Modern Rodent Fauna of the Lower Omo Valley, Ethiopia

MODERN RODENT FAUNA OF THE LOWER OMO VALLEY, ETHIOPIA

BERNARD HUBERT Laboratoire de Zoologie Appliquee, O.R.S.T.O.M., B.P. 1386, Dakar, Senegal

Y ABSTRACT I One hundred and seventy rodents, comprising 15 species of rodent population of the several biotopes of the areas-Sahelian nine genera and five families (Sciuridae, Gerbillidae, Criceto- savanna, arid zones (paleontological beds), and forested blocks .P ' myidae, Muridae, and Gliridae), have been captured in the lower comprising a continuous stratum of large trees and associated Omo Valley. These represent a good sampling of the modern bushes and shrubs.

INTRODUCTION One hundred and seventy rodents, comprising 15 However, their ecological characteristics are well- species of nine genera and five families, were cap- defined. A future study, based on karyotypes, will tured in the lower Omo Valley in the summer of permit the systematics of species to be more firmly 1973 during the paleontological expedition orga- established. nized by Yves Coppens. These represent a good Animals have been captured in various ways- sampling of the modern rodent population of the through excavation of their burrows; by hand at several biotopes of the areas. night with the aid of a light; by traps set at burrows The specific designation of certain species men- or in lines and baited with peanut butter; or by large tioned here cannot always be regarded as definitive, metal cans sunk half to three-quarters of a m into because systematic revisions may be required. the ground. / I RESULTS Species List Gliridae Sciuridae Graphiurus nzurinus Desmarest, 1822 ' Heliosciurus gambianus omensis (Thomas, 1909)* *Captured by A. Duff-Mackay (National Mu- Xerus rutilus (Cretzschmar, 1826) seum, Nairobi, in 1967). Gerbillidae **Captured by F. Rhodain and R. Houin. Tatera nigricauda (Peters, 1878) Tatera initzuscula Osgood, 1936 The presence of at least two other species is in- Taterillus harringtoni (Thomas, 1906) directly documented. An immense network of bur- Gerbilhs pulvinatus Rhoads, 1896 rows, 3 to 5 cm in diameter, situated at a depth of Gerbillus pusillm Peters, 1878 about a meter, occurs on the surface of the Kibish Cricetomyidae Formation sediments, and may well testify to the Saccostonzus mearnsi Heller, 1910 presence of a bathyergid mole-rat. Some nests, Muridae filled with dry leaves, and unlike those of Graphiu- Mastomys erythroleucus (Temminck, 1853) rus or of Thanznonzys, were found situated either in Arvicantlzis niloticus (Desmarest, 1822) hollow limbs or in the empty trunks of certain trees Arvicanthis somalicus Thomas, 1902 in the gallery forest and could well belong to a spe- Acomys percevali Dollman, 1911 cies of Thallornys, a genus already documented Acomys wilsoni Thomas, 1892 elsewhere in Ethiopia (Petter, 1973). Thamnomys dolichurus (Thomas and Wrough- ton, 1908)""

DISCUSSION

Geographic Distribution mys erythroleucus, Thamnomys dolichurus, Gra- Some species are widely distributed throughout phiurits murinus). Others are restricted to eastern sub-Saharan Africa (Arvicanthis niloticus, Masto- Africa (Xerus rutilus, Tatera nigricauda, Taterillus 109 110 BULLETIN CARNEGIE MUSEUM OF NATURAL HISTORY NO. 6

Table 1 .-Number of rodents collected in the five ecological zones of the trapped area in the Omo Valley, Ethiopia. The number of trap-nights is indicated for each zone, but is not signijkant because sonte species were collected by digging their burrows or by sunken can-traps.

Riverine areas Arid zones Without trees, With trees with grass J Species captured & shrubs & thickets Slopes Bottom Plateau Xerus rutilus 5 Tatera nigricauda 4 11 8 Tatera minuscula 1 Taterillus harringtoni 1 3 Gerbillus pulvinatus 27 Gerbillus pusillus 6 Saccostomus mearnsi 9 Mastontys erythroleucus 2 3 Arvicanthis niloticus 11 2 Arvicanthis somalicus 7 5 Acomys percevali 4 3 16 3 Acomys wilsoni 2 16 9 Total specimens 19 53 3 31 62 Number of trap-nights 640 1,880 290 630 1,711 harringtoni, Saccostomus mearnsi, Arvicanthis so- dolichuriis have been captured. Native villages sit- malicus, Acomys wilsoni). Others appear to be uated here harbor Arvicanthis niloticus and Mas- more or less endemic and limited in distribution to tomys erythroleiicus. There is also the riverine southern Ethiopia and northern Kenya (Heliosciu- zone, largely without trees, but with grasses and rus gambianus omensis, Tatera minuscula, Gerbil- very dense thicket. Here Mastomys erythroleucus, lus pulvinatus, Gerbillus pusillus, Acomys percev- Saccostomus mearnsi, Acomys wilsoni, and Tatera ali). 1 nigricauda occur. The soils of this zone are heavy and clay-like and often appear as vertisols. The lat- Ecological Aspects er zone is flooded at more or less regular intervals These taxa differ in their biology. Certain species in keeping with the Omo flood regime. dig burrows and are active nocturnally (Mastomys The Shungura Formation exposures may be con- erythroleucus) or diurnally (Xerus rutilus). Others sidered separately and to have three aspects. The are active both day and night and construct nests slopes of the exposures are comprised of detritus of dry vegetation at the foot of bushes or in high and eroded Plio-Pleistocene sediments. This very grasses (Arvicanthis spp.). Still others are arboreal arid facies, practically without vegetation, harbors and active during the day (Heliosciurus gambianus Acomys percevali. The cuestas of these exposures, omensis) or the night, and construct nests of dry which are covered with tuffs, and the bottoms, leaves in hollow limbs (Graphiurus murinus) or at- which separate them and contain recently eroded tach them to bushes or lianas (Thamnomys dolichu- sediments, still have a very arid vegetation com- rus). Their nests do not seem to be occupied prised of open shrubs (Euphorbias, Adenium obe- throughout the year. sum, Cissus rotundifolin, Dasysphiera prostrata, Three major habitats may be usefully distin- Commiphora africana, Adenia venenata, Carallu- guished (see Table 1). The riverine area has two ma russelliana, Sesamothamniis brisseanus, and aspects-with trees and shrubs (including Trichilia Maerua crassifolia) and rare herbaceous plants (In- roka, Tapura Jischeri, Ficus sycomorus, Celtis in- digofera sp., some Cyperaceae). In these situations tegrifolia, Ziziphus rubescens, Cordia gharuf) Gerbillus pusillus, Taterillus harringtoni, Tatera forming a continuous canopy, and with lianas (Sis- minuscula, Arvicanthis somalicus, and Xerus riiti- sus, Sabia) present. This zone appears to have a lus occur. poor rodent fauna; only Heliosciurus gambianus Finally, the plateau has several soil types, more omensis, Graphiurus murinus, and Thamnomys or less rich in clay, and these have a considerable 1978 HUBERT-RODENTS FROM OMO VALLEY, ETHIOPIA 111

influence on the local composition of the vegeta- the other hand, in the Kalam area, where rain was tion, generally that of a more or less open shrub or lacking, there was no evidence of reproductive ac- bush savanna (Acacia paolii, A. refcieizs, A. hor- tivity among those rodent populations. rida, Grewia terias, Cadaba glaizdulosa, C. gilletti, Sciatula Izortacarztha, Serriconiopsis pallida) with Taxonomic Comnzents herbs (Blepharis persica, Polygala erioptera, Ipo- The modern rodent fauna of the Lower Omo Val- V mea belpharosepala) as well as numerous grasses ley is typical for this area in Africa, where different (Aristida, Cerzclzrus, Chloris, Dactylocterziuni, and types of habitat are represented-Sahelian savanna, others). The plateau rodents are Xerus rutilus, Aco- arid zones, and forested blocks comprising a con- mys wilsoni, and Arvicaritlzis somalicus, which oc- tinuous stratum of large trees and associated bush cur everywhere, and Gerbillus pusillus and G. pul- and shrubs. vinatus which occupy areas of lighter, more sandy There are two squirrels, one arboreal (H. g. soil than those where Taterillus Izarrirzgtorii and Ta- ornensis, described from this place) and one fos- tera nigricauda are found. sorial (X. rutilus). Two species of Tatera are pres- The distribution of these various rodent species, ent, one widely distributed in eastern Africa (T. izi- which utilize the part of the environment which best gricauda), and a very small one, endemic to this agrees with their ecological preferences, is further area, T. rninuscula (which was described by Osgood affected by another limiting factor-dryness- in 1936 from specimens collected in a nearby area, which is more or less strongly pronounced accord- the Webbi Shebelli in southeastern Ethiopia, and ing to the year and the season. Except for the gal- was never officially collected since that time). Ta- lery forest species, the rodent fauna is rather char- terillus harrirzgtoni occurs in all this part of eastern acteristic of dry, and even arid zones. The low Africa (Robbins, 1975) where it lives together in the incidence of species in the gallery forest is a result Sahelian savanna with two species of gerbils, a me- of recurrent dry periods that prevent the occurrence dium-sized hairy-footed species, G. pulvinatus (Hu- of species that are linked to a permanently moist bert, 1978; Schlitter, personal communica- environment and in addition the occasional flooding tion) and a small one with naked sole, subgenus of the entire area, thereby restricting immigration Hendecapleura, Gerbillus pusillus (Roche, 1976). by burrowing or purely terrestrial species. Saccostonzus rnearnsi is the northern form of this genus, occuring in Ethiopia, Uganda, Kenya, and Reproduction Tanzania (Hubert, 1978). The Mastonzys seems to Observations made during the summer of 1973 be referable to Mastorrzys erythroleucus, a large broadly confirm the conclusions of previous work- species characterized by a light gray belly, a brown- ers (Dieterlen, 1967; Happold, 1966; Poulet, 1972; yellowish back, and a karyotype of 38 chromo- Hubert, 1977) on the reproduction of tropical ro- somes, occurring in western and central Africa (Pet- dents in sub-Saharan Africa. Reproduction occurs ter, 1957 and 1978). Two species of Arvicaizthis are some weeks after onset of the rainy season. The represented-A. riiloticrts, a large species, which is onset of rains in early July 1973, in the north Shun- found in wet areas in the savannas over all Africa, gura area, saw active reproduction of the principal and A. sornalicus, a smaller one with a paler fur, rodent species represented there. In August, 80% from the arid zones of eastern Africa. The two spe- of the trapped females were sexually active-either cies of Acornys were reported from this area by in estrous, gestating, or lactating. Farther south, on Matthey (1968).

CONCLUSIONS It is worth noting that four of the nine rodent of the heterogeneity of the environment, in space genera present in the lower Omo basin are repre- and in time. This strategy, reflected in more or less sented each by two species adapted to slightly dif- marked specialization of different species, adapted ferent habitats. Here is an adaptative evolutionary to particular conditions, contrasts with that of spe- process of some plasticity frequently found in var- cies substantially less strict in' their ecological re- ious animal species. It permits maximum utilization quirements. However, those latter species are ca- 112 BULLETIN CARNEGIE MUSEUM OF NATURAL HISTORY NO. 6 pable of very rapid occupation of various habitats Omo basin is rich, reflecting the different habitats given some favorable change of a seasonal or other present, which is characteristic of the African tropi- unusual nature. cal zones, even the dry zones. Thus, the modern rodent fauna of the Loweí-

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