ThefamilyCaenolestidae contains the seven surviving species ofshrew opossum: small,shrew-likemarsupials that are confined to theAndes mountains ofSouth America.[1] The order is thought to have diverged from the ancestral marsupial line very early. They were once included in the superorder but it is now known that Ameridelphia isparaphyletic, having given rise toAustralidelphia, and thus could be considered anevolutionary grade.[2] Genetic studies indicate that they are the second mostbasal order of marsupials, after thedidelphimorphs.[2] As recently as 20 million years ago, at least seven genera were in South America. Today, just three genera remain. They live in inaccessible forest and grassland regions of the High Andes.
Shrews were entirely absent from South America until theGreat American Interchange three million years ago, and are currently present only in the northwestern part of the continent. Traditionally, it was thought that shrew opossums lost ground to these and otherplacental invaders that fill the sameecological niches. Evidence suggests, however, that both groups not only overlap, but do not seem to be in direct competition, and the marsupials' larger size seems to imply that they prey on shrews and rodents.[3] Several opossums, such asMonodelphis, also occupy small insectivore niches.
Shrew opossums (also known asrat opossums orcaenolestids) are about the size of a small rat (9–14 cm long), with thin limbs, a long, pointed snout and a slender, hairy tail. They are largely carnivorous, being active hunters ofinsects,earthworms, and small vertebrates. They have small eyes and poor sight, and hunt in the early evening and at night, using their hearing and long, sensitivewhiskers to locate prey. They seem to spend much of their lives in burrows and on surface runways. Like several other marsupials, they do not have a pouch, and it appears that females do not carry the young constantly, possibly leaving them in the burrow.[4]
Largely because of their rugged, inaccessible habitat, they are very poorly known and have traditionally been considered rare. Several ecological factors, including density of forest, contribute to the part of the forests the shrew opossums occupy. Recent studies suggest they may be more common than had been thought. Theirkaryotype has also been described through contemporary research in order to better understand this organism.[5]
However, Bublitz[citation needed] suggested in 1987 there were actually twoLestoros andRhyncholestes species (those listed here plusL. gracilis andR. continentalis). This is, however, not accepted by most scientists.[citation needed]
^Kelt, Douglas A.; Martínez, David R. (1989). "Notes on Distribution and Ecology of Two Marsupials Endemic to the Valdivian Forests of Southern South America".Journal of Mammalogy.70 (1):220–224.doi:10.2307/1381695.JSTOR1381695.
