Abstract:
West African Mountains of the Cameroon Volcanic Line harbour two montane-endemic species of laminatedtoothed
rats (Otomys), which represent the most westerly occurrence of the genus. We explore here through mtDNA
sequencing and cranial morphometrics the taxonomic status and phylogenetic relationships of O. burtoni (Mt
Cameroon) and O. occidentalis (Mts Oku and Gotel). We conclude that both species are valid and can be
discriminated by molecular data, as well as quantitative and qualitative cranial characters. From molecular data,
O. occidentalis and O. burtoni are closest neighbours (p-distance = 7.5–8.5%) and weakly associated sister species
(suggesting a single West African radiation) and both are sister clades to a well supported clade of central, East
and northeast African members of the O. typus s.l. and O. tropicalis s.l. species complexes from mountain ranges
comprising the East African ‘Montane Circle’ and Ethiopian Highlands. Re-evaluation of the evolutionary origins
of the allopatric Otomys populations in equatorial Africa is undertaken in light of fossil evidence of a southern
African origin of the genus. We can conclude that Otomys reached the Cameroon Volcanic Line via corridors of
temperate grasslands during the Late Pliocene. Our data support the hypothesis that, following major peripatric
speciation events at around 2.3 to 2.03 Ma (from East Africa into West and North Africa respectively), further
speciation occurred across neighbouring mountain ranges in West, Central-East and North-East Africa. Estimated
molecular dates of speciation events in Otomys reveal close congruence with well-constrained geochronological
estimates, pertinently the uplift of the Albertine Rift in the Early Pleistocene. These regional analyses reveal how
peripatric speciation events established narrow-range endemics of Otomys on principal stratovolcanoes across the
East African plateau and Cameroon.
