Odontometrical divergence in the Gerbe’s vole Microtus ( Terricola ) pyrenaicus gerbei (G ERBE , 1879) in comparison to the Pyrenean vole M . ( Terricola ) pyrenaicus pyrenaicus

This study con ﬁ rms the odontometrical divergence of the ﬁ rst lower molar in the Gerbe’s vole Microtus ( Terricola ) pyrena-icus gerbei in comparison with the Pyrenean vole M. pyreneaicus pyrenaicus as well as the interest of genetic analyses to resolve its systematic position with regard to Microtus ( Terricola ) pyrenaicus .

The comparative odontometrical analysis of the fi rst lower molar (M 1 ), the most informative tooth in the study of the evolution of voles, in populations of Gerbe's vole (Loire-Atlantique and Maine et Loire) and Pyrenean vole (Brunet-Lecomte & al., 1995) had shown a signifi cant divergence between the nominative subspecies M. pyrenaicus pyrenaicus and the subspecies M. pyrenaicus gerbei distributed at the northwestern margin of the species distribution.The obtaining of a new material from Vendée (South of the Loire Region) of the Gerbe's vole, compared with geographical populations of the Pyrenean vole from Navarre, Southwest and Massif Central justify to make a new study in the aim to confi rm the previously obtained results.

Material and method
The studied material was composed of a population of M. pyrenaicus gerbei (62 M 1 ) from Loire-Atlantique (15 M 1 ), Maine et Loire (11 M 1 ), Western Vendée (12 M 1 ), South Vendée (16 M 1 ) and Eastern Vendée (8 M 1 ).It was compared to 8 geographical populations of M. (T.) pyrenaicus pyrenaicus from following Region (Spain) or Departments (France): Navarre (Spain, N = 55), Gironde and Landes (France, N = 47), Gers Because of the non-normality or of the non-compliance with the equality of the variances between the populations of the studied criteria, Kruskal-Wallis non-parametric test was used to compare the criteria, followed by a Dunn's test for post-hoc multiple comparisons when necessary (Hinton, 2004) between the population of M. pyrenaicus gerbei with each geographical population of M. pyrenaicus pyrenaicus.

Results
The description for each parameter is given in Tab. 1 to Tab. 5.
Kruskal-Wallis test showed populations were signifi cant different (p < 0.0001) for each studied parameter.
The 5 local sub-populations of M. pyrenaicus gerbei showed a strong homogeneity with the exception of that of the Loire-Atlantique characterised by a tilt of the pitymyan rhombus and a ratio length/width close to that observed in populations of M. pyrenaicus pyrenaicus.

Conclusion
This study confi rms the signifi cant odontometrical divergence of the M 1 criteria between M. pyrenaicus gerbei and M. pyrenaicus pyrenaicus previously reported (Brunet-Lecomte et al., 1995).
In 1880 already in their article describing exactly the taxon gerbei, Gerbe & de L'Isle considered this one as a different species from M. pyrenaicus, in particular on the basis of skull criteria (brain-case convexer, frontal bones less fl attened, nasal bones more indented in back and occipital foramen wider).The odontometrical divergences, which cannot allow to resolve this question, highlight nevertheless all the relevance of this one.Only a genetic analysis will allow to know if the taxon gerbei is a subspecies of M. pyrenaicus either a different species.A pitymyan rhombus less tilted    and a large ratio length/width are more often observed in Pleistocene species than in present-day species of Terricola (Brunet-Lecomte & Paunesco, 2004& 2008).The Loire Basin was able to be a refugium area during Middle or Upper Pleistocene cold periods for some micromammals like Microtus and activate a genetic isolation of gerbei taxon from M. pyrenaicus pyrenaicus or another Pleistocene Terricola species.
Description of the development of the anterior part of the fi rst lower molar (%) in Microtus (Terricola) pyrenaicus populations.N = sample size, SD = standard deviation, SEM = standard error of the mean, Min = minimum, Max = maximum.Description of the tilt of the pitymyan rhombus (mm) in Microtus (Terricola) pyrenaicus populations.N = sample size, SD = standard de viation, S EM = standard e rror of t he m ean, M in = minimum, M ax = maximum.Description of the closure of the anterior loop (%) in Microtus (Terricola) pyrenaicus populations.N = sample size, SD = standard de viation, S EM = standard e rror of t he m ean, M in = minimum, M ax = maximum.Description of the ratio of length/width of the fi rst lower molar in Microtus (Terricola) pyrenaicus populations.N = sample size, SD = standard deviation, SEM = standard error of the mean, Min = minimum, Max = maximum.Result of Dunn's test for post hoc multiple comparisons between Microtus (Terricola) pyrenaicus gerbei population and each Microtus (Terricola) pyrenaicus pyrenaicus geographical population.S = Signifi cant (p < 0.05), NS = Not Signifi cant (P ≥ 0.05).Tilt of the pitymyan rhombus (mm) of the fi rst lower molar: Mean ± Standard Error of the Mean (SEM) in Microtus (Terricola) py renaicus populations.