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Synopsis In response to rapidly changing environmental conditions, many organisms are experiencing shifts in geographic ranges and in the timing and expression of key life-history traits, which have important effects on fitness. However, the physiological mechanisms that mediate these phenotypic responses, such as endocrine and other signaling pathways are not well understood. This information will be critical for predicting organismal responses to climate change because physiological mechanisms are often highly responsive to environmental cues and influence the phenotypic variation available to selection. Additionally, they often integrate suites of correlated traits and are thus expected to influence the evolutionary response to selection. The overarching goals of this symposium were to gain novel insights into the physiological mechanisms that underlie organismal responses to rapidly changing environmental conditions and to identify gaps in knowledge and experimental approaches to advance the field. Here we review and discuss the symposium contributions and the research themes that emerged as important foci for future studies. 

Abstract Phenotypic expression is often constrained by functional conflicts between traits, and the resulting trade-offs impose limits on phenotypic and taxonomic diversity. However, the underlying mechanisms that maintain trade-offs or allow organisms to resolve them via phenotypic plasticity are often challenging to detect. The trade-off between gas exchange and water loss across respiratory surfaces represents a fundamental trade-off that constrains phenotypic diversity in terrestrial life. Here, we investigate plastic mechanisms that mitigate this trade-off in lungless salamanders that breathe exclusively across their skin. Our field and laboratory experiments identified plastic responses to environmental variation in water loss and oxygen uptake, and gene expression analyses identified putative pathways that regulate this trade-off. Although the trade-off was generally strong, its strength covaried with environmental conditions. At the molecular level, antagonistic pleiotropy in multiple biological pathways (e.g., vasoconstriction and upregulation of aerobic respiration) putatively produce the trade-off, while other pathways mitigate the trade-off by affecting a single trait (e.g., oxygen binding affinity, melanin synthesis). However, organisms are likely to encounter novel trade-offs in the process of bypassing another. Our study provides evidence that alternative pathways allow organisms to mitigate pleiotropic conflicts, which ultimately may allow greater phenotypic diversity and persistence in novel environments. 

Synopsis Terrestrial environments pose many challenges to organisms, but perhaps one of the greatest is the need to breathe while maintaining water balance. Breathing air requires thin, moist respiratory surfaces, and thus the conditions necessary for gas exchange are also responsible for high rates of water loss that lead to desiccation. Across the diversity of terrestrial life, water loss acts as a universal cost of gas exchange and thus imposes limits on respiration. Amphibians are known for being vulnerable to rapid desiccation, in part because they rely on thin, permeable skin for cutaneous respiration. Yet, we have a limited understanding of the relationship between water loss and gas exchange within and among amphibian species. In this study, we evaluated the hydric costs of respiration in amphibians using the transpiration ratio, which is defined as the ratio of water loss (mol H2O d−1) to gas uptake (mol O2 d−1). A high ratio suggests greater hydric costs relative to the amount of gas uptake. We compared the transpiration ratio of amphibians with that of other terrestrial organisms to determine whether amphibians had greater hydric costs of gas uptake relative to plants, insects, birds, and mammals. We also evaluated the effects of temperature, humidity, and body mass on the transpiration ratio both within and among amphibian species. We found that hydric costs of respiration in amphibians were two to four orders of magnitude higher than the hydric costs of plants, insects, birds, and mammals. We also discovered that larger amphibians had lower hydric costs than smaller amphibians, at both the species- and individual-level. Amphibians also reduced the hydric costs of respiration at warm temperatures, potentially reflecting adaptive strategies to avoid dehydration while also meeting the demands of higher metabolic rates. Our results suggest that cutaneous respiration is an inefficient mode of respiration that produces the highest hydric costs of respiration yet to be measured in terrestrial plants and animals. Yet, amphibians largely avoid these costs by selecting aquatic or moist environments, which may facilitate more independent evolution of water loss and gas exchange.