Winter is a key driver of individual performance, community composition, and ecological interactions in terrestrial habitats. Although climate change research tends to focus on performance in the growing season, climate change is also modifying winter conditions rapidly.Changes to winter temperatures, the variability of winter conditions, and winter snow cover can interact to induce cold injury, alter energy and water balance, advance or retard phenology, and modify community interactions. Species vary in their susceptibility to these winter drivers, hampering efforts to predict biological responses to climate change. Existing frameworks for predicting the impacts of climate change do not incorporate the complexity of organismal responses to winter. Here, we synthesise organismal responses to winter climate change, and use this synthesis to build a framework to predict exposure and sensitivity to negative impacts, and that can be used to estimate the vulnerability of species to winter climate change. We describe the importance of relationships between winter conditions and performance during the growing season in determining fitness, and demonstrate how summer and winter processes are linked.Incorporating winter into current models will require concerted effort from theoreticians and empiricists, and the expansion of current growing season studies to incorporate winter.

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The time required to recover from cold-induced paralysis (chill-coma) is a common measure of insect cold tolerance used to test central questions in thermal biology and predict the effects of climate change on insect populations. The onset of chill-coma in the fall field cricket (Gryllus pennsylvanicus, Orthoptera: Gryllidae) is accompanied by a progressive drift of Na + and water from the hemolymph to the gut, but the physiological mechanisms underlying recovery from chill-coma are not understood for any insect. Using a combination of gravimetric methods and atomic absorption spectroscopy, we demonstrate that recovery from chill-coma involves a reestablishment of hemolymph ion content and volume driven by removal of Na + and water from the gut. Recovery is associated with a transient elevation of metabolic rate, the time span of which increases with increasing cold exposure duration and closely matches the duration of complete osmotic recovery. Thus, complete recovery from chill-coma is metabolically costly and encompasses a longer period than is required for the recovery of muscle potentials and movement. These findings provide evidence that physiological mechanisms of hemolymph ion content and volume regulation, such as ion-motive ATPase activity, are instrumental in chill-coma recovery and may underlie natural variation in insect cold tolerance.ionoregulation | metabolism | osmotic homeostasis | thermal limits | stress resistance

Among-population variation in insect thermal performance is important for understanding patterns and mechanisms of evolution and predicting insect responses to altered climate regimes in future or novel environments. Here we review and discuss several key examples of among-population variation in insect thermal performance, including latitudinal gradients in chill coma recovery time, variation in energy consumption and metabolic biochemistry, rapid changes in thermal biology with range expansion in invasive and introduced species, and potential constraints on variation in thermal performance traits. This review highlights that while there is substantial evidence for among-population variation that is generally correlated with local climate regimes, neither the underlying mechanisms nor the implications for whole-animal fitness in the field are well understood. We also discuss the potential limitations of interpreting evolved variation among populations and argue for a genes-to-environment approach to population-level variation in thermal biology of insects.

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