Paucituberculata

Shrew opossums

(Caenolestidae)

Class Mammalia

Order Paucituberculata

Family Caenolestidae

Number of families 1

Thumbnail description
Small, shrewlike animals with small eyes, shaggy fur, and a long tail; females lack a pouch

Size
Head-body 3.5–5 in (9–13 cm); tail 2.5–5 in (6.5–13 cm); weight 0.7–1.5 oz (20–40 g)

Number of genera, species
2 genera; 5 species

Habitat
Temperate rainforest and alpine scrub bordering high altitude paramo meadow

Conservation status
Vulnerable: 1 species

Distribution
Western South America, including parts of Ecuador, Colombia, Venezuela, Peru, Chile, and Argentina

Evolution and systematics

The order Paucituberculata is represented by just a single living family, the Caenolestidae, with just two genera. The handful of living species is all that remains of what appears to have been a once-abundant marsupial dynasty during the Oligocene epoch, approximately 25 million years ago. There are seven extinct families, some of which were described from fossils before the living caenolestids were discovered, including the Groberiidae. Some of the earliest fossils, members of the family Polydolopidae, date back to the Palaeocene, more than 60 million years ago.

The decline of the paucituberculates began in the Oligocene, and it gathered pace in the Miocene, when the continents of North and South America were briefly joined. There was an influx of placental mammals from the north into what had been for many millions of years a bastion of marsupial diversity. The newcomers included rodents and primates whose descendants have since thrived at the expense of many ousted marsupials.

A number of similarities with other American marsupials has led some authorities to consider the shrew opossums to be no more than a subgroup of the order Didelphimorphia. However, molecular evidence supports the classification used here—it may be that at one time the Paucituberculata were as diverse as the extant Australian order Diprotodontia.

Until recently, there were thought to be three living caenolestid genera. The third, Lestoros, contained the species, L. inca, and is now considered part of the larger genus, Caenolestes.

Physical characteristics

Shrew opossums are all rather similar looking. The largest specimens (usually males) are no more than 10 in (25 cm) long, half of which is tail, except in the Chilean shrew opossum, whose tail is relatively short. The face is long and tapering, with a pointed snout, long whiskers, and very small eyes. The ears are quite large and project well beyond the animal's fur. The overall appearance is comparable to a rat or overgrown shrew.

Distribution

The living shrew opossum species all derive from western South America, from the coast to the high Andes.

Habitat

The preferred habit of most shrew opossums is densely vegetated and humid–temperate rainforest, montane woodland, or the lush high altitude meadows of the Andean paramo. Such habitats offer cover and an abundance of suitable nest sites. They also support a rich invertebrate fauna and thus present good feeding opportunities.

Behavior

The shrew opossums' small size and the remote nature of their habitats mean they have little contact with humans; consequently, details of their ecology and behavior are not well known. They are all largely nocturnal or crepuscular, and spend the day resting in hollow logs or holes around the roots of trees. They appear to live alone, and travel around the home range on regular runways. They are not territorial, and more than one individual use the paths. They are proficient climbers, and will scramble into the branches of shrubs, using the tail as a prop and a counterbalance, but not for grasping.

Feeding ecology and diet

The diet is predominantly insects and other invertebrates such as earthworms, which the shrew opossums find by rummaging in the surface litter and investigating likely nooks and crannies along their regular runways. Like placental shrews, they hunt mainly by smell; they also have sharp hearing, but their eyesight is relatively poor. Various species have also been reported feeding opportunistically on fruit, scavenging the flesh of dead animals, and killing and eating the young of other mammals such as rats found in nests.

No torpid or hibernating specimens have been recorded, but there is good evidence that shrew opossums can become torpid in times of food shortage. The Chilean shrew opossum builds up reserves of fat in its tail in late summer, which help it survive the cold Andean winter.

Reproductive biology

Very little is known about caenolestid reproduction, including mating system. Normal litter sizes can be guessed at from the number of teats—four in members of the genus, Caenolestes, five or seven in the Rhyncholestes species. Females do not have a pouch, so nursing young must latch on firmly to the mother's teat in order to survive. Once milk is flowing, the teat swells in the infant's mouth so that it cannot easily become detached. When the young become too big to be carried beneath the mother's body, she presumably leaves them in a nest while she goes out to feed, returning periodically to suckle them.

Conservation status

The Chilean shrew opossum (Rhyncholestes raphanurus) is listed as Vulnerable by the IUCN. Information regarding shrew opossum population density is very limited, since few specimens have ever been collected and field sightings are not well known. However, the temperate rainforest in which the Chilean shrew opossum lives is shrinking quickly, and other species in the same habitat have undergone a significant decline. Caenolestes shrew opossums are probably more secure.

Significance to humans

None known.

Species Accounts

List of Species

Silky shrew opossum

Caenolestes fuliginosus

taxonomy

Hyracodon fuliginosus (Tomes, 1863), Ecuador.

other common names

English: Ecuador shrew opossum; French: Caenolestidé d'Ecuador; German: Ekuador-Opossumaus; Spanish: Ratón musarana de los Andes.

physical characteristics

Head and body 3.5–5.5 in (9–13 cm) long; tail 3.5–5.5 in (9–13 cm) long; fur brown, soft, and shaggy, toes bear small sharp claws.

distribution

Alpine regions of northern Colombia, Ecuador, and western Venezuela.

habitat

Cool, humid thickets in alpine forests and meadows.

behavior

Nocturnal, terrestrial, fast-moving; an excellent climber.

feeding ecology and diet

Mostly insect larvae, will also take young vertebrate prey and carrion.

reproductive biology

Not well known, but young are probably born between June and September in litters of two to four.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Chilean shrew opossum

Rhyncholestes raphanurus

taxonomy

Rhyncholestes raphanurus Osgood, 1924, Chiloe Island, Biobio, Chile.

other common names

French: Caenolestidé du Chili; German: Chile-Opossumaus; Spanish: Comadrejita trompuda.

physical characteristics

Head and body 4–5 in (10–13 cm) long; tail 2.5–3.5 in (6–9 cm); may fatten prior to onset of winter. Female has five or seven teats.

distribution

South central Chile and Chiloe Island.

habitat

Temperate rainforest.

behavior

Nocturnal, terrestrial, burrows through surface litter; may enter torpor in winter.

feeding ecology and diet

Insects, worms, and other invertebrates caught by rummaging in topsoil and leaf litter.

reproductive biology

Not well understood, but females are apparently capable of breeding at any time of year.

conservation status

Listed as Vulnerable by IUCN.

significance to humans

None known.

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, Australia: 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.

Macdonald, D. The New Encyclopedia of Mammals. Oxford University Press, 2001.

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

Amy-Jane Beer, PhD