AbstractUnderstanding how phenotypic plasticity is generated, whether through shared or divergent molecular mechanisms, is critical for predicting organismal responses to environmental change. Similar phenotypes can arise from different pathways, which may vary in costs, trade-offs, and consequences for performance. This study examined whether physiological plasticity in a woodland salamander (Plethodon metcalfi) was shaped by common or distinct gene expression patterns across populations distributed along an elevational gradient. In a laboratory acclimation experiment, I measured plasticity in skin resistance to water loss and metabolic rate and used transcriptomic analyses to identify the underlying molecular mechanisms. Plasticity in skin resistance to water loss did not vary with elevation, yet the expression of molecular pathways associated with this plasticity differed depending on elevational origin. Low-elevation populations also tended to exhibit greater independent regulation of metabolic rate, despite the constraints imposed by the trade-off between metabolic rate and skin resistance. These findings reveal a distinct phenomenon: geographic divergence in plasticity mechanisms, where similar plastic phenotypes emerge from different molecular pathways. Despite being similar in magnitude, plasticity in some populations may be locally adapted through alternative mechanisms that differ in their costs and benefits, complicating our ability to predict responses to environmental change.

Climate change is shifting when animals breed [C. Parmesan, G. Yohe, Nature 421, 37-42 (2003) and S. J. Thackeray et al., Nature 535, 241-245 (2016)], but it is not clear why some populations keep pace with warming while others fall behind [L. D. Bailey et al., Nat. Commun. 13, 2112 (2022) and J. M. Samplonius et al., Glob. Change Biol. 24, 3780-3790 (2018)]. Differences could arise from variation in sensitivity to temperature [L. D. Bailey et al., Nat. Commun. 13, 2112 (2022)] or constraints on the ability to respond to temperature. Without knowing whether populations differ in sensitivity-or in their ability to act on that sensitivity-we cannot identify which are most at risk. Using 1,555 population-years from 123 populations of tree swallows (Tachycineta bicolor), we show that populations have similar sensitivity to local temperature, advancing breeding by about one day per degree of warming. However, northern populations face tighter time constraints and greater exposure to recent warming. Northern populations have advanced laying dates the most, but still experience stronger selection for earlier breeding, especially in warm years; they have also declined most in breeding abundance. These findings suggest that vulnerability to climate change can arise not just from different sensitivity to warming, but from when and where populations can respond effectively. By disentangling sensitivity from timing constraints, our results are consistent with a general mechanism by which even uniformly responsive species can show uneven impacts of climate change across their ranges.

Population growth models are essential tools for predicting population demographic trends and assessing population viability over time. Population growth models are often parameterized using demographic data that account for imperfect detection; however, historical datasets—collected prior to the widespread incorporation of imperfect detection—may still yield valuable insights into long-term population dynamics. Here, we used a deterministic age structured model and a stochastic individual based model, which were both parameterized with age-specific vital rates, to estimate population growth in a woodland salamander (Plethodon metcalfi). To parameterize our model, we used long-term historical demographic data collected by Hairston (1983) to estimate population growth. Our objectives were to evaluate differences between the population growth models and assess whether historical demographic data can produce realistic estimates of population dynamics. We found that the individual based model predicted a declining population size, whereas the age structured model predicted an increasing population size. However, sensitivity analyses revealed that minimal changes to survival or fecundity were sufficient to produce stable populations in both models, reflecting observations of these populations in the wild. Therefore, our results suggest that historical data can be informative even in the absence of detection-corrected estimates in cryptic species. We also found that both models were capable of predicting age distributions similar to those observed in nature. Together, these results emphasize the importance of model selection and utility of historical datasets in forecasting population resilience under variable environmental conditions.

The external appearance of frog skin varies among species and across body regions. Although this variation has long been recognized, it remains an understudied aspect of frog diversity. Further, the evolutionary processes driving this variation are unclear because previous work has been largely qualitative. Here we quantify the skin texture of 187 species spanning 45 of the 57 frog families using standardized gel-based profilometry. Using phylogenetic comparative models we explore the extent to which skin texture differs among body regions, how these differences have evolved across major frog clades, and whether microhabitat, climate, and body size help explain texture patterns. We find that the ventral posterior region, which functions in water uptake and rehydration, tends to be rougher than other body regions, yet skin texture evolves at similar rates across the body. Microhabitat, particularly arboreality, is linked to greater skin texture variation among body regions. Among species occupying more terrestrial microhabitats (e.g., arboreal, burrowing, leaflitter), we find that body size and local climate has little effect on skin texture. By comparing skin texture across a wide range of species and environments worldwide, our study tests hypotheses about external skin diversity in frogs that have previously received limited comparative evaluation and highlights texture as an informative component of amphibian skin biology.

1. Amphibians are frequently identified as highly vulnerable to climate change, yet the mechanisms driving this sensitivity remain uncertain. Approaches that explicitly link physiological mechanisms to environmental variation provide powerful tools for forecasting climate vulnerability. However, their reliability depends on assumptions that accurately reflect the conditions amphibians experience in nature. 2. Here, we evaluate the physiological mechanisms most often invoked to explain amphibian climate vulnerability, including overheating, desiccation, energetic constraints and seasonal dormancy and assess the ecological realism of current modelling frameworks. 3. We show that broad-scale assessments often overlook widespread behavioural buffering, such as nocturnal activity, subterranean refuge use and close association with saturated microhabitats, which substantially reduce exposure to extreme temperatures and water loss. We also identify opportunities to improve mechanistic models by incorporating microclimatic heterogeneity, behavioural avoidance of stressful conditions and hydric landscape dynamics. 4. Doing so will clarify when and where climate change poses a true physiological threat and provide greater insight into the processes shaping amphibian extinction risk. Grounding mechanistic forecasts in ecological reality is essential for avoiding mischaracterization of risk and for directing limited resources towards amphibian research, species and regions most in need.

Most animals have a complex life cycle that allows organisms to evolve distinct phenotypes at different life stages, termed developmental or adaptive decoupling. Support for developmental decoupling has been found in a wide range of morphological, physiological, behavioral, and molecular traits, largely from studies on insects and amphibians. However, few studies have focused on morphological structures that are continuously present and developing throughout metamorphosis, which may not evolve as independently as structures only present in one life stage or structures that completely regenerate during metamorphosis. Most anurans undergo metamorphosis, transitioning from larval (tadpole) to adult stages, while remaining continuously active. Critical organs, such as the skin, are present and developing before, during, and after metamorphosis. We measured and compared dorsal and ventral skin layer thicknesses of both tadpoles and adults in 10 anuran species. Our results support developmental decoupling of anuran skin thickness across metamorphosis, agreeing with prior studies of anuran morphological traits. In testing this hypothesis, we found interesting skin thickness variation among tadpole and adult species that was not fully explained by body size, which prompts future studies to test skin adaptation hypotheses within each life stage.

Morphological evolution can be explosive, producing visually spectacular adaptive radiations like Caribbean anoles, Malagasy vangas, and African Rift Lake cichlids. Yet morphological stasis, the long-term retention of a conserved body plan, is often observed across evolutionary radiations. Woodland salamanders ( Plethodon ) are a classic example of such “nonadaptive” radiation, characterized by prolific speciation alongside morphological stasis (i.e., limited morphological divergence), often attributed to phylogenetic conservatism in their climatic and microhabitat niches. However, the multidimensional nature of phenotypes and the niche means that adaptive evolution in less apparent traits can occur even when morphology appears static. We investigated whether woodland salamanders exhibit adaptive divergence in a less conspicuous phenotypic axis—specifically, physiology—and compared patterns and rates of trait evolution to those of morphological traits. We found that most physiological traits are associated with climatic variation and exhibit elevated rates of evolution, high trait disparity, and more frequent shifts in adaptive optima than morphological traits. In particular, skin resistance to water loss, metabolic rate, and cold tolerance exhibit evolutionary signatures of adaptive radiation. Notably, morphology is not entirely static: Some traits show climatic associations, several exhibit localized shifts, and evolutionary rates exceed those of slower evolving physiological traits, such as heat tolerance. Biological systems, as evidenced by woodland salamanders, are not exclusively “conserved” or “labile” in their evolution, and this system illustrates how the same features that limit morphological divergence may also facilitate physiological evolution. Woodland salamanders exemplify how adaptive radiation can proceed despite outward similarity.

Ecological opportunity (EO) is an important catalyst for evolution. Whereas theory often centers around a lineage encountering a source of EO in isolation, in practice they experience numerous sources of opportunity, either concurrently or sequentially. Such multiplicity can obscure the macroevolutionary signature of EO. Here, we test the effects of elevation (a proxy of the "mountain effect") and an array of functional innovations on the evolutionary history of plethodontid salamanders, a diverse and charismatic radiation of lungless amphibians. Functional innovations unlock access to novel microhabitats, ultimately enabling sub-lineages to occupy a diverse range of ecological niches, particularly in lowland areas where those niches are more abundant. Consistent with expanded ecological opportunity, such transitions to lower elevation result in rapid phenotypic evolution. At high elevation, by contrast, rates of phenotypic evolution and phenotypic disparity decline, reflecting a loss of phenotypically extreme ecological specialists. Transitions in elevation and the origin of innovations appear largely coincident among lungless salamanders, suggesting myriad sources of EO. The magnitude of the "mountain effect" on evolutionary rates (∼10-fold) is on par or greatly exceeds that of islands, lakes, and coral reefs on other iconic vertebrate radiations. Therefore, we find that elevation acts as a major ecological moderator and, in concert with functional innovations, shapes the ecological and phenotypic diversity of lungless salamanders.

Gas exchange requires a permeable respiratory surface, yet the same permeability also creates a pathway for water loss, posing a particular challenge for organisms whose respiratory surfaces are directly exposed to the terrestrial environment. Woodland salamanders (Plethodon) are a diverse radiation of lungless, terrestrial amphibians that rely entirely on cutaneous respiration, coupling gas exchange and water loss through a shared surface. Whether this shared surface produces a consistent relationship between water loss and metabolic rate across species, or whether the two traits can diverge despite their functional link, remains unresolved. Here, we quantified metabolic rate, water loss rate, and the ratio between these traits across 30 Plethodon species and related these traits to elevational distributions, geographic range size, and climatic conditions across species' ranges. We also compared the relationship between gas exchange and water loss in Plethodon to that observed across a broader set of amphibian taxa. Metabolic rate and water loss were not tightly correlated across Plethodon, despite a positive association across amphibians more broadly. Instead, physiological traits correlated with different aspects of species' elevational limits, range sizes, and climatic niches. Metabolic rate was most consistently associated with elevation and climate, with lower values observed in species occupying higher, cooler environments. Water loss alone showed weaker and less consistent environmental associations, but the transpiration ratio varied with elevational limits such that species extending into lower-elevation environments exhibited greater hydric efficiency. These patterns were also reflected in phylogenetic path analyses, which supported lower elevational limit as a mediator linking physiology to geographic range size. These results suggest that although cutaneous respiration imposes broad hydric costs across amphibians, Plethodon species do not show a single, shared pattern of physiological coupling between gas exchange and water loss.

Understanding how anthropogenic change impacts metabolic physiology is crucial for predicting species survival and ecosystem dynamics. Microplastics are ubiquitous in both aquatic and terrestrial environments and can disrupt organismal physiology. We used Drosophila melanogaster as a model species to identify the metabolic effects of dietary exposure to 1 µm polystyrene microplastic (MP) and 50 nm nanoplastic (NP) particles. We exposed flies to ecologically relevant and equivalent doses (1.4×1011 particles day-1 kg-1 larvae for MPs; 1.2×1018 particles day-1 kg-1 larvae for NPs) from egg to adult eclosion and used flow-through respirometry to investigate changes in the volume of carbon dioxide production and evaporative water loss rate. We observed that MP exposure disrupted the relationship between carbon dioxide production and water loss rate - suggesting the use of alternative metabolic pathways - while NP exposure did not. Such responses could have implications for physiological function, ecological interactions and evolutionary trajectories amid ongoing environmental change.

Life-history traits evolve to optimize fitness trade-offs across an organism’s life cycle. Potentially to mediate the trade-off between survival and fecundity, multiple animal groups have independently evolved live birth (viviparity), including at least 70 transitions in lizards alone. In lizards, viviparity is thought to evolve as a mechanism to improve embryonic development in cold climates (cold climate hypothesis, or CCH), possibly at the expense of the mother’s survival. Past comparative studies often align with the CCH’s predictions, but they usually treat core features of the hypothesis as implicit and, most importantly, infer process from pattern rather than testing causal mechanisms. To address this, we developed a process-based model that integrates behavior, thermal physiology, life history, and climate to predict optimal gestation length in lizards globally. We generated a comprehensive trait database of 89 lizard populations that vary in parity mode, and we used ecophysiological modelling to test our model’s predictive power. Our model produced accurate predictions, strongly supporting the hypothesis that cold climates favor the evolution of viviparity in lizards and revealing the ecological contexts and underlying mechanisms by which this life history strategy evolves.

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.

Research Highlight: Edwards, O. M., Zhai, L., Reichert, M. S., Shaughnessy, C. A., Ozment, L., & Zhang, B. (2024). Physiological and morphological traits affect contemporary range expansion and implications for species distribution modelling in an amphibian species. Journal of Animal Ecology, . Range expansion can have profound ecological and evolutionary consequences that feedback on the expansion process itself. With global climate change causing widespread species range shifts to higher latitudes and altitudes, it is essential that we better understand these dynamics during native range expansion in the wild. In a recent study on poleward-spreading treefrogs (Hyla cinerea), Edwards et al. (2024) measured how morphological and physiological traits differed between populations from the recently expanded and historic range. They found that range-edge frogs had increased cold tolerance and longer legs associated with better dispersal, which could strongly affect the rate and geographic limits of expansion. Edwards et al. then show how species distribution models fit separately to the historic and expanded range more accurately predict habitat suitability near the historic range boundary. This paper provides a timely and compelling example of rapid differentiation in dispersal and niche traits during native range expansion, and explores ways in which we can model species range shifts while accounting for this phenotypic variation in space and time.

AbstractMultitrait analyses can be used to measure the differential performance of phenotypic traits in species complexes. Hybridization within these complexes can result in a mismatch between mitochondrial and nuclear DNA that may lead to reduced performance and acclimation capacity in hybrids. To test the effect of this mismatch on physiology, we compared physiological performance and acclimation capacity of metabolic rate (V̇˙CO2) and total resistance to water loss (rT) between two sexual Ambystoma species and a closely related unisexual lineage. We also separated unisexuals by their unique biotypes to determine how physiology varies with subgenomic composition. We found that unisexual biotypes exhibited phenotypes more like their related sexual species than other unisexuals. We also found a trade-off between rT and V̇˙CO2, with increasing rT resulting in a decrease in V̇˙CO2. Although we did not find evidence for mitonuclear mismatch, our results indicate that the genomic composition of hybrids may be a suitable predictor of hybrid trait performance. Multitrait analyses are imperative for understanding variation in phenotypic diversity, providing insight into how this diversity affects species responses to environmental change.

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.

Scholander-Irving curves describe the relationship between ambient temperature and metabolic rate and are fundamental to understanding the energetic demands of homeothermy. However, Scholander-Irving curves are typically measured in dry air, which is not representative of the humidity many organisms experience in nature. Consequently, it is unclear (1) whether Scholander-Irving curves (especially below thermoneutrality) are altered by humidity, given the effects of humidity on thermal properties of air, and (2) whether physiological responses associated with Scholander-Irving curves in the lab reflect organismal performance in humid field conditions. We used laboratory experiments and biophysical models to test the effects of humidity on the thermoregulatory physiology of tree swallows (Tachycineta bicolor). We also tested whether physiological responses measured under lab conditions were correlated with field body temperatures and nestling provisioning rates. We found that humidity reduced rates of evaporative water loss but did not have large effects on body temperature or metabolic rate, suggesting that swallows can decouple evaporative cooling, body temperature and metabolic rate. Although the effect of humidity on metabolic rate in the lab was small, our biophysical models indicated that energetic costs of thermoregulation were ∼8% greater in simulations that used metabolic rates from birds in humid compared with dry conditions. Finally, we found mixed evidence that physiological responses measured in the lab under humid or dry conditions were associated with body temperature and nest provisioning rates in the field. Our results help clarify the effect of humidity on endotherm thermoregulation, which may help forecast organismal responses to environmental change.

Hybridization between species affects biodiversity and population sustainability in numerous ways, many of which depend on the fitness of the hybrid relative to the parental species. Hybrids can exhibit fitter phenotypes compared to the parental lineages, and this ‘hybrid vigour’ can then lead to the extinction of one or both parental lines. In this study, we analysed the relationship between water loss and gas exchange to compare physiological performance among three tiger salamander genotypes—the native California tiger salamander (CTS), the invasive barred tiger salamanders (BTS) and CTS × BTS hybrids across multiple temperatures (13.5°C, 20.5°C and 23.5°C). We developed a new index of performance, the water‐gas exchange ratio (WGER), which we define as the ratio of gas exchange to evaporative water loss (μL V O 2 /μL H 2 O). The ratio describes the ability of an organism to support energetically costly activities with high levels of gas exchange while simultaneously limiting water loss to lower desiccation risk. We used flow through respirometry to measure the thermal sensitivity of metabolic rate and resistance to water loss of each salamander genotype to compare indices of physiological performance. We found that temperature had a significant effect on metabolic rate and resistance to water loss, with both traits increasing as temperatures warmed. Across genotypes, we found that hybrids have a higher WGER than the native CTS, owing to a higher metabolic rate despite having a lower resistance to water loss. These results provide a greater insight into the physiological mechanisms driving hybrid vigour and offer a potential explanation for the rapid spread of salamander hybrids. More broadly, our introduction of the WGER may allow for species‐ or lineage‐wide comparisons of physiological performance across changing environmental conditions, highlighting the insight that can be gleaned from multitrait analysis of organism performance. Read the free Plain Language Summary for this article on the Journal blog.

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.