A review of thermoregulation and physiological performance in reptiles: what is the role of phenotypic flexibility?

Sexual color ornamentation, microhabitat choice, and thermal physiology in the common wall lizard (Podarcis muralis)

Common wall lizards (Podarcis muralis) in Italy show a striking variation in body coloration across the landscape, with highly exaggerated black and green colors in hot and dry climates and brown and white colors in cool and wet climates. Males are more intensely colored than females, and previous work has suggested that the maintenance of variation in coloration across the landscape reflects climatic effects on the strength of male-male competition, and through this sexual selection. However climatic effects on the intensity of male-male competition would need to be exceptionally strong to fully explain the geographic patterns of color variation. Thus, additional processes may contribute to the maintenance of color variation. Here we test the hypothesis that selection for green and black ornamentation in the context of male-male competition is opposed by selection against ornamentation because the genes involved in the regulation of coloration have pleiotropic effects on thermal physiology, such that ornamentation is selected against in cool climates. Field observations revealed no association between body coloration and microhabitat use or field active body temperatures. Consistent with these field data, lizards at the extreme ends of the phenotypic distribution for body coloration did not show any differences in critical minimum temperature, preferred body temperature, temperature-dependent metabolic rate, or evaporative water loss when tested in the laboratory. Combined, these results provide no evidence that genes that underlie sexual ornamentation are selected against in cool climate because of pleiotropic effects on thermal biology.

Seasonal variation of behavioural thermoregulation in a fossorial salamander (Ambystoma maculatum)

Temperature seasonality plays a pivotal role in shaping the thermal biology of ectotherms. However, we still have a limited understanding of how ectotherms maintain thermal balance in the face of varying temperatures, especially in fossorial species. Due to thermal buffering underground, thermal ecology theory predicts relaxed selection pressure over thermoregulation in fossorial ectotherms. As a result, fossorial ectotherms typically show low thermoregulatory precision and low evidence of thermotactic behaviours in laboratory thermal gradients. Here, we evaluated how temperature selection (T sel) and associated behaviours differed between seasons in a fossorial amphibian, the spotted salamander (Ambystoma maculatum). By comparing thermoregulatory parameters between the active and overwintering seasons, we show that A. maculatum engages in active behavioural thermoregulation despite being fossorial. In both seasons, T sel was consistently offset higher than acclimatization temperatures. Thermoregulation differed between seasons, with salamanders having higher T sel and showing greater evidence of thermophilic behaviours in the active compared with the overwintering season. Additionally, our work lends support to experimental assumptions commonly made but seldom tested in thermal biology studies. Ultimately, our study demonstrates that the combination of careful behavioural and thermal biology measurements is a necessary step to better understand the mechanisms that underlie body temperature control in amphibians.

Alpine viper in changing climate: thermal ecology and prospects of a cold-adapted reptile in the warming Mediterranean

In a rapidly changing thermal environment, reptiles are primarily dependent on in situ adaptation because of their limited ability to disperse and the restricted opportunity to shift their ranges. However, the rapid pace of climate change may surpass these adaptation capabilities or elevate energy expenditures. Therefore, understanding the variability in thermal traits at both individual and population scales is crucial, offering insights into reptiles' vulnerability to climate change. We studied the thermal ecology of the endangered Greek meadow viper (Vipera graeca), an endemic venomous snake of fragmented alpine-subalpine meadows above 1600 m of the Pindos mountain range in Greece and Albania, to assess its susceptibility to anticipated changes in the alpine thermal environment. We measured preferred body temperature in artificial thermal gradient, field body temperatures of 74 individuals in five populations encompassing the entire geographic range of the species, and collected data on the available of temperatures for thermoregulation. We found that the preferred body temperature (Tp) differed only between the northernmost and the southernmost populations and increased with female body size but did not depend on sex or the gravidity status of females. Tp increased with latitude but was unaffected by the phylogenetic position of the populations. We also found high accuracy of thermoregulation in V. graeca populations and variation in the thermal quality of habitats throughout the range. The overall effectiveness of thermoregulation was high, indicating that V. graeca successfully achieves its target temperatures and exploits the thermal landscape. Current climatic conditions limit the activity period by an estimated 1278 h per year, which is expected to increase considerably under future climate scenarios. Restricted time available for thermoregulation, foraging and reproduction will represent a serious threat to the fitness of individuals and the persistence of populations in addition to habitat loss due to mining, tourism or skiing and habitat degradation due to overgrazing in the shrinking mountaintop habitats of V. graeca.

Temperature-dependent exercise recovery is not associated with behavioral thermoregulation in a salmonid fish

The relationship between behavioral thermoregulation and physiological recovery following exhaustive exercise is not well understood. Behavioral thermoregulation could be beneficial for exercise recovery; for example, selection of cooler temperatures could reduce maintenance metabolic cost to preserve aerobic scope for recovery cost, or selection of warmer temperatures could accelerate recovery of exercise metabolites. While post-exercise behavioral thermoregulation has been observed in lizards and frogs, little is known about its importance in fish. We examined the influence of post-exercise recovery temperature on metabolic rate, thermal preference, and metabolite concentrations in juvenile brook char (Salvelinus fontinalis). Fish were acclimated to and exercised at 15 °C, then recovered at either 15 °C or 10 °C while their metabolic rate was measured via respirometry. Metabolite concentrations were measured in fish after exercise at 15 °C and recovery under one of three thermal treatments (to simulate various behavioral thermoregulation scenarios): (i) 6 h recovery at 15 °C, (ii) 6 h recovery at 10 °C, or (iii) 3 h recovery at 10 °C followed by 3 h recovery at 15 °C. Thermal preference was quantified using a static temperature preference system (15 °C vs. 10 °C). Metabolic rates returned to resting faster at 10 °C compared with 15 °C, although at 10 °C there was a tradeoff of delayed metabolite recovery. Specifically, post-exercise plasma osmolality, plasma lactate, and muscle lactate remained elevated for the entire period in fish recovering at 10 °C, whereas these parameters returned to resting levels by 6 h in fish from the other two recovery groups. Regardless, fish did not exhibit clear behavioral thermoregulation (i.e., fish overall did not consistently prefer one temperature) to prioritize either physiological recovery process. The advantage of metabolic rate recovery at cooler temperatures may balance against the advantage of metabolite recovery at warmer temperatures, lessening the usefulness of behavioral thermoregulation as a post-exercise recovery strategy in fish.

Effects of thermophily-relevant temperature variation and sex on digestive performance in pythons

Different physiological performances are often optimized at slightly varying temperatures, which can lead to ectotherms selecting higher body temperatures during certain physiological efforts (e.g., digestion, reproduction). Such thermophilic responses can lead to temperature-based tradeoffs between two physiological activities with differing optimal temperatures or between optimizing a physiological activity and water balance, as water loss is elevated at higher temperatures. For example, ectotherms will often select a higher body temperature after consuming a meal, but the extent to which body temperature is elevated after eating is affected by its hydric state. Despite this known hydration state-based suppression of thermophily associated with digestion, the impact of this reduced body temperature on digestion performance is unknown. Accordingly, we determined whether small, thermophily-relevant changes in body temperature impact digestive efficiency or passage time and whether sex influenced the extent of the effect. Eighteen (9 female and 9 male) Children's pythons (Antaresia childreni) each consumed a meal at three temperatures (29 °C, 30 °C, and 31 °C), and gut passage time and digestive efficiency were determined. We found that neither metric was affected by temperature over the range tested. However, digestive efficiency was significantly impacted by the interaction between sex and temperature with males having significantly lower digestive efficiency than females at 31 °C, but not 29 °C or 30 °C. Our results provide insight into the effects of temperature on digestive physiology across narrow temperature ranges as well as demonstrate a sex-based difference in digestive physiology.

How seasonality influences the thermal biology of lizards with different thermoregulatory strategies: a meta-analysis

Ectotherms that maintain thermal balance in the face of varying climates should be able to colonise a wide range of habitats. In lizards, thermoregulation usually appears as a variety of behaviours that buffer external influences over physiology. Basking species rely on solar radiation to raise body temperatures and usually show high thermoregulatory precision. By contrast, species that do not bask are often constrained by climatic conditions in their habitats, thus having lower thermoregulatory precision. While much focus has been given to the effects of mean habitat temperatures, relatively less is known about how seasonality affects the thermal biology of lizards on a macroecological scale. Considering the current climate crisis, assessing how lizards cope with temporal variations in environmental temperature is essential to understand better how these organisms will fare under climate change. Activity body temperatures (Tb ) represent the internal temperature of an animal measured in nature during its active period (i.e. realised thermal niche), and preferred body temperatures (Tpref ) are those selected by an animal in a laboratory thermal gradient that lacks thermoregulatory costs (i.e. fundamental thermal niche). Both traits form the bulk of thermal ecology research and are often studied in the context of seasonality. In this study, we used a meta-analysis to test how environmental temperature seasonality influences the seasonal variation in the Tb and Tpref of lizards that differ in thermoregulatory strategy (basking versus non-basking). Based on 333 effect sizes from 137 species, we found that Tb varied over a greater magnitude than Tpref across seasons. Variations in Tb were not influenced by environmental temperature seasonality; however, body size and thermoregulatory strategy mediated Tb responses. Specifically, larger species were subjected to greater seasonal variations in Tb , and basking species endured greater seasonal variations in Tb compared to non-basking species. On the other hand, the seasonal variation in Tpref increased with environmental temperature seasonality regardless of body size. Thermoregulatory strategy also influenced Tpref , suggesting that behaviour has an important role in mediating Tpref responses to seasonal variations in the thermal landscape. After controlling for phylogenetic effects, we showed that Tb and Tpref varied significantly across lizard families. Taken together, our results support the notion that the relationship between thermal biology responses and climatic parameters can be taxon and trait dependent. Our results also showcase the importance of considering ecological and behavioural aspects in macroecological studies. We further highlight current systematic, geographical, and knowledge gaps in thermal ecology research. Our work should benefit those who aim to understand more fully how seasonality shapes thermal biology in lizards, ultimately contributing to the goal of elucidating the evolution of temperature-sensitive traits in ectotherms.

Glyphosate-based herbicide (GBH) challenged thermoregulation in lizards (Eremias argus), compensatory warming could mitigate this effect

Chemical pollution and global warming are two major threats to reptiles, and these two factors can interact with each other. Glyphosate have attracted worldwide attention due to their ubiquitous occurrence, yet their impact on reptiles remains unknown. We designed a crossover experiment with different external GBH exposures (control/GBH) x different environmental temperatures (current climate treatment/warmer climate treatment) over 60 days to simulate environmental exposure in the Mongolian Racerunner lizard (Eremias argus). Preferred body temperature and active body temperature data were collected to calculate the accuracy of thermoregulation, while liver detoxification metabolic enzymes, oxidative stress system function, and the non-targeted metabolome of the brain tissue were assessed. Warmer-treated lizards adjusted their physiological levels and behavioral strategies in response to increased ambient temperatures and maintained body temperature homeostasis at moderate thermal perturbations. GBH-treated lizards suffered from oxidative damage to the brain tissue and abnormal histidine metabolism, thus their thermoregulatory accuracy reduced. Interestingly, at elevated ambient temperatures, GBH treatment did not affect on their thermoregulatory, possibly through several temperature-dependent detoxification mechanisms. Importantly, this data suggested that the subtle toxicological effects of GBH may threaten increasingly thermoregulation behavior of E. argus with species-wide repercussions, as climate change and exposure time extension.

Can animals tune tissue mechanics in response to changing environments caused by anthropogenic impacts?

Anthropogenic climate change and pollution are impacting environments across the globe. This Review summarises the potential impact of such anthropogenic effects on animal tissue mechanics, given the consequences for animal locomotor performance and behaviour. More specifically, in light of current literature, this Review focuses on evaluating the acute and chronic effects of temperature on the mechanical function of muscle tissues. For ectotherms, maximal muscle performance typically occurs at temperatures approximating the natural environment of the species. However, species vary in their ability to acclimate to chronic changes in temperature, which is likely to have longer-term effects on species range. Some species undergo periods of dormancy to avoid extreme temperature or drought. Whilst the skeletal muscle of such species generally appears to be adapted to minimise muscle atrophy and maintain performance for emergence from dormancy, the increased occurrence of extreme climatic conditions may reduce the survival of individuals in such environments. This Review also considers the likely impact of anthropogenic pollutants, such as hormones and heavy metals, on animal tissue mechanics, noting the relative paucity of literature directly investigating this key area. Future work needs to determine the direct effects of anthropogenic environmental changes on animal tissues and related changes in locomotor performance and behaviour, including accounting for currently unknown interactions between environmental factors, e.g. temperature and pollutants.

Changes in clinicomorphometrical findings, lipid profiles, hepatorenal indices and oxidant/antioxidant status as thermoregulatory adaptive mechanisms in poikilothermic Dabb lizard (Uromastyx aegyptia)

Wildlife has exposed to various environmental stressors. Reptiles (ectothermic) are highly susceptible to climatic changes due to their behaviour, physiology, and life history that were so heavily reliant on the ambient environmental temperature. The present work aims to monitor different biochemical and haematological indices of Dabb lizards (Uromastyx aegyptia) at various thermal gradients as well as their adaptation to oxidative stress. This has been reflected through assessment of their impact on some adaptive physiological traits i.e. thermoregulation, and muscle metabolic biomarkers, blood pictures and oxidant/antioxidant status. This experiment is carried out on non-hibernating adult male Dabb lizards (U. aegyptia; n = 24) of age of 18-24 months. These Dabb lizards are divided into four equal groups (n = 6 for each one) where they are exposed to different thermal treatments for one week as following; control group [Exposed to terrarium temperature 38-39 °C], low temperature exposed group [Exposed to 12-14 °C], Gp. C; moderate temperature exposed group [Exposed to 41-43 °C] and high temperature exposed group [Exposed to 43-45 °C]. Each independent group (n = 6) are kept at separated glass terraria. The investigated lizards are monitored for body temperature, morphometric measurements i.e. body weight (g) and total body length (cm; TBL), muscle biochemical analysis, haematological pictures indices and serum biochemical assays including mainly oxidant/antioxidants biomarkers throughout the current experiment. The results state that the thermoregulatory behaviour of Dabb varies with the increase of concentration of muscular metabolic enzymes. In low temperature exposed group, the increase in red blood corpuscles (RBCs), haemoglobin concentrations (Hb), white blood cell (WBC), serum antioxidant biomarkers and anaerobic Lactate dehydrogenase (LDH) enzyme are associated with a marked reduction in serum levels of total cholesterol (TC), triglycerides (TGs), total proteins (TPs), albumin, glucose and electrolytes. In moderate temperature exposed group, a significant elevation in serum values of TC, TGs, TPs, glucose, urea and uric acids levels are mentioned. In high temperature exposed Dabb group, a remarkable increase in blood values of RBCs, Hb, haematocrit value (HCT), WBC, T. chol., TGs, TPs, glucose, urea, uric acids, triiodothyronine (T3) and thyroxine (T4) levels are also observed. Moreover, significant increases in muscular anaerobic/aerobic metabolic enzymes as well as stimulation of antioxidant defence system have been reported. Different significant correlations have been stated between variably estimated laboratory indices in the investigated Dabb lizards under different thermal treatments. The study concludes that the Dabb lizards have a strong antioxidant defence system and undergo physiological thermoregulatory adaptive mechanisms, that involve biochemical and metabolic acclimatization as a response to environmental temperature changes that act as a protective mechanism against oxidative stress as well as maintained homeostatic responses and normal physiological functions.

Climate aridity and habitat drive geographical variation in morphology and thermo-hydroregulation strategies of a widespread lizard species

Thermo-hydroregulation strategies involve concurrent changes in functional traits related to energy, water balance and thermoregulation and play a key role in determining life-history traits and population demography of terrestrial ectotherms. Local thermal and hydric conditions should be important drivers of the geographical variation of thermo-hydroregulation strategies, but we lack studies that examine these changes across climatic gradients in different habitat types. Here, we investigated intraspecific variation of morphology and thermo-hydroregulation traits in the widespread European common lizard (Zootoca vivipara louislantzi) across a multidimensional environmental gradient involving independent variation in air temperature and rainfall and differences in habitat features (access to free-standing water and forest cover). We sampled adult males for morphology, resting metabolic rate, total and cutaneous evaporative water loss and thermal preferences in 15 populations from the rear to the leading edge of the distribution across an elevational gradient ranging from sea level to 1750 m. Besides a decrease in adult body size with increasing environmental temperatures, we found little effect of thermal conditions on thermo-hydroregulation strategies. In particular, relict lowland populations from the warm rear edge showed no specific ecophysiological adaptations. Instead, body mass, body condition and resting metabolic rate were positively associated with a rainfall gradient, while forest cover and water access in the habitat throughout the season also influenced cutaneous evaporative water loss. Our study emphasizes the importance of rainfall and habitat features rather than thermal conditions for geographical variation in lizard morphology and physiology.

Anna's hummingbird (Calypte anna) physiological response to novel thermal and hypoxic conditions at high elevations

Many species have not tracked their thermal niches upslope as predicted by climate change, potentially because higher elevations are associated with abiotic challenges beyond temperature. To better predict whether organisms can continue to move upslope with rising temperatures, we need to understand their physiological performance when subjected to novel high-elevation conditions. Here, we captured Anna's hummingbirds - a species expanding their elevational distribution in concordance with rising temperatures - from across their current elevational distribution and tested their physiological response to novel abiotic conditions. First, at a central aviary within their current elevational range, we measured hovering metabolic rate to assess their response to oxygen conditions and torpor use to assess their response to thermal conditions. Second, we transported the hummingbirds to a location 1200 m above their current elevational range limit to test for an acute response to novel oxygen and thermal conditions. Hummingbirds exhibited lower hovering metabolic rates above their current elevational range limit, suggesting lower oxygen availability may reduce performance after an acute exposure. Alternatively, hummingbirds showed a facultative response to thermal conditions by using torpor more frequently and for longer. Finally, post-experimental dissection found that hummingbirds originating from higher elevations within their range had larger hearts, a potential plastic response to hypoxic environments. Overall, our results suggest lower oxygen availability and low air pressure may be difficult challenges to overcome for hummingbirds shifting upslope as a consequence of rising temperatures, especially if there is little to no long-term acclimatization. Future studies should investigate how chronic exposure and acclimatization to novel conditions, as opposed to acute experiments, may result in alternative outcomes that help organisms better respond to abiotic challenges associated with climate-induced range shifts.

A Multilevel Assessment of Plasticity in Response to High-Altitude Environment for Agama Lizards

Upslope range shifting has been documented in diverse species in response to global warming. Plasticity, which refers to the ability of organisms to alter their phenotypes in changing environments, is crucial for the survival of those that newly migrated to a high-altitude environment. The scope and mechanisms of plasticity across biological levels, however, have rarely been examined. We used two agama lizards (genus Phrynocephalus) as model systems and a transplant experiment to comprehensively assess their plasticity on multiple organization levels. Two low-altitude (934 m) agama species, Phrynocephalus axillaris (oviparous) and P. forsythii (viviparous), were transplanted to a high-altitude site (3,400 m). After acclimation for 6 weeks in seminatural enclosures, plasticity was measured from bite force, tail display behavior, gene expression, and metabolome. Both lizards were capable of acclimating to the high-altitude environment without sacrificing their performance in bite force, but they also showed high plasticity in tail display behavior by either decreasing the intensity of a specific display component (P. forsythii) or by the trade-off between display components (P. axillaris). Genes and metabolites associated with lipids, especially fatty acid metabolism, exhibited significant differentiation in expression, compared to individuals from their native habitats. Improved fatty acid storage and metabolism appeared to be a common response among animals at high altitudes. Despite distinct reproductive modes that may differ in response to physiological pressure, the two lizards demonstrated high concordance in plasticity when they faced a novel environment at high altitudes. Taken together, lizards likely acclimate to high-altitude environments by reducing behavioral activity and increasing energy efficiency after range shifting. Our results provide new insights into our understanding of phenotypic plasticity and its importance in today’s changing climate.

Impact of developmental temperatures on thermal plasticity and repeatability of metabolic rate

Phenotypic plasticity is an important mechanism that allows populations to adjust to changing environments. Early life experiences can have lasting impacts on how individuals respond to environmental variation later in life (i.e., individual reaction norms), altering the capacity for populations to respond to selection. Here, we incubated lizard embryos (Lampropholis delicata) at two fluctuating developmental temperatures (cold = 23 ºC + / − 3 ºC, hot = 29 ºC + / − 3 ºC, ncold = 26, nhot = 25) to understand how it affected metabolic plasticity to temperature later in life. We repeatedly measured individual reaction norms across six temperatures 10 times over ~ 3.5 months (nobs = 3,818) to estimate the repeatability of average metabolic rate (intercept) and thermal plasticity (slope). The intercept and the slope of the population-level reaction norm was not affected by developmental temperature. Repeatability of average metabolic rate was, on average, 10% lower in hot incubated lizards but stable across all temperatures. The slope of the thermal reaction norm was overall moderately repeatable (R = 0.44, 95% CI = 0.035 – 0.93) suggesting that individual metabolic rate changed consistently with short-term changes in temperature, although credible intervals were quite broad. Importantly, reaction norm repeatability did not depend on early developmental temperature. Identifying factors affecting among-individual variation in thermal plasticity will be increasingly more important for terrestrial ectotherms living in changing climate. Our work implies that thermal metabolic plasticity is robust to early developmental temperatures and has the capacity to evolve, despite there being less consistent variation in metabolic rate under hot environments.

Effects of hypoxic conditions during the plateau period on pre- and posthatch broiler performance

Adequate ambient temperature and oxygenation are necessary to maintain normal embryonic development of broilers; however, hypoxia challenge during incubation can aid in improving regulatory plasticity and lead to different phenotypes later in life. This study aimed to examine the effects of moderate hypoxia (O2 17%) during the plateau phase on the embryonic physiological parameters and on posthatch performance (growth rate, feed consumption and feed conversion) up to the age of poultry marketing. The study included examined embryos exposed to O2 17% for 12 h per day (h/d) from E16 through E18 (designated as 12H), or O2 17% continuously, from E16 through E17 (designated as 48H) and a standard incubation control group (21% O2). Physiological and morphological parameters of embryos and hatched chicks were measured. Male Chicks from all 3 treatment groups were raised under recommended temperature regime, and body weight, feed intake and FCR were recorded on a weekly basis. The intermittent hypoxia protocol (12H), allowed embryos to properly adapt to the shortage of oxygen, compensate for the gap in body mass that developed following the first exposure window, and hatch with characteristics similar to those of the control embryos. In contrast, while the 48H embryos were able to adapt to the hypoxic stress, the prolonged exposure prevented them from catching up with both control and 12H embryos. Broilers that were subjected to hypoxia showed hatchling body weights and growth rates similar to those of controls, throughout the entire growth phase. During the fifth wk, lower feed consumption was observed in the 12H and 48H groups and became significantly lower than the control chicks in the sixth wk of growth. Following hypoxia exposure, chicks managed to reach normal body weight with less feed, with the 12H group demonstrating lower and more efficient FCR during the last 2 wk of growth. Broiler embryos reacted to plateau-phase hypoxia challenge with numerous physiological and metabolic modifications. The prudent alterations in metabolism and cardiovascular system during exposure to hypoxia and posthatch, resulted in more efficient energy utilization in broilers, which may have a long-lasting enhancing effect on posthatching thermotolerance and sustainability in chicks reared under sub-optimal environmental conditions.

Thermal biology of two sympatric Lacertid lizards (Lacerta diplochondrodes and Parvilacerta parva) from Western Anatolia

Sympatric lizard species present convenient models for studying differentiation in thermal behavior and the role of morphological differences in their thermal biology. Here we studied the thermal biology of two sympatric lizard species which occur sympatrically in the Phrygian Valley of Western Anatolia. These two species differ in body size, with Lacerta diplochondrodes being larger than Parvilacerta parva. The surface body temperatures of the individuals belonging to both species were recorded when active in the field. Additionally, several environmental parameters including solar radiation, substrate temperature, air temperature and wind speed were monitored to investigate the relative effects of these abiotic parameters on the thermal biology of the two species. The surface body temperature and temperature excess (difference between body and substrate temperature) of the two species, while being relatively close to each other, showed seasonal differences. Solar radiation, substrate temperature and air temperature were the main factors influencing their thermal biology. Additionally, although body size did not have a direct effect on body temperature or temperature excess, the interaction between body size and solar radiation on temperature excess was significant. In conclusion, our study partially supports the conservation of body temperature of related lizard species.

Sex steroids are correlated with environmental factors and body condition during the reproductive cycle in females of the lizard Sceloporus torquatus

Reproduction is regulated by multiple factors that influence physiology and behavior to ensure the continuity of species. However, more work is needed to examine the complex relationships between environmental factors and endocrine transducers that modulate reproductive cycles, particularly in lizards. Here, we aimed to characterize the variation in plasma sex steroid levels in different stages of the reproductive cycle in the lizard Sceloporus torquatus and assess whether sex steroid levels were related to environmental factors (temperature, photoperiod, precipitation, and relative humidity) and body condition. Plasma concentrations of estradiol (E₂) and progesterone (P₄) from blood samples were quantified by enzyme-linked immunosorbent assay (ELISA) and radioimmunoanalysis (RIA), respectively. Our results indicate that sex steroid concentrations were positively related to follicular development but negatively related to temperature and precipitation. E₂ increased as the follicles grew, and its concentrations were highest in the preovulatory phase. P₄ showed a similar pattern and persisted during pregnancy. Changes in body condition were non-significant and mainly unrelated to the reproductive stage and plasma sex steroids. Our findings indicate that sex steroids change depending on the season and reproductive stage. We observed high concentrations of E₂ and P₄ in the late vitellogenic and preovulatory stages, probably because of their role in promoting vitellogenesis and ovulation. Additionally, we observed that follicular development is correlated with temperature and photoperiod. To better understand the mechanisms underlying reproduction, future studies of captive populations where environmental factors can be manipulated are needed.

A chromosome-level genome assembly for the eastern fence lizard (Sceloporus undulatus), a reptile model for physiological and evolutionary ecology

Background

High-quality genomic resources facilitate investigations into behavioral ecology, morphological and physiological adaptations, and the evolution of genomic architecture. Lizards in the genus Sceloporus have a long history as important ecological, evolutionary, and physiological models, making them a valuable target for the development of genomic resources.

Findings

We present a high-quality chromosome-level reference genome assembly, SceUnd1.0 (using 10X Genomics Chromium, HiC, and Pacific Biosciences data), and tissue/developmental stage transcriptomes for the eastern fence lizard, Sceloporus undulatus. We performed synteny analysis with other snake and lizard assemblies to identify broad patterns of chromosome evolution including the fusion of micro- and macrochromosomes. We also used this new assembly to provide improved reference-based genome assemblies for 34 additional Sceloporus species. Finally, we used RNAseq and whole-genome resequencing data to compare 3 assemblies, each representing an increased level of cost and effort: Supernova Assembly with data from 10X Genomics Chromium, HiRise Assembly that added data from HiC, and PBJelly Assembly that added data from Pacific Biosciences sequencing. We found that the Supernova Assembly contained the full genome and was a suitable reference for RNAseq and single-nucleotide polymorphism calling, but the chromosome-level scaffolds provided by the addition of HiC data allowed synteny and whole-genome association mapping analyses. The subsequent addition of PacBio data doubled the contig N50 but provided negligible gains in scaffold length.

Conclusions

These new genomic resources provide valuable tools for advanced molecular analysis of an organism that has become a model in physiology and evolutionary ecology.

Thermoregulation of Liolaemus aparicioi (Iguania: Liolaemidae) along a 1000 m elevational gradient in La Paz Valley, La Paz, Bolivia

Lizard species have diverse behavioral and physiological responses to thermo-environmental conditions, which allow them to inhabit a broad range of latitudes and elevations. Because the availability of suitable thermal resources is limited and more variable at high-elevation environments than at lower elevations, we expect high-elevation lizards to be constrained in their thermoregulation relative to lizards at lower elevations by the fewer available thermal resources to reach optimal temperatures (colder environment). We studied the thermal biology of an endemic and Critically Endangered lizard, Liolaemus aparicioi, to assess its thermal responses along a 1000 m elevational gradient in La Paz Valley from May to August of 2015 (dry season). We took field body and microhabitat temperatures at capture sites (substrate and air above ground), and body size (snout-vent length and mass) of individuals at Taypichullo (3000 m asl), Gran Jardín de la Revolución Municipal Park (3500 m asl), and Taucachi (4000 m asl) localities. Operative temperatures were taken from calibrated models deployed in different available microhabitats. Preferred temperatures and thermal tolerance limits were determined in laboratory settings for lizards from each locality. Field body, microhabitat, and operative temperatures decreased with increasing elevation and differed between sexes. Lizards at the high elevation locality had the lowest thermoregulatory efficiency as compared with the mid and lower elevation localities. In laboratory measurements, while the preferred temperatures varied between sexes, pooled preferred temperatures and thermal tolerances were similar in all localities. Although thermal resources at high elevation can limit thermoregulatory possibilities in L. aparicioi, behavioral microhabitat use, time allocated to thermoregulation, and physiological adjustments seem to be possible strategies to counteract thermal costs along elevational gradients.

Recovery from discrete wound severities in side-blotched lizards (Uta stansburiana): implications for energy budget, locomotor performance, and oxidative stress

Wounding events (predation attempts, competitive combat) result in injuries and/or infections that induce integrated immune responses for the recovery process. Despite the survival benefits of immunity in this context, the costs incurred may require investment to be diverted from traits contributing to immediate and/or future survival, such as locomotor performance and oxidative status. Yet, whether trait constraints manifest likely depends on wound severity and the implications for energy budget. For this study, food intake, body mass, sprint speed, and oxidative indices (reactive oxygen metabolites, antioxidant capacity) were monitored in male side-blotched lizards (Uta stansburiana) healing from cutaneous wounds of discrete sizes (control, small, large). Results indicate that larger wounds induced faster healing, reduced food consumption, and led to greater oxidative stress over time. Granted wounding did not differentially affect body mass or sprint speed overall, small-wounded lizards with greater wound area healed had faster sprint speeds while large-wounded lizards with greater wound area healed had slower sprint speeds. During recovery from either wound severity, however, healing and sprint performance did not correspond with food consumption, body mass loss, nor oxidative status. These findings provide support that energy budget, locomotor performance, and oxidative status of a reptile are linked to wound recovery to an extent, albeit dependent on wound severity.

Alligator presence influences colony site selection of long-legged wading birds through large scale facilitative nest protector relationship

Positive ecological relationships, such as facilitation, are an important force in community organization. The effects of facilitative relationships can be strong enough to cause changes in the distributions of species and in many cases have evolved as a response to predation pressure, however, very little is known about this potential trend in vertebrate facilitative relationships. Predation is an important selective pressure that may strongly influence breeding site selection by nesting birds. The American Alligator (Alligator mississippiensis) facilitates a safer nesting location for wading birds (Ciconiiformes and Pelecaniformes) by deterring mammalian nest predators from breeding sites. However, alligators do not occur throughout the breeding range of most wading birds, and it is unclear whether alligator presence affects colony site selection. We predicted that nesting wading birds change colony site preferences when alligators are not present to serve as nest protectors. Within the northern fringe of alligator distribution we compared colony characteristics in locations where alligator presence was either likely or unlikely while controlling for availability of habitat. Wading birds preferred islands that were farther from the mainland and farther from landmasses > 5 ha when alligator presence was unlikely compared to when alligators were likely. These findings indicate that wading birds are seeking nesting locations that are less accessible to mammalian predators when alligators are not present, and that this requirement is relaxed when alligators are present. This study illustrates how a landscape-scale difference between realized and fundamental niche can result from a facilitative relationship in vertebrates.

Baroreflex gain and time of pressure decay at different body temperatures in the tegu lizard, Salvator merianae

Ectotherms may experience large body temperature (T_b) variations. Higher T_b have been reported to increase baroreflex sensitivity in ectotherm tetrapods. At lower T_b, pulse interval (PI) increases and diastolic pressure decays for longer, possibly resulting in lower end-diastolic pressures and mean arterial pressures (P_m). Additionally, compensatory baroreflex-related heart rate modulation (i.e. the cardiac branch of the baroreflex response) is delayed due to increased PI. Thus, low T_b is potentially detrimental, leading to cardiovascular malfunctioning. This raises the question on how P_m is regulated in such an adverse condition. We investigated the baroreflex compensations that enables tegu lizards, Salvator merianae, to maintain blood pressure homeostasis in a wide T_b range. Lizards had their femoral artery cannulated and pressure signals recorded at 15°C, 25°C and 35°C. We used the sequence method to analyse the heart rate baroreflex-related corrections to spontaneous pressure fluctuations at each temperature. Vascular adjustments (i.e. the peripheral branch) were assessed by calculating the time constant for arterial pressure decay (τ)—resultant from the action of both vascular resistance and compliance—by fitting the diastolic pressure descent to the two-element Windkessel equation. We observed that at lower T_b, lizards increased baroreflex gain at the operating point (G_op) and τ, indicating that the diastolic pressure decays at a slower rate. G_op normalized to P_m and PI, as well as the ratio τ/PI, did not change, indicating that both baroreflex gain and rate of pressure decay are adjusted according to PI lengthening. Consequently, pressure parameters and the oscillatory power fraction (an index of wasted cardiac energy) were unaltered by T_b, indicating that both G_op and τ modulation are crucial for cardiovascular homeostasis.

Temporal variation in thermal plasticity in a free-ranging subalpine lizard

Thermally variable environments are particularly challenging for ectotherms as physiological functions are thermo-dependent. As a consequence, ectotherms in highly seasonal environments are predicted to have greater thermal plasticity. However, much of our understanding of thermal plasticity comes from controlled experiments in a laboratory setting. Relatively fewer studies investigate thermal plasticity in free-ranging animals living in their natural environment. We investigated the presence of thermal plasticity within a single activity season in adult males of a natural high elevation population of White's skink (Liopholis whitii) in south-eastern Australia. This species lives in a permanent home site (rock crevice and/or burrow), facilitating the repeated capture of the same individuals across the activity season. We monitored the thermal variation across the field site and over the activity season, and tested thermal tolerances and performance of male L. whitii on three occasions across their activity season. Maximum and average temperatures varied across the field site, and temperatures gradually increased across the study period. Evidence of temporal plasticity was identified in the critical thermal minimum and thermal tolerance breadth, but not in the critical thermal maximum. Thermal performance was also found to be plastic, but no temporal patterns were evident. Our temporal plasticity results are consistent which much of the previous literature, but this is one of the first studies to identify these patterns in a free-ranging population. In addition, our results indicate that performance may be more plastic than previous literature suggests. Overall, our study highlights the need to pair laboratory and field studies in order to understand thermal plasticity in an ecologically relevant context.

The highest kingdom of Anolis: Thermal biology of the Andean lizard Anolis heterodermus (Squamata: Dactyloidae) over an elevational gradient in the Eastern Cordillera of Colombia

Vertebrate ectotherms may deal with changes of environmental temperatures by behavioral and/or physiological mechanisms. Reptiles inhabiting tropical highlands face extreme fluctuating daily temperatures, and extreme values and intervals of fluctuations vary with altitude. Anolis heterodermus occurs between 1800 m to 3750 m elevation in the tropical Andes, and is the Anolis species found at the highest altitude known. We evaluated which strategies populations from elevations of 2200 m, 2650 m and 3400 m use to cope with environmental temperatures. We measured body, preferred, critical maximum and minimum temperatures, and sprint speed at different body temperatures of individuals, as well as operative temperatures. Anolis heterodermus exhibits behavioral adjustments in response to changes in environmental temperatures across altitudes. Likewise, physiological traits exhibit intrapopulation variations, but they are similar among populations, tended to the "static" side of the evolution of thermal traits spectrum. The thermoregulatory behavioral strategy in this species is extremely plastic, and lizards adjust even to fluctuating environmental conditions from day to day. Unlike other Anolis species, at low thermal quality of the habitat, lizards are thermoconformers, particularly at the highest altitudes, where cloudy days can intensify this strategy even more. Our study reveals that the pattern of strategies for dealing with thermal ambient variations and their relation to extinction risks in the tropics that are caused by global warming is perhaps more complex for lizards than previously thought.

Resting metabolic rates increase with elevation in a mountain-dwelling lizard

Individuals that inhabit broad elevational ranges may experience unique environmental challenges. Because temperature decreases with increased elevation, the ectotherms living at high elevations have to manage limited activity time and high thermoregulatory effort. The resting metabolic rate (RMR) of a postabsorptive animal is related to its total energy requirements as well as many other fitness traits. Mesquite lizards (Sceloporus grammicus) living on La Malinche Volcano, Mexico, inhabit a wide elevational range with some populations apparently thriving above the tree line. We measured the RMR of lizards from different elevations (i.e., 2,600, 3,200, and 4,100 m) at four ecologically relevant temperatures (i.e., 15, 25, 30, and 35 °C) and found that RMR of mesquite lizards increased with temperature and body mass. More importantly, lizards from the high-elevation population had mass specific RMR that was higher at all temperatures. While the higher RMRs of high-elevation populations imply higher metabolic costs at a given temperature these lizards were also smaller. Both of these traits may allow these high elevation populations to thrive in the face of the thermal challenges imposed by their environment.

Acclimatization in the physiological performance of an introduced ectotherm

Phenotypic flexibility may facilitate range expansion by allowing organisms to maintain high levels of performance when introduced to novel environments. Phenotypic flexibility, such as reversible acclimatization, permits organisms to achieve high performance over a wide range of environmental conditions, without the costly allocation or acquisition tradeoffs associated with behavioral thermoregulation, which may expedite range expansion in introduced species. The northern curly-tailed lizard, Leiocephalus carinatus, was introduced to the USA in the 1940s and is now established in southern Florida. We measured bite force and the thermal sensitivity of sprinting of L. carinatus during the winter and spring to determine how morphology and performance varied seasonally. We found evidence of seasonal variation in several aspects of physiological performance. Lizards sampled in spring sprinted faster and tolerated higher temperatures, while lizards sampled in winter had high performance over a wider range of temperatures. Furthermore, seasonal differences in physiology were only detected after generating thermal reaction norms. Both sprint and bite force performance did not differ seasonally when solely comparing performance at a common temperature. No seasonal relationships between morphology and performance were detected. Our results suggest that L. carinatus may use reversible acclimatization to maintain high levels of performance across seasons not typically experienced within their native range. Thermal physiology plasticity may ameliorate the impacts of sub-optimal temperatures on performance without the cost of behavioral thermoregulation. Our work highlights the importance of utilizing reaction norms when evaluating performance and the potential ecological impacts of introduced species.

Diel and seasonal variations in the thermal biology of San Cristobal Lava Lizards (Microlophus bivittatus)

Thermal biology, and therefore energy acquisition and survival, of ectotherms can be affected by diel and seasonal patterns of environmental temperatures. Galápagos Lava Lizards live in seasonal environments that are characterized by a warm and wet period when reproductive activity is maximal, and cooler and drier period. With the use of radiotelemetric techniques to record lizard surface temperatures (T_s), we studied the thermal ecology of the San Cristóbal Lava Lizard (Microlophus bivittatus) during both the warm and cool seasons over two years. During the diel activity period and when operative temperatures exceeded T_set-min, at least on rock faces without canopy, 52% or less of the T_s observations fell within the laboratory-determined T_set range (36-40 °C). Therefore, lizards may have avoided very warm midday temperatures in shaded microhabitats and the lag times in changes in T_s values occurred as operative temperatures rose rapidly during late morning warming phase. Lizards effectively thermoregulated during a year with moderate warm season temperatures and during a cool season that was unseasonably warm. In contrast, lizards less effectively thermoregulated during the warmest and coolest years of the study. We did not detect intersexual differences in thermoregulation although males may thermoregulate less effectively than do females during the cool season although we were unable to detect significant differences using our nonparametric statistical techniques.

Ecological and evolutionary consequences of metabolic rate plasticity in response to environmental change

Basal or standard metabolic rate reflects the minimum amount of energy required to maintain body processes, while the maximum metabolic rate sets the ceiling for aerobic work. There is typically up to three-fold intraspecific variation in both minimal and maximal rates of metabolism, even after controlling for size, sex and age; these differences are consistent over time within a given context, but both minimal and maximal metabolic rates are plastic and can vary in response to changing environments. Here we explore the causes of intraspecific and phenotypic variation at the organ, tissue and mitochondrial levels. We highlight the growing evidence that individuals differ predictably in the flexibility of their metabolic rates and in the extent to which they can suppress minimal metabolism when food is limiting but increase the capacity for aerobic metabolism when a high work rate is beneficial. It is unclear why this intraspecific variation in metabolic flexibility persists-possibly because of trade-offs with the flexibility of other traits-but it has consequences for the ability of populations to respond to a changing world. It is clear that metabolic rates are targets of selection, but more research is needed on the fitness consequences of rates of metabolism and their plasticity at different life stages, especially in natural conditions. This article is part of the theme issue 'The role of plasticity in phenotypic adaptation to rapid environmental change'.

The impact of ectoparasitism on thermoregulation in Yarrow’s Spiny Lizards (Sceloporus jarrovii)

Parasites are ubiquitous and can have large impacts on the fitness of their hosts. The effects of ectoparasites on physiology, behaviour, and immune function suggest that they could be part of the factors which impact thermoregulation. We tested the hypothesis that ectoparasites impact thermoregulation in Yarrow’s Spiny Lizards (Sceloporus jarrovii Cope in Yarrow, 1875) living along an elevational gradient. We predicted a positive association between ectoparasite load and body temperature (Tb), and a negative association between ectoparasite load and effectiveness of thermoregulation (de – db index). We also predicted that the impacts of ectoparasites would be greatest at high elevation where thermal quality of the environment is low because the costs of thermoregulation increase with elevation and these costs can impact thermal immune responses. We found a significant association between the number of chiggers (Trombiculoidea) harboured by lizards and Tb that depended on elevation, but no association between ectoparasite load and de – db index. The mean chigger infection rate was associated with a ΔTb of +0.18 °C at low elevation (consistent with fever) and of –1.07 °C at high elevation (consistent with hypothermia). These findings suggest that parasitism by chiggers impacts lizard Tb in a way that depends on environmental thermal quality.

Thermal and salinity effects on locomotor performance of Thoropa taophora tadpoles (Anura, Cycloramphidae)

It is well known that environmental temperature influences several biological functions of ectotherms, notably in amphibians. The high permeability of anuran skin, associated with the effect of elevated environmental temperature, potentiates the dehydration process and this combination may restrict locomotor performance. Thoropa taophora is an endemic species from the Atlantic Rainforest whose tadpoles are semiterrestrial and predominantly diurnal, and are found in rocky seashores where they are exposed to sea spray and high temperatures. In this study we investigated how temperature and salinity conditions affect the locomotor performance in Thoropa taophora tadpoles. We also assessed how different osmotic concentrations affect the activity of the metabolic pathways that support muscle function. We measured the sprint speed of tadpoles of various sizes at different temperatures and salinities in the field. We also measured the activity of the enzymes pyruvate kinase (PK), lactate dehydrogenase (LDH) and citrate synthase (CS) in different temperatures and osmotic concentrations, and calculated the thermal sensitivity and the activity constants for each osmolality. Our results showed that, in general, sprint speed decreased with increasing temperature and salinity. However, whereas the effect of increased salinity was similar in smaller and larger tadpoles, increased temperature had a higher negative impact on sprint speed of larger tadpoles, thus indicating low thermal sensitivity of small tadpoles. PK and LDH thermal sensitivities and LDH constant of activity decreased as the osmolality increased. In conclusion, the locomotor capacity of tadpoles was decreased by temperature and salinity, which may be related to a decrease in anaerobic metabolism both in terms of sensitivity and total energy turnover through enzymatic activity. We discuss the ecological consequences, including the potential impacts on predator escape behavior promoted by changes in metabolism and locomotor performance in an early stage of development of this species.

Rice Leaf Folder Larvae Alter Their Shelter-Building Behavior and Shelter Structure in Response to Heat Stress

Behavioral thermoregulation is a key strategy for insects to cope with heat stress. The rice leaf folder Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae) larvae usually fold one leaf to construct a leaf shelter. The larvae are vulnerable to heat stress, and the temperature in summer is often beyond the optimal range of them. Shelters confer protection against environmental stress but unclear whether larvae will alter shelter-building behavior when encountering heat stress. We observed the shelter-building behavior of larvae during and after heat shock, and then examined the shape and structure of shelters. Larvae spent more time in selecting a site and building a shelter during and after heat shock than at the optimal temperature. More than 70% of larvae folded two or three leaves to build a shelter during and after heat shock, but more than 60% of larvae only folded one leaf at the optimal temperature. Larvae built more single-leaf longitudinal shelters at the optimal temperature, but they built more multileaf overlapping shelters during and after heat stress. Larvae constructed a short leaf shelter using a small amount of silk binds when they were exposed to 40°C for 4 h. The rice leaf folder larvae can alter their shelter-building behavior and shelter structure in response to heat stress.

Differential plasticity of membrane fatty acids in northern and southern populations of the eastern newt (Notophthalmus viridescens)

Seasonal changes in membrane composition and metabolic activity allow many temperate ectotherms to contend with changes in body temperature, but few studies have investigated whether the plasticity of these traits has diverged within a single species. Therefore, we studied the effects of thermal acclimation on the membrane fatty acid composition and the activities of cytochrome c oxidase (CCO) and citrate synthase (CS) in the skeletal muscle and liver of eastern newts from Maine and Florida. Newts were acclimated to either 6 °C or 28 °C for 12 weeks prior to experiments. Cold acclimation resulted in a lower saturated fatty acid (SFA) content in the muscle membranes of both populations. SFA content in liver was lower in cold compared to warm-acclimated newts from Florida, but acclimation did not affect SFA content in liver membranes of the Maine population. In liver, cold acclimation resulted in a higher monounsaturated fatty acid (MUFA) content in the Florida population and a higher polyunsaturated fatty acid (PUFA) content in the Maine population. Regardless of acclimation conditions, the muscle and liver membranes of the Maine population had higher SFA and PUFA contents compared to those of the Florida population. MUFA content of muscle and liver membranes was higher in the Florida population compared to the Maine population. The effect of acclimation on CCO and CS activity was tissue-specific. In muscle, CCO and CS activities were higher in cold compared to warm-acclimated newts in both populations, and CS and CCO activities were higher in the Maine compared to the Florida population. In liver, CCO and CS activity were unaffected by acclimation in the Florida population, but activity was lower in cold compared to warm-acclimated Maine newts. These results demonstrate that the phenotypic plasticity of these traits in response to seasonal change has diverged between northern and southern populations.

The likely effects of thermal climate change on vertebrate skeletal muscle mechanics with possible consequences for animal movement and behaviour

Climate change can involve alteration in the local temperature that an animal is exposed to, which in turn may affect skeletal muscle temperature. The underlying effects of temperature on the mechanical performance of skeletal muscle can affect organismal performance in key activities, such as locomotion and fitness-related behaviours, including prey capture and predator avoidance. The contractile performance of skeletal muscle is optimized within a specific thermal range. An increased muscle temperature can initially cause substantial improvements in force production, faster rates of force generation, relaxation, shortening, and production of power output. However, if muscle temperature becomes too high, then maximal force production and power output can decrease. Any deleterious effects of temperature change on muscle mechanics could be exacerbated by other climatic changes, such as drought, altered water, or airflow regimes that affect the environment the animal needs to move through. Many species will change their location on a daily, or even seasonal basis, to modulate the temperature that they are exposed to, thereby improving the mechanical performance of their muscle. Some species undergo seasonal acclimation to optimize muscle mechanics to longer-term changes in temperature or undergo dormancy to avoid extreme climatic conditions. As local climate alters, species either cope with the change, adapt, avoid extreme climate, move, or undergo localized extinction events. Given that such outcomes will be determined by organismal performance within the thermal environment, the effects of climate change on muscle mechanics could have a major impact on the ability of a population to survive in a particular location.

Comparative Transcriptomics of Seasonal Phenotypic Flexibility in Two North American Songbirds

Phenotypic flexibility allows organisms to reversibly alter their phenotypes to match the changing demands of seasonal environments. Because phenotypic flexibility is mediated, at least in part, by changes in gene regulation, comparative transcriptomic studies can provide insights into the mechanistic underpinnings of seasonal phenotypic flexibility, and the extent to which regulatory responses to changing seasons are conserved across species. To begin to address these questions, we sampled individuals of two resident North American songbird species, American goldfinch (Spinus tristis) and black-capped chickadee (Poecile atricapillus) in summer and winter to measure seasonal variation in pectoralis transcriptomic profiles and to identify conserved and species-specific elements of these seasonal profiles. We found that very few genes exhibited divergent responses to changes in season between species, and instead, a core set of over 1200 genes responded to season concordantly in both species. Moreover, several key metabolic pathways, regulatory networks, and gene functional classes were commonly recruited to induce seasonal phenotypic shifts in these species. The seasonal transcriptomic responses mirror winter increases in pectoralis mass and cellular metabolic intensity documented in previous studies of both species, suggesting that these seasonal phenotypic responses are due in part to changes in gene expression. Despite growing evidence of muscle nonshivering thermogenesis (NST) in young precocial birds, we did not find strong evidence of upregulation of genes putatively involved in NST during winter in either species, suggesting that seasonal modification of muscular NST is not a prominent contributor to winter increases in thermogenic capacity for adult passerine birds. Together, these results provide the first comprehensive overview of potential common regulatory mechanisms underlying seasonally flexible phenotypes in wild, free-ranging birds.

Thermal biology of the spotted snow skink, Niveoscincus ocellatus, along an altitudinal gradient

Body temperatures in ectotherms are strongly affected by their thermal environment. Ectotherms respond to variation in the thermal environment either by modification of behavioural thermoregulation to maintain their optimal body temperature or by shifting their optimal body temperature. In this study, the body temperatures of males of three populations of spotted snow skinks, Niveoscincus ocellatus, living along an altitudinal gradient (low, mid, and high altitude) were studied in the field and laboratory in spring, summer, and autumn, representing the full activity period of this species. The environmental variation across both sites and seasons affected their field active body temperatures. At the low and mid altitude, N. ocellatus had a higher mean body temperature than at the high altitude. Animals achieved their thermal preference at the low and mid altitude sites in all seasons. At the high altitude, however, N. ocellatus struggled to reach its preferred body temperatures, especially in autumn. The lower body temperature at the high-altitude site is likely due to limited thermal opportunity and/or an effect of avoiding the costs associated with increased intensity of basking.

Plasticity and cross-tolerance to heterogeneous environments: divergent stress responses co-evolved in an African fruit fly

Plastic adjustments of physiological tolerance to a particular stressor can result in fitness benefits for resistance that might manifest not only in that same environment but also be advantageous when faced with alternative environmental stressors, a phenomenon termed 'cross-tolerance'. The nature and magnitude of cross-tolerance responses can provide important insights into the underlying genetic architecture, potential constraints on or versatility of an organism's stress responses. In this study, we tested for cross-tolerance to a suite of abiotic factors that likely contribute to setting insect population dynamics and geographic range limits: heat, cold, desiccation and starvation resistance in adult Ceratitis rosa following acclimation to all these isolated individual conditions prior to stress assays. Traits of stress resistance scored included critical thermal (activity) limits, chill coma recovery time (CCRT), heat knockdown time (HKDT), desiccation and starvation resistance. In agreement with other studies, we found that acclimation to one stress typically increased resistance for that same stress experienced later in life. A more novel outcome, however, is that here we also found substantial evidence for cross-tolerance. For example, we found an improvement in heat tolerance (critical thermal maxima, CT_max ) following starvation or desiccation hardening and improved desiccation resistance following cold acclimation, indicating pronounced cross-tolerance to these environmental stressors for the traits examined. We also found that two different traits of the same stress resistance differed in their responsiveness to the same stress conditions (e.g. HKDT was less cross-resistant than CT_max ). The results of this study have two major implications that are of broader importance: (i) that these traits likely co-evolved to cope with diverse or simultaneous stressors, and (ii) that a set of common underlying physiological mechanisms might exist between apparently divergent stress responses in this species. This species may prove to be a valuable model for future work on the evolutionary and mechanistic basis of cross-tolerance.