Voltinism-associated differences in winter survival across latitudes: integrating growth, physiology, and food intake.

Species that span large latitudinal gradients face strong differences in voltinism and in winter conditions within their range. Latitudinal gradients in winter survival and especially their underlying mechanisms and association with voltinism patterns are poorly studied. We tested in the damselfly Enallagma cyathigerum whether high-latitude populations were better in dealing with the longer winters compared to central- and low-latitude populations and whether this was associated with changes in voltinism. We thereby evaluated whether higher initial levels and/or lower reductions during winter of energy storage (measured as fat content) and investment in immune function [measured as the activity of phenoloxidase (PO)], and/or stronger compensatory responses in food intake contributed to the higher winter survival in high-latitude populations. To this end, we simulated a long high-latitude winter at 4 °C under manipulated food conditions. Across food levels, winter survival was highest in Swedish larvae, intermediate in Belgian larvae, and lowest in Spanish larvae, indicating latitude-specific thermal adaptation that could be partly linked to differences in voltinism. The semi-voltine Swedish larvae were growing slower before winter and as a result accumulated the highest fat content and PO activity when the winter started compared to the univoltine, faster growing Belgian, and Spanish larvae. Fat content and PO activity declined during the winter, yet equally across latitudes, and were not buffered by compensatory food intake. Our data identified possible underlying physiological mechanisms of winter survival and support the hypothesis that widespread latitude-associated voltinism shifts may be a selective factor contributing to latitudinal shifts in winter survival.