Bivariance in the size structure of sable and pine marten populations: gene flow or introgression?

Abstract – A comparative analysis of the size structure of 24 population groups of sable and pine marten is performed using two museum pools of craniometric data: the first is based on a set of 17 standard features, and the second is based on a new feature Δ (“delta”), proposed by us to determine the two species by the skull. Each pool contains more than 3 thousand specimens. Three groups of features have been identified: four features by which the sable is always larger than the marten; five features by which the marten surpasses the sable at any point of the range; the main group is represented by 7 features with special variation: in transgression areas their values are higher for sable, and in “pure” areas, on the contrary, for pine marten. In hybridization areas, the size parameters of sable increase, and, on the contrary, for marten decrease, leading to their partial convergence. Each species demonstrates two variants of the size structure: one is realized in allopatric groups (the sable has smaller sizes), the second is observed in sympatry areas (the sable, on the contrary, is larger), but the convergence of the parameters is not absolute – statistically significant differences in size between the species are preserved. The described phenomenon of bivariant size structure is most likely the result of limited introgression in the sympatry zone. For the Δ trait, a high level of intersexual differences was revealed, exceeding the usual values by 3–4 times: 28% in the marten and 30.5% in the sable. In this dimension, the marten exceeds the sable by 33–36%.