Arvicanthis nairobae J. A. Allen 1909

Arvicanthis nairobae J. A. Allen 1909, Bull. Am. Mus . Nat. Hist., 26: 168.

Type Locality: Kenya, Nairobi.

Vernacular Names: East African Arvicanthis.

Synonyms: Arvicanthis chanleri Dollman 1911; Arvicanthis nubilans Wroughton 1909; Arvicanthis pallescens Dollman 1914; Arvicanthis praeceps Wroughton 1909; Arvicanthis rumruti Dollman 1911; Arvicanthis virescens Heller 1914 .

Distribution: Recorded from S Ethiopia (Sidamo) through Kenya in the Rift Valley south to the Dodoma region in EC Tanzania; western and southern limits unresolved (based largely upon our study of specimens; the Ethiopian record and some specimens from W Kenya were identified using discriminant function analysis by Fadda and Corti, 2001). An unidentified sample of Arvicanthis from SW Uganda clusters with Tanzanian samples of A. nairobae in the analysis of mtDNA cytochrome b sequences by Ducroz et al. (1998), which indicates the species may range farther west than described here.

Conservation: IUCN – Lower Risk (lc).

Discussion: 2n = 62, FN = 78 (Castiglia et al., 2003 a; Fadda et al., 2001). The name nairobae is the oldest applicable to samples from Kenya and E Tanzania containing animals smaller in body size and generally brighter and buffier in pelage tones and hues than those larger and darker specimens we have identified as A. niloticus from W Tanzania and Uganda. Some specimens of A. nairobae closely resemble those of A. neumanni in pelage coloration, and could be mistaken for it, but are larger in body size. Corbet and Yalden (1972) even suggested that chanleri might represent A. neumanni (they used somalicus), but the holotype is larger and fits within the range of variation seen among samples of A. nairobae (our study of holotypes and larger samples in AMNH, BMNH, and USNM). Records of sympatry between A. nairobae and A. neumanni are documented by series from Mount Lololokwi ("an isolated mountain east of the Mathews Range, about midway between Mount Kenia and Mount Marsabit," Hollister, 1919; specimens in USNM), and the Dodoma region of Tanzania (samples in AMNH), and the two are probably broadly sympatric throughout their ranges in Kenya and Tanzania.

The morphological and geographic relationships between A. nairobae and A. niloticus require resolution, which could be accomplished by critical systematic review of the A. niloticus-A. nairobae complex. For example, A. nairobae is found with A. neumanni in the Dodoma region of Tanzania and they are the only species of Arvicanthis (represented by specimens) from E Tanzania. In our samples from the Tabora area to the west of Dodoma we could detect only one species, which is larger in body size and has much darker pelage than samples from Dodoma. It is this species (which we identify as A. niloticus) that ranges south into Zambia and north through W Tanzania and Uganda. According to the geometric morphometrical analysis by Fadda and Corti (2001), however, two morphological types occur in that region, which they designated as A. sp. and A sp.1; a third form, A. sp2, had a geographic distribution similar to A. nairobae and included the holotype. Studying the distribution of characters from samples collected along a transect between the Dodoma and Tabora regions would likely elucidate character and geographic relationships among the different populations.

Ducroz et al. (1998) provided another view of possible relationships between A. nairobae and A. niloticus in their phylogenetic analysis of mtDNA cytochrome b sequences. Samples of Arvicanthis from West Africa identified by Volobouev et al. (2002 a) as A. ansorgei, those from Central African Republic and Benin (the latter identified as A. rufinus; Volobouev et al., 2002 a), and samples from East Africa representing A. nairobae formed a cluster ("clade 1") separate from that composed of A. niloticus, A. abyssinicus, and A. neumanni ("clade 2"), which is congruent with chromosomal data (e.g., Castiglia et al., 2003 a) because "clade 1" is composed of species with a high FN. Within "clade 1" the West African lineage was phylogenetically more closely allied to the East African linage than to the samples from Benin and Central African Republic. Analyses of cytogenetic data has affirmed the specific integrity of A. nairobae and its closer phylogenetic relationship to West African A. ansorgei and A. rufinus than to the East African lineage consisting of A. niloticus, A. nairobae, A. abyssinicus, and A. blicki (Castiglia et al., 2003 a) . The inclusion of rumruti and nubilans in the synonymy above is based upon their identification as A. nairobae by Fadda and Corti’s (2001) morphometric analysis.