THE EFFECT OF HIBERNATION POSITION ON WINTER SURVIVAL OF THE ROCK-DWELLING LAND SNAILS CHONDRINA-CLIENTA AND BALEA-PERVERSA ON OLAND, SWEDEN

The effect of hibernation position on winter survival was examined for four years in the rock-dwelling land snails Chondrina clienta and Balea perversa inhabiting exposed stone walls on the Baltic island of Oland, Sweden. Individuals of both species hibernated either singly or aggregated in groups, attached to the lower surface of horizontally arranged pieces of limestone. B. perversa hibernated more frequently in aggregations than did C. clienta, and in both species adults occurred more frequently in aggregations than did juveniles. For both age classes of C. clienta in winter 1987/88 and for juveniles of B. perversa in 1988/89, individuals that hibernated in large aggregations had a higher survival rate than individuals hibernating singly or in small aggregations. No differential survival with respect to hibernation position was found in the winter of 1986/87 when the lowest temperatures were recorded (minimum temperature -16.4-degrees-C). During this extremely cold winter mean survival rates of 33.9% were recorded for C. clienta and 73.3% for B. perversa. The succeeding three winters were mild, with mean survival rates of 83.2% in C. clienta and 91.1% in B. perversa. In both species adults had a higher survival rate than juveniles. After a period of extreme cold (-16-degrees-C) survival of C. clienta inhabiting a snow-covered pile of stones was significantly higher than that of conspecifics on a snow-free stone wall, suggesting that snow cover has an insulating effect.