SHREW OPOSSUMS: Paucituberculata

PHYSICAL CHARACTERISTICS

The New World marsupial order Paucituberculata contains only one surviving family, the Caenolestidae, or shrew opossums (the true shrews, family Soricidae, are not marsupial but placental mammals). Shrew opossums are mouse-sized marsupial mammals with dense, dark gray or brown fur. Also called mouse opossums and rat opossums, their appearance is more suggestive of shrews. Shrew opossums have long, conical, pointed snouts and tiny, beady eyes. There are three genera (JEN-uh-rah; plural of genus, JEE-nus) and five species: the living members of order Paucituberculata and family Caenolestidae are relicts, little-changed survivors of a once far more diverse assemblage of genera and species. Information about them is spotty, since the animals are nocturnal and secretive, and live for the most part in remote, difficult terrain.

Head and body length across species runs from 3.5 to 5.5 inches (9 to 14 centimeters), and tail length is 3.5 to 5.5 inches (9 to 14 centimeters), the tail length being about the same as the head and body. Males are larger than females, males weighing from 1 to 1.5 ounces (25 to 41 grams), females from 0.5 to 1 ounce (16.5 to 25.5 grams). A shrew opossum has five digits on each foot, the two small outer toes of the forefeet with blunt nails, the remaining three equipped with curved, sharp, strong claws. The hind feet have strong, curved claws on all toes except the so-called great toe, which is small and carries a small nail.

The fur of shrew opossums is thick and soft, covering the entire body and less dense on the tail. The coat has a disorderly look because different areas of the coat have different textures. The fur may be dark on the dorsal (upper) parts of the body, with lighter-colored under parts, or dark all over the body. Dark colors vary from gray-brown, black-brown, to near-black. The ears are shaped much like those of typical mice, and large enough to protrude well above the thick fur. Hearing is acute, as is the tactile (touch) sense of the long whiskers.

The upper lips bear small flaps of skin on both sides, a feature found only in the Caenolestidae. The Chilean opossum has these and similar flaps on its lower lips. Their function is, so far, unknown. They may be barriers to prevent blood, pieces of flesh, and dirt from collecting on the sides of the jaws. The tail is about as long as the combined head and body. The tail fur, sparser than on the body, is the same color as the upper pelt, but may include a white tip in some individuals. The tail is not prehensile, meaning it is not able to curl around and grasp objects. The tail of the Chilean shrew opossum swells up with stored fat for the southern winter months. Females do not have pouches and have four nipples, except for females of the Chilean shrew opossum, which have seven nipples, the seventh located on the midline of the underbelly.

The rostrum, or the front part of the skull including the jaws, is long and tapering. Each of the lower incisors has only one cusp, or protruding bump on its crown, unlike most mammal teeth, hence the order name, Paucituberculata, meaning, in Latin, "few bumps," since this feature is found in all species, living and extinct, in the order.

GEOGRAPHIC RANGE

The gray-bellied shrew opossum, the blackish shrew opossum, and the silky shrew opossum are found in isolated, separated populations in the mountains of the Western Andes, from Colombia and Venezuela in the north and southwards through Ecuador, Peru, and Argentina. The Incan shrew opossum inhabits southern Peru, while the Chilean shrew opossum lives along the south-central coast of Chile and on the close offshore Chilean island of Chiloé, plus another population in Argentina.

HABITAT

The gray-bellied shrew opossum, the blackish shrew opossum, the silky shrew opossum, and the Incan shrew opossum live in dense vegetation in cool, rainy mountain forests and meadows, from 4,500 to 12,000 feet (1,500 to 4,000 meters) above sea level. The Chilean shrew opossum inhabits rainy lowland temperate rainforest along the Chilean coast from sea level to 2,270 feet (1,135 meters), preferring dense forest with mossy trees and logs, and soaking wet forest floors.

DYNASTY IN EXILE

Like the few poor refugees of a downfallen, once mighty civilization, the five living shrew opossum species, lurking in the undergrowth of temperate forests in South America, are but a pale afterglow of their former splendor. From seventeen to twenty-four million years ago, the order Paucituberculata included four other families besides Caenolestidae, while the caenolestids were the most abundant marsupials in terms of species during that era. The extinct Paucituberculata species had teeth specialized in unique ways, and lived in both temperate and tropical biomes throughout most of the South American continent.

The extinct families of Paucituberculata are Paleothentidae, Abderitidae, Polydolopidae, and Argyrolagidae. Species of Paleothentinae had developed an enlarged forward lower molar with a shearing crest. Abderitinae species took that molar further and equipped it with a forward-facing, sharp, high, serrated blade. The species of Polydolopidae carried a number of enlarged, forward-directed lower incisors and a pair of large shearing teeth of uncertain origin. The Polydolopidae were small marsupials, the largest no bigger than a rabbit, that behaved and were equipped like rodents. The rodent-like Argyrolagidae resembled small kangaroos, with two-toed hind feet and a long, well-muscled tail. They moved about by hopping and leaping, much like kangaroos. Family Paleothentidae evolved an impressive nine genera and nineteen species.

DIET

Shrew opossums forage nocturnally, at night, on the ground, and are carnivorous (meat-eaters) with some herbivory (plant-eating). They eat insects, earthworms, small vertebrates, fruits, other plant food, and fungi, in forest floor growth and in alpine meadows, traveling among feeding areas by means of trails through ground vegetation that they maintain by constant use. A shrew opossum uses its lower canines to stab and skewer prey, then uses its sharp premolars to slice the prey into pieces.

BEHAVIOR AND REPRODUCTION

Little is known about the particulars of breeding among shrew opossum species. Field researchers have found lactating (producing milk for young) females of the Chilean shrew opossum in February, March, May, October, November, and December, suggesting a breeding season from December through May, and no breeding from June through September, the coldest months in the Southern Hemisphere. Breeding season for the silky shrew opossum, which lives in a less stressful climate, is thought to begin in July.

Since shrew opossums are marsupials, the unborn young remain in the females' uterus (YOO-ter-us; womb) only a few days, then are born in an incomplete state, to be suckled by the mother until they complete development. Suckling shrew opossums cling to their mother as she moves about. Litters probably number up to four individuals.

If alarmed, a shrew opossum will hop forward repeatedly on all fours, a mode of locomotion unique to the Caenolestidae. Shrew opossums have also been observed climbing trees, though not foraging in trees. The animals rest during the day in hollow logs and burrows. Despite their fattened tails, Chilean shrew opossums have been observed running across packed snow in midwinter.

SHREW OPOSSUMS AND PEOPLE

Tiny, secretive, and living in remote regions, shrew opossums have very little interaction with humanity and pose no threats.

CONSERVATION STATUS

The World Conservation Union (IUCN) lists the Chilean shrew opossum as Vulnerable (facing a high risk of extinction), due to deforestation (removing trees) from logging.

SILKY SHREW OPOSSUM (Caenolestes fuliginosus): SPECIES ACCOUNT

Physical characteristics: The silky shrew opossum is probably the best known of the shrew opossum species. Its head and body length ranges from 3.7 to 5.3 inches (9.3 to 13.5 centimeters), the tail 3.7 to 5.3 inches (9.3 to 12.7 centimeters). The fur on the dorsal (back) body is soft and thick, and colored a dark brown gradually giving way to lighter brown on the lower body and underbelly.

Geographic range: The silky shrew opossum inhabits the western Andes of northern and western Colombia, extreme western Venezuela, and Ecuador.

Habitat: This shrew opossum is nocturnal and terrestrial, preferring cool, wet areas with heavy vegetation. The species is found in alpine scrub forests and meadow zones of the Andes, at altitudes from 4,500 to 12,000 feet (1,500 to 4,000 meters).

Diet: Silky shrew opossums eat mostly caterpillars, centipedes, and spiders, varied with fruit.

Behavior and reproduction: The breeding season is believed to be July, because animals caught in August were suckling (nursing, or feeding breast milk) their young.

Silky shrew opossums run by bounding, front feet and rear feet working as units and alternating. If threatened, an individual will open its jaws wide and hiss. The tail is not prehensile (able to grab or hold things), but the animal will use it as a sort of third leg when sitting upright.

Silky shrew opossums and people: There is little to no interaction between silky shrew opossums and humans.

Conservation status: The silky shrew opossum has no special conservation status. ∎

FOR MORE INFORMATION

Books:

Lee, Anthony K., and Andrew Cockburn. Evolutionary Ecology of Marsupials. New York: Cambridge University Press, 1985.

Nowak, Ronald M. Walker's Mammals of the World, 6th ed. Baltimore: Johns Hopkins University Press, 1999.

Simpson, George Gaylord. Splendid Isolation: The Curious History of South American Mammals. New Haven, CT: Yale University Press, 1980.

Szalay, Frederick S. Evolutionary History of the Marsupials and an Analysis of Osteological Characters. New York: Cambridge University Press, 1995.

Periodicals:

Bown, T. M., and J. G. Fleagle. "New Colhuehuapian and Santacrucian Microbiotheriidae and Caenolestidae From Patagonian Argentina." Journal of Vertebrate Paleontology 14, no. 3 (1994): 18.

Cifelli, Richard L., and Brian M. Davis. "Marsupial Origins." (Paleontology). Science 302, no. 5652 (2003): 1899–1900.

Flynn, J. J., and A. R. Wyss. "New Marsupials From the Eocene-Oligocene Transition of the Andean Main Range, Chile." Journal of Vertebrate Paleontology 19, no. 3 (1999): 533–549.

Marshall, L. G. "Systematics of the South American Marsupial Family Caenolestidae." Fieldiana Geology (1980): 145.

Meserve, P. L., , B. K. Lang, and B. D. Patterson. "Trophic Relations of Small Mammals in a Chilean Temperate Rainforest." Journal of Mammalogy 69 (1988): 721–730.

Patterson, B. D. "Dominance of South American Marsupials (Scientific Correspondence)." Nature 337, no. 6204 (1989): 215.

Patterson, B. D., and M. H. Gallardo. "Rhyncholestes raphanurus." Mammalian Species 286 (1987): 1–5.

Sanchez-Villagra, M. R. "The Phylogenetic Relationships of Argyrolagid Marsupials." Zoological Journal of the Linnean Society 131, no. 4 (2001): 48–496.