Ectothermic Species Research Articles

Environmental and biological influences on the distribution and population dynamics of polar cod (Boreogadus saida) in the US Chukchi Sea

Polar cod (Boreogadus saida) is the most abundant fish in the US Arctic (US Chukchi and Beaufort seas), where commercial fishing is currently prohibited because data to assess the sustainability of potential fisheries are lacking. Using data from fisheries oceanographic surveys in the US Chukchi Sea during 2012 and 2013 (August–September), we examined drivers of distribution and abundance, and assessed the current status and dynamics of polar cod. We estimated age structure, abundance, biomass, natural mortality, and reproductive potential using data from these surveys, combined with available estimates of biological parameters from the literature. High densities of age-0 polar cod were observed in the northeastern Chukchi Sea, while older polar cod (age-1+) were more widely distributed throughout the survey area. Results indicate that temperature is an important driver for the distribution of both age-0 and age-1+ polar cod with age-0 polar cod less likely to be present in warmer waters (≥ 8 °C), which supports past research on drivers of fish and other ectotherm species distribution. Estimates of egg production and early survival suggest that the numbers of mature polar cod present in the survey area during summer are unlikely to produce the observed high abundances of age-0 polar cod in the US Chukchi Sea. This could imply that either mature polar cod migrate to the Chukchi Sea to spawn in the winter, that age-0 fish are advected from outside the study area, or that adult polar cod abundance is underestimated. Additional systematic surveys and further research are needed to resolve the origins of age-0 polar cod, identify nursery areas, and estimate early life survival.

Variation of thermal plasticity in growth and reproduction patterns: Importance of ancestral and developmental temperatures.

Temperature is an important ecological driver modulating life history traits of organisms, such as growth and reproduction. With the ongoing global warming, understanding the mechanisms underlying the effect of temperature on size and resource allocation trade-off is crucial. The temperature-size rule (TSR) describes plastic growth patterns in populations of ectothermic species under different thermal environments, whereby warming results in faster initial growth but lower size at maturity. However, the evolution of the TSR remains poorly understood. Here we conducted an experiment with populations of the medaka fish Oryzias latipes maintained at two temperatures for successive generations to investigate changes in the growth pattern of the TSR. After rearing six generations at cold (20 °C) and warm (30 °C) temperature, we conducted common garden experiments on the seventh generation where we compare growth trajectories and reproduction patterns in four different groups of fish: (i) fish reared at cold temperature over all seven generations (cold past and present), (ii) fish reared at warm temperature over six generations and at cold temperature at the seventh generation (warm past and cold present), (iii) fish reared at warm temperature over all seven generations (warm past and present) and (iv) fish reared at cold temperature over six generations and at warm temperature at the seventh generation (cold past and warm present). For each treatment, we monitored growth curves and reproduction, and investigated changes in model parameters and reproduction up to 350 days after hatching. Our study showed changes in TSR patterns according to ancestral and developmental temperatures. Developmental temperature mainly impacted age at maturity and asymptotic size, whereas size at maturity was driven more by the dissimilarity between developmental and ancestral temperatures. Our results also highlight a loss of plasticity in temperature-size and reproductive patterns for the fish that were reared under warm condition over six generations.

Physiological and biochemical responses to thermal stress vary among genotypes in the parasitic wasp Nasonia vitripennis

In ectotherm species such as insects, thermal fluctuations represent a major environmental factor driving development, survival and reproduction of individuals. Reproductive traits are particularly sensitive to heat stress that can induce a permanent sterility, or at least hypofertility, of adult males. This study aims to compare physiological and biochemical responses associated to male performances to an exposure of 24 h to moderately high temperature (36 °C) among three inbred lines of N. vitripennis (AsymC, Cor, Oul). Cor males showed very specific metabolic adjustments compared to the two other lines. By contrast, Oul males showed stronger phenotypic adjustment of its life cycle, and produced metabolic water to compensate water loss by heat stress. Finally, AsymC males had probably more difficulties to acclimate at 36 °C, even for a short period, as their adult longevity was significantly reduced. Thus, the ability of developmental plasticity in N. vitripennis males exposed to heat stress appears to be dependent of their genotypes.

Thermal landscape change as a driver of ectotherm responses to plant invasions.

A growing body of research demonstrates the impacts of invasive alien plants on native animals, but few studies consider thermal effects as a driver of the responses of native organisms. As invasive alien plants establish and alter the composition and arrangement of plant communities, the thermal landscapes available to ectotherms also change. Our study reviews the research undertaken to date on the thermal effects of alien plant invasions on native reptiles, amphibians, insects and arachnids. The 37 studies published between 1970 and early 2019 portray an overall detrimental effect of invasive plants on thermal landscapes, ectothermic individuals' performance and species abundance, diversity and composition. With a case study of a lizard species, we illustrate the use of thermal ecology tools in plant invasion research and test the generality of alien plant effects: changes in thermoregulation behaviour in invaded landscapes varied depending on the level of invasion and lizard traits. Together, the literature review and case study show that thermal effects of alien plants on ectotherms can be substantial albeit context-dependent. Further research should cover multiple combinations of native/invasive plant growth forms, invasion stages and ectotherm traits. More attention is also needed to test causality along the chain of effects from thermal landscapes to individuals, populations and communities.

Crickets become behaviourally more stable when raised under higher temperatures

Developmental plasticity “prepares” individuals to environments experienced during adulthood. Labile traits, such as behaviour, vary within and among individuals owing to (i) reversible plasticity and (ii) developmental plasticity and genetic make-up, respectively. Here, we test whether developmental environments affect the expression of both within- and among-individual variation in behaviour. In ectothermic species, low temperatures are associated with a reduced expression of behaviour and can also limit the expression of reversible plasticity and among-individual differentiation, for example, by restricting the expression of additive genetic variation. We focused on exploratory behaviour since activity-related behaviours are assumed to be temperature-dependent in ectotherms. Specifically, low temperatures can restrict the physiological machinery needed for movement. To test our predictions, we raised field crickets, Gryllus bimaculatus, at low versus high temperatures throughout ontogeny and compared behavioural means and variance components (i.e. among- and within-individual variances) across treatments. We also compared the coefficients of variation for each variance component across treatments to assess whether environmental effects on variance were independent of the effects on mean behaviour. As predicted, individuals raised at high temperatures were more explorative and exhibited more among- and within-individual variance compared to individuals raised at low temperatures. Interestingly, individuals raised under high temperatures became more stable in their behaviour when controlling for treatment effects on average behaviour. Our results show experimentally that different developmental temperatures trigger different amounts of behavioural “stability” in exploratory behaviour, thereby suggesting a key role for temperature in affecting the ecological processes associated with exploratory behaviour in ectothermic species. Individuals differ relative to one another in their mean behavioural expression (i.e. “animal personality”), while, at the same time, individuals change their behaviour from one instance to the next (i.e. reversible plasticity). Expression of both above-mentioned components should strongly depend on environments experienced throughout ontogeny. Temperature represents a key environmental factor in most ectothermic animals as it directly affects the ability to express behaviour. Here, we experimentally test whether temperature experienced throughout ontogeny has permanent effects on the expression of individual differences and reversible plasticity in behaviour later in life. We show that individuals become more stable, i.e. express less reversible plasticity, in risky exploratory behaviour when raised under high temperatures. Our results thus suggest a key role for temperature in affecting the ecological processes associated with risky behaviours in ectothermic species.

Rate dynamics of ectotherm responses to thermal stress.

Critical thermal limits (CTLs) show much variation associated with the experimental rate of temperature change used in their estimation. Understanding the full range of variation in rate effects on CTLs and their underlying basis is thus essential if methodological noise is not to overwhelm or bias the ecological signal. We consider the effects of rate variation from multiple intraspecific assessments and provide a comprehensive empirical analysis of the rate effects on both the critical thermal maximum (CTmax) and critical thermal minimum (CTmin) for 47 species of ectotherms, exploring which of the available theoretical models best explains this variation. We find substantial interspecific variation in rate effects, which takes four different forms (increase, decline, no change, mixed), with phylogenetic signal in effects on CTmax, but not CTmin. Exponential and zero exponential failure rate models best explain the rate effects on CTmax. The majority of the empirical rate variation in CTmin could not be explained by the failure rate models. Our work demonstrates that rate effects cannot be ignored in comparative analyses, and suggests that incorporation of the failure rate models into such analyses is a useful further avenue for exploration of the fundamental basis and implications of such variation.

Physiological acclimation and persistence of ectothermic species under extreme heat events

AbstractAimTo test if physiological acclimation can buffer species against increasing extreme heat due to climate change.LocationGlobal.Time period1960 to 2015.Major taxa studiedAmphibians, arthropods, brachiopods, cnidarians, echinoderms, fishes, molluscs, reptiles.MethodsWe draw together new and existing data quantifying the warm acclimation response in 319 species as the acclimation response ratio (ARR): the increase in upper thermal limit per degree increase in experimental temperature. We develop worst‐case scenario climate projections to calculate the number of years and generations gained by ARR until loss of thermal safety. We further compute a vulnerability score that integrates across variables estimating exposure to climate change and species‐specific tolerance through traits, including physiological plasticity, generation time and latitudinal range extent.ResultsARR is highly variable, but with marked differences across taxa, habitats and latitude. Polar terrestrial arthropods show high ARRs [95% upper confidence limit (UCL95%) = 0.68], as do some polar aquatic invertebrates that were acclimated for extended durations (ARR > 0.4). While this physiological plasticity buys 100s of years until thermal safety is lost, combination with long generation times leads to decreased potential for evolutionary adaptation. Additionally, 27% of marine polar invertebrates have no capacity for acclimation and reptiles and amphibians have minimal ARR (UCL95% = 0.16). Low physiological plasticity, long generations times and restricted latitudinal ranges combine to distinguish reptiles, amphibians and polar invertebrates as being highly vulnerable amongst ectotherms.Main conclusionsIn some taxa the combined effects of acclimation capacity and generation time can provide 100s of years and generations before thermal safety is lost. The accuracy of assessments of vulnerability to climate change will be improved by considering multiple aspects of species’ biology that, in combination may increase persistence under extreme heat events, and increase the probability for evolutionary rescue.

The subtropical–temperate transition along the east coast of South Africa shapes the thermal physiology of the truncated mangrove snail Cerithidea decollata (Gastropoda: Caenogastropoda, Potamididae)

The east and south coasts of South Africa are characterised by a transition from subtropical to warm-temperate conditions. This transition in environmental temperatures may shape the physiological tolerance of ectothermic species inhabiting harsh environments, such as the intertidal zone. A subtropical (Mngazana Estuary) and a warm-temperate (Knysna Estuary) population of the truncated mangrove snail Cerithidea decollata were selected to investigate thermal tolerance (LT50) and performance (oxygen consumption) across increasing and decreasing air temperatures (at a rate of 2 °C h−1) under controlled laboratory conditions. Animals from both populations showed a considerable thermal tolerance, surviving temperatures of over 50 °C. The thermal performances of the two populations under temperature change showed similar trends, with individuals from the Mngazana Estuary displaying higher rates of metabolism than those from the Knysna Estuary. Both thermal tolerance and performance in increasing/decreasing temperatures suggested that this snail shows intraspecific differences in thermal physiology. We discuss the importance of including the effect of microhabitat variability and behavioural thermoregulation when investigating the effect of climatic transition on the thermal physiology of intertidal species. This would enhance our knowledge of the interactions between organisms and their environment, helping to evaluate the likelihood that a species can maintain itself in a changing landscape.

Upper thermal limits and warming safety margins of coastal marine species – Indicator baseline for future reference

The threat of global warming has driven recent efforts of estimating upper thermal limits of ectothermic species all over the world. The investigation of thermal limits is crucial for the understanding of climate change ecology, since it provides insight into how climate will shape future species distributions. This work estimated the Critical Thermal Maxima (CTMax) of 42 coastal species (Gastropoda, Crustacea, Teleostei, Echinodermata and Cnidaria) in a tropical and a temperate area. The thermal safety margin (CTMax-Maximum Habitat Temperature) and future thermal safety margin (CTMax-(Maximum Habitat Temperature + 3 °C)) of each species was estimated for two alternative habitats, shallow coastal waters and tide pools. The CTMax of tropical species was higher than that of temperate species and no difference was found among the taxonomic groups tested. Thermal safety margins were larger for temperate species, than for tropical species, and considerably larger for shallow waters than for tide pools. Most tropical species had negative safety margins in tide pools (with only two exceptions), while most temperate species had positive safety margins (with only three exceptions). Future thermal safety margins for tide pools were negative for all tropical organisms and also for most of their temperate counterparts. This work adds to the data collection already available for the study areas, raising the number of species with known upper thermal limits to 100. These estimations do not take into account phenotypical acclimation, nor genetic adaptation, to future temperatures, which are likely to occur. However, they constitute important values for future reference, allowing insights into the adaptation capacity of coastal species, as well as basis to explain future distribution shifts and/or local extinctions.

Anthropogenic thermal gradient in managed landscapes determines physiological performance and explains the edge-biased distribution of ectothermic arthropods

The intensive management of tree plantations has replaced and fragmented native forests worldwide. A direct consequence of this activity is the creation of new edges, which generate changes in microclimatic conditions in the adjacent forest as well as in the neighboring clearcut stands left over after harvesting by clearcut logging. Thus, newly created anthropogenic thermal gradients could influence ectothermic responses regarding abundance and physiology of native species in fragmented landscapes. Interestingly, the consequences of these changes have not been studied in insects. To test the effects of the anthropogenic thermal gradient on the abundance and physiology of ectothermic species, we chose the fragmented Maulino forest and the ground-dwelling beetle Ceroglossus chilensis (Coleoptera: Carabidae) as our biological model, working with seven fragments of native forest surrounded by active and clearcut pine plantation stands. We measured temperature variables and beetle abundance in different months using transects perpendicular to the edge and used general linear modelling to explain abundance with temperature. In addition, we assessed the preferential body temperatures (Tb) and Rollover Speed (RoS) of C. chilensis at five different temperatures in controlled conditions to explain field abundance. We found that mean ambient temperatures were up to 42% higher in clearcut stands than in native forest fragments during summer. However, ambient temperatures in autumn were similar between native forest fragments and clearcut stands. The abundance of C. chilensis, is concentrated near the edge in the native forest (between 2 and 10 m to interior of native forest), regardless of month sampled. The abundance of C. chilensis was higher at the edge than inside native forest interiors or clearcut stands, regardless of the month. Ambient temperature surpasses preferential body temperature (14–15.99 °C) and physiological performance of C. chilensis (15 °C), particularly during summer. Ceroglossus chilensis abundance at the edge of clearcut stands could be favored by understory structural complexity in understory vegetation, creating suitable microclimatic conditions and increasing abundance of prey resource. In essence, we found that the higher temperatures found near the edge inside forest interiors might enable individuals of C. chilensis to display thermoregulatory behavior by concentrating activity at these particular regions, resulting in increased abundances and more concentrated occurrences, helping to explain the edge-biased distribution. Thus, we offer evidence that physiological performance might be a key functional trait to explain abundance of ectothermic species in fragmented forest landscapes. We submit to decision makers linked to the forestry industry, that managing the understory of young tree plantations and increasing the structural complexity at remnant forest edges to cushion high temperatures in clearcut stands would enable more effective conservation planning of thermosensitive ectotherms in disturbed forest landscapes.

Thermal behaviour of edible crab Cancer pagurus Linnaeus, 1758 in coastal Norway

Ocean warming drives latitudinal shifts in the distribution of ectotherm species. The rate and magnitude of such shifts are constrained by physiology and behavioural thermoregulation. Here, we investigated the thermal preference and lower critical temperature (CTmin) in female edible crab Cancer pagurus, a decapod crustacean with an ongoing northward dispersal along the Norwegian coast. The temperature selected by individual crabs from a northern (latitude ~69°N) and southern (latitude ~62°N) location was examined in a horizontal gradient (5.5-14.5°C) under a simulated day and night light regime. Irrespective of origin, crabs showed pronounced responses to the light cycle – during the day crabs stayed inactive in the warm end of the gradient but during night they actively explored the entire gradient. A preferred temperature of ~13 °C (measured as mode of loggings) was identified for crabs at both locations. Righting reflex experiments of crabs exposed to a rapid temperature drop (7 - 1 °C at -0.1 °C/min) identified a CTmin of ~1.3 °C (i.e., the temperature at which 50% of crabs failed to right from an up-side-down position), and with no significant difference between locations (p > 0.05). Our results provide important information about the functional characteristics of edible crab, and are discussed in context of the biology and ongoing northward dispersal of the species.

Back to the future: conserving functional and phylogenetic diversity in amphibian-climate refuges

Climate refuges have been used by several species over historical climate change. Ectothermic species often display good models for climate change studies because they are highly sensitive to temperature. Analysis of species loss with ecosystem and evolutionary values helps to understand environmental processes and climate change consequences. We determined the functional and phylogenetic diversity of amphibians in the Atlantic Forest hotspot, using multiple models representing present and future conditions. Through a novel approach, we predict species’ threat status by 2080, following the IUCN’s criterion B1. Our results estimate a drastic reduction in species richness, ecosystem functioning and evolutionary history at low latitudes and altitudes. We show that species will tend to disperse to the areas with milder temperatures (i.e., high latitudes/altitudes). Some of these areas are the same climate refuges that have been suggested for the Late Pleistocene. We highlight that 60% of amphibians can become threatened under predicted-future conditions. This work advances the knowledge on climate refuges for amphibian ecology and evolution, supporting complementary tools for conservation strategies.

The response of nitric oxide system to high Altitude in Phrynocephalus erythrurus on Qinghai-Tibetan plateau.

Nitric oxide (NO), produced by nitric oxide synthases (NOS) from L-arginine, plays important roles in a wide range of physiological processes. However, little is known about ectothermic species. To investigate the response of NO/NOS system in adaptation to different altitudes in Phrynocephalus genus, the red tail toad-headed lizard Phrynocephalus erythrurus that live at 4500-5300 m on the Qinghai-Tibet Plateau and another low altitude living lizard Phrynocephalus przewalskii were selected in the present study. The results of mRNA expression and activity of NOSs, as well as NO metabolite levels in different tissues of the two lizards indicate that nNOS mRNA levels in cardiac and skeletal muscle were notably elevated in P. erythrurus, and iNOS expression was also increased markedly (up to 4-fold) in cardiac muscle. There was no significant difference in eNOS mRNA level in tested tissues between two species. However, the total NOS activity in skeletal muscle of P. erythrurus was slightly lower than that of P. przewalskii (p < .05) while no difference in other tissues. Similarly, lower iNOS activity (p < .01) was found in cardiac and skeletal muscle in P. erythrurus compared to P. przewalskii. In addition, the NO metabolite levels were dramatically lower in P. erythrurus in all tested tissues. We propose that higher nNOS and iNOS mRNA expression, lower iNOS activity and NO metabolite levels may represent physiological characteristics in nitric oxide system, which may contribute to high-altitude adaptation in P. erythrurus.

Vulnerability of the marine ecosystem to climate change impacts in the Arabian Gulf—an urgent need for more research

Climate change, represented by ever-rising ocean temperatures, is a mounting threat to the marine ecosystem and its services. This is most evident in the longitudinal and depth-related migrations of the ectothermic species. Although the impacts of climate change on the marine ecosystem of the Arabian Gulf are expected to be exacerbated—owing to its semi-enclosed basin that limits species range shift, extreme environmental conditions, overfishing, and pollution—very few studies have been carried out to evaluate such impacts. Here, we conduct a systematic review of literature over the period 1950–2018 to assess the status of knowledge about climate change impacts on the Arabian Gulf's marine ecosystem and fisheries resources. We found that this region suffers a significant research gap in this critical subject, with only a handful of studies that explicitly addresses the effects of climate change. Our finding raises an urgent need for initiating long-term monitoring programs, along with establishing effective transboundary institutions to advance the current knowledge in climate change.

Temperature alters multimodal signaling and mating success in an ectotherm

Temperature affects ectotherms in a variety of ways. These effects can be especially complex in sexual behaviors, as different sexes may be affected differently by temperature. We examined this in the jumping spider, Habronattus clypeatus. In this species, males court females using visual and vibratory signals. We tested whether key intersexual behaviors would change with temperature in similar, predictable ways across males and females. We first measured temperature and apparent activity of individuals across the day. We found that H. clypeatus are active across a wide range of temperatures (11–56 °C) and are most active at times of day when temperature ranges from 13 to 46 °C. Next, we performed mating experiments across behaviorally relevant temperatures. Females were more likely to allow males to progress to later stages of courtship and had higher mating rates at higher temperatures. Male visual and vibratory courtship behaviors generally became faster, higher-pitched, and lower in amplitude at higher temperatures. This relationship between temperature and signal aspects attained a roughly curvilinear shape, with an asymptote around 40 °C. Intriguingly, mating rates in the lab were highest at temperatures potentially above those during peak spider activity in the field. Our results suggest that temperature’s effects on behavior are complex and can affect males and females differently. This work emphasizes that understanding temperature effects on mating is critical to understanding sexual selection patterns particularly in species which use complex signals. Temperature affects communication in most ectothermic species. Previous research has shown that temperature changes courter signals and chooser choice. However, this has never been investigated in species that use multimodal signals. We investigate how signals and choice change across temperatures in a desert-dwelling jumping spider. Using field temperature/activity modeling and a series of courtship experiments in the lab, we show that male signals and female choice change with temperature across biologically relevant ranges. Our results suggest that the temperatures at which mating is most likely occur at times of the day when animals are least active. These counterintuitive results highlight the importance of understanding how behavior in a controlled lab environment corresponds to natural field conditions as well as the importance of examining the effects of naturally occurring environmental variation on behavior.

Miami heat: Urban heat islands influence the thermal suitability of habitats for ectotherms.

The urban heat island effect, where urban areas exhibit higher temperatures than less-developed suburban and natural habitats, occurs in cities across the globe and is well understood from a physical perspective and at broad spatial scales. However, very little is known about how thermal variation caused by urbanization influences the ability of organisms to live in cities. Ectotherms are sensitive to environmental changes that affect thermal conditions, and therefore, increased urban temperatures may pose significant challenges to thermoregulation and alter temperature-dependent activity. To evaluate whether these changes to the thermal environment affect the persistence and dispersal of ectothermic species in urban areas, we studied two species of Anolis lizards (Anolis cristatellus and Anolis sagrei) introduced to Miami-Dade County, FL, USA, where they occur in both urban and natural habitats. We calculated canopy openness and measured operative temperature (Te ), which estimates the distribution of body temperatures in a non-thermoregulating population, in four urban and four natural sites. We also captured lizards throughout the day and recorded their internal body temperature (Tb ). We found that urban areas had more open canopies and higher Te compared to natural habitats. Laboratory trials showed that A.cristatellus preferred lower temperatures than A.sagrei. Urban sites currently occupied by each species appear to lower thermoregulatory costs for both species, but only A.sagreihad field Tb that were more often within their preferred temperature range in urban habitats compared to natural areas. Furthermore, based on available Te within each species' preferred temperature range, urban sites with only A.sagrei appear less suitable for A.cristatellus, whereas natural sites with only A.cristatellus are less suitable for A.sagrei. These results highlight how the thermal properties of urban areas contribute to patterns of persistence and dispersal, particularly relevant for studying species invasions worldwide.

Thermal pace-of-life strategies improve phenological predictions in ectotherms

Phenological variability among populations is widespread in nature. A few predictive phenological models integrate intrapopulational variability, but none has ever explored the individual strategies potentially occurring within a population. The “pace-of-life” syndrome accounts for such individual strategies, but has yet to be explored under a phenological context. Here we integrated, for the first time, the slow-fast thermal strategies stemming from the “pace-of-life” into a mechanistic predictive framework. We obtained 4619 phenological observations of an important crop pest in the Bolivian Andes by individually following 840 individuals under five rearing temperatures and across nine life stages. The model calibrated with the observed individual “pace-of-life” strategies showed a higher accuracy in phenological predictions than when accounting for intrapopulational variability alone. We further explored our framework with generated data and suggest that ectotherm species with a high number of life stages and with slow and/or fast individuals should exhibit a greater variance of populational phenology, resulting in a potentially longer time window of interaction with other species. We believe that the “pace-of-life” framework is a promising approach to improve phenological prediction across a wide array of species.

Limited differentiation of fundamental thermal niches within the killifish assemblage from shallow temporary waters

The coexistence of ectothermic species is enabled among other factors by the differentiation of their thermal niches. While this phenomenon is well described from deep temperate lakes, it is unclear whether the same pattern applies to temporary pools. In this study, we examined fundamental thermal niches in three coexisting annual killifish species Nothobranchius furzeri, N. orthonotus and N. pienaari from temporary pools in southern Mozambique. We hypothesized that the disparate thermal requirements of the three congeneric species are a candidate niche component that facilitates their local coexistence. We estimated species’ thermal requirements as preferred body temperatures (Tpref) in a horizontal thermal gradient. Under thermal gradient conditions, sympatric killifish maintained their body temperatures within similar Tpref ranges despite some variation in mean Tpref. The daily variation in water temperature in their native habitats enables killifish to thermoregulate at least for part of the diurnal cycle. We conclude that the coexistence of African annual killifish species is possible without the differentiation of their fundamental thermal niches.

Natural selection on plasticity of thermal traits in a highly seasonal environment.

For ectothermic species with broad geographical distributions, latitudinal/altitudinal variation in environmental temperatures (averages and extremes) is expected to shape the evolution of physiological tolerances and the acclimation capacity (i.e., degree of phenotypic plasticity) of natural populations. This can create geographical gradients of selection in which environments with greater thermal variability (e.g., seasonality) tend to favor individuals that maximize performance across a broader range of temperatures compared to more stable environments. Although thermal acclimation capacity plays a fundamental role in this context, it is unknown whether natural selection targets this trait in natural populations. Additionally, understanding whether and how selection acts on thermal physiological plasticity is also highly relevant to climate change and biological conservation. Here, we addressed such an important gap in our knowledge in the northernmost population of the four‐eyed frog, Pleurodema thaul. We measured plastic responses of critical thermal limits for activity, behavioral thermal preference, and thermal sensitivity of metabolism to acclimation at 10 and 20°C. We monitored survival during three separate recapture efforts and used mark‐recapture integrated into an information‐theoretic approach to evaluate the relationship between survivals as a function of the plasticity of thermal traits. Overall, we found no evidence that thermal acclimation in this population is being targeted by directional selection, although there might be signals of selection on individual traits. According to the most supported models, survival increased in individuals with higher tolerance to cold when cold‐acclimated, probably because daily low extremes are frequent during the cooler periods of the year. Furthermore, survival increased with body size. However, in both cases, the directional selection estimates were nonsignificant, and the constraints of our experimental design prevented us from evaluating more complex models (i.e., nonlinear selection).

Effect of temperature on the locomotor performance of species in a lizard assemblage in the Puna region of Argentina.

Locomotion is relevant to the ecology of reptiles because of its presumed influence on an organism's Darwinian fitness. Moreover, in ectothermic species, physiological performance capacity is affected by body temperature. We analyzed two components of locomotor performance in three species of lizards, Phymaturus extrilidus, Liolaemus parvus, and Liolaemus ruibali, in the Puna environment of Argentina. First, we estimated the thermal sensitivity of locomotion by measuring sprint speed at four different body temperatures. We included two measures of sprint speed: initial velocity and long sprint for sustained runs. Based on these data, we calculated the optimal temperature for performance and the optimal performance breadth. We also estimated endurance capacity at a single temperature. Maximum sprint speed for L. parvus was greater than L. ruibali and P. extrilidus in both initial velocity and long sprint. In contrast, L. parvus exhibited lower levels of endurance than L. ruibali and P. extrilidus. However, endurance in L. ruibali exceeded that of P. extrilidus. The species differed in the optimal temperature for the initial velocity with the lowest for L. ruibali (31.8°C) followed by P. extrilidus (33.25°C) and then L. parvus (36.25°C). The optimal temperature for long sprint varied between 32 and 36°C for all species. We found that all species attained maximum performance at body temperatures commonly experienced during daily activity, which was higher than the thermal quality of the environment. We found evidence for thermal sensitivity in locomotor performance in these species. However, we also show that the broad thermal breadth of performance suggests that the lizards are capable of sustaining near optimal levels of locomotor performance at ambient temperatures that would appear to be suboptimal. Thus, this lizard assemblage is capable of coping with the highly variable climatic conditions in the Puna region of Argentina.