Testing the heat-invariant and cold-variability tolerance hypotheses across geographic gradients

Changes in temperature across geographic gradients can occur on a wide temporal range, from fluctuations within hours as a result of day-night to those over many years. These events will drive many organisms towards their physiological limits of thermal tolerance. Recently, many reports support a limited scope for adaptive evolutionary responses to high temperatures, meaning a conserved heat tolerance among ectotherms in general. We address this problem and tested the heat and cold tolerance invariant-variant hypotheses in terrestrial isopods. We studied five different populations of Porcellio laevis and three populations of Porcellio scaber, spanning 30° S latitudinal gradient in Chile. The heat tolerance of woodlice was conserved with little variation along latitude and environmental temperatures, but cold tolerance decreases significantly with environmental temperatures and latitudes. Indeed, a significant and negative correlation was observed between cold tolerance and latitude. Also, significant and positive correlations were observed among cold tolerance and environmental temperatures. Conversely, heat tolerance was not significantly correlated with any of the environmental temperatures tested neither with latitude. This macrophysiological pattern indicated that heat and cold-tolerances of species and populations not always change across geographical gradients meaning that thermal tolerance responses to high temperatures may be evolutionary constrained.