Seasonal and interannual survivorship in the common shrew: the early bird catches the worm

Abstract

In a 7-year capture-recapture analysis, we estimated effects of population density and weather conditions on the mortality rate in five neighbouring populations located in different habitats in Central Russia. Non-resident shrews had little chance of surviving to reproduction. The average lifespan of resident young shrews first seen in June, July and August was 4.7, 4.5 and 3.2 months, respectively, and the longest one was 15 months. In spring, the number of males that overwintered exceeded that of females; subsequently, the former diminished faster leading to a female-biased sex ratio in late summer. Young common shrews survived the winter usually close to their summer home ranges, although several > 100 m shifts were recorded. The number of newly discovered overwintering shrews positively correlated (r = 0.93, P < 0.008) with the expansion of home ranges after wintering. The pattern of the resident population had formed by the end of July; later, only a few new residents were detected, although the dispersal rate remained high. The major contribution of the shrews that overwintered locally to the genetic pattern explains the previously observed deficit of heterozygotes at three of the studied sites. Although the interannual dynamics of mortality varied among the habitats, overall mortality was high in the 2 years with the thinnest late-autumn snow cover. An association of the mortality rate with population abundance was found only in the most favourable habitat. The occasional high mortality and asynchronous survival give immigrants an opportunity from time to time enhance the genetic diversity of local populations.