You never really hear much about shrew-opossums or rat-opossums, the small group of living, South American marsupials properly called caenolestids or caenolestoids. Small (c 20-30 cm long in total), long-tailed, mostly dark brown, and predominantly faunivorous and nocturnal, they inhabit the grasslands and forests of the western side of the Andes. They’re said to be semi-fossorial and well able to dig burrows with their strong claws. Incidentally, the most frequently used name for the group – shrew-opossums – might not be a particularly good one, seeing as they don’t look much like shrews, don’t live like shrews, and don’t act like shrews. And they're not technically opossums, either, but perhaps we can let that go.

All members of this group are included in the one family-level clade Caenolestidae which is itself included within the higher-level groups Caenolestoidea and Paucituberculata. Paucituberculata was conventionally taken to include caenolestids alone but the extinct palaeothentoids and several additional taxa also appear to be part of the clade (Goin et al. 2009). If you’re wondering what the hell a palaeothentoid is... I suppose I’ll have to come back to them at another time.

Caenolestidae is not a large group: there are only three extant genera (Caenolestes, Lestoros and Rhyncholestes), the first of which contains about five species while the other two are monospecific [adjacent photo of Rhyncholestes uploaded by Kennethgrima]. The Caenolestes species inhabit Venezuela, Colombia, Ecuador and extreme northern Peru, Lestoros inhabits southern Peru, and Rhyncholestes occurs in southern Chile and Argentina (Albuja & Patterson 1996). Two are recently described: C. condorensis Albuja & Patterson, 1996 and C. sangay Ojala-Barbour et al., 2013. Several fossil taxa are known, the oldest of which is Stilotherium from the Middle Miocene (Goin et al 2007).

Shrew-opossums are poorly known as goes ecology and behaviour. Like all groups for which this is the case, the same small handful of facts tend to be repeated whenever the animals concerned are discussed. So, we’re always told that the shrew-opossum snout is long, that they have a large number of tactile whiskers, and that their eyes are proportionally small and that their vision is poor. Then there are their teeth: they have a reduced number of incisors (four, as opposed to five, are present on each side), the middle two of which in the lower jaw are extraordinarily large and procumbent (that is, they project forwards and outwards). These teeth are sometimes termed ‘gliriform’, meaning that they resemble those of rodents. These gliriform teeth have been mentioned a few times in discussions of other animals that also have procumbent anterior teeth (example: Madagascan Cretaceous theropod Masiakasaurus). How do shrew-opossums use those incredible teeth? I’m not sure that anyone knows but the idea that they might be used to impale worms and other prey has been mooted. Obvious gaps are present between the upper canines and premolars.

Arthropods (including caterpillars, centipedes and arachnids) are among the most abundant recorded prey items, but they’re also known to eat worms, fungi and fruit. Some species also prey occasionally on vertebrates: one C. caniventer specimen ate a bird (Patterson 2008), C. fuliginosus has been reported to “efficiently kill newborn rats” (Nowak 1999) and the possibility that they might prey on sympatric shrews and other small mammals has been mentioned. I’m sure I read of a case where a Caenolestes killed and ate a grass mouse (Akodon) but cannot find it in the literature right now.

A comment made in an oft-used review (O’Connell 2001) and paraphrased in several other sources is that shrew-opossums have proved more common than expected when mammalogists make a point of looking for them. The origin (or, one of the origins) of this contention comes from Albuja & Patterson (1996), who wrote “... when suitable habitats have been sampled, caenolestids are not difficult to collect and are often as abundant and readily caught as other sympatric small mammals” (p. 41).

What else do we know about shrew-opossums? They’re sexually dimorphic, males being larger than females [adjacent photo of Lestoros uploaded by Kennethgrima]. The stomach is divided, containing three compartments (Patterson 2008). Now, you may not have had much cause to ever think about the morphology of the marsupial stomach, and I’ll admit that I haven’t either... but, while I know that kangaroos of various sorts have complex stomachs with two major compartments, I wonder how any of this compares to what we know of marsupial stomachs in general. And, hey, why would faunivorous animals that mostly eat invertebrates need a multi-chambered stomach in any case? As for other aspects of soft tissue anatomy, they also have weird lips with strange lateral flaps; meanwhile, males have a corkscrew-shaped, bifid penis (Patterson 2008). Bifid penises are all over the place in marsupials. Oh, there is no pouch.

A bounding form of locomotion has been documented in the Incan shrew-opossum L. inca, and – in at least some species – the tail is used as a fat store during the winter, expanding notably in size as this fat is deposited.

In the several Caenolestes species, those inhabiting cool highlands are relatively gracile and have a silky pelage while those from subtropical, more lowland places are more robust and with a coarser pelage (Albuja & Patterson 1996). The staphylinid beetle Chilamblyopinus piceus has a (probably mutualistic) relationship with Rhyncholestes, just as it does with the rodent Akodon.

While (as usual) there’s lot more to say, a final point worth noting is that there are surely a great many physiological, morphological, behavioural and ecological peculiarities and specialisations left to discover in these animals. Remember that they are not just generalist, bug-eating ‘marsupial rats’, but that some species are peculiar, anatomically weird denizens of cool highlands, and even of elfin forests, temperate rainforests, and heath-type uplands where the substrate is formed of Cretaceous ash.

Tet Zoo does not (yet) have a particularly good back-catalogue of articles on marsupials/metatherians, but see...

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Albuja V. L. & Patterson, B. D. 1996. A new species of northern shrew-opossum (Paucituberculata: Caenolestidae) from the Cordillera del Condor, Ecuador. Journal of Mammalogy 77, 41-53.

Goin, F. J., Candela, A. M., Abello, M. A., & Oliveria, E. V. 2009. Earliest South American paucituberculatans and their significance in understanding of ‘pseudodiprotodont' marsupial radiations. Zoological Journal of the Linnean Society 155, 867-884.

Goin, F. J., Sánchez-Villagra, M. R., Abello, A. & Kay, R. F. 2007. A new generalized paucituberculatan marsupial from the Oligocene of Bolivia and the origin of ‘shrew-like' opossums. Palaeontology 50, 1267-1276.

O’Connell, M. A. 2001. American opossums. In MacDonald, D. (ed) The New Encyclopedia of Mammals. Oxford University Press, Oxford, pp. 808-813.

Nowak, R. M. 1999. Walker’s Mammals of the World, Volume II (Sixth Edition). The John Hopkins University Press, Baltimore and London.

Patterson, B. D. 2008. Order Paucituberculata Ameghino, 1894. In Gardner, A. L. (ed) Mammals of South America, Volume 1: Marsupials, Xenarthrans, Shrews, and Bats. University of Chicago Press, Chicago, pp. 119-124.