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Metabolic impacts of climate warming Organisms living at mid- to high latitudes in the Northern Hemisphere have been predicted to be potentially the most affected by climate warming, as that is where temperatures have risen most rapidly. But Michael Dillon and colleagues now turn the spotlight onto the prospects for ectotherms — 'cold blooded' animals that regulate their body temperatures by exchanging heat with their surroundings — living in the tropics. Temperature rise does not have a linear effect on an organism's biology, and estimated warming-induced changes in metabolic rate for tropical ectotherms are found to be larger than, or equivalent in magnitude to, those observed in temperate climates. This work may have profound implications both locally and globally, because the tropics are an important engine of primary productivity and contain a large proportion of the world's biodiversity. Temperature increase does not have a linear effect on an organism's biology. These authors use observed global temperature change to calculate the change in metabolic rate for ectotherms. Despite smaller temperature increases in the tropics, these areas, which contain the largest proportion of biodiversity, are likely to experience just as much change in metabolic rate. Documented shifts in geographical ranges 1 , 2 , seasonal phenology 3 , 4 , community interactions 5 , genetics 3 , 6 and extinctions 7 have been attributed to recent global warming 8 , 9 , 10 . Many such biotic shifts have been detected at mid- to high latitudes in the Northern Hemisphere 4 , 9 , 10 —a latitudinal pattern that is expected 4 , 8 , 10 , 11 because warming is fastest in these regions 8 . In contrast, shifts in tropical regions are expected to be less marked 4 , 8 , 10 , 11 because warming is less pronounced there 8 . However, biotic impacts of warming are mediated through physiology, and metabolic rate, which is a fundamental measure of physiological activity and ecological impact, increases exponentially rather than linearly with temperature in ectotherms 12 . Therefore, tropical ectotherms (with warm baseline temperatures) should experience larger absolute shifts in metabolic rate than the magnitude of tropical temperature change itself would suggest, but the impact of climate warming on metabolic rate has never been quantified on a global scale. Here we show that estimated changes in terrestrial metabolic rates in the tropics are large, are equivalent in magnitude to those in the north temperate-zone regions, and are in fact far greater than those in the Arctic, even though tropical temperature change has been relatively small. Because of temperature’s nonlinear effects on metabolism, tropical organisms, which constitute much of Earth’s biodiversity, should be profoundly affected by recent and projected climate warming 2 , 13 , 14 .

For insects, the timing of many life history events (phenology) depends on temperature cues. Body size is a critical mediator of insect responses to temperature, so may also influence phenology. The determinants of spring emergence of bumble bee queens are not well understood, but body size is likely important for several reasons. In fall, queens accumulate energy stores to fuel overwinter survival. Accumulation of fat stores prior to and depletion of fat stores during overwintering are likely size-dependent: larger queens can accumulate more lipids and have lower mass-specific metabolic rates. Therefore, larger queens and queens in relatively better condition may have delayed depletion of energy stores, allowing for later spring emergence. To test whether timing of spring emergence is associated with body size and condition, we captured 295 Bombus huntii queens in Laramie, WY, during the 2020 and 2021 growing seasons, weighed them, and measured intertegular width (a size metric unaffected by variation in feeding and hydration state). Early emerging queens were smaller than later emerging queens across years. Mass relative to intertegular width increased as the season progressed suggesting, as predicted, that body condition influences the timing of spring emergence for these crucial pollinators.

An analysis of a high-resolution global temperature data set shows that temperate and polar regions are becoming more tropical in their temperature variation profiles, potentially affecting organisms and impacting human agriculture and health. Warming mean temperatures over the past century 1 have probably shifted distributions 2 , altered phenologies 3 , increased extinction risks 4 , 5 , and impacted agriculture 6 and human health 7 . However, knowledge of mean temperatures alone does not provide a complete understanding either of changes in the climate itself or of how changing climate will affect organisms 8 , 9 , 10 , 11 . Temporal temperature variation, primarily driven by daily and annual temperature cycles, has profound effects on organism physiology 8 , 9 and ecology 12 , yet changes in temperature cycling over the past 40 years are still poorly understood 1 , 13 . Here we estimate global changes in the magnitudes of diurnal and annual temperature cycles from 1975 to 2013 from an analysis of over 1.4 billion hourly temperature measurements from 7,906 weather stations. Increases in daily temperature variation since 1975 in polar (1.4 °C), temperate (1.0 °C) and tropical (0.3 °C) regions parallel increases in mean temperature. Concurrently, magnitudes of annual temperature cycles decreased by 0.6 °C in polar regions, increased by 0.4 °C in temperate regions, and remained largely unchanged in tropical regions. Stronger increases in daily temperature cycling relative to changes in annual temperature cycling in temperate and polar regions mean that, with respect to diurnal and annual cycling, the world is flattening as temperate and polar regions converge on tropical temperature cycling profiles.

Development time is a critical life-history trait that has profound effects on organism fitness and on population growth rates. For ectotherms, development time is strongly influenced by temperature and is predicted to scale with body mass to the quarter power based on 1) the ontogenetic growth model of the metabolic theory of ecology which describes a bioenergetic balance between tissue maintenance and growth given the scaling relationship between metabolism and body size, and 2) numerous studies, primarily of vertebrate endotherms, that largely support this prediction. However, few studies have investigated the allometry of development time among invertebrates, including insects. Abundant data on development of diverse insects provides an ideal opportunity to better understand the scaling of development time in this ecologically and economically important group. Insects develop more quickly at warmer temperatures until reaching a minimum development time at some optimal temperature, after which development slows. We evaluated the allometry of insect development time by compiling estimates of minimum development time and optimal developmental temperature for 361 insect species from 16 orders with body mass varying over nearly 6 orders of magnitude. Allometric scaling exponents varied with the statistical approach: standardized major axis regression supported the predicted quarter-power scaling relationship, but ordinary and phylogenetic generalized least squares did not. Regardless of the statistical approach, body size alone explained less than 28% of the variation in development time. Models that also included optimal temperature explained over 50% of the variation in development time. Warm-adapted insects developed more quickly, regardless of body size, supporting the \"hotter is better\" hypothesis that posits that ectotherms have a limited ability to evolutionarily compensate for the depressing effects of low temperatures on rates of biological processes. The remaining unexplained variation in development time likely reflects additional ecological and evolutionary differences among insect species.

Summary Plasticity is a near‐ubiquitous feature of the thermal physiology of ectothermic organisms. Understanding the significance of plasticity in evolutionary and ecological contexts requires determining whether observed plasticity is beneficial, and, if so, to what extent plasticity can compensate for environmental change (i.e. is plasticity ‘complete’ or ‘incomplete’). Using site‐specific daily temperature records spanning several decades, we test the ‘beneficial acclimation hypothesis’ by calculating the number of days predicted to exceed the heat tolerance limits of 103 terrestrial ectotherm populations when incorporating observed taxon‐specific plasticity and when plasticity is ignored. We find that heat tolerance plasticity can reduce the predicted number of overheating events, although the effects are clade specific with plasticity providing greater benefits to amphibians than to reptiles or arthropods. Despite these benefits, heat tolerance plasticity is incomplete; as temperatures rise, plasticity cannot prevent an increase in the risk of overheating. Specifically, we find that a one degree increase in mean maximum temperature leads to approximately one additional day of predicted overheating during the warmest months of the year, on average. We conducted a broad‐scale analysis of the degree to which plasticity in heat tolerance reduces the probability that ectotherms experience temperatures beyond their physiological heat limits. Our results indicate that plasticity in heat tolerance benefits ectotherms, but, consistent with previous studies, those benefits are inadequate to provide complete compensation for global climate change. Lay Summary

Studies have typically used shifts in mean temperatures to make predictions about the biotic impacts of climate change. Though shifts in mean temperatures correlate with changes in phenology and distributions, other hidden, or cryptic, changes in temperature, such as temperature variation and extreme temperatures, could pose greater risks to species and ecological communities. Yet, these cryptic temperature changes have received relatively little attention because mean temperatures are readily available and the organism-appropriate temperature response is often elusive. An alternative to using mean temperatures is to view organisms as physiological filters of hourly temperature data. We explored three classes of physiological filters: (1) nonlinear thermal responses using performance curves of insect fitness, (2) cumulative thermal effects using degree-day models for corn emergence, and (3) threshold temperature effects using critical thermal maxima and minima for diverse ectotherms. For all three physiological filters, we determined the change in biological impacts of hourly temperature data from a standard reference period (1961–90) to a current period (2005–10). We then examined how well mean temperature changes during the same time period predicted the biotic impacts we determined from hourly temperature data. In all cases, mean temperature alone provided poor predictions of the impacts of climate change. These results suggest that incorporating high frequency temperature data can provide better predictions for how species will respond to temperature change.

Global temperature changes have emphasized the need to understand how species adapt to thermal stress across their ranges. Genetic mechanisms may contribute to variation in thermal tolerance, providing evidence for how organisms adapt to local environments. We determine physiological thermal limits and characterize genome-wide transcriptional changes at these limits in bumble bees using laboratory-reared Bombus vosnesenskii workers. We analyze bees reared from latitudinal (35.7–45.7°N) and altitudinal (7–2154 m) extremes of the species’ range to correlate thermal tolerance and gene expression among populations from different climates. We find that critical thermal minima (CT MIN ) exhibit strong associations with local minimums at the location of queen origin, while critical thermal maximum (CT MAX ) was invariant among populations. Concordant patterns are apparent in gene expression data, with regional differentiation following cold exposure, and expression shifts invariant among populations under high temperatures. Furthermore, we identify several modules of co-expressed genes that tightly correlate with critical thermal limits and temperature at the region of origin. Our results reveal that local adaptation in thermal limits and gene expression may facilitate cold tolerance across a species range, whereas high temperature responses are likely constrained, both of which may have implications for climate change responses of bumble bees.

Over the last few decades, biologists have made substantial progress in understanding relationships between changing climates and organism performance. Much of this work has focused on temperature because it is the best kept of climatic records, in many locations it is predicted to keep rising into the future, and it has profound effects on the physiology, performance, and ecology of organisms, especially ectothermic organisms which make up the vast majority of life on Earth. Nevertheless, much of the existing literature on temperature–organism interactions relies on mean temperatures. In reality, most organisms do not directly experience mean temperatures; rather, they experience variation in temperature over many time scales, from seconds to years. We propose to shift the focus more directly on patterns of temperature variation, rather than on means per se, and present a framework both for analyzing temporal patterns of temperature variation and for incorporating those patterns into predictions about organismal biology. In particular, we advocate using the Fourier transform to decompose temperature time series into their component sinusoids, thus allowing transformations between the time and frequency domains. This approach provides (1) standardized ways of visualizing the contributions that different frequencies make to total temporal variation; (2) the ability to assess how patterns of temperature variation have changed over the past half century and may change into the future; and (3) clear approaches to manipulating temporal time series to ask “what if” questions about the potential effects of future climates. We first summarize global patterns of change in temperature variation over the past 40 years; we find meaningful changes in variation at the half day to yearly times scales. We then demonstrate the utility of the Fourier framework by exploring how power added to different frequencies alters the overall incidence of long-term waves of high and low temperatures, and find that power added to the lowest frequencies greatly increases the probability of long-term heat and cold waves. Finally, we review what is known about the time scales over which organismal thermal performance curves change in response to variation in the thermal environment. We conclude that integrating information characterizing both the frequency spectra of temperature time series and the time scales of resulting physiological change offers a powerful new avenue for relating climate, and climate change, to the future performance of ectothermic organisms.

Organizational climate is a key determinant of diverse aspects of success in work settings, including in academia. Power dynamics in higher education can result in inequitable experiences of workplace climate, potentially harming the well-being and productivity of employees. Quantifying experiences of climate across employment categories can help identify changes necessary to create a more equitable workplace for all. We developed and administered a climate survey within our academic workplace—the Department of Zoology and Physiology at the University of Wyoming—to evaluate experiences of climate across three employment categories: faculty, graduate students, and staff. Our survey included a combination of closed-response (e.g., Likert-scale) and open-ended questions. Most department members (82%) completed the survey, which was administered in fall 2021. Faculty generally reported more positive experiences than staff. Graduate students often fell between these two groups, though in some survey sections (e.g., mental health and well-being) students reported the most negative experiences of departmental climate. Three common themes emerged from the analysis of open-ended responses: equity, community, and accountability. We discuss how these themes correspond to concrete action items for improving our departmental climate, some of which have been implemented already, while others constitute future initiatives and/or require a collective push towards systemic change in academia. Finally, service work of this type often falls outside of job descriptions, requiring individuals to either work more or trade-off productivity in other areas that are formally evaluated. With the goal of minimizing this burden for others, we detail our process and provide the materials and framework necessary to streamline this process for other departments aiming to evaluate workplace climate as a key first step in building a positive work environment for all employees.

Homeothermy requires increased metabolic rates as temperatures decline below the thermoneutral zone, so homeotherms typically select microhabitats within or near their thermoneutral zones during periods of inactivity. However, many mammals and birds are heterotherms that relax internal controls on body temperature and go into torpor when maintaining a high, stable body temperature, which is energetically costly. Such heterotherms should be less tied to microhabitats near their thermoneutral zones and, because heterotherms spend more time in torpor and expend less energy at colder temperatures, heterotherms may even select microhabitats in which temperatures are well below their thermoneutral zones. We studied how temperature and daily torpor influence the selection of microhabitats (i.e., diurnal roosts) by a heterothermic bat (Myotis thysanodes). We (1) quantified the relationship between ambient temperature and daily duration of torpor, (2) simulated daily energy expenditure over a range of microhabitat temperatures, and (3) quantified the influence of microhabitat temperature on microhabitat selection. In addition, warm microhabitats substantially reduced the energy expenditure of simulated homeothermic bats, and heterothermic bats modulated their use of daily torpor to maintain a constant level of energy expenditure across microhabitats of different temperatures. Daily torpor expanded the range of energetically economical microhabitats, such that microhabitat selection was independent of microhabitat temperature. Our work adds to a growing literature documenting the functions of torpor beyond its historical conceptualization as a last-resort measure to save energy during periods of extended or acute energetic stress.