Thermal and moisture habitat preferences do not maximize jumping performance in frogs

Abstract

1. Amphibians are suffering population declines globally, and understanding how environmental parameters influence their thermal and moisture preferences and performance at various tasks is crucial to understanding how these animals will be influenced by climate change. 2. Body temperature and hydration affect organismal performance at many fitness-related tasks. Since amphibians are ectotherms with highly water-permeable skin, environmental temperature and moisture directly affect their body temperature and hydration. Therefore, amphibians should select habitats with the optimal combination of temperature and moisture to perform tasks necessary for survival. However, interactions between environmental temperature and moisture can influence habitat selection and task performance in different and often unpredictable ways, and this has only infrequently been considered. 3. We tested for interactions between environmental temperature, moisture and organismal hydration on temperature and moisture preferences and jumping performance in green frogs (Lithobates clamitans) in the laboratory, using thermal and moisture gradients, and high-speed video and force plate data. We then integrated the laboratory experiments with field data. 4. In the thermal and moisture gradients, frogs selected environmental conditions that minimized cutaneous evaporative water loss, hydroregulating more stringently than thermoregulating. These results are consistent with frogs in the field, which had highly variable body temperatures, but were always hydrated above 95% of their standard mass. However, conditions that minimized evaporative water loss frequently did not maximize jumping performance because warmer temperatures conferred greater performance. 5. The ecology of L. clamitans may explain the discrepancy between their preferences and jumping performance optima because the frogs remain in wet environments that serve as refuges from dehydration. In parts of their range where frogs are subjected to warmer and drier conditions, they are likely to select microhabitats that minimize the risk of dehydration, possibly at the expense of their ability to forage and escape from predators.

An empirical test for a zone of canalization in thermal reaction norms

“…Such a mismatch between the average environmental temperature experienced by a population and the optimal temperature is commonly observed (e.g. Campbell et al ., ; Lamb & Gerber, ; Dell et al ., ; Mitchell & Bergmann, ). This may result from the left‐skewed shape of the reaction norm, causing fitness costs of experiencing temperatures above the optimum (causing mortality events) to greatly exceed those of experiencing temperatures below the optimum (Martin & Huey, ).…”

“…Such a mismatch between the average environmental temperature experienced by a population and the optimal temperature is commonly observed (e.g. Campbell et al, 1974;Lamb & Gerber, 1985;Dell et al, 2011;Mitchell Table 1 Variance components of thermal reaction norm parameters for three life-history traits in a population of Daphnia magna. Variance components were obtained from mixed-effect models.…”

“…Size at maturation (lg 2 ) 13.02 8.62 2.86 -Somatic growth rate (day À2 ) 0.56 9 10 À4 11.31 9 10 À4 1.06 9 10 À4 4.97 9 10 À6 Age at maturation (ln days) 2 0.00333 0.00750 0.02648 0.00155 Bergmann, 2016). This may result from the leftskewed shape of the reaction norm, causing fitness costs of experiencing temperatures above the optimum (causing mortality events) to greatly exceed those of experiencing temperatures below the optimum (Martin & Huey, 2008).…”

“…Gavrilets & Scheiner, ; de Jong, ), linear models may be particularly relevant for understanding the evolution of thermal reaction norms because the range of temperatures normally experienced by ectotherms is commonly below the optimum temperature, especially for temperate ectotherms (e.g. Campbell et al ., ; Deutsch et al ., ; Kingsolver, ; Dell et al ., ; Nilsson‐Örtman et al ., ; Thomas et al ., ; Richter‐Boix et al ., ; Mitchell & Bergmann, ; Amarasekare & Johnson, ). Therefore, the evolution of thermal reaction norms for these organisms should essentially concern the temperature range where performance increases monotonically and close to linearly.…”

When water interacts with temperature: Ecological and evolutionary implications of thermo‐hydroregulation in terrestrial ectotherms

“…Ectotherms have evolved diverse indirect means to control rates of water loss such as mucus secretion in anurans (Lillywhite, 1971), changes in the F I G U R E 2 Performance curves. Examples of such curves can be found in studies of maximal locomotor capacities in dehydrated amphibians (Anderson & Andrade, 2017;Mitchell & Bergmann, 2016;Preest & Pough, 1989) and of muscular and cognitive performances in endotherms (Cheuvront & Kenefick, 2014). (a) Hypothetical thermal performance curve.…”