The Monito del Monte ("little mountain monkey," Dromiciops australis) is a semi-arboreal South American marsupial which is thought to be more closely related to the marsupials of Australasia than to those of the Americas. Also known as the Colocolo, it is only a little larger than a mouse: about 8 to 13 cm long with a thick-based, moderately prehensile tail about the same length again. Weight varies between 17 and 31 grams. It has a coat of short, dense, silky fur, brown on the upper side with a number of ashy white patches, and paler underneath. The ears are short and rounded, and there are black rings around the eyes.
Monitos del Monte are found only in the mountains of Chile and Argentina, preferring dense, humid forests, particularly where there are areas of Chilean Bamboo. It is reported to be reasonably common within its restricted range. Pairs make nests of leaves and sticks, about 20 cm in diameter and lined with grass or moss, in a variety of places: under rocks, in hollow trees, on branches, or suspended in thick shrubbery.
Monitos del Monte are largely carnivorous. Most of their diet is insects and other small invertebrates, although they may also take some fruit. They are nocturnal, and excellent climbers with both feet and tail, but equally at home on the ground. They have an extraordinary ability to rapidly accumulate a reserve of fat in the tail, being able to double their body weight within a week. This reserve is enough to sustain them through periods of cold weather, during which they hibernate.
Mating takes place in the spring and early summer, and results in a litter of between one and five young, which are carried in the well-developed pouch. Sexual maturity is reached in the second year.
The Monito del Monte is the only member of its family (Microbiotheriidae) and the only surviving member of an ancient order, the Microbiotheria, which is known from Oligocene and Miocene times in South America. Although once thought to be a member of the Didelphidae (the order that contains the Virginia Opossum), an accumulation of both anatomical and molecular evidence in recent years has led to the conclusion that this tiny South American marsupial is not a member of the Ameridelphia (the magnorder that contains all other American marsupials) at all, but is instead most closely related to the Australidelphia, or Australasian marsupials. The distant ancestors of the Monito del Monte, it is thought, either remained in what is now South America while others entered Antarctica and eventually Australia during the time when all three continents were joned as part of Gondwana, or else were part of the Australian marsupial fauna which reinvaded South America.