Microbiotheria

Monitos del Monte

(Microbiotheriidae)

Class Mammalia

Order Microbiotheria

Family Microbiotheriidae

Number of families 1

Thumbnail description
Small, mouse-like, South American nocturnal marsupials

Size
Head and body length 3–5 in (8–13 cm); tail length 3.5–5.2 in (9–13 cm); weight 16–42g (0.5–1.4 oz)

Number of genera, species
1 genus; 1 species

Habitat
Occupies dense, humid forest

Conservation status
Vulnerable

Distribution
Chile and Argentina

Evolution and systematics

The mouse-like monito del monte is a small, inconspicuous species of marsupial from South America. Its unprepossessing appearance belies its huge zoological importance. This drab little mammal is a relic of a bygone era—all that remains of a once-prominent group that may have given rise to the entire diverse marsupial fauna that now lives on the other side of the world in Australia. The Microbiotheridae are therefore considered part of the otherwise exclusively Australian group known as the cohort Australidelphia.

Of the seven species of microbiothere that have been described, six members of the genus Microbiotherium are long extinct and are known only from fossil remains. This order of marsupials has only one living family, the Microbiotheriidae, with a single representative, Dromiciops gliroides, the diminutive monito del monte, also known in its native Chile as the colocolo. Dromiciops has several very primitive traits and is only distantly related to other South American marsupials (opossums). In fact, it has more in common with some Australian marsupials, in particular the carnivorous dasyurids, from which it must have been separated for at least 40 million years. The monito del monte is thus thought to be the only surviving member of an early offshoot of the marsupial lineage that lived on the ancient supercontinent of Gondwana, to which Australia, Antarctica, and South America were once joined. A microbiotherian marsupial could have been the ancestor of the diverse marsupial fauna seen in Australia today. While the descendants of ancient micorbiotheres were doing well in Australia, the initially successful marsupial radiation in South America came to an end when placental mammals arrived from the north. Like most other South American marsupial orders, the little microbiotheres were mostly edged out—all but Dromiciops, which alone survives to this day.

The taxonomy of this species is Dromiciops gliroides (Thomas, 1894), Biobio, Chile, "Huite, NE Chiloe Island."

Physical characteristics

The monito del monte is small (head and body less than 5 in [13 cm] long) and superficially mouse-like, with a pointed

face, small rounded ears, and large eyes. Its furry tail is prehensile, up to 5 in (16 cm) long and sometimes very thick, especially around the base where fat accumulates prior to hibernation. There is a patch of naked skin on the underside of the tail tip, which helps improves traction when the tail is used to grasp branches. The paws are similar to those of opossums, each having opposable toes adapted for gripping. The face is short and pointed, with very large eyes ringed with dark fur. The ears are oval and erect. The body is covered in soft brownish gray fur, which fades to buff-white on the belly and sometimes shows a pattern of swirls on the shoulders and back.

Distribution

The monito del monte is found only in the southern Andes mountains between the latitudes of 36° and 43°S in Chile, and just over the border into southwestern Argentina. This very limited distribution makes the Microbiotheria the least widespread of all living mammalian orders.

Habitat

This small, nimble marsupial lives for the most part off the ground, in trees and shrubs and especially in thickets of Chilean Chusquea bamboo. These grow best in the cool, humid forests of the Andes foothills. The species' common name means "mouse of the mountain."

Behavior

Monitos del monte are nocturnal and active most of the year in milder parts of their range. However, prolonged periods of cold winter weather or food shortage induce hibernation during which the animals survive on reserves of fat stored in the base of the tail. They spend most of their life aboveground, climbing with great skill using hands, feet, and tail to grasp branches and stems. They build intricate but sturdy nests of twigs and bamboo leaves woven into a ball. Preferred nest sites are tree holes or dense thickets, but rocky crevices and hollow fallen logs are also used, and nests are sometimes built suspended aboveground in trailing liana vines. The bamboo leaves with which the nests are made are waterproof, so the lining material of soft moss and grass remains dry and snug. Mosses are also sometime used to adorn the outside of the nest as well, helping to make it less conspicuous.

Individuals typically live in pairs or in small groups that usually comprise a mother and up to four young of the current year. Neighbors and family members communicate by sound; the most distinctive call is a long, trilling sound ending in a soft, hoarse cough.

Feeding ecology and diet

Monitos mainly eat insect grubs and pupae, also flies and small lizards that they collect from leaves and from cracks and crevices in tree bark. They grab prey with their nimble hands. They will also eat fruits, especially when preparing to hibernate in the fall.

Reproductive biology

The breeding season is in spring, and males and females mate in October soon after emerging from hibernation. The first litters appear in November, but continue to be born until January. Litters of between one and four young are normal; there are some records of five but, since the female only has four teats, the chances are the fifth will not survive. The young spend the first few weeks of life in their mother's pouch, attached to a teat. As soon as they are old enough to maintain their own body heat, the female leaves them in the nest while she goes out to feed. A little later, the young family accompanies the mother, at first riding on her back, and then learning to climb and feed themselves. Once independent, they often remain close to their mother for the rest of the year. They are sexually mature at one year old.

Conservation status

The monito del monte was once considered a common animal in parts of its range, but it is increasingly under pressure from habitat loss due to deforestation and development. The species' very restricted range means that even small losses can be significant. Having survived apparently little change for at least 20 million years, it would be a tragedy if the monito del monte was to become extinct as a result of human negligence—its loss would represent the demise of an entire order of animals. The species is listed as Vulnerable by the IUCN.

Significance to humans

The monito del monte is the subject of some extreme superstitions. Some Andean people regard it as a bearer of ill fortune. Being nimble and inquisitive, and interested in stored food such as fruit, monitos inevitably enter houses from time to time. They do little real harm, but in some places a "colocolo" indoors is believed to be such bad luck that the only way to avert disaster is to move out and destroy the house completely. In actual fact, these animals probably do more good than harm by feeding on a variety of insect pests.

Resources

Books

Aplin, K. P., and M. Archer. "Recent Advances in Marsupial Systematics with a New Syncretic Classification." In Possums and Opossums: Studies in Evolution. Sydney: Surrey Beatty and Sons & Royal Zoological Society of New South Wales, 1987.

Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. Mammalogy: Adaptation, Diversity, and Ecology. Boston: WCB McGraw-Hill, 1999.

Nowak, R. "Order Microbiotheria." In Walker's Mammals of the World. Vol. I, 6th ed. Baltimore and London: Johns Hopkins University Press, 1999.

Amy-Jane Beer, PhD