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Behavioral Thermoregulation in Lizards: Strategies for Achieving Preferred Temperature

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... Furthermore, the dependence of physiological performance on temperature, is expected to constrain adaptive shifts in T p , which would be manifested by a lower evolutionary rate. In addition, we expect that T p , despite being a conservative trait, coevolves with T b in Liolaemidae as observed in other families within the order Squamata (Black et al. 2019). But, we predict the rate of evolution will be slower for T p than T b . ...
... The correlation between T p with T b suggests evolutionary constraint between the traits as observed for several species of lizards, including Liolaemus (Labra et al. 2009;Black et al. 2019;García-Porta et al. 2019). Consistent with our prediction and despite the family-wide diversification in several thermal traits, rates of environmental change may outpace rates of trait evolution (Miles 1994), considering that there is a difference of ∼3°C to 4°C between field active T b and T p in the family Liolaemidae. ...
... Consistent with our prediction and despite the family-wide diversification in several thermal traits, rates of environmental change may outpace rates of trait evolution (Miles 1994), considering that there is a difference of ∼3°C to 4°C between field active T b and T p in the family Liolaemidae. This difference is higher than what was observed for most lizard families (Black et al. 2019). Lizards in Patagonia show T p values higher than the T b lizards attain in nature (Labra et al. 2008;Medina et al. 2012;Duran et al. 2018;Cabezas-Cartes et al. 2019;Ibargüengoytía et al. 2020). ...
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The diversity of habitats generated by the Andes uplift resulted a mosaic of heterogeneous environments in South America for species to evolve a variety of ecological and physiological specializations. Species in the lizard family Liolaemidae occupy a myriad of habitats in the Andes. Here we analyze the tempo and mode of evolution in the thermal biology of liolaemids. We assessed whether there is evidence of local adaptation (lability) or conservatism (stasis) in thermal traits. We tested the hypothesis that abiotic factors (e.g., geography, climate) rather than intrinsic factors (egg‐laying [oviparous] or live‐bearing [viviparous], substrate affinity) explain variation in field active body temperature (Tb), preferred temperature (Tp), hours of restriction of activity and potential hours of activity. Although most traits exhibited high phylogenetic signal, we found variation in thermal biology was shaped by geography, climate and ecological diversity. Ancestral character reconstruction showed shifts in Tb tracked environmental change in the past ∼20 thousand years. Thermal preference is 3°C higher than Tb, yet exhibited a lower rate of evolution than Tb and air temperature. Viviparous Liolaemus have lower Tbs than oviparous species, whereas Tp is high for both modes of reproduction, a key difference that results in a thermal buffer for viviparous species to cope with global warming. The rapid increase in environmental temperatures expected in the next 50–80 years in combination with anthropogenic loss of habitats are projected to cause extirpations and extinctions in oviparous species. This article is protected by copyright. All rights reserved
... Shuttling to and from the retreat allows basking, a process used by squamates to gain heat from the sun by exposing their whole or some body parts to the source of heat (Black et al., 2019). ...
... What signals the onset of shuttling remains unclear. In most squamates, it is believed to be based on the thermal threshold of the squamates (Black et al., 2019). This threshold is highly predictable at the species level and is termed the voluntary thermal maximum (VTmax), a set point when squamates voluntarily avoid a heat source and seek shelter elsewhere (Camachoa & Rusch, 2017). ...
Thesis
Ectotherms, especially terrestrial squamates, rely on suitable microclimatic conditions to maintain critical physiological processes. Selection of appropriate microhabitat determines how well squamates will function in fluctuating thermal environments. Understanding the use of microhabitat by cool-climate lizards, including the long-lived gecko, Woodworthia “Otago/Southland”, will give insight into the future implications of global climate change for New Zealand lizards. These viviparous, rock-dwelling geckos from southern New Zealand (N 45° S) live in an environment with an air temperature frequently lower than their preferred body temperature. They are nocturnally foraging, and females in a mid-elevation population at Macraes have the most extended gestation length found in reptiles. Factors determining microhabitat use, and how changes in current weather conditions might influence the use of microhabitat by Otago/Southland geckos remain unclear. I investigated microhabitat use by Otago/Southland geckos of different life-history groups at Macraes, including the influence of current weather conditions in different seasons, using a combination of laboratory- and field-based approaches. Body temperatures (Tb) can be used to predict microhabitat use and activity time of squamates and require accurate measurements using a reliable device, especially in field situations. I tested the accuracy of an inexpensive, hand-held mini-infrared thermometer (mini-IRT) in measuring skin surface temperature of small lizards or their models. I assessed the effect of model width on the accuracy of the measurement and compared the measurement of skin surface temperatures using the mini-IRT with a thermal infrared camera and thermocouple thermometer. Mean discrepancy for skin surface temperatures collected with a mini-IRT was low for Otago/Southland geckos with a relatively wider abdominal width, but higher for the sympatric McCann’s skink (Oligosoma maccanni) with a narrower abdominal width. I concluded that the mini-IRT is acceptable for field studies on small lizards with an abdominal width ≥ 7 mm. For rock-dwelling squamates, microhabitats may be abandoned on hot days if the Tb reaches a threshold that cannot be tolerated voluntarily. In my second study, I identified the upper voluntary thermal limit (VTmax) of Otago/Southland geckos, namely the body temperature (Tb) at which lizards are forced to abandon their daytime retreats when heated. Using a new laboratory protocol, I developed for determining VTmax in a retreat-dwelling species, I compared Tb at VTmax, and duration of heating, between two source groups with different thermal histories and among three reproductive groups. I also assessed total evaporative water loss (EWL) during heating for the wild geckos. I found VTmax and duration of heating varied between source groups (and thus potentials with prior thermal experience), but not among reproductive groups. Non-pregnant females lost more water than other reproductive groups. In the wild, pregnant female geckos with attached biologgers reached a Tb matching VTmax, and the temperatures of some separately monitored microhabitats exceeded VTmax in hot weather, implying that some retreats must be abandoned to avoid overheating. The results suggest that Otago/Southland geckos at Macraes may abandon retreats more frequently if climate warming persists, implying a trade-off between retention of the originally occupied shelter and ongoing water loss due to overheating. The selection of appropriate retreats affects the microhabitat use of Otago/Southland geckos in their field environment. Using a thermal infrared camera and field survey, I found that geckos inhabit daytime retreats under rock slabs that are warmer, wider, thicker, closer to vegetation and with less depression on the lower substrate than slabs that are unoccupied. Also, 2-y thermal profiles of initially occupied retreats indicated that for thin slabs, the temperatures of the underside of retreat slab or Tretreat-top, and the surface of the rocky substrate beneath the slabs, Tretreat-bottom, exceeded VTmax on hot days in summer. In contrast, thick rock slabs and deep crevice temperatures were cooler and lower than the geckos’ preferred body temperature over time. Path analysis indicated that field Tb was positively correlated with maximum Trock-surface, maximum Tretreat-bottom, and air temperature. Also, daytime field Tb varied across seasons, life-history groups and with the time of capture. This study demonstrates the importance of microclimate conditions in influencing retreat-site choice. As in other parts of the world, air temperature at Macraes is predicted to increase with global climate change. This increase may be beneficial to cool-climate lizards in creating a thermal environment that better supports physiological processes and activity. Using a combination of trail camera and nighttime field survey, I assessed the seasonal diurno-nocturnal activity (daytime basking and nighttime emergence) of the geckos, concurrently with field operative temperature (Te) exposed to the open and nighttime field Tb using a thermal infrared camera. The activity patterns showed that Otago/Southland geckos at Macraes are thermal generalists, active over a wide range of body temperatures, from a low of 1.4 °C at night, to 31.9°C by day. Air temperature positively influenced the Te available to emerged geckos, the geckos’ emergence activity by day and night, and their field Tb. However, higher wind speed reduced their diurnal and nocturnal activity. I identified that geckos that basked during the day in spring are exclusively pregnant females. My study also, for the first time, recorded evidence of basking and nocturnal emergence (at reduced levels) in winter. The results suggest that present weather conditions, the activity of Woodworthia “Otago/Southland” geckos was high. Also, Otago/Southland geckos are capable of activity at current low temperatures in winter, and sparsely use loose rock slabs in winter. These observations suggest that greater opportunities to bask in winter are likely to increase under future climate scenarios. Collectively, my thesis results provide evidence of weather-dependent effects and life-history traits on microhabitat use by Otago/Southland geckos at Macraes. The findings suggest that future climate heating may provide some initial benefits to these geckos, but longer-term outcomes likely hinge on the magnitude of heating and the availability of suitable refugia below the voluntary thermal maximum.
... Typically, these traits are optimally efficient within a narrow range of T b (Angilletta et al., 2002). Thus, the ability to maintain a relatively constant T b , despite the thermally heterogeneous environment, has direct effects on survivorship and, therefore, fitness (Black et al., 2019). ...
... Another strategy to cope with thermal heterogeneity includes the plasticity and adaptability of different traits (thermoregulatory behaviour, thermal sensitivity) to the changing abiotic circumstances (Angilletta, 2009;Black et al., 2019;Pigliucci, 2001). In particular, the potential of a genome to produce a range of phenotypes in response to distinct environmental conditions is known as phenotypic plasticity (Pigliucci, 2001). ...
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In ectotherms, temperature exerts a strong influence on the performance of physiological and ecological traits. One approach to understand the impact of rising temperatures on animals and their ability to cope with climate change is to quantify variation in thermal-sensitive traits. Here, we examined the thermal biology, the temperature dependence and the thermal plasticity of bite force (endurance and magnitude) in Diplolaemus leopardinus, an aggressive and territorial lizard, endemic to Mendoza province, Argentina. Our results indicated that this lizard behaves like a moderate thermoregulator which uses the rocks of its environment as the main heat source. Bite endurance was not influenced by head morphometry and body temperature, whereas bite force was influenced by head length and jaw length, and exhibited thermal dependence. Before thermal acclimation treatments, the maximum bite force for D. leopardinus occured at the lowest body temperature and fell sharply with increasing body temperature. After acclimation treatments, lizards acclimated at higher temperatures exhibited greater bite force. Bite force showed phenotypic plasticity, which reveals that leopard iguanas are able to maintain (and even improve) their bite force under a rising-temperature scenario.
... Thermoregulation in reptiles, particularly those with relatively small body size, is mostly behavioural (Black et al., 2019) accomplished through shuttling among locations with variable solar radiation levels, wind speeds, or substrate temperatures, and through postural changes (Cowles and Bogert, 1944;Huey, 1982). For lizards in equatorial, lowland environments extreme high temperatures can limit physiological performance and diel activity as well as present potentially lethal temperatures (Huey and Slatkin, 1976;Angilletta, 2009). ...
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We studied thermoregulation in Pacific Knobbed Iguanas (Microlophus occipitalis Peters 1871) in a sand dune ecosystem in coastal Ecuador. During the daily activity period, operative temperatures (Te) on sand or on debris in direct sunlight far exceeded the upper limit of the lizard thermoregulatory setpoint range reported for Microlophus (Tset-max = 40°C). However, the operative temperatures of other microhabitats, whether in shade or in direct sunlight, were largely below Tset-max throughout most of the day. Average lizard surface temperatures (Ts), as recorded remotely using radiotelemetry, increased rapidly during late morning, exceeded Tset-min (36 ºC) slightly between roughly 10:00–16:00 h, and thereafter declined slowly into the evening and early morning hours of the following day. Lizards maintained Ts within the Tset range about 66% of the time when it would be permitted by mean Te. We did not detect differences in thermal biology between males or females.
... According to the IPCC (2013), heat waves and drought will intensify in both duration and frequency in the coming years, increasing the already high thermal stress and water deficiency experienced by organisms. The rising temperatures predicted are of major concern for ectotherms because they directly impact the metabolic rate, water balance, digestion, growth, reproduction, activity, survival and therefore the fitness of this group (Black et al., 2019). As physiological performance mediates a species' resilience to warming exposure (Bozinovic et al., 2011), physiological plasticity could greatly reduce the susceptibility to climate change (Stillman, 2003;Seebacher et al., 2015), especially if the potential to carry out behavioral adjustments is reduced (e.g. ...
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The rising temperature predicted is of main concern for ectotherms because its direct impact on their behavior and physiology. Since physiological performance mediates a species’ resilience to warming exposure, physiological plasticity could greatly reduce the susceptibility to climate change. We studied the degree to which Diplolaemus leopardinus’ lizards are able to adjust behavioral and physiological traits in response to short periods of temperature change. We used a split cross design to measure acclimation response of preferred body temperature (Tp), and thermal performance curve of resting metabolic rate (RMR) and evaporative water loss (EWL). Our results showed that plasticity differs among traits; whereas Tp and EWL showed lower values in warm conditions, RMR increased the temperature at which its value is highest. Moreover, RMR was affected by thermal history, showing a great increase in response to cold exposure in the group initially acclimated to warm. The reduction of EWL and the increase in optimal temperature will give lizards the potential to partially mitigate the impact of rising temperatures in the energy cost and water balance. However, the decrease in Tp and the sensitivity to the warm thermal history in RMR could be detrimental to the energy net gain increasing the species vulnerability, especially considering the increase of heat waves predicted for the next fifty years. The integration of acclimation responses in behavioral and physiological traits provides a better understanding of the range of possible responses of lizards to cope with the upcoming climatic and environmental modifications expected due to climate change.
... Microhabitat selection by individuals may allow populations to maximize the geographic range 82 . In this sense, shrubs play a significant role in thermal ecology of the species. ...
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Thermoregulation is critical for ectotherms as it allows them to maintain their body temperature close to an optimum for ecological performance. Thermoregulation includes a range of behaviors that aim at regulating body temperature within a range centered around the thermal preference. Thermal preference is typically measured in a thermal gradient in fully-hydrated and post-absorptive animals. Short-term effects of the hydric environment on thermal preferences in such set-ups have been rarely quantified in dry-skinned ectotherms, despite accumulating evidence that dehydration might trade-off with behavioral thermoregulation. Using experiments performed under controlled conditions in climatic chambers, we demonstrate that thermal preferences of a ground-dwelling, actively foraging lizard ( Zootoca vivipara ) are weakly decreased by a daily restriction in free-standing water availability (less than 0.5°C contrast). The influence of air humidity during the day on thermal preferences depends on time of the day and sex of the lizard, and is generally weaker than those of of free-standing water (less than 1°C contrast). This shows that short-term dehydration can influence, albeit weakly, thermal preferences under some circumstances in this species. Environmental humidity conditions are important methodological factors to consider in the analysis of thermal preferences.
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Phylogenetic generalised least squares (PGLS) is one of the most commonly employed phylogenetic comparative methods. The technique, a modification of generalised least squares, uses knowledge of phylogenetic relationships to produce an estimate of expected covariance in cross-species data. Closely related species are assumed to have more similar traits because of their shared ancestry and hence produce more similar residuals from the least squares regression line. By taking into account the expected covariance structure of these residuals, modified slope and intercept estimates are generated that can account for interspecific autocorrelation due to phylogeny. Here, we provide a basic conceptual background to PGLS, for those unfamiliar with the approach. We describe the requirements for a PGLS analysis and highlight the packages that can be used to implement the method. We show how phylogeny is used to calculate the expected covariance structure in the data and how this is applied to the generalised least squares regression equation. We demonstrate how PGLS can incorporate information about phylogenetic signal, the extent to which closely related species truly are similar, and how it controls for this signal appropriately, thereby negating concerns about unnecessarily ‘correcting’ for phylogeny. In addition to discussing the appropriate way to present the results of PGLS analyses, we highlight some common misconceptions about the approach and commonly encountered problems with the method. These include misunderstandings about what phylogenetic signal refers to in the context of PGLS (residuals errors, not the traits themselves), and issues associated with unknown or uncertain phylogeny.
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The ability to change colour rapidly is widespread among ectotherms and has various functions including camouflage, communication and thermoregulation. The process of colour change can occur as an aperiodic event or be rhythmic, induced by cyclic environmental factors or regulated by internal oscillators. Despite the importance of colour change in reptile ecology, few studies have investigated the occurrence of a circadian rhythm in lizard pigmentation. Additionally, although colour change also entails changes in near-infrared reflectance, which may affect thermoregulation, little research has examined this part of the spectrum. We tested whether the bearded dragon lizard, Pogona vitticeps, displays an endogenous circadian rhythm in pigmentation changes that could be entrained by light/dark (LD) cycles and how light affected the relative change in reflectance in both ultraviolet-visible and near-infrared spectra. We subjected 11 lizards to four photoperiodic regimens: LD 12∶12; LD 6∶18; LD 18∶6 and DD; and measured their dorsal skin reflectance at 3-hour intervals for 72 hours after a habituation period. A proportion of lizards displayed a significant rhythm under constant darkness, with maximum reflectance occurring in the subjective night. This endogenous rhythm synchronised to the different artificial LD cycles, with maximum reflectance occurring during dark phases, but did not vary in amplitude. In addition, the total ultraviolet-visible reflectance in relation to the total near-infrared reflectance was significantly higher during dark phases than during light phases. We conclude that P. vitticeps exhibits a circadian pigmentation rhythm of constant amplitude, regulated by internal oscillators and that can be entrained by light/dark cycles.
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We studied the body temperature of the Saxicolous Lizard (Tropidurus torquatus) in a rocky outcrop of Minas Gerais State, Brazil. The influence of seasonality and the importance of microbabitat temperatures (substrate and air) for T. torquatus body temperature regulation were investigated. The mean body temperature of active T. torquatus was 31.2 ± 3.1°C (21.0-37.0°C) and was subject to seasonal variation probably resulting from the adjustment of lizards to changes in the local thermal environment.
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Signals used in female choice should honestly ad-vertise the benefits that males can provide, with direct benefits often argued as being more important than indirect benefits. However, the nature of direct benefits in species without paternal care or nuptial gifts is poorly understood. Previous studies on lizards suggest that females decide where to settle and assumedly who to mate with based on information contained in scent marks from territorial males. Access to high-quality thermal resources is crucial for female reproduc-tive success. Females may therefore be able to detect and exploit thermal-induced variation in the chemical composition of male scent marks when assessing the quality of his territory. We show that the amount of time male wall lizards (Podarcis muralis) are allowed to bask significantly alters the chemical composition of their femoral secretions used in scent marking. The direction of the change is consistent with adaptive plas-ticity to maintain signalling efficacy under warm conditions that increase evaporation of femoral secretions. The com-pounds affected by basking experience included those previ-ously associated with male quality or shown to mediate male– male competition in lizards. However, whilst female lizards could discriminate between scent marks of males that had experienced different basking conditions, they did not prefer-entially associate with the scent from males from high-quality thermal conditions. These results highlight the potential im-portance of a previously neglected environmental effect on chemical signalling. We suggest thermal effects may have significant consequences for scent-mark composition in vari-able environments, with potential repercussions on olfactory communication in lizards.
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We studied several aspects of the thermal biology of two coexisting saxicolous species of the genus Phymaturus (Liolaemidae) at the Payunia, a volcanic region in Argentina where harsh conditions prevail. We marked individuals in grids and assessed the environmental conditions when the individuals showed their maximum activity (i.e. when more than 50% of marked individuals were active), the time spent basking, and the postural behaviour in relation to different cloudiness conditions. We found that temperatures recorded when the species exhibit their peaks of activity were higher for Phymaturus payuniae, the smaller species, during the summer at one of the three studied sites. Phymaturus roigorum exhibited the longest basking periods, but during these longer periods cloudy conditions prevailed. Both species adopted a head-up posture when conditions were sunny and a head-down posture when under cloudy conditions. This is the first contribution where thermal aspects of two sympatric species of Phymaturus are evaluated.
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1.1. We compared preferred body temperatures (Tbs) of gravid, nongravid and postparturient diamondback water snakes (Nerodia rhombifera) in linear thigmothermal gradients2.2. Gravid females had a significantly higher mean preferred Tb (27.7 ± 0.3°C) than nongravid (24.8 ± 0.8°C), postparturient winter (22.3 ± 2.2°C) and postparturient summer (24.1 ± 1.2°C) snakes3.3. Gravid snakes spent longer periods at higher temperatures and had no diel cycle in Tb, but nongravid and postparturient winter snakes had higher Tb in the scotophase than in the photophase4.4. The selected Tbs of gravid snakes varied less than that of nongravid and postparturient groups
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Thermal trait variation is of fundamental importance to forecasting the impacts of environmental change on lizard diversity. Here, we review the literature for patterns of variation in traits of upper and lower sub-lethal temperature limits, temperature preference and active body temperature in the field, in relation to space, time and phylogeny. Through time, we focus on the direction and magnitude of trait change within days, among seasons and as a consequence of acclimation. Across space, we examine altitudinal and latitudinal patterns, incorporating inter-specific analyses at regional and global scales. This synthesis highlights the consistency or lack thereof, of thermal trait responses, the relative magnitude of change among traits and several knowledge gaps identified in the relationships examined. We suggest that physiological information is becoming essential for forecasting environmental change sensitivity of lizards by providing estimates of plasticity and evolutionary scope.
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Previous work on phenotypic plasticity in hatchling reptiles has dealt almost exclusively with lizards and turtles from temperate zone habitats, in taxa where the only maternal control over incubation regimes is exerted via nest site selection. In contrast, water pythons (Liasis fuscus) in northern Australia are tropical snakes that show facultative maternal brooding, with shivering thermogenesis to warm the clutch. Thus, incubation temperatures of this species are influenced both by nest site selection and by maternal care. We experimentally simulated three thermal regimes typical of different types of natural nests in our study population. These were (1) hot, stable temperatures typical of nests laid in the burrows of varanid lizards (constant 32°C); (2) lower and more variable temperatures typical of nests laid inside tree root boles, either with maternal attendance (diel range 27.1°32.9°C); or (3) in root boles but without maternal attendance (24.3°-32.9°C). We incubated 187 eggs from 15 clutches obtained from field-caught gravid pythons, using a split-clutch design to quantify influences on morphology (offspring size and shape), locomotor performance (swimming ability), and behavior (escape tactics, propensity to strike, willingness to feed in captivity). The thermal regime during incubation strongly affected incubation periods, body sizes, body shapes (mass and tail length relative to snout-vent length), initial growth rates, escape behavior, and willingness to feed. We also detected strong maternal effects on all of these traits, and interactions between maternal effects and incubation regimes. Thus, a female python's 'decisions' as to where she lays her eggs and whether she remains and broods them through development have major consequences for the phenotypes of her offspring.
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For blood flow to be an effective agent for the control of heat exchange, it must occur in a region of the body where conduction resistance in the tissues is relatively high, and in an environment where external resistance to heat exchange is relatively low. If either of these conditions is not met, control of heat exchange by blood flow is not possible. Very small reptiles should not be able to control heat exchange by blood flow in any environment, unless they control blood flow specifically to appendages. Very large reptiles should be able to control heat exchange by blood flow only under certain conditions, such as in water, very high winds, or intense radiative heating. Otherwise, they should have little control. An optimum body size should exist for a reptile's ability to control heat exchange using blood flow. In air, this optimum body size for alligators appears to be about 5 kg. Theoretically, the optimum size should be substantially larger than 5 kg for reptiles heating and cooling in water.
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Analyzes the historical evolution of thermal sensitivity of locomotion in iguanid lizards. Taxa that experience high body temperatures in nature have evolved high optimal temperatures for sprinting. Critical thermal maxima are coadapted with optimal temperatures but not with critical thermal minima. Thus some but not all aspects of thermal sensitivity are coadapted. The authors then describe selection experiments that help reveal potential genetic constraints on the future evolution of thermal sensitivity in Drosophila. Thermal sensitivity responds rapidly to selection, and tolerance of extreme high temperature appears genetically correlated with performance at intermediate temperature. The authors next describe how the shape of thermal performance curves may affect evolutionary responses of thermal sensitivity to a gradual shift in the thermal environment. Theoretical predictions depend crucially on the relationship between the genetic variation in optimal temperature and the performance breadth. If genetic variation is independent of breadth, then populations with an intermediate value of performance breadth will tolerate the greatest rate of environmental change. If a trade-off exists between maximum performance and breadth of performance, then thermal specialists will be favored over thermal generalists in a rapidly changing environment, but if genetic variation increases with increasing breadth, then populations of thermal generalists will tolerate the greatest rates of environmental change. -from Authors
Book
Introduction.- The Skin as a Source of Temperature Signals.- The Inner Body as a Source of Temperature Signals.- The Neuronal Basis of Temperature Reception.- Heat Production and Heat Balance of the Body.- Physics of Heat Exchange with the Environment.- External and Internal Insulation.- The Temperature Field of the Body Core.- Behavioural Control of Heat Exchange with the Environment.- Autonomic Control of Dry Heat Loss from the Skin.- Autonomic Control of Evaporative Heat Loss.- Interaction of Various Body Temperatures in Control of Thermoregulatory Responses.- The Central Interface Between Afferent Temperature Signals and Efferent Drives.- Short-Term Temperature Regulation in Various Environments: Inputs and Responses.- Exercise in the Heat: the Ultimate Challenge.- Changes of Set-Point.- Adaptation to Cold.- Adaptation to Heat - Pathophysiology of Temperature Regulation.- References.- Subject Index.
Article
Many terrestrial ectotherms are capable of rapid colour change, yet it is unclear how these animals accommodate the multiple functions of colour, particularly camouflage, communication and thermoregulation, especially when functions require very different colours. Thermal benefits of colour change depend on an animal's absorptance of solar energy in both UV–visible (300-700 nm) and near-infrared (NIR; 700-2600 nm) wavelengths, yet colour research has focused almost exclusively on the former. Here, we show that wild-caught bearded dragon lizards (Pogona vitticeps) exhibit substantial UV–visible and NIR skin reflectance change in response to temperature for dorsal but not ventral (throat and upper chest) body regions. By contrast, lizards showed the greatest temperature-independent colour change on the beard and upper chest during social interactions and as a result of circadian colour change. Biophysical simulations of heat transfer predicted that the maximum temperature-dependent change in dorsal reflectivity could reduce the time taken to reach active body temperature by an average of 22 min per active day, saving 85 h of basking time throughout the activity season. Our results confirm that colour change may serve a thermoregulatory function, and competing thermoregulation and signalling requirements may be met by partitioning colour change to different body regions in different circumstances. © 2016 The Author(s) Published by the Royal Society. All rights reserved.
Article
Dehydration to 80% initial body weight had no effect on the skin and core temperatures at which desert iguanas exited the hot or cold side of an electronic shuttle box. Hyperosmotic NaCl loading resulted in a significant decrease in the upper threshold skin temperature (44.23 ± 0.07 to 43.82 ± 0.08 C) and both the lower threshold skin (37.52 ± 0.16 to 34.93 ± 0.20 C) and core (39.28 ± 0.10 to 37.05 ± 0.25 C) temperatures. Hypoosmotic NaCl loading resulted in a significantly higher upper threshold skin temperature (42.84 ± 0.09 to 43.44 ± 0.08 C) but significant decreases in lower threshold skin (34.93 ± 0.11 to 32.11 ± 0.14 C) and core (35.34 ± 0.11 to 33.97 ± 0.14 C) temperatures. While changes in thermoregulatory behavior can be correlated with changes in plasma osmolality, shifts in the lower behavioral threshold during osmotic manipulation may be due to ionic rather than osmotic influences.
Article
A mathematical analysis of continuously proportional and neutral zone (dual threshold) temperature regulation leads to criteria for experimentally distinguishing between these differing controls. Colonic, skin, and head temperatures were measured in a lizard as it shuttled between hot and cold environments. An analysis of body temperature when hot-environment temperature was 65 C shows that the lizard was thermoregulating (P < .05). A further analysis of behavior at a hotenvironment temperature of 47 C shows that the body temperatures initiating exit from the hot and cool environments were significantly (P < .05) different. Furthermore, shuttling ceased when cool-environment temperature was increased to 36 C or hot-environment temperature was lowered to 43 C. Therefore, it was concluded (P < .05) that thermoregulation was not single set-point continuously proportional. Our results, when combined with those of field and gradient observations, support a dual stochastic threshold (DST) control hypothesis. Exit skin temperature was unaffected by different hot-environment temperatures; however, as hot-environment temperature was increased exit colonic and brain temperatures decreased linearly, suggesting that peripheral temperature was regulated. We conclude (P < .001) that deep body temperature is not the major directly regulated variable.
Article
To study behavioral thermoregulation in the lizard, Tiliqua scincoides, two animals were implanted with two thermodes astraddle the preoptic region of the brainstem. These and four others without thermodes wandered between warm and cold environments, keeping the brain or colonic temperature cycling about a mean preferred temperature of 29.3 C during activity. When inactive, they voluntarily moved to a cool place and let their internal temperatures drop. When put into a hot test chamber, the lizards returned to a neutral environment when their colonic temperatures reached a reproducible level, designated the colonic exit temperature. Increasing the temperature of the test environment decreased the colonic exit temperature, the difference being highly significant when increasing the ambient temperature from 40 to 55 C. Warming the rostral brainstem by circulating water of 42 C through the thermodes also lowered the colonic exit temperature significantly at an ambient temperature of 40 and 45 C. Cooling the brainstem by 25 C water caused an increase of the colonic exit temperatures. We believe that the behavioral regulation of the internal temperature in Tiliqua scincoides is activated by a combination of brain and peripheral temperatures.
Article
The desert iguana (Dipsosaurus dorsalis) precisely thermoregulates in an operant conditioning apparatus, where it must walk 1 m to obtain a short-duration thermal reinforcement. The preferred body temperature depends on the duration of the thermal reinforcement (30-180 s) and on the ambient temperature (20 C or 30 C). Variation in preferred body temperature is not due to physiological constraintsexcept perhaps in the shortest-duration reinforcement at 20 C-but reflects motivational variation. The precision of thermoregulation depends on the thermal reinforcement duration, the ambient temperature, and the extent of the elevation of body temperature above ambient air temperature. The body temperature distributions were negatively skewed except under the short-duration thermal regimens. The stochastic dual-limit model of lizard behavioral thermoregulation closely predicts the observed frequency distribution of body temperatures only when the cost for thermoregulation is low (i.e., when there is high thermal reinforcement or high ambient temperature).
Article
We measured basking duration of fed and nonfed yellow slider turtles (Pseudemys scripta) in a constant-temperature environmental chamber. We acclimated and tested turtles at four temperatures (10, 20, 30, and 35 C), once in spring/summer and once in fall/winter. In the spring/summer there was a statistically significant difference (P < .01) between basking durations of fed and nonfed turtles and between basking times of males and females at all acclimation temperatures except 35 C, when basking time was minimal for both fed and nonfed animals. Fed females basked much longer than fed males (P < .05), and both groups of fed turtles basked much longer than nonfed animals. These differences were greatest at 10 and 20 C but were also apparent at 30 C. In the fall/winter there were no statistically significant differences between the basking times of fed and nonfed turtles or between the basking times of males or females. There were, however, statistically significant differences (P < .01) between basking times at all four acclimation temperatures. We suggest that basking in turtles is a complex behavior that is affected by the nutritional status, acclimation, season, and sex of the individual.
Article
Heath has proposed that thermoregulatory behavior of lizards is controlled by maximum and minimum limit temperatures (high and low setpoints) separated by a nonthermoregulatory zone. On precursory examination this hypothesis seems incompatible with the unimodal distribution of body temperatures of lizards in the field and in continuous thermal gradient experiments. Evidence from two-temperature selection experiments indicates that the high and low limit temperatures are stochastically distributed. Based on this observation, we have formulated a stochastic mathematical model of the system which predicts the distribution of body temperatures of lizards confined in a temperature gradient. According to the model, an increase in the variance of the limit temperatures results in decrease in the variance of the unimodel distribution of body temperatures in a continuous gradient. As the variance of the lower limit increases relative to that of the upper limit, the model predicts a negatively skewed distribution of body temperatures, a common observation in continuous gradient experiments.
Article
Temperature pervasively impacts the phenotypes and distributions of organisms. These thermal effects generate strong selective pressures on behaviour, physiology, and life history when environmental temperatures vary over space and time. Despite this fact, progress toward a quantitative theory of thermal adaptation has lagged behind empirical descriptions of patterns and processes. This book draws on current evolutionary paradigms (optimization, quantitative genetics, and genetic algorithms) to establish a theory of thermal adaptation. It initially focuses on simple models that describe the evolution of thermosensitivity, thermoregulation, or acclimation. Later chapters focus on more complex models describing the coadaptation of traits or the coevolution of species. Throughout the book, various lines of evidence are used to question the major assumptions of these models. Furthermore, the predictions of these models are confronted with experimental and comparative data. Empirical examples represent a wide range of taxa, including bacteria, plants, fungi, and animals. The result is a synthesis of theoretical and empirical studies of thermal biology that offers insights about evolutionary processes.
Article
Question: Associations of body size and of body temperature with fitness have complex relationships for ectotherms, but three general patterns are known. Bigger is better: Larger body size is frequently associated with greater fitness within populations. Hotter is smaller: Smaller adult body sizes typically result from development at higher temperatures. Hotter is better: Greater maximal performance at the optimal temperature is frequently associated with higher optimal temperatures. How do we - or even can we - reconcile these three apparently conflicting empirical patterns about temperature, size, and fitness of ectotherms? Methods: We summarize available evidence supporting or contradicting these three rules. We present a conceptual framework that describes how developmental and adult body temperatures affect causal connections among size, performance, and key components of fitness. Findings: There is strong empirical support for Bigger is better and Hotter is smaller (≥79% of studies/estimates), primarily for terrestrial insects, reptiles, and annual plants. Evidence regarding Hotter is better is still limited (and primarily from terrestrial insects), but most available information supports the rule. Analyses of counterexamples are particularly instructive. The rules operate at different levels. Bigger is better describes phenotypic variation within populations. Hotter is smaller describes phenotypic plasticity of a genotype. Hotter is better describes evolved variation in reaction norms among genotypes or between species. Conclusions: We unify these three rules into a path diagram that describes how temperature impacts critical rate processes throughout the life cycle. Adult body size and development time are key traits that are not only consequences of temperature-dependent processes, but also are causes of variation in fitness. An unresolved issue involves how to determine the appropriate fitness metric for a particular ecological context (population and environment). For example, the intrinsic rate of population increase (r) is strongly influenced by generation time (and development time), whereas net reproductive rate (R0) is strongly influenced by fecundity (and size). Because the relative strengths of different paths contributing to fitness change differ for these fitness metrics, the choice of metric can affect whether Hotter is better is 'better' than Bigger is better.
Article
The influence of body temperature (12.5-37°C) and thermal acclimation (20° and 30°C) on sprint speed in a thigmothermic lizard, Xantusia vigilis, was measured. Body temperature has a profound influence on sprint speed, which is maximal at 33°-34°C and declines exponentially at lower body temperatures. Neither sprint speed nor its thermal dependence was modified by thermal acclimation over the body temperature range of 15.0-34°C. Acclimation temperature did affect sprint speed at more extreme temperatures: 20°C-acclimated lizards ran faster at 12.5° and slower at 37°C than did 30°C-acclimated animals. -from Authors
Article
Although the effects of temperature on ectotherm physiology have been generally recognized, consequences for animal performance in the field have seldom been assessed. We experimentally investigated the thermal dependence within the body temperature range 20-35-degrees-C, of distinct behavioural and physiological functions related to feeding (sprint speed, prey-catching rate, prey-handling rate, gut-passage rate, voluntary food intake, mass change) in the lizard Lacerta vivipara Jacquin. All functions examined were strongly dependent on body temperature (T(b)); their rate generally increased rapidly within the range 20-30-degrees-C and then levelled. Exceptions were gut-passage rate and the amount of weight change, which exhibited a decline at 35-degrees-C. Some small but consistent differences in thermal sensitivities between performance functions were evident. Gut-passage rate and weight change were maximal at T(b) almost-equal-to 30-degrees-C, which is slightly lower than the optimal temperature of the other functions (32-34-degrees-C). Passage rate had the widest, whereas prey-catching rate and weight change had the narrowest 80% thermal performance breadth. These results indicate that no single T(b) maximizes all behavioural and physiological processes and therefore supports the 'multiple optima' hypothesis. We predicted performance levels in the field by integrating results of the thermal sensitivity experiments with data on T(b)s of field-active L. vivipara. Lizards seem to be able to achieve > 80% of their maximum capacity for most of the time, at least during sunny weather. Under cloudy/variable conditions, when lizards suffer impaired possibilities to thermoregulate, performance levels were reduced. This depression is less accentuated for gut-throughput rate and weight change. We discuss our results in the context of the overall thermal biology of this cool-climate lizard.
Article
A. pulchra is a fossorial lizard that selected a defined range of temperatures in laboratory thermal gradients. The mean thermal preferendum was 24-25 C, depending on moisture conditions. Lizards selected significantly higher temperatures when the test chambers were evenly moist. Our data indicate that A. pulchra selects temperatures which are lower than known diurnal, basking lizards. Such a low preference may permit increased activity in a subterranean environment where ambient temperatures are often cool. They appear to have an eurythermal strategy.
Article
The aims of this paper are to compare the thermal ecology of four species of varanid lizards that occupy a range of habitats and climatic regions, and to assess the efficacy of methods for evaluating the extent to which ectothermic animals exploit their thermal environments. Hertz et al. (1993) have proposed several indices of thermoregulation, and these are evaluated with respect to our data from varanid lizards. The thermoregulatory characteristics of three tropical monitor lizards (Varanus panoptes, V. gouldii, and the semiaquatic V. mertensi), and the temperate-zone V. rosenbergi were studied throughout the year. Radiotelemetry was used to measure the body temperatures (Tb's) of free-ranging animals, and microclimatic data were collected to determine the range of possible Tb's that an animal could achieve. Operative temperatures (Tb's) were estimated by biophysical models for each set of animal characteristics and microclimatic conditions. The Tb's selected by animals in a laboratory thermal gradient were used to determine the set-point range of Tb's that the animals voluntarily select. Plots that superimpose Tb's, Te's, and the set-point range across the day are extremely useful for describing the thermoregulatory characteristics of ectotherms. These plots can be used to determine the extent to which the animals exploit their thermal environment: we define an index of thermal exploitation (Ex) as the time in which Tb's are within the set-point range, divided by the time available for the animal to have its Tb's within the set-point range. Only V. mertensi was active throughout the year. In general, during seasons of inactivity, the Tb's of inactive species fell outside the set-point range, but during periods of activity all species selected Tb's within their set-point range. The temperate-zone species (V. rosenbergi) thermoregulates very carefully during periods when environmental conditions allow the animals to attain the set-point range, and V. gouldii also thermoregulates carefully in the wet season. V. mertensi selects Tb's that are significantly lower than the other species both in the field and in the laboratory, and thermoregulatory indices of this species were intermediate relative to the other species. The amount of time spent in locomotion each day was not correlated with the indices of thermoregulation: the most active species, V. panoptes, was, with respect to several indices, the least careful thermoregulator. The type of question that is being addressed, with respect to the interactions between an animal's thermal environment and its thermoregulatory behavior, determines the appropriateness of the various indices of thermoregulation. The Ex index describes the thermoregulatory characteristics of ecotherms in a heterogeneous thermal environment, and in such an environment a large amount of information can easily be interpreted graphically. This index is less useful in a thermally homogeneous environment.
Article
In an effort to better understand the function of physiological color change in anurans, we measured color change responses of the green treefrog, Hyla cinerea, to background brightness and temperature. We used our results to compare color and color change responses of males and females and to test two hypotheses for the function of physiological color change in anurans: (1) that color change serves to prevent detection by visual predators, and (2) that color change functions in thermoregulation or water balance. We found that (1) males are generally darker than females but the sexes exhibit similar color change responses, (2) treefrogs become lighter on brighter backgrounds, (3) treefrogs become lighter at higher temperatures, and (4) there is a background-by-temperature interaction such that color change responses to one variable depend on the level of the other. Our results suggest that color change may function both in predator avoidance and in thermoregulation or water balance. However, further investigations of color change responses to differently hued backgrounds, of the magnitude and biological significance of color-induced changes in body temperature and rates of evaporative water loss, and of color change in free-ranging animals are needed.
Article
The effect of acute desiccation upon the preferred body temperatures and activity levels of the lizard Sceloporus undulatus (Iguanidae) collected from two habitats differing in thermal regime and water availability was investigated. In thermal gradients, desiccated lizards selected significantly lower body temperatures than did hydrated lizards. There was a significant negative relationship between the body temperatures selected during activity and the degree of desiccation. Relative to hydrated lizards, desiccated lizards also showed a significant decrease in activity, tending to remain buried in the substrate of the thermal gradient. Lizards collected from two different habitats differed significantly in response to desiccation when active but not when inactive. Reductions in body temperature and activity presumably function to decrease water loss.
Article
Selection of an increased body temperature (T-b) following injection of dead bacteria and placement into a photothermal gradient was first reported for reptiles in the lizard Dipsosaurus dorsalis. Subsequent to the study on D. dorsalis, a fever response was demonstrated in other lizard species, in turtles and in a crocodilian, however, the only previous study conducted with snakes did not show significant increases in selected T-b following injection of dead bacteria or endotoxin. The objective of the current study was to examine six species of snakes, representing two geographic regions (California and Pennsylvania) and different habitat types (desert and mesic environment) for the presence or absence of a fever response after injection with a single dosage of dead Aeromonas sobria bacteria. Body temperatures for individuals of each species were measured for 24 h under saline injection conditions and for 48 h under bacteria injection conditions using 36 gauge type K thermocouples and a computerized data acquisition system. A substrate type thermal gradient provided a range of available surface temperatures from 13 to 40 C. Two of the desert species, Pltuophis melanoleucus and Arizona elegans, did exhibit a fever response during at least one of the four 12 h periods following bacteria injection. One of the mesic species, Thamnophis sirtalis, exhibited a significant hypothermia in response to bacteria injection. One desert species, Lampropeltis getulus and two mesic species, Elaphe obsoleta and Nerodia sipedon, did not exhibit any change in T-b following bacteria injection. The results of this study indicate that the behavioral fever response can now be confirmed in snakes. Short acclimation periods, inadequate selection of a proper pyrogen and the use of a photothermal gradient may have contributed to negative results in the previous study on snakes. Use of only a single dosage of bacteria may have produced negative results in some species during this study.
Article
Environmental factors strongly influence phenotypic variation within populations. The environment contributes to this variation in two ways: (1) by acting as a determinant of phenotypic variation (i.e., plastic responses) and (2) as an agent of selection that "chooses" among existing phenotypes. Understanding how these two environmental forces contribute to phenotypic variation is a major goal in the field of evolutionary biology and a primary objective of my research program. The objective of this article is to provide a framework to guide studies of environmental sources of phenotypic variation (specifically, developmental plasticity and maternal effects, and their adaptive significance). Two case studies from my research on reptiles are used to illustrate the general approaches I have taken to address these conceptual topics. Some key points for advancing our understanding of environmental influences on phenotypic variation include (1) merging laboratory-based research that identifies specific environmental effects with field studies to validate ecological relevance; (2) using controlled experimental approaches that mimic complex environments found in nature; (3) integrating data across biological fields (e.g., genetics, morphology, physiology, behavior, and ecology) under an evolutionary framework to provide novel insights into the underlying mechanisms that generate phenotypic variation; (4) assessing fitness consequences using measurements of survival and/or reproductive success across ontogeny (from embryos to adults) and under multiple ecologically-meaningful contexts; and (5) quantifying the strength and form of natural selection in multiple populations over multiple periods of time to understand the spatial and temporal consistency of phenotypic selection. Research programs that focus on organisms that are amenable to these approaches will provide the most promise for advancing our understanding of the environmental factors that generate the remarkable phenotypic diversity observed within populations.