Anderson's Mouse Opossum
(Marmosa andersoni)
A small mouse-like marsupial with a prehensile – or grasping – tail, and no pouch. Little is known about this rare and cryptic species. It is nocturnal and thought to inhabit moist areas of montane forest in the Andes. Its diet consists of a wide range of invertebrates, small vertebrates and eggs. Known from only seven specimens, this Critically Endangered opossum is threatened with extinction by habitat loss caused by the continued clearing of brush in the Andes.
Urgent Conservation Actions
Status surveys to establish population size and distribution.
Southern Peru.
In the past there was a popular misconception that opossums copulated through the nose and that the young were later blown out through the nostrils into the pouch! This idea may have originated from the fact that males have a forked penis, and females have a tendency to lick the pouch area before birth.
Evolutionary Distinctiveness
Order: Didelphimorphia
Family: Didelphidae
Didelphid (American opossum) remains are known from North American fossil deposits dating back 70-80 million years ago. These small marsupials are thought to have entered South America and Europe from North America, but by 10-20 million years ago they had become extinct in both of these regions. Some species re-entered North America when the Isthmus of Panama reformed, connecting North and South America, around 3 million years ago. The family Didelphidae includes 87 species in 17 genera. There are currently 9 recognised species of Marmosa (mouse opossums). However, recent molecular evidence indicates that M. andersoni is sufficiently distinct from other species in the genus to be placed in a separate unique genus (Stegomarmosa).
Head and body length: approx. 85-190 mm
Tail length: approx 125-230 mm
Weight: 28-38 g
A small, mouse-like marsupial with fine, velvety fur and prominent black eye-rings. The upper parts are reddish-brown, and the underparts are paler, usually a yellowish-grey colour. The tail is scaly and strongly prehensile. Most of its ventral surface is covered with unusually long silvery bristles that form a well-developed and characteristic fringe. Like other mouse-opossums, this species does not posses a pouch for its young.
Little is known about the ecology of this species. It is nocturnal and may be arboreal. Closely related species mostly eat insects and fruits, although lizards, bird eggs and small rodents are also eaten. Mouse opossums build nests of leaves and twigs in trees, or shelter in abandoned bird nests. They have also been seen in ground nests, in holes, in hollow logs, and under rocks. They are generally solitary creatures, usually hunting and nesting alone. Some species breed throughout the year while others are seasonal. Life expectancy is probably under 1 year in the wild.
Found in hilly lowland forest. Mouse opossums are generally found near streams or in other moist areas.
Until recently this species was known from only a single specimen collected more than 50 years ago in south-eastern Peru. In 1997-98 six additional specimens were collected at two little-separated localities almost 200 km north-west of the type locality.
Population Estimate
Unknown. Known only from the type specimen, collected more than 50 years ago, and six additional individuals caught in 1997-1998.
Population Trend
Classified as Critically Endangered (CR B1+2c) on the 2007 IUCN Red List of Threatened Species.
The main threat to this species is habitat loss, caused by widespread clearing of brush in the Andes region. Mouse opossums are probably not directly threatened by people, because of their small size and nocturnal habits.
Conservation Underway
There do not appear to be any conservation measures in place for this species.
Conservation Proposed
The species was originally classified as Critically Endangered in 1996, on the basis of being: “Severely endangered or known to exist at only a single location” and “Continuing decline, observed, inferred or projected, in…area, extent and/or quality of habitat.” The discovery of the six additional specimens has increased the known localities from one to three, but has not appreciably changed the extent of scientists’ knowledge concerning these criteria with respect to Anderson’s mouse opossum. All the localities are within a narrow strip along the base of the Andes, in very similar pre-montane forests below 1000 m. However, the actual distribution and abundance of this animal may be quite different than the limited information currently available would seem to indicate. The actual conservation status of this species, like that of so many known from very few specimens, is unknown. For this reason, further surveys are required to find out more about the population size and distribution are recommended.
MacDonald, D. (ed.). 2002. The New Encyclopedia of Mammals. Oxford University Press. Oxford.

New World Marsupial Specialist Group. 1996. Marmosa andersoni. In: IUCN 2007. 2007 IUCN Red List of Threatened Species. Downloaded on 13 May 2008.

Nowak, R. M. 1999. Walker’s Mammals of the World. Sixth edition. The Johns Hopkins University Press, Baltimore and London.

Distribution map based on data provided by the IUCN Global Mammal Assessment.

Solari, S. and Pine, R. H. 2008. Rediscovery and redescription of Marmosa (Stegamarmosa) andersoni Pine (Mammalia: Didelphimorphia: Didelphidae), an endemic Peruvian mouse opossum, with a reassessment of its affinities. Zootaxa 1756: 49-61.View paper.

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