The body weight of capercaillie, Tetrao urogallus: spatial variability in Northern Eurasia

Abstract

Published data on the body weight of capercaillie, Tetrao urogallus , from the region mostly situated north of 60° latitude were analyzed. With the addition of new estimates from some points in the species range one can see that the study area is almost exclusively populated by large individuals (cocks ≥ 4.0 kg, hens ≥ 2.0 kg). Taking all the populations included in the analysis as a whole, the body weight of birds changed independently of the geographic latitude and longitude. Apparently, the intra-species taxonomic heterogeneity in the study area was the main source of the spatial variability of body weight. If we remove the influence of the ≪taxonomy≫ factor by excluding the populations chiefly represented by small-size birds, the weight of males demonstrates a significant direct dependence on longitude and latitude, the one of females shows a slight direct correlation with geographical latitude only. These correlations allow interpreting the spatial variability of body weight in accordance with Bergmann's rule (for both sexes) and/or the rule of optimum (for males only). The nature of inter-population changes in the asymmetry of the frequency distributions of male body weight indicates the direction of the resultant effect of all (multidirectional) vectors of natural selection in accordance with these rules.