Dromiciops

The monito del monte (''Dromiciops gliroides''), or colocolo opossum, is a diminutive species of marsupial native only to south-western South America (Argentina and Chile). It is the only extant species in the ancient order Microbiotheria, and the sole New World representative of the superorder Australidelphia, being more closely related to Australian marsupials than to other American marsupials. The species is nocturnal and arboreal, and lives in thickets of South American mountain bamboo in the Valdivian temperate forests of the southern Andes, aided by its partially prehensile tail. It consumes an omnivorous diet based on insects and fruit.

Taxonomy and etymology

''Dromiciops gliroides'' is the sole extant member of the order Microbiotheria. It was first described by British zoologist Oldfield Thomas in 1894. The generic name ''Dromiciops'' is based on the resemblance of the monito del monte to the eastern pygmy possum (''Cercartetus nanus''), one of the synonyms of which is ''Dromicia nana''. The specific name ''gliroides'' is a combination of the Latin ''glis, gliris'' ("dormouse", more generally "rodent") and Greek ''oides'' ("similar to"). The name ''australis'' in a synonym (''D. australis'') refers to the southern distribution of the animal. The common name monito del monte is Spanish for "little monkey of the bush".

In his 1943 ''Mammals of Chile'', American zoologist Wilfred Hudson Osgood identified two subspecies of the monito del monte:

*''Dromiciops gliroides australis'' F. Philippi, 1893: It occurs in the Valdivian temperate rain forest in southcentral Chile.
*''Dromiciops gliroides gliroides'' Thomas, 1894: It occurs in the northeastern Chiloé Island.

Phylogeny and biogeography

South American marsupials have long been suspected to be ancestral to those of Australia, consistent with the fact that the two continents were connected via Antarctica in the early Cenozoic. Australia's earliest known marsupial is ''Djarthia'', a primitive mouse-like animal that lived in the early Eocene about 55 million years ago (mya). ''Djarthia'' had been identified as the earliest known australidelphian, and this research suggested that the monito del monte was the last of a clade that included ''Djarthia''. This relationship suggests that the ancestors of the monito del monte might have reached South America by back-migration from Australia. The time of divergence between the monito del monte and Australian marsupials was estimated to have been 46 mya.

''Dromiciops'' is thought to have evolved from members of the genus '' Microbiotherium'', known from the early Miocene of South America, with some authors considering the genera indistinguishable. All other genera, like '' Pachybiotherium'', had become extinct by the late Miocene.

However, in 2010, analysis of retrotransposon insertion sites in the nuclear DNA of a variety of marsupials, while confirming the placement of the monito del monte in Australidelphia, also clarified that its lineage is the most basal of that superorder. The study further confirmed that the most basal of all marsupial orders are the other two South American lineages (Didelphimorphia and Paucituberculata, with the former probably branching first). This conclusion indicates that Australidelphia arose in South America (along with the ancestors of all other living marsupials), and probably reached Australia in a single dispersal event after Microbiotheria split off. Fossils of another Eocene australidelphian, the microbiotherian '' Woodburnodon casei'', have been described from the Antarctic Peninsula, and fossils of a related early Eocene woodburnodontid have been found in Patagonia.

Habitat

Monitos del monte live in the dense forests of highland Argentina and Chile, mainly in trees, where they construct spherical nests of water-resistant colihue leaves. These leaves are then lined with moss or grass, and placed in well-protected areas of the tree, such as underbrush, tree cavities, or fallen timber. The nests are sometimes covered with gray moss as a form of camouflage. These nests provide the monito del monte with some protection from cold, both when it is active and when it hibernates.

Fragmentation of Valdivian temperate rainforests into non-contiguous areas is known to reduce the abundance of monitos del monte in a given area, but has little or no impact on whether it occurs in an area or not.

Morphology

Monitos del monte are small marsupials that look like mice. ''Dromiciops'' have the same dental formula as ''Didelphids'': , a total of 50 teeth. Their size ranges from . They have short and dense fur that is primarily brown-gray with patches of white at their shoulders and back, and their underside is more of a cream or light gray color. Monitos del monte also have distinct black rings around their eyes. Their small furred ears are well-rounded and their rostrums are short. The head to body length is around , and their tail length is between . Their tails are somewhat prehensile and mostly furred with the exception of of the underside. The naked underside of their tails may contribute to increasing friction when the mammal is on a tree. The base of their tails also functions as a fat storage organ which they use during winter hibernation. In a week, Monitos del monte can store enough fat to double their body size.

Sexual dimorphism

At the end of the summer, female monitos del monte tend to be larger and heavier than males. The tails of the sexes also vary in size during this time; females have a thicker tail, which is where they store fat; the difference suggests that females need more energy than males during hibernation. The sexual dimorphism is only seen during this time and not year-round.

Reproduction

Monitos del monte have a monogamous mating system. The females have well-formed, fur-lined pouch containing four teats. They normally reproduce in the spring once a year and can have a litter size varying from one to five. They can feed a maximum of four offspring, so if there are five young, one will not survive. When the young are mature enough to leave the pouch, approximately five months, they are nursed in a distinctive nest. They are then carried on the mother's back. The young remain in association with the mother after weaning. Males and females both reach sexual maturity after two years.

Habits

The monito del monte is adapted to arboreal life; its tail and paws are prehensile. It is largely nocturnal and, depending on the ambient and internal temperature, and on the availability of food, it spends much of the day in a state of torpor. Such behaviour enables it to survive periods of extreme weather and food shortage, conserving energy instead of foraging to no effect.

The animal covers its nest with moss for concealment, and for insulation and protection from bad weather.

Diet

The monito del monte depends on consuming both insects and fruit, with either component individually being nutritionally unbalanced. Fruit consumed comes from 16 species of plant, with the mistletoe species '' Tristerix corymbosus'' being a preferred source of fruit. A study performed in the temperate forests of southern Argentina showed a mutualistic seed dispersal relationship between ''D. gliroides'' and ''Tristerix corymbosus.'' The monito del monte is the sole dispersal agent for this plant, and without it the plant would likely become extinct. The monito del monte eats the fruit of ''T. corymbosus'', and germination takes place in the gut.

Conservation

For the past few years the number of ''Dromiciops'' has declined, and the species is now classified as "near threatened". Many factors contribute to the decline:

The monito del monte is not the only organism which will be affected if it becomes endangered. ''Dromiciops'' illustrate parasite-host specificity with the tick '' Ixodes neuquenensis''. This tick can only be found on the monito del monte, so it depends on the survival of this nearly endangered mammal. '' T. corymbosus'' also depends on the survival of this species, because without the seed dispersal agency of the monito del monte, it would not be able to reproduce.

Currently, there are minimal conservation efforts. Ecological studies are being conducted in the Chiloé Island that might help future conservation efforts. ''Dromiciops'' has been found in the Los Ruiles National Reserve and the Valdivian Coastal Reserve, which are protected areas in Chile and the Nothofagus forest of Parque Nacional Los Alerces, Chubut, Southern Argentina.

References

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{{Taxonbar, from=Q691991

Microbiotheriidae
Mammals described in 1894
EDGE species
Marsupials of Argentina
Marsupials of Chile
Mammals of Patagonia
Mammals of the Andes
Taxa named by Oldfield Thomas
Fauna of the Valdivian temperate forests