In vertebrates, changes in surface temperature following exposure to an acute stressor are thought to be promising indicators of the physiological stress response that may be captured noninvasively by infrared thermography. However, the efficacy of using stress‐induced changes in surface temperature as indicators of physiological stress‐responsiveness requires: (1) an understanding of how such responses vary across the body, (2) a magnitude of local, stress‐induced thermal responses that is large enough to discriminate and quantify differences among individuals with conventional technologies, and (3) knowledge of how susceptible measurements across different body regions are to systematic error. In birds, temperature of the bare tissues surrounding the eye (the periorbital, or “eye,” region) and covering the bill have each been speculated as possible predictors of stress physiological state. Using the domestic pigeon (Columba livia domestica; n = 9), we show that stress‐induced changes in surface temperature are most pronounced at the bill and that thermal responses at only the bill have sufficient resolution to detect and quantify differences in responsiveness among individuals. More importantly, we show that surface temperature estimates at the eye region experience greater error due to changes in bird orientation than those at the bill. Such error concealed detection of stress‐induced thermal responses at the eye region. Our results highlight that: (1) in some species, bill temperature may serve as a more robust indicator of autonomic stress‐responsiveness than eye region temperature, and (2) future studies should account for spatial orientation of study individuals if inference is to be drawn from infrared thermographic images.

Adult lake trout held at two temperatures were interbred to study the influence of parental thermal environments on the next generation’s thermal physiology. Offspring performance reflected both their own rearing environment and parental influences, although parental effects on offspring physiology were limited and not always beneficial.

Brook trout (Salvelinus fontinalis) exhibit within-generation and transgenerational plasticity for thermal performance, although neither response appears sufficient to cope with long-term climate change effects.

No abstract

Ecogeographical rules, describing common trends in animal form across space and time, have provided key insights into the primary factors driving species persistence on our planet. Among the most well-known ecogeographical rules are James’ rule (an intraspecific variant of Bergmann’s rule) and Allen’s rule, with each correlating ambient temperature to the size and shape of endotherms within a species. These two rules have recently gained renewed research attention, largely with the goal of understanding how they emerge (e.g. via natural selection or phenotypic plasticity), and thus, whether they may emerge quickly enough to facilitate species persistence in a warming world. Yet despite this attention, the precise proximate and ultimate drivers of James’ and Allen’s rules remain unresolved. In this paper, we review over a century of empirical literature surrounding these rules and ask whether each could be explained by plastic effects of developmental temperature on adult phenotype among endotherms. Across birds and mammals, studies strongly support developmental plasticity as a driver of James’ and Allen’s rules, particularly with regards to Allen’s rule and responses to heat. However, we find that plastic contributions toward each are non-linear and probably depend on: (1) efficiency of energy use at given ambient temperatures (James’ rule), and (2) thermal advantages at given ambient temperatures (Allen’s rule). These findings suggest that, among endotherms, rapid changes in body shape and size will continue to occur, but generalising the direction of responses across populations (e.g. as “shrinking” or “shape-shifting”) is likely naive.

Ecogeographical rules, describing common trends in animal form across space and time, have provided key insights into the primary factors driving species diversity on our planet. Among the most well-known ecogeographical rules are Bergmann's Rule and Allen's rule, with each correlating ambient temperature to the size and shape of endotherms within a species. These two rules have recently attracted renewed research attention, largely with the goal of understanding how they emerge (e.g., via natural selection or phenotypic plasticity), and thus, whether they may emerge quickly enough to aid adaptations to a warming world. Yet despite this attention, the precise proximate and ultimate drivers of Bergmann's and Allen's rules remain unresolved. In this conceptual paper, we articulate novel and classic hypotheses for understanding whether and how plastic responses to developmental temperatures might contributed to each rule. Next, we compare over a century of empirical literature surrounding Bergmann's and Allen's Rules against our hypotheses to uncover likely avenues by which developmental plasticity might drive temperature-phenotype correlations. Across birds and mammals, studies strongly support developmental plasticity as a driver of Bergmann's and Allen's rules, particularly with regards to Allen's rule and responses to heat. However, plastic contributions toward each appear largely non-linear and dependent upon: (1) efficiency of energy use (Bergmann's rule), and (2) thermal advantages (Allen's Rule) at given ambient temperatures. These findings suggest that, among endotherms, rapid changes in body shape and size will continue to occur, but generalizing the direction of responses across populations is likely naive.

In response to stressors, many species limit reproduction, reduce investment in immunity, and even forgo somatic repair to free energy for use toward coping. Often, however, the energetic costs of simply regulating body temperature can exceed those required for each of these processes, yet whether thermoregulation is also eschewed in response to stressors is not known. In this study, we used a phylogenetically-controlled meta-regression to test whether changes in body temperature that accompany the stress response - a common phenomenon among terrestrial vertebrates - represent the outcome of an energy allocation trade-off between thermoregulation and responding to stressors in these species. Our results, obtained from 65 studies and across 24 species, support this trade-off hypothesis by showing that body temperatures tended to drift toward ambient following stress exposures, but particularly in species with high costs of thermoregulation and little energy available for use (i.e those with low body mass and high resting metabolism respectively). These cross-species findings indicate that terrestrial endotherms may trade-off investment toward thermoregulation for that toward the stress response. With such findings in mind, evaluating how these endotherms may cope with warming climates and concurrent stressors should be key priority in the coming decade.

Environmental temperatures associated with climate change are rising too rapidly for many species to adapt, threatening the persistence of taxa with limited capacities for thermal acclimation. We investigated the capacity for within- and transgenerational responses to increasing environmental temperatures in brook trout (Salvelinus fontinalis), a cold-adapted salmonid. Adult fish were acclimated to temperatures within (10□) and above (21□) their thermal optimum for six months before spawning, then mated in a full factorial breeding design to produce offspring from cold- and warm-acclimated parents as well as bidirectional crosses between parents from both temperature treatments. Offspring families were subdivided and reared at two acclimation temperatures (15□ and 19□) representing their current environment and a projected climate change scenario. Offspring thermal physiology was measured as the rate of oxygen consumption (MO2) during an acute change in temperature (+2□ h-1) to observe their MO2-temperature relationship. We also recorded resting MO2, the highest achieved (peak) MO2, and critical thermal maximum (CTM) as performance metrics. Within-generation plasticity was greater than transgenerational plasticity, with offspring acclimation temperature having demonstrable effects on peak MO2 and CTM. Transgenerational plasticity was evident as an elevated resting MO2 and the MO2-temperature relationship in offspring from warm-acclimated parents. Both parents contributed to offspring thermal responses, although the paternal effect was stronger. Although brook trout exhibit both within- and transgenerational plasticity for thermal physiology, it is unlikely that these will be sufficient for coping with long-term changes to environmental temperatures resulting from climate change.SummaryBrook trout (Salvelinus fontinalis) exhibit within-generation and transgenerational plasticity for thermal performance, although neither response appears sufficient to cope with long-term climate change effects.