1. Critical thermal limits represent an important component of an organism's capacity to cope with future temperature changes. Understanding the drivers of variation in these traits may uncover patterns in physiological vulnerability to climate change. Local temperature extremes have emerged as a major driver of thermal limits, although their effects can be mediated by the exploitation of fine‐scale spatial variation in temperature through behavioural thermoregulation. 2. Here, we investigated thermal limits along elevation gradients within and between two cold‐water frog species (Ascaphus spp.), one with a coastal distribution (A. truei) and the other with a continental range (A. montanus). We quantified thermal limits for over 700 tadpoles, representing multiple populations from each species. We combined local temporal and fine‐scale spatial temperature data to quantify local thermal landscapes (i.e., thermalscapes), including the opportunity for behavioural thermoregulation. 3. Lower thermal limits for either species could not be reached experimentally without the water freezing, suggesting that cold tolerance is <0.3°C. By contrast, upper thermal limits varied among populations, but this variation only reflected local temperature extremes in A. montanus, perhaps as a consequence of the greater variation in stream temperatures across its range. Lastly, we found minimal fine‐scale spatial variability in temperature, suggesting limited opportunity for behavioural thermoregulation and thus increased vulnerability to warming for all populations. 4. By quantifying local thermalscapes, we uncovered different trends in the relative vulnerability of populations across elevation for each species. In A. truei, physiological vulnerability decreased with elevation, whereas in A. montanus, all populations were equally physiologically vulnerable. These results highlight how similar environments can differentially shape physiological tolerance and patterns of vulnerability of species, and in turn impact their vulnerability to future warming.

Trade-offs shaping behavioral variation are often influenced by the environment. We investigated the role that the environment plays in mediating trade-offs using a widespread frog with a conspicuous mating display, Pseudacris crucifer. We first demonstrated, using playback and desiccation experiments, that calling site selection involves a trade-off between sound transmission and desiccation. We then determined the influence of local environmental conditions on the intensity of the trade-off by examining range-wide behavioral and environmental data. We showed that the benefit of improved call transmission is positively influenced by vegetation density and ground cover. Behavioral data are consistent with this relationship: sites with a greater transmission benefit have increased prevalence of arboreally calling males. We also found that the prevalence of arboreal calling behavior increases with relative humidity and air temperature, suggesting an influence of these environmental variables on the desiccation cost of arboreal calling. This study provides a clear example of the role of the environment in mediating trade-off intensities and shaping critical behavioral traits. Local environment mediates the intensity of a trade-off associated with arboreal calling behavior in a treefrog. Combining observational and experimental approaches, we show that arboreal calling behavior increases the transmission of a mating call while potentially subjecting individuals to a rate of desiccation six times greater than terrestrial calling. Local environmental conditions influence both the benefit and the cost of this trade-off, subjecting different populations to varying trade-off intensities and shaping arboreal calling behavior.

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Anuran tadpoles exhibit extraordinary diversity in mouthpart morphology, reflecting phylogenetic relationships, life history specializations, and ecological adaptations. In this study, we investigate patterns and potential environmental drivers of variation in labial teeth counts across and within the coastal tailed frog (Ascaphus truei) and the Rocky Mountain tailed frog (A. montanus), which together constitute the basal frog family Ascaphidae. Tailed frogs occupy cold, fast-flowing, high gradient streams in mesic forests of western North America, with differences in environmental conditions that reflect their coastal and continental distributions. Despite a very strong relationship between the number of labial teeth and developmental (Gosner) stage, we found significant relationships between tooth counts and site-specific environmental predictors after controlling for stage. The most important environmental predictor varied between the species, with percent canopy cover for A. truei and average benthic substrate size for A. montanus. There were also differences in the overall tooth counts between species, with A. truei having more teeth in all rows compared to A. montanus. These findings demonstrate that differences in oral morphology in response to local environmental conditions can override the homogenizing effects of gene flow in both Ascaphus species, likely through a combination of local adaptation and phenotypic plasticity. Follow-up investigations should include the assessment of community level variability across sites, and experimental tests to confirm functional relationships between environmental variation and oral morphology, in addition to partitioning genetic from plastic mechanisms.

Estimates of organismal thermal tolerance are frequently used to assess physiological risk from warming, yet the assumption that these estimates are predictive of mortality has been called into question. We tested this assumption in the cold-water-specialist frog, Ascaphus montanus . For seven populations, we used dynamic experimental assays to measure tadpole critical thermal maximum (CTmax) and measured mortality from chronic thermal stress for 3 days at different temperatures. We tested the relationship between previously estimated population CTmax and observed mortality, as well as the strength of CTmax as a predictor of mortality compared to local stream temperatures capturing varying timescales. Populations with higher CTmax experienced significantly less mortality in the warmest temperature treatment (25°C). We also found that population CTmax outperformed stream temperature metrics as the top predictor of observed mortality. These results demonstrate a clear link between CTmax and mortality from thermal stress, contributing evidence that CTmax is a relevant metric for physiological vulnerability assessments.