Abstract
Species’ tolerance limits determine their capacity to tolerate climatic extremes and limit their potential distributions. Interspecific variation in thermal tolerances is often proposed to indicate climatic vulnerability and is, therefore, the subject of many recent meta-studies on differential capacities of species from climatically different habitats to deal with climate change. Most studies on thermal tolerances do not acclimate animals or use inconsistent, and insufficient, acclimation times, limiting our knowledge of the shape, duration and extent of acclimation responses. Consequently patterns in thermal tolerances observed in meta-analyses, based on data from the literature are based on inconsistent, partial acclimation and true trends may be obscured. In this study we describe time-course of complete acclimation of critical thermal minima in the tropical ectotherm Carlia longipes and compare it to the average acclimation response of other reptiles, estimated from published data, to assess how much acclimation time may contribute to observed differences in thermal limits. Carlia longipes decreased their lower critical thermal limits by 2.4°C and completed 95% of acclimation in 17 weeks. Wild populations did not mirror this acclimation process over the winter. Other reptiles appear to decrease cold tolerance more quickly (95% in 7 weeks) and to a greater extent, with an estimated average acclimation response of 6.1°C. However, without data on tolerances after longer acclimation times available, our capacity to estimate final acclimation state is very limited. Based on the subset of data available for meta-analysis, much of the variation in cold tolerance observed in the literature can be attributed to acclimation time. Our results indicate that (i) acclimation responses can be slow and substantial, even in tropical species, and (ii) interspecific differences in acclimation speed and extent may obscure trends assessed in some meta-studies. Cold tolerances of wild animals are representative of cumulative responses to recent environments, while lengthy acclimation is necessary for controlled comparisons of physiological tolerances. Measures of inconsistent, intermediate acclimation states, as reported by many studies, represent neither the realised nor the potential tolerance in that population, are very likely underestimates of species’ physiological capacities and may consequently be of limited value.
Highlights
Climate patterns within species’ geographic distributions shape their physiological traits and tolerances [1,2,3]
When acclimated to a constant low temperature regime of 18–22°C, critical thermal minimum temperatures (CTmin) in C. longipes rapidly decreased over the first few weeks and subsequently approached a predicted asymptote of 6.8–7.6°C, representative of a drop of 2.4°C compared to that of animals collected from the wild (9.67 ± 0.89°C; mean ± SE)
The acclimation response of Carlia longipes (Fig 1), as well as that of other reptiles (Fig 4), was substantial, prolonged and explained the bulk of the interspecific differences observed in the literature
Summary
Climate patterns within species’ geographic distributions shape their physiological traits and tolerances [1,2,3]. Ectotherms are limited by colder temperatures at high latitudes [14,15,16] and many species that extend into temperate regions have only achieved these range expansions through mechanisms such as viviparity [14, 17], brumation periods over winter months [18], metabolic compensation or inverse acclimation of metabolic rates at low temperatures [19], lowered critical minimum temperatures or other specialised cold adaptations [20, 21]. Tolerances of low temperatures, appear to show a clear trend across latitude [9, 13, 22, 23], reflecting geographic trends in minimum temperatures [10, 22]
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