Taxonomic position and biogeography of Mus callewaerti, the largest species of the subgenus Nannomys (Rodentia: Muridae)

The Callewaert’s mouse (Mus callewaerti) is shown as an ancient lineage of the African endemic subgenus Nannomys. Described in 1925 as a large-bodied species with proodont (forward pointing) incisors, it was long known only from a handful of localities in Angola and southern Democratic Republic of the Congo. Here, it is revealed identical with a genetically distinctive Nannomys, provisionally called Mus sp. “Nyika” in previous studies and reported from Nyika Plateau (Malawi) and the Angolan Escarpment. The skull shape analysis clearly associated the holotype of M. callewaerti with other specimens ascribed to the species (including the genotyped ones). It also pinpointed diagnostic features distinguishing M. callewaerti from other large bodied Nannomys, especially its sympatric Mus triton, for which the species was repeatedly mistaken. Mus callewaerti is presumably insectivorous and rare or not easy to capture. The divergence between its Malawian and Angolan populations is relatively shallow, dated to 0.32 million years ago, which suggests that at least in the past the species could be widespread. Mus callewaerti is known from grassy, locally moisty habitats. Together with a handful of other rodent taxa it provides evidence of persistence and historic connection of these habitats across the Zambezian region.