2.1 Shrews, study area and trapping
Sorex araneus (Figure 1) were collected between 1953 and 2004 in Białowieża National Park, preserved in alcohol and stored in the zoological collection of the Mammal Research Institute, Polish Academy of Sciences, in Białowieża. Shrews were trapped in a mesic broadleaved forest with a transition to a moist broadleaved forest, mainly in compartment No. 371 (52°44’ N, 23°52’ E) but also in adjacent Nos. 369 and 370 (see Dehnel 1949 for a map; all these compartments are in the strict reserve). This type of forest is classified as a subcontinental oak-lime-hornbeam forest Tilio-Carpinetum . The mean age of the trees exceeded 100 years, and the amount of dead wood on the ground averaged 50 m3 ha-1.
S. araneus were caught with pitfall traps (cylinders 30 cm deep and cones 45 cm deep). After 1965, wooden live traps were additionally used. We selected and measured the skulls of shrews from the three main stages of Dehnel’s phenomenon: summer individuals in their first calendar year (hereafter “juveniles”) caught from July to August, winter individuals (“subadults”) caught from December to February, and mature spring/summer individuals in their second calendar year (“adults”) caught from April to August. After 1981, juvenile individuals were trapped only in July. There was no winter trapping at all after 1985. We also measured three individuals each from June (juveniles), September (juveniles) and November (subadults) from 1955-1985. The sex of all individuals (determined by dissection) was noted on their labels.