The Mouse Shrews of Africa

Shrews are one of the less appreciated groups of mammals. Small (some are among the smallest mammals on earth), skulking, they are often overlooked but are nevertheless represented by a diversity of species in many parts of the world. Among this diversity are the mouse shrews of the genus Myosorex.

Forest shrew Myosorex varius, from Roberts (1951).

Myosorex is a genus of nearly twenty known species of shrew, some of which have only been identified very recently, with more probably yet to be described. They are known in the modern fauna only from sub-Saharan Africa, though the fossil record indicates they once extended as far north as Spain (Furió et al. 2007). Mouse shrews differ from most other living shrew genera (except the closely related Congosorex) in the presence of a tiny vestigial tooth in the lower jaw behind the first antemolar (the tooth behind the incisors, so called in shrews because it is unclear whether it corresponds to a canine or premolar in relation to the teeth of other mammals). Because their teeth lack the red pigment found in shrews of the subfamily Soricinae, Myosorex have historically been classified with the white-tooth shrews of the Crocidurinae. However, the presence of white teeth is, of course, a primitive feature of questionable significance phylogenetically. Instead, more recent authors have pointed to the retention of the second antemolar and other features to support recognising Myosorex and related African shrew genera as relictual members of a third subfamily, the Myosoricinae, that may also include a number of earlier fossil shrews (this group has also been known as the 'Crocidosoricinae' but 'Myosoricinae' is the name with priority; the argument by Furió et al., 2007, that the latter name cannot be used for the broader subfamily because it was originally used only for the African genera has no standing under current nomenclatorial rules).

Foraging forest shrew, copyright Johnny Wilson.

In the modern fauna, Myosorex species have a very scattered distribution. Species found in central and eastern Africa are restricted to high mountains, among moist, densely vegetated environments. Species found in southern Africa are often found in similar habitats. However, the species M. varius is also found in drier locations at lower altitudes, closer to the South African coast. Nevertheless, it is still restricted to areas with high seasonal rainfall, or (in part of Western Cape Province) zones dominated by succulent vegetation where low levels of actual rainfall may be compensated for by precipitation from mist (Meester 1958). The distribution of the genus as a whole is marked by a broad gap of over 1600 km separating the northern limit of species in South Africa and Zimbabwe from their nearest neighbours to the north in the DRC and Kenya, a gap that they were presumably only able to cross in the past when climate conditions were more amenable.

This sensitivity to environment means that Myosorex species may be very vulnerable to changes in habitat. Several species are restricted to limited ranges and several are recognised as potentially endangered. The prospect of climate change makes this vulnerability even worse: as levels of rainfall decrease, mouse shrews will be forced to retreat to ever higher altitudes, and there's only so high they can go before running out of mountain.

REFERENCES

Furió, M., A. Santos-Cubedo, R. Minwer-Barakat & J. Agustí. 2007. Evolutionary history of the African soricid Myosorex (Insectivora, Mammalia) out of Africa. Journal of Vertebrate Paleontology 27 (4): 1018–1032.

Meester, J. 1958. Variation in the shrew genus Myosorex in southern Africa. Journal of Mammalogy 39 (3): 325–339.