The Tet Zoo rodent t-shirt. I says 'One in 2.2 mammals is a rodent. DEAL WITH IT'.

Sometimes, I pick up Volume II of Walker’s Mammals of the World, go to page 1400 or 1500 or thereabouts and look at all the obscure Old World rats and mice. You might have done the same thing. If you have, you might have been left with the same general feeling as me: of being overwhelmed by the sheer number and variety of all those muroid rodents, and the wish that you knew these animals off by heart, and knew more about their biology, behaviour and evolutionary history. One of my aims over recent months has been to cover more rodents at Tet Zoo. I’ve mostly failed, but here’s yet another effort to redress the balance. Oh, and – if you aren’t much interested in rodents, this article isn’t for you. Nor are the other ones that will be devoted to other obscure rodents. If you do like rodents, maybe you should purchase the tasteful Tet Zoo t-shirt shown here.

Here, we’re going to look at the small number of muroid species grouped together in Cricetomyinae, the so-called African pouched rats. The pouches, of course, are cheek pouches, used by the rats to transport food and bedding material. And some of these rats are very good at transporting things. A nest belonging to a Greater long-tailed pouched rat Beamys major contained 1383 Mwabve tree seeds, the whole lot weighing 1.2 kg. Seeing as B. major can carry eight Mwabwe seeds at a time (Hanney 1977), this collection represents over 170 collecting trips. The cheek pouches of cricetomyines have seemingly evolved independently of those present in hamsters and other rodent groups. The cheek pouches of Cricetomys (the giant pouched rats) are sometimes inflated with air when the animals are acting defensively.

Beautiful rendition of Cricetomys gambianus by Willems van der Merwe, used with permission.

Classic or ‘core’ cricetomyine species are strongly nocturnal, greyish or brownish and slow-moving, forest-dwelling rodents that are good at climbing. The namesake of the group – Cricetomys – is by far the best known member, and it’s relatively familiar as African muroids go. Two species are most often recognised in Cricetomys: the Gambian pouched rat C. gambianus (occurring from Senegal and Sudan south to South Africa) and Emin’s pouched rat C. emini (occurring from Sierra Leone to Lake Tanganika in the east; it’s also present on Bioko). Two additional species are recognised by some authors (the Southern giant pouched rat C. ansorgei and Kivu pouched rat C. kivuensis). These are big rodents, reaching 45 cm in body length and 46 cm in tail length, and with big males reaching 2.8 kg (Nowak 1999). The eyes are proportionally small, the head long and narrow, and the tail is long and scaly. They are omnivorous, eating everything from palm fruits and kernels to snails, insects and crabs.

A tame, trained giant pouched rat being hand-fed a reward. Image by Gooutside; in public domain.

An especially interesting aspect of Cricetomys biology is the fact that it has become a human commensal in places, inhabiting sewers alongside Rattus species. They have even entered houses and, reportedly, attacked and even killed human children on a few occasions (there are two 2011 reports from Johannesburg where children were either injured or killed and partially eaten). The exceptional sense of smell and tameability of these rodents is well known: many articles online explain how they’ve been trained to seek out landmines while others have been trained to recognise medical conditions, like tuberculosis.

Beamys, the lesser pouched rats, long-tailed pouched rats or hamster-rats, is another member of Cricetomyinae and is not all that different in overall appearance from Cricetomys. Only one species of Beamys was recognised until recently (this being B. hindei); a second species (B. major, named in 1914) was recently resurrected from synonymy. Beamys is a mid-sized rodent (in total length, the largest individuals are 34 cm long or so) that is like Cricetomys in being long-tailed and mostly associated with dense woodlands and thickets, though it seems to be reliant on areas with soft sand or soil and nearby water. Riverbanks are thus typically places where lesser pouched rats are found. Burrows as long as an incredible 9 m are constructed. These can be quite complicated, with various side-passages and chambers serving as nesting and food storage regions, a blind-ended region near the nest being a latrine and the vertical entrance tunnel being plugged with earth when the rat is at home (Hanney 1977).

Schematic burrow system for Beamys (from Hanney 1977). E = escape tunnel, L = latrine, N = nest, P = soil plug.

Beamys is greyish or brownish with limp fur, a white underside and a mottled distal tail. The tail is further unusual in being triangular in cross-section (as opposed to the more normal subcircular): the ventral surface is flat and the dorsal surface is keeled (Nowak 1999).

Incidentally, both Cricetomys and Beamys carry the weird, blind, wingless Hemimerus earwigs. These earwigs aren’t parasites but commensals, apparently.

Finally, the most aberrant of cricetomyines are the Saccostomus species of eastern and southern Africa, variously termed pouched mice or African pouched rats. These aren’t forest rodents like Cricetomys or Beamys, instead frequenting savannahs, sandy plains, cultivated regions and open forests. These are cute, short-snouted, robustly built rodents that (at most) can have a body length of 19 cm and a tail of 8 cm. The fur is soft and silky, and of variable length amongst populations. The cheek pouches are proportionally huge, the legs and tail are short, and the ears are short and rounded. Overall, these rodents are not especially similar to Cricetomys or Beamys and don’t look especially ‘rat-like’. They’re actually superficially hamster-like and consequently Cricetomyinae is said by some authors to include hamster-like species as well as rat-like ones. [Photo below by Ryan van Huyssteen.]

Saccostomus campestris (or, a representative of a population conventionally referred to this species). Photo by Ryan van Huyssteen, CC BY-SA 4.0. Incidentally, I have some doubts about the identification - any African rodent experts out there who can confirm that this really is Saccostomus? UPDATE: see comments below. This animal may be Mus minutoides.

Following a period of debate that extended from the 1950s to the early 1990s, it came to be agreed that there were just two Saccostomus species: S. campestris of southern Africa and S. mearnsi of north-eastern Africa. These differ in the proportional size of the ears and tail, length of the toothrow and in colour and pelt form. However, Corti et al. (2004) reported extensive genetic variation within these animals that, they suggest, shows how both traditional species are actively evolving species complexes that include numerous lineages that we should recognise as species. In other words, the existing nomenclature doesn’t accurate reflect history or diversity and change is needed.

Left maxillary toothrow of the fossil species Saccostomus major from the middle Pliocene of Laetolil, Tanzania. Scale bar = 1 mm. Image from Denys (2011).

Due predominantly to the presence of those big cheek pouches and general similarities in lifestyle and ecology, Saccostomus has typically been classified alongside Cricetomys. However, some molecular studies (Jansa & Weksler 2004, Nakamura et al. 2013) have found Saccostomus to be allied with the dendromurines (the African climbing mice and kin), a possibility that I always thought likely based on the fact that such dendromurines as Leimacomys, Steatomys and Malacothrix are short-snouted, short-tailed, robust little rodents, superficially similar to Saccostomus. If this result is accurate, cricetomyines as conventionally considered are not monophyletic.

A cricetomyine montage. Cricetomys at top, Beamys in the middle, Saccostomus at bottom. As discussed in the text, Saccostomus may well not be allied to the other two. Illustration by Darren Naish.

Fossils referred to Saccostomus are known from the Lower and Upper Pliocene of both eastern and southern Africa; some have been named as distinct, extinct species (S. major and S. geraadsi) while others represent S. mearnsi or a similar species (Denys 2011). Molecular clock data suggests an origin at about 10 million years ago, during the Late Miocene (Denis 1999). This isn’t necessarily inconsistent with the hypothesis of a sister-group relationship between Saccostomus and other cricetomyines (since Beamys and Cricetomys are thought to have diverged between about 9 and 6 million years ago), but it was used by Corti et al. (2004) as evidence for a particularly ancient divergence of Saccostomus. In fact, the Saccostomus lineage might – Corti et al. (2004) suggest – be the oldest surviving rodent lineage 'genus' in eastern and southern Africa after Anomalurus [reason for correction: see comment 2 below]. Ah yes, the anomalures... I really must write about them some time.

A Beamys + Cricetomys clade does appear genuine, and has been recovered as sister to Dendromurinae, both groups being deeply nested within a major assemblage dominated by the Madagascan nesomyines (Jansa & Weksler 2004; see also Michaux et al. 2001, Steppan et al. 2004). This ‘nesomyine lineage’ muroid clade has most frequently been regarded as a subset of the giant ‘family’ Muridae. However, some authors use a revised nomenclature where the name Muridae is more restricted in scope, in which case dendromurines, cricetomyines and nesomyines belong to a group called Nesomyidae. And we’ll come back to this issue another time, since I want to talk about other members of this clade in future articles.

For previous Tet Zoo articles on other rodents, see...

Refs - -

Corti, M., Castiglia, R., Annesi, F. & Verheyen, W. 2004. Mitochondrial sequences and karyotypes reveal hidden diversity in African pouched mice (subfamily Cricetomyinae, genus Saccostomus). Journal of Zoology 262, 413-424.

Denis, C. 1999. Of mice and men. Evolution in east and south Africa during Plio-Pleistocene times. In Bromage, T. G. & Schrenk, F. (eds) African Biogeography, Climate Change, and Human Evolution. New York: Oxford University Press, New York, pp. 226-252.

Denys, C. 2011. Rodents. In Harrison, T. (ed) Paleontology and Geology of Laetoli: Human Evolution in Context. Volume 2: Fossil Hominins and the Associated Fauna. Springer, Berlin, pp. 15-53.

Hanney, P. W. 1977. Rodents: Their Lives and Habits. David & Charles, Newton Abbot, London, Vancouver.

Jansa, S. A. & Weksler, M. 2004. Phylogeny of muroid rodents: relationships within and among major lineages as determined by IRBP gene sequences. Molecular Phylogenetics and Evolution 31, 256-276.

Michaux, J., Reyes, A. & Catzeflis, F. 2001. Evolutionary history of the most speciose mammals: molecular phylogeny of muroid rodents. Molecular Biology and Evolution 17, 280-293.

Nakamura, I., Hang’Ombe, B. M., Sawa, H., Kobayashi, S., Orba, Y., Ishi, A., Thomas, Y., Isozumi, R., Yoshimatsu, K., Mweene, A. S., Takada, A., Sugimoto, C. & Arikawa, J. 2013. Cross-reactivity of secondary antibodies against African rodents and application for sero-surveillance. Journal of Veterinary Medical Science 75, 819-825.

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

Steppan, S. J., Adkins, R. A. & Anderson, J. 2004. Phylogeny and divergence date estimates of rapid radiations in muroid rodents based on multiple nuclear genes. Systematic Biology 53, 533-553.