Subgenus Microtus, socialis species group (Zagorodnyuk, 1990) or subgenus Sumeriomys (Gromov and Polyakov, 1977; Pavlinov and Rossolimo, 1987; Pavlinov et al., 1995a). Corbet (1978c) initially included guentheri in M. socialis but later (1984) recognized its specific distinctness, a status earlier demonstrated by Neuhäuser (1936), Felten et al. (1971), and Morlok (1978). Harrison and Bates (1991) continued to unite guentheri with M. socialis. C- and G-banding chromosomal patterns in SE Bulgarian reported by Belcheva et al. (1980), and standard karyotype documented for SE Turkey by Çolak et al. (1997); molecular and cytogenetic characteristics of satellite DNA and heterochromatin reported by Modi (1993). Greater breadth of chromosomal data integrated by Zima and Kral (1984a) and Zima (in litt., 2002); 2n = 54, FN = 52-56 in M. guentheri and 2n and FN = 62 for M. socialis. Cranial traits clearly distinguish M. guentheri from M. socialis (Kock and Nader, 1983; Kock et al., 1972); Zykov and Zagorodnyuk (1988) morphometrically separated it from M. socialis and M. paradoxus; and Kefelioğlu and Kryštufek (1999) contrasted it with Turkish M. socialis and other species using chromosomal and multivariate analyses. Allozymic analysis (40 loci) employing samples of M. guentheri, M. arvalis, and Chionomys nivalis revealed strong, species-level divergence among the three (Markov et al., 1995). Closest relatives of M. guentheri are M. socialis, M. paradoxus, and apparently M. qazvinensis (see those accounts). Microtus paradoxus, recorded from the mountains of NE Iran and adjacent Kopet Dag Mtns in S Turkmenistan, is similar to M. guentheri in body size and pelage coloration and texture. Whether it represents the easternmost distribution of M. guentheri or a separate species requires resolution, as does the status of other taxa listed within the synonymy. See Golenishchev et al. (2000b) for map of most synonym type localities.
Morphological and geographic definition of the species has been unclear due partly to substantial geographic variation and partly to its confusion with M. irani (Kock and Nader, 1983). Yiğit and Çolak (2002) morphologically and morphometrically examined Turkish samples and concluded that two species are present: M. guentheri in SE Turkey and M. lydius (type locality Izmur; Blackler, 1916), with M. l. lydius in coastal W Turkey and the larger-bodied M. l. ankaraensis throughout C Anatolia. Chromosomal traits do not separate M. lydius from M. guentheri, as was previously noted by Kefelioğlu (1995) and Çolak et al. (1997), but they have been considered separable based on bacular morphology and pelage coloration (brown versus yellowish brown). Kryštufek and colleagues (in litt., 2002) revisited the problem by analyzing 13 samples from Macedonia, Greece, throughout Turkey, Lebanon, Israel, NW Iran, and N Libya, including holotypes of hartingi, lydius, philistinus, mustersi and topotypes of martinoi (= macedonicus), ankaraensis, and guentheri; their preliminary results revealed two indistinct clusters, joined by intermediate samples and suggesting continuous morphological variation among the taxa represented. They regarded these "intergrading samples as belonging to guentheri, which is supported by available evidence on the karyotype."
The taxon mustersi (Hinton, 1926b), described as distinct from but morphologically closely related to philistinus from Israel (= M. guentheri), is the only living arvicoline in Africa, known by only a few specimens collected at or near the type locality. Described as a species and often recognized as such (G. M. Allen. 1939; Ellerman, 1941; Ranck, 1968); or included in M. guentheri (Ellerman and Morrison-Scott, 1951; Toschi, 1954), M. socialis (Corbet, 1978c; Harrison and Bates, 1991), or M. irani (Musser and Carleton, 1993). Libyan voles are a zoogeographic enigma, "particularly since they are not known from coastal Egypt, the only conceivable dispersal route," wrote Ranck (1968:73), who speculated that the population is a relict "from the period when the more boreal climates of the Pleistocene prevailed over North Africa. The Cyrenaican Plateau by virtue of its higher elevation has retained at least a vestige of these boreal elements, whereas the remainder of coastal Africa has become much drier and more sparsely vegetated and no longer contains habitats suitable for voles." The morphological identity of mustersi with M. guentheri should be tested by chromosomal and molecular analyses. Recorded in Middle Palaeolithic deposits in Lebanon, where M. guentheri is now common (Kersten, 1992), perhaps reaching the E Mediterranean region from the north during middle Pleistocene (Tchernov, 1988). Bate (1937) described machintoni based on fossils from Palestine, but Tchernov (1968) considered it part of a chronocline within M. guentheri.