Arvicanthis Division. Morphological features tie Arvicanthis to species of Pelomys, Mylomys, Rhabdomys, and Lemniscomys (Musser, 1987b), which has been confirmed by results from mtDNA sequences (cytochrome b, 12S and 16S rRNA genes), with the addition of Desmomys, by Ducroz et al. (2001). Their results indicate three distinct lineages: one containing Arvicanthis, Mylomys, and Pelomys; another with Desmomys and Rhabdomys; and a third containing only Lemniscomys. The Asian Golunda, provisionally (Musser, 1987b) or certainly (Jacobs, 1978; Misonne, 1969; Sabatier, 1982) allied with this clade, is not part of it according to molecular data (Ducroz et al., 2001; Lecompte, 2003; see Golunda account).

Opinions on the number of species in Arvicanthis have varied from one (Misonne, 1974) to several (G. M. Allen, 1939; Corbet and Hill, 1991; Dollman, 1911; Ellerman, 1941; Musser and Carleton, 1993; Rousseau, 1983). The first and still useful review of Arvicanthis was made by Dollman (1911). Several recent attempts to assess variability in the genus using breeding studies (F. Petter et al., 1969; Philippi, 1994), morphometric analyses (Bekele et al., 1993; Corti and Fadda, 1996; Fadda and Corti, 1998, 2001; Rousseau, 1983), spermatozoal morphology (Baskevich and Lavrenchenkio, 1995), chromosomal evidence (Baskevich and Lavrenchenko, 2000; Capanna and Civitelli, 1988; Castiglia et al., 2003a; Civitelli et al., 1995a; Corti et al., 1996b; Garagna et al., 1999; Orlov et al., 1992a; Volobouev et al., 1987, 1988a, 2002a, b; and references cited there), electrophoretic patterns of blood proteins (Kaminski et al., 1984), allozymic variation (Capula et al., 1997; Milishnikov et al., 1992), analysis of mitochondrial (cytochrome b, 12S and 16S rRNA) gene sequences (Ducroz et al., 1998, 2001), and a combination of data sources (Capanna et al., 1996b; Ducroz et al., 1997) have provided insights into species-diversity within Arvicanthis, as have results obtained based upon our examination of museum specimens (focusing on qualitative traits as well as external and craniodental measurements), and study of the discussions by Dollman (1911), Hollister (1919), Osgood (1936), and Corbet and Yalden (1972). Results of the careful chromosomal, molecular, and interbreeding studies by Volobouev et al. (1988a, 2002a, b), Ducroz (1998), and Ducroz et al. (1997, 1998) have been especially significant in uncovering the species diversity of Arvicanthis in West Africa.

Of the seven species we list here, A. abyssinicus and A. blicki are endemic to the Ethiopian Plateau and easily diagnosed and recognizable. Two, A. ansorgei and A. rufinus, have been recorded only from West Africa and the full extent of their geographic distributions have yet to be resolved. The East African A. nairobae is distinctive but its actual range and phylogenetic relationship to other species requires investigation. The small-bodied A. neumanni is also endemic to East Africa and exhibits significant geographic variation possibly reflecting the presence of another species in the southern part of its range. Arvicanthis niloticus has the most expansive geographic distribution of any Arvicanthis and as defined here may still consist of more than one entity. Several samples of Arvicanthis have karyotypes different from any reported for other species and remain taxonomically unidentified. Capanna and Civitelli (1988) recorded a karyotype with 2n = 44 from a sample they identified as A. niloticus from Somalia, which contrasts with the 2n = 62 of A. niloticus or documented diploid number for any other sample of Arvicanthis. Two unidentified Ethiopian samples were reported by Orlov et al. (1992a): 2n = 60, FN = 78 (S Ethiopia) and 2n = 56, FN = 60 (SW Ethiopia); the former might apply to A. nairobae, which occurs in the same region of S Ethiopia (Fadda and Corti, 2001); Tanzanian specimens of A. nairobae, for example, exhibit 2n = 62, FN = 78 (Castiglia et al., 2003a; Fadda et al., 2001b).

Living species of Arvicanthis occur only in subsaharan Africa and the SW portion of the Arabian peninsula, but the genus is represented in middle to late Pleistocene cave deposits in Israel by A. ectos (Bate, 1942b; Tchernov, 1968), and is not recorded as fossils in cave sites younger than 80,000-70,000 years before present (Tchernov, 1986, 1992, 1994, and references cited in therein). Isolated molars identified as A. niloticus were obtained from late Pleistocene sediments in N Morocco, which is outside the modern range of the genus (Ouahbi et al., 2001), and from the late Pleistocene of East Africa (Jaeger, 1976; Wesselman, 1984; see review by Denys, 1999). Arvicanthis sp. was recovered from late Pleistocene beds in Tunisia (Mein and Pickford, 1992). The earliest fossil representing Arvicanthis comes from 3-million year old sediments (late Pliocene) in the Omo Valley of Ethiopia (Wesselman, 1984). Based upon complete mtDNA cytochrome b sequences, Ducroz et al. (1998) estimated Arvicanthis and Lemniscomys diverged from a common ancestor 5.5-7.7 million years ago. The split between Arvicanthis and Mylomys + Pelomys would have occurred between 5 and 6.3 millions years ago (Ducroz et al., 2001). Subsequently the North African species may have split from the other Arvicanthis 4.3-6.2 million years ago, and the separation of A. niloticus, A. abyssinicus, A. blicki, and A. neumanni was a Plio-Pleistocene event (Capanna et al., 1996b; see extended discussion in Ducroz et al., 1998, 2001).