogenetic Relationships in the Didelphid Marsupial Genus Thylamys Based on Mitochondrial DNA Sequences and Morphology. Bulletin of the American Museum of Natural History, 346: 1-67.

Palma, R. 1997. Thylamys elegans. Mammalian Species, 572: 1-4.

Voss, R., P. Myers, F. Catzeflis, A. Carmignotto, J. Barreiro. 2009. The Six Opossums of Felix de Azara: Identification, Taxonomic History, Neotype Designations, and Nomenclatural Recommendations. Bulletin of the American Museum of Natural History, 331: 406-433.