Winter torpor in Helix pomatia: regulated defence mechanism or forced inactivity?

Abstract

We examined Helix pomatia L., 1758 snails to answer two questions. First, is their winter dormancy facilitated by cold-seeking behaviour, or is it rather a passive phenomenon imposed by cold weather? Second, what thermal conditions are necessary to arouse these snails out of torpor in midwinter and in spring? Snails, collected from natural habitats, were placed in a temperature-gradient apparatus. Their thermal preference was recorded for 48 h under natural photoperiod in early spring, midsummer, and autumn. The autumn recording was continued until the end of November on non-starved and starved snails. Two additional sessions were performed on torpid snails in January (latencies of arousals from torpor at various temperatures and immediate thermal preferences were recorded) and in February (they were kept permanently in darkness at 5 °C and their spontaneous arousals were examined). Temperature selected in spring (23.60 ± 0.15 °C) was significantly lower than that in summer (26.90 ± 0.05; P < 0.05) and in autumn (27.50 ± 0.10 °C; P < 0.01). The prolonged autumn recording did not show cold-seeking behaviour either in non-starved or in starved snails. Threshold temperature inducing midwinter arousals was 10 °C and the aroused snails immediately selected temperatures indistinguishable from those in summer and autumn. Spontaneous arousals from torpor in the snails continuously exposed to cold occurred within a period of arousals in the field. In conclusion, winter torpor of the snails displays unique properties, i.e., its start and maintenance are passive phenomena, while its spring interruption is a precisely controlled, endogenous mechanism.