Regarded as conspecific with O. irroratus by Bohmann (1952) and accordingly recognized in regional treatments (Delany, 1975; Kingdon, 1974b); however, others have recognized eastern African tropicalis as morphologically divergent and specifically distinct from the southern African O. irroratus (De Graaff, 1981; Meester et al., 1986; Misonne, 1974; Musser and Carleton, 1993; Taylor and Kumirai, 2001).

Even removed from O. irroratus and divorced of burtoni and dollmani (see those accounts), at least three assemblages are apparent among the populations embraced by this nominal species. The taxon tropicalis in the strict sense inhabits middle to upper slopes, 2300-4000 m (specimens in USNM), of Mt Kenya and Aberdare Mtns. Examples of the elgonis complex (also faradjius, ghigii?, giloensis, nubilus, vivax) exhibit a dark russet-brown pelage that is somewhat sleek and moderately long; tail relatively longer; skull flatter, nasals narrower and flaring less abruptly; M3 with 7 laminae, upper incisors wider and medial sulcus of lower incisor faint. As noted by Hollister (1919), this dark-brown form occurs on the lower western slopes of Mount Kenya (2150 m; USNM 164277), seemingly distinct from populations of tropicalis found on middle to upper slopes. Elsewhere, specimens exhibiting this morphology occur sympatrically with thomasi (here = O. orestes) at Molo, Kenya (FMNH 16693, 16695), and with squalus (here = O. orestes) in the Aberdare Mtns (series in USNM), and are altitudinally parapatric with O. typus on Mount Albasso (FMNH 28165), with O. dartmouthi in the Ruwenzori Mtns (series in AMNH, FMNH), and with O. jacksoni on Mount Elgon (FMNH 25379, 25380; Wroughton, 1906). A third moiety (rubeculus) consists of populations in Western Rift mtns that are large-bodied and have lower incisors with a moderately deep medial groove. Variation in pelage color, size, and cranial and dental characteristics within and among these assemblages is appreciable, however, and the complex will require careful revision and multiple data sources to assess the number and distributions of species. Dense altitudinal transects, especially on Mt Kenya and the Abedare Mtns, would be helpful.