A unifying, eco‐physiological framework for animal dormancy

A tenet of macroecology is that physiological processes of organisms are linked to large-scale geographical patterns in environmental conditions. Species at higher latitudes experience greater seasonal temperature variation and are consequently predicted to withstand greater temperature extremes. We tested for relationships between breadths of thermal tolerance in ectothermic animals and the latitude of specimen location using all available data, while accounting for habitat, hemisphere, methodological differences and taxonomic affinity. We found that thermal tolerance breadths generally increase with latitude, and do so at a greater rate in the Northern Hemisphere. In terrestrial ectotherms, upper thermal limits vary little while lower thermal limits decrease with latitude. By contrast, marine species display a coherent poleward decrease in both upper and lower thermal limits. Our findings provide comprehensive global support for hypotheses generated from studies at smaller taxonomic subsets and geographical scales. Our results further indicate differences between terrestrial and marine ectotherms in how thermal physiology varies with latitude that may relate to the degree of temperature variability experienced on land and in the ocean.

To date, all altered patterns of seasonal interactions observed in insects, birds, amphibians, and plants associated with global warming during the latter half of the 20th century are explicable as variable expressions of plastic phenotypes. Over the last 30 years, the genetically controlled photoperiodic response of the pitcher-plant mosquito, Wyeomyia smithii, has shifted toward shorter, more southern daylengths as growing seasons have become longer. This shift is detectable over a time interval as short as 5 years. Faster evolutionary response has occurred in northern populations where selection is stronger and genetic variation is greater than in southern populations. W. smithii represents an example of actual genetic differentiation of a seasonality trait that is consistent with an adaptive evolutionary response to recent global warming.

Daily torpor and hibernation are the most powerful measures of endotherms to reduce their energy expenditure. During entrance into these torpid states metabolic rate is suppressed to a fraction of euthermic metabolism, paralleled by reductions in ventilation and heart rate. Body temperature gradually decreases towards the level of ambient temperature. In deep torpor body temperature as well as metabolic rate are controlled at a hypothermic and hypometabolic level. Torpid states are terminated by an arousal where metabolic rate spontaneously returns to normal levels again and euthermic body temperature is established by a burst of heat production. In recent years some of the cellular mechanisms which contribute to hypometabolism have been disclosed. Transcription, translation, as well as protein synthesis are largely suppressed. Cell proliferation in highly proliferating epithelia like the intestine is suspended. ATP production from glucose is reduced and lipids serve as the major substrate for remaining energy requirements. All these changes are rapidly reverted to normometabolism during arousal. Hibernation and daily torpor are found in small mammals inhabiting temperate as well as tropical climates. It indicates that this behaviour is not primarily aimed for cold defense, instead points to a general role of hypometabolism, as a measure to cope with a timely limited or seasonal bottleneck of energy supply.