Elegant fat-tailed mouse opossum
The elegant fat-tailed mouse opossum (Thylamys elegans), also known as the Chilean mouse opossum, is an opossum from central Chile. The type species of Thylamys, it was first described by English naturalist George Robert Waterhouse in 1839. This medium-sized opossum is characterized by black rings around the eyes, white limbs, gray to light brown coat, lighter flanks and underbelly and a thick  long tail covered with hairs. It is crepuscular (active mainly around twilight) and lives in nests in tree hollows or under rocks and roots. This opossum feeds mainly on arthropods and larvae apart from fruits. Litter size is typically between 11 and 13. The elegant fat-tailed opossum can occur in a variety of habitats – from cloud forests to chaparrals. The IUCN classifies the opossum as least concern.

Taxonomy and etymology 
The elegant fat-tailed mouse opossum is the type species of Thylamys, and is placed in the family Didelphidae. It was first described by English naturalist George Robert Waterhouse as Didelphis elegans in 1839. It was given its present binomial name by English zoologist John Edward Gray in 1843.

The cladogram below, based on a 2016 study, shows the phylogenetic relationships of the elegant fat-tailed mouse opossum.

The generic name is composed of the Greek words thylas ("pouch") and mys ("mouse"), and the specific name elegans means "elegant" in Latin. Alternate names for this mouse opossum include Chilean mouse opossum, comadreja, llaca, marmosa chilena, marmosa elegante and yaca.

Description 
The elegant fat-tailed mouse opossum is a medium-sized opossum characterized by white limbs, gray to light brown coat, lighter flanks and underbelly, and a thick  long tail covered with hairs. A prominent facial feature is the black ring around either eye; the rings slightly extend toward the nose. The coat color varies geographically. The tail can thicken due to accumulation of fat; the diameter of the tail can reach  where it is connected to the body. The head-and-body length is ; the hindfeet measure , while ears measure .

This opossum can exhibit torpor, a mechanism that allows the opossum to significantly reduce its food and energy requirements. This is similar to hibernation, except that it is only done for a short period of time. Additionally, when water is scarce, the urine tends to become very concentrated.

Ecology and behavior
The elegant fat-tailed mouse opossum is crepuscular (active mainly around twilight). It lives in nests (abandoned by birds and rodents, or built by itself) in tree hollows, under rocks and roots. It is arboreal (tree-living) as well as terrestrial (land-living). The opossum feeds mainly on arthropods and larvae, along with fruits, small vertebrates and carrion. This opossum can use its prehensile tail, regardless of its thickness, to efficiently climb and grasp branches – an ability not as developed in other small Chilean mammals. Nests are generally occupied by one individual; in southern Chile, the home range of this opossum is  large in July, and shrinks to  in December. Predators of this opossum include the culpeo fox, burrowing owl and the great horned owl.

Both sexes mature by the first year. A female can have one to two litters in the annual breeding season, typically from September to March. Up to 17 embryos can be produced, but the number of survivors will depend on the number of nipples functioning (typically 11 to 13).

Distribution and status 
The elegant fat-tailed mouse opossum occurs in a variety of habitats – from cloud forests to chaparrals, up to an altitude of  above the sea level. It occurs to the west of the Andes in central Chile. The IUCN lists this opossum as least concern given its wide distribution and presumably large numbers, but deforestation and agricultural expansion have led to decline in populations in some parts of the range.

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Opossums
Marsupials of South America
Mammals of Patagonia
Mammals of the Andes
Mammals of Argentina
Mammals of Chile
Taxa named by George Robert Waterhouse
Mammals described in 1839
Taxonomy articles created by Polbot
Endemic fauna of Chile