Shrew-opossums, our strangest mammals

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The shrew-opossum Caenolestes sangay, not exactly cute and cuddly! Photo: Jorge Brito.

Most mammals, including us, are placental mammals. There are two smaller groups of mammals: egg-laying monotremes like the platypus, and marsupials like the opossum and kangaroo. These groups diverged more than a hundred million years ago from the lineage that became the placental mammals, and though they are minor players in the world today, both were more important in the distant past.  Marsupials in particular were once much more important and much more diverse. Marsupials apparently originated in the northern continent that became Asia and North America. About 65Mya marsupials moved from North America into South America, which at this time was also connected to Antarctica and Australia. Around 50-35Mya, at least one species of marsupial made it to what is now Australia via Antarctica, setting the stage for the later diversification of marsupials on that continent as it moved away from Antarctica and into its splendid isolation in the remoteness of the Pacific Ocean.

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South American saber-toothed marsupial carnivore Thylacosmilus. Photo: Wikipedia CC.

Fossil evidence shows that ancient South America of 10-40Mya had a rich and ecologically diverse marsupial fauna. Some of them were the size of bears, and others were large predators with two saber-like teeth like those of the famous saber-toothed cats. Some were hopping animals similar to the kangaroo rat, some resembled the present-day North American opossum, and some were arboreal animals resembling primates. There was also a rich and varied group of small and mid-sized rat-like marsupials belonging to the order Paucituberculata, which included both carnivorous and plant-eating genera.

Over time, these strange marsupials slowly disappeared. Only a few species in the order Paucituberculata, and one species (or species complex) in the order Microbiotheria (which may have been  a reverse migrant from the early marsupial diversification in Australia), survive today.

Our reserves protect two of these survivors, the “shrew-opossums” Caenolestes convelatus in our Dracula Reserve and Caenolestes sangay in our Cerro Candelaria Reserve (see Technical Note 1 below). Both shrew-opossums are in the order Paucituberculata and both are mainly predators, feeding on insects, other arthropods, worms, frogs, and small mammals, but they also sometimes eat fruit and fungi. They have two distinctive lower incisors that point straight ahead, like daggers. Caenolestes sangay is a new species described in 2013 by a group of scientists that included our collaborator Jorge Brito. It is exciting to add a previously unknown descendant of this lonely lineage, which diverged from other marsupials 55Mya.

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Caenolestes sangay skull, note the dagger-like lower incisors. From Ojala-Barbour et al (2013) A new species of shrew-opossum (Paucituberculata: Caenolestidae) with a phylogeny of extant caenolestids, Journal of Mammology 94: 967-982.

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The shrew-opossum Caenolestes sangay. Photo: Jorge Brito.

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The shrew-opossum from our Dracula Reserve, Caenolestes convelatus. Photo: Jorge Brito.

In our Dracula and Cerro Candelaria reserves, the resident species of Caenolestes is the sole representative of its order, and this makes its conservation especially important. Conservationists tend to think in terms of species diversity, but we should also pay attention to higher-level diversity. All else being equal, a reserve that contained sloths, manatees, monkeys, bats, and deer would be far more important than a reserve that protected only a set of rodents, even if the number of species were the same in each of the two reserves. A reserve with one species of rat and one species of shrew-opossum is far more diverse and important than an otherwise identical reserve with two species of rat and no species of shrew-opossum. The first reserve protects more unique evolutionary history than the second. I believe this should be the guiding principle of conservation: maximize the amount of unique evolutionary history protected.

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Phylogenetic tree of the major mammal groups (orders). The order Paucituberculata, which contains the shrew-opossums, is highlighted in red. Modified from https://www.palaeontologyonline.com/articles/2012/fossil-focus-marsupials/

The amount of unique evolutionary history represented in a given locality is called its “phylogenetic diversity”. In this age of DNA analysis we have reasonably accurate phylogenetic trees for many plant and animal groups. For any given natural group — mammals, for example — the simplest measure of the amount of unique evolutionary history protected at a locality is the total length of all the branches in the phylogenetic tree (including the “trunk” that connects the group to the rest of the organisms in the reserve) of the species found there (see Technical Note 2 for other ways of measuring this). In the case of our shrew-opossum, it has been evolving on its own unique branch for at least 55 million years, so it contributes quite a lot of  phylogenetic diversity to our Cerro Candelaria and Dracula reserves. The shrew-opossums are among the most interesting mammals in our reserves, even though almost no one has ever heard of them.

Lou Jost, Fundacion EcoMinga

Technical notes:

  1. The name “Shrew-opossum” can be misleading. Strictly speakimg, the opossums are marsupials in a different order than this animal. I think a better English name for these would be “marsupial shrew”.
  1. My colleagues Anne Chao, CH Chiu, and I have developed some more advanced measures of phylogenetic diversity and differentiation: Chao A, Chiu CH, Jost L (2010) Phylogenetic diversity measures based on Hill numbers, Philosophical Transactions of the Royal Society B 365:3599–3609 https://www.researchgate.net/publication/47566303_Phylogenetic_diversity_measures_based_on_Hill_numbers

3 thoughts on “Shrew-opossums, our strangest mammals

  1. Love the article! Also, the first photo and caption gave me a good chuckle. Viva la marsupial shrew! Gracias para todo ustedes hacen a cuidar la planeta Fundacion Ecominga.

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