Morphometrics of the wood mouse (Apodemus sylvaticus, Mammalia, Rodentia) in the Mediterranean

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Abstract
Sexual dimorphism, ontogenetic variation and allometric shape changes related to variation in size were considered and eliminated before performing discriminant analyses on geographic variation of Apodemus sylvaticus in central Mediterranean areas. Preliminary multivariate analyses of A. sylvaticus populations showed influences of sex and age in the ordination results. Giantism, i.e. size increase and shape‐related change, was more pronounced on islands like Pantelleria and Marettimo. By principal component analysis, we found an allometric factor which proved statistically correlated to insularity parameters (area, distance from mainland, and altitude) but not to the number of predator and competitor species in the seven islands. Application of Burnaby's procedure permitted a ‘size‐free’ multivariate analysis, (canonical variate analysis and related Mahalanobis distances, their UPGMA clustering and minimum spanning tree), which showed ordination of populations independent of within‐group size variation and sample composition. The resulting pattern of geographic variation was considered an example of independent divergence of island populations. The wood mouse in Algeria and Tunisia is morphometrically related to the northern Italian population; a second cluster grouped the seven islands and the Calabrian population. The latter and that of Marettimo appear to be the more phenetically separated populations. Pantelleria and Malta form a distinct sub‐group separated from Sardinia and Sicily. The minimum spanning tree joined the north‐eastern Sicilian populations to those of the small islands and to Calabria, whereas the south‐western populations proved to be connected to those of Tunisia. These relationships in skull morphometrics may result from adaptation to similar ecological conditions or can probably be traced to Holocene colonization events.