Aethomys Division. An isolated member of an apparently monophyletic Subsaharan murine radiation as estimated by albumin immunology (Watts and Baverstock, 1995a), but included in an Arvicanthis Division by Misonne (1969), who focused on molar occlusal patterns. Analyses of mtDNA sequences (cytochrome b and 12S and 16S gene fragments) placed Aethomys as the basal member of an African murine clade containing Grammomys, Hybomys, Dasymys, Leminiscomys, Rhabdomys, Desmomys, Pelomys, Mylomys, and Arvicanthis (Ducroz et al., 2001). The affinity with Arvicanthis, Hybomys, and Grammomys is supported by DNA/DNA hybridization (Chevret, 1994). Aethomys was originally proposed as subgenus of Epimys (=Rattus), then elevated to generic rank (see G. M. Allen, 1939:267). The genus was first reviewed by Ellerman (1941), then Davis (1975b), and most recently Chimimba (1998) and Chimimba et al. (1999; also see below), but some species-complexes still require careful systematic revision, particularly taxa extralimital to the Southern African Subregion, to understand species-diversity and their geographic distributions, intraspecific variation, and interspecific phylogenetic relationships. The large-bodied, short-tailed members of the A. kaiseri complex (A. hindei, A. kaiseri, A. thomasi, and A. stannarius), for example, may comprise a monophyletic unit, is considered such by Crawford-Cabral (1998, 1999; as a superspecies) based upon morphology, but integrity of the grouping requires testing by analyses of molecular and other kinds of data.

Micaelamys, diagnosed as a subgenus of Rattus by Ellerman (1941) primarily by dental traits, has been traditionally and is currently used as a subgenus for A. granti and A. namaquensis, with few exceptions (Senut et al., 1992). The two species are set apart from all other Aethomys by length of tail relative to body length and pelage coloration (Davis, 1975b), cranial morphology (Chimimba, 1997; Chimimba et al., 1999), dental traits (Ellerman, 1941; Misonne, 1969), chromosomal characteristics (Matthey, 1954, 1958, 1964; Visser and Robinson, 1986), spermatozoal structure and bacular features (Visser and Robinson, 1987). Results from analysis of microcomplement fixation of albumin indicated A. namaquensis (subgenus Micaelamys) to be farther from A. chrysophilus (subgenus Aethomys) in immunological distance units than would be expected for congeneric species (Watts and Baverstock, 1995a). These results were reinforced by phylogenetic analyses of mtDNA cytochrome b, 12S and 16S ribosomal RNA sequences, which separated A. namaquensis and A. chrysophilus into separate clades, each associated with species in other genera of the African murines sampled (Ducroz et al., 2001). A more recent phylogenetic analyses of cytochrome b sequences by Castiglia et al. (2003b) placed A. chrysophilus and A. kaiseri in a monophyletic clade near Grammomys dolichurus in all trees but excluded A. namaquensis, associating it with either Lemniscomys or basal to members of an arvicanthine clade. We treat Micaelamys (containing M. granti and M. namaquensis) as a distinct genus: chromosomal, morphological, and molecular data clearly define it as a separate monophyletic cluster that may or may not be closely related to species of Aethomys (at least those used in the studies cited above). We also include Micaelamys with Aethomys in the same Division, but that allocation is provisional. In phylogenetic analyses of mtDNA cytochrome b sequences, for example, Aethomys (represented by A. chrysophilus and A. kaiseri) is closest to Grammomys, while Micaelamys (M. namaquensis) joins either Lemniscomys or is basal to an arvicanthine clade depending on the analyses (Castiglia et al., 2003b; Ducroz et al., 2001). Analyses of combined mtDNA cytochrome b, 12S and 16S ribosomal RNA sequences identified A. chrysophilus and G. dolichurus as a single clade, and placed M. namaquensis basal to that clade, a Dasymys/Hybomys clade, and an arvicanthine clade (Ducroz et al., 2001). Unequivocal phylogenetic affinity of Aethomys and Micaelamys within the evolutionary radiation of African murines has yet to be uncovered, and until such resolution we isolate them in their own Division. Phylogenetic analyses employing a broader sampling of not only the speciose Aethomys but of endemic African murine genera using molecular, chromosomal, and morphological data sets may resolve the phylogenetic relationships of Aethomys and Micaelamys. Based on their results from cytochrome b sequences, Castiglia et al. (2003b) estimated time of divergence between Aethomys (sampling A. chrysophilus and A. kaiseri) and Grammomys dolichurus as about 8 million years ago.

Chromosomal information available for various species (which includes those in Aethomys and Micaelamys: Baker et al., 1988c; Castiglia et al., 2003b; Matthey, 1954, 1958, 1964; Viegas-Péquignot et al., 1986; Visser and Robinson, 1986). Chimimba and Dippenaar (1994) morphometrically assessed sexual dimorphism and age variation in A. chrysophilus and M. namaquensis (the latter as Aethomys) and subsequently used statistical procedures to select the most discriminating set of morphometric characters to use in systematically revising several species of Aethomys (Chimimba and Dippenaar, 1995). Study of the correlation between diet and molar cusp patterns in A. chrysophilus and M. namaquensis by Denys (1994a, the latter as Aethomys) allowed her to determine possible diets of extinct Pliocene species. Spermatozoal morphology of several species described by Breed (1995a), and used in survey of relationship among body mass, testes mass, and sperm size within murine rodents (Breed and Taylor, 2000). Morphometric and morphological definitions of southern African species, along with a taxonomic synthesis for this group of Aethomys (which included those we place in Micaelamys), provided by Chimimba (1998) and Chimimba et al. (1999). Assessments of intraspecific variation and its significance in several southern African species produced by Chimimba (2000, 2001a, b) and Chimimba et al. (1999). Evolutionary history of Aethomys (not Micaelamy) as documented by fossils extends back to the Pliocene and Pleistocene of East Africa (Denys, 1987b, 1994a; Jaeger, 1976; Jaeger and Wesselman, 1976; Wesselman, 1984), early Pliocene-late Pleistocene of South Africa (Avery, 1995, 1998, 2000; Denys, 1990c), and Pleistocene of Namibia (Senut et al., 1992); see review by Denys (1999).