Small Mammals
East Africa contains a very high proportion (20%) of the world’s placental land mammal species. Of the 14 extant orders of land mammals 12 are represented in East Africa with the order Rodentia encompassing 27.5% of the total number of mammal species (Kingdon 1974). The rodents of East Africa are relatively well known. As there are several reliable identification guides in circulation (Kingdon 1974, Delany 1974, Meester and Setser 1977), they are well enough known so that most fundamental identification debates have been resolved. The African rodents are thought to have originated in the Cretaceous, more than 70 million years ago, but due to poor early fossil records little is known about them until the Eocene (50 million years ago) (Kingdon 1974).
East and central Africa possess a number of high mountains, some of which support permanent glaciers. The Rwenzori are the only mountains out of the large East African group that are not volcanic in origin (Coe 1969). The mountains of eastern Uganda and western Kenya are Miocene (about 20 million years old) and are associated with rifting; Mount Elgon is the largest of these. The more easterly mountains are surrounded for the most part by semi-arid savanna, a factor which produces a more considerable degree of isolation than is the case for the Rwenzori where montane forest clothing its slopes gives way at its base to rain forest which has acted as a reservoir for many montane species (Coe 1969). The Miocene mountains are a relatively recent addition to the East African landscape, appearing long after the rodents of the region were already highly speciose, their colonisation by lowland species is therefore also comparatively recent on an evolutionary time scale. Since their formation, the Miocene mountains have been isolated for relatively long periods and may have acted as refuges and centres of radiation over several eras of climatic change and through the various faunal invasions of Africa from Eurasia; they now contain large numbers of relict species and many rare and endemic species. The long-term evolutionary significance of montane forest may therefore be greater than its present very limited distribution suggests (Kingdon 1974).
The rat-like muroid rodents are divided into three families; the Rhizomyidae, Cricetidae and Muridae. The latter two families form the focus of this study and are of particular interest. These two groups each contain species which appear to have evolved to fill similar niches and are possibly in direct competition with each other. This appears to be leading to the decline of several cricetid species because of the superior competitive abilities of the murids (Kingdon 1974). On an evolutionary time scale the murids are a relatively recent addition to the rodents of Africa, first appearing (already differentiated) in the fossil record of the Pleistocene. The murid mice in Africa show a high level of resemblance which, supported by evidence from their fossil record, suggests that they have not undergone much divergence. Sub-Saharan Africa and south east Asia are the only areas of the world where the cricetids appear to be in retreat, elsewhere they are the dominant rodent type. Kingdon (1974) presents “a good deal of evidence to show that the murids (in Africa) are in the last stages of taking over from all but the most specialised cricetids”.
The Dendromurines are an exclusively African group of highly specialised cricetid rodents which appear to be sufficiently specialised to escape competition from the murids (Kingdon 1974). Many members of the other five subfamilies composing the Cricetidae are declining and rare but several have found refuge upon East Africas mountains. Examples include the extremely rare Delanymys brooksi and most members of the Otomyinae.
The Muridae are separated into four divisions; the Omnivores (Rattus division), Herbivores (Arvicanthis division), Specialists (Lopuromys division) and the Climbers (Thallomys division). Amongst the Muridae replacement of older genera by more recent ones appears to be an ongoing process. Fossil records show that Rhabdomys pumilio was once widely distributed in eastern and southern Africa but today it is restricted in its distribution and the only population in East Africa is found on Mount Elgon (Kingdon 1974, Delany 1975).