Modeling energetic and theoretical costs of thermoregulatory strategy

Poikilothermic ectotherms have evolved behaviours that help them maintain or regulate their body temperature (T (b)) around a preferred or 'set point' temperature (T (set)). Thermoregulatory behaviors may range from body positioning to optimize heat gain to shuttling among preferred microhabitats to find appropriate environmental temperatures. We have modelled movement patterns between an active and non-active shuttling behaviour within a habitat (as a biased random walk) to investigate the potential cost of two thermoregulatory strategies. Generally, small-bodied ectotherms actively thermoregulate while large-bodied ectotherms may passively thermoconform to their environment. We were interested in the potential energetic cost for a large-bodied ectotherm if it were forced to actively thermoregulate rather than thermoconform. We therefore modelled movements and the resulting and comparative energetic costs in precisely maintaining a T (set) for a small-bodied versus large-bodied ectotherm to study and evaluate the thermoregulatory strategy.     

Evolutionary stasis and lability in thermal physiology in a group of tropical lizards

Understanding how quickly physiological traits evolve is a topic of great interest, particularly in the context of how organisms can adapt in response to climate warming. Adjustment to novel thermal habitats may occur either through behavioural adjustments, physiological adaptation or both. Here, we test whether rates of evolution differ among physiological traits in the cybotoids, a clade of tropical Anolis lizards distributed in markedly different thermal environments on the Caribbean island of Hispaniola. We find that cold tolerance evolves considerably faster than heat tolerance, a difference that results because behavioural thermoregulation more effectively shields these organisms from selection on upper than lower temperature tolerances. Specifically, because lizards in very different environments behaviourally thermoregulate during the day to similar body temperatures, divergent selection on body temperature and heat tolerance is precluded, whereas night-time temperatures can only be partially buffered by behaviour, thereby exposing organisms to selection on cold tolerance. We discuss how exposure to selection on physiology influences divergence among tropical organisms and its implications for adaptive evolutionary response to climate warming.     

Comparative thermal ecology of two lizards

Summary In a habitat judged to be energetically costly for thermoregulation, mean body temperatures (MBT's) ofAnolis sagrei are significantly higher than those ofA. distichus. As indexed by the slope of the regression of body temperatures (T _b ) on substrate temperature (T _s ),A. sagrei is more dependent upon environmental temperatures thanA. distichus.In a habitat judged to be less costly for thermoregulation and where interspecific competition for perch sites may be less, MBT's ofA. sagrei are significantly higher, proportionally more lizards occupy sunny perches, and the slope of the regression of T _b on T _s is significantly less, than for conspecifics in the costly habitat.As indexed by length-specific fat body weights, well-nourished lizards in the costly habitat have T _b 's which are independent of environmental temperature; T _b 's of poorly-nourished lizards are highly dependent upon environmental temperature. This relationship does not hold for lizards in the low-cost habitat.These results corroborate the hypothesis that energetic costs are important in controlling the extent to which lizards thermoregulate. In high-cost habitats lizards thermoregulate less precisely than in low-cost habitats. Lizards that exploit the habitat as if it were highly productive thermoregulate more precisely than lizards that exploit the environment as if it were of low productivity.     

Living in sympatry: The effect of habitat partitioning on the thermoregulation of three Mediterranean lizards

The ability for effective, accurate and precise thermoregulation is of paramount importance for ectotherms. Sympatric lizards often partition their niche and select different microhabitats. These microhabitats, however, usually differ in their thermal conditions and lizards have to adapt their thermoregulation behavior accordingly. Here, we evaluated the impact of habitat partitioning on the thermal biology of three syntopic, congeneric lacertids (Podarcis peloponnesiacus, P. tauricus and P. muralis) from central Peloponnese, Greece. We assessed thermoregulation effectiveness (E) using the three standard thermal parameters: body (T_b), operative (T_e) and preferred (T_pref) temperatures. We hypothesized that the microhabitats used by each species would differ in thermal quality. We also predicted that all species would effectively thermoregulate, as they inhabit a thermally challenging mountain habitat. As expected, the partition of the habitat had an effect on the thermoregulation of lizards since microhabitats had different thermal qualities. All three species were effective and accurate thermoregulators but one of them achieved smaller E values as a result of the lower T_b in the field. This discrepancy could be attributed to the cooler (but more benign) thermal microhabitats that this species occupies.     

Viviparity Advantages in the Lizard <Emphasis Type="Italic">Liolaemus sarmientoi</Emphasis> from the End of the World

Two hypotheses have prevailed to explain the evolution of viviparity in reptiles: the first proposed that viviparity evolved in response to cold-climates because the possibility of pregnant females to thermoregulate at higher temperatures than embryos could experience in a nest in nature. The second hypothesis posits that the advantage of viviparity is based on the possibility of females to maintain stable body temperatures during development, enhancing offspring fitness. With the aim to contribute to understanding the origins of viviparity in reptiles, we experimentally subjected pregnant females of the austral lizard Liolaemus sarmientoi to two temperature treatments until parturition: one that simulated environmental temperatures for a potential nest (17–25?°C) and another that allowed females to thermoregulate at their preferred body temperature (17–45?°C). Then, we analysed newborn body conditions and their locomotor performance to estimate their fitness. In addition, we measured the body temperature in the field and the preferred temperature in the laboratory of pregnant and non-pregnant females. Pregnant females thermoregulated to achieve higher temperatures than the environmental temperatures, and also thermoregulated within a narrower range than non-pregnant females. This could have allowed embryos to develop in higher and more stable temperatures than they would experience in a nest in nature. Thus, offspring developed at the female preferred temperature showed greater fitness and were born earlier in the season than those developed at lower environmental temperatures. Herein, we show that results are in agreement with the two hypotheses of the origin of viviparity for one of the southernmost lizards of the world.     

Evidence for parallel ecological speciation in scincid lizards of the Eumeces skiltonianus species group (Squamata: Scincidae)

We identify instances of parallel morphological evolution in North American scincid lizards of the Eumeces skiltonianus species group and provide evidence that this system is consistent with a model of ecological speciation. The group consists of three putative species divided among two morphotypes, the small-bodied and striped E. skiltonianus and E. lagunensis versus the large-bodied and typically uniform-colored E. gilberti. Members of the group pass through markedly similar phenotypic stages during early development, but differ with respect to where terminal morphology occurs along the developmental sequence. The morphotypes also differ in habitat preference, with the large-bodied gilberti form generally inhabiting lower elevations and drier environments than the smaller, striped morphs. We inferred the phylogenetic relationships of 53 skiltonianus group populations using mtDNA sequence data from the ND4 protein-coding gene and three flanking tRNAs (900 bp total). Sampling encompassed nearly the entire geographic range of the group, and all currently recognized species and subspecies were included. Our results provide strong evidence for parallel origins of three clades characterized by the gilberti morphotype, two of which are nested within the more geographically widespread E. skiltonianus. Eumeces lagunensis was also nested among populations of E. skiltonianus. Comparative analyses using independent contrasts show that evolutionary changes in body size are correlated with differences in adult color pattern. The independently derived association of gilberti morphology with warm, arid environments suggests that phenotypic divergence is the result of adaptation to contrasting selection regimes. We provide evidence that body size was likely the target of natural selection, and that divergences in color pattern and mate recognition are probable secondary consequences of evolving large body size.     

The impact of insularity on the thermoregulation of a Mediterranean lizard

The overall biology of ectotherms is strongly affected by the thermal quality of the environment. The particular conditions prevailing on islands have a strong effect on numerous features of animal life. In this study we compared mainland and island populations of the lizard Lacerta trilineata and hypothesized that insularity would affect the thermoregulatory strategy. Continental habitats were of lower thermal quality, experiencing more intense fluctuations and had higher values of operative temperatures. Nevertheless mainland lizards selected for higher body temperatures in the lab and showed more effective thermoregulation during summer than their island peers. Lizards achieved similar body temperatures in the field in both types of habitat, underlining the importance of predation as a potential factor to mainland lizards that failed to reach their higher thermal preferences. Both island and mainland populations of L. trilineata have been adapted to their thermal environment, supporting the labile view on the evolution of thermal physiology for this species.     

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.     

Thermal ecology and physiology of an elongate and semi-fossorial arthropod,the bark centipede

Organismal performance is strongly linked to temperature because of the fundamental thermal dependence of chemical reaction rates. However, the relationship between the environment and body temperature can be altered by morphology and ecology. In particular, body size and body shape can impact thermal inertia, as high surface area to volume ratios will possess low thermal mass. Habitat type can also influence thermal physiology by altering the opportunity for thermoregulation. We studied the thermal ecology and physiology of an elongate invertebrate, the bark centipede (Scolopocryptops sexspinosus). We characterized field body temperature and environmental temperature distributions, measured thermal tolerance limits, and constructed thermal performance curves for a population in southern Georgia. We found evidence that bark centipedes behaviorally thermoregulate, despite living in sheltered microhabitats, and that performance was maintained over a broad range of temperatures (over 20 °C). However, both the thermal optimum for performance and upper thermal tolerance were much higher than mean body temperature in the field. Together, these results suggest that centipedes can thermoregulate and maintain performance over a broad range of temperatures but are sensitive to extreme temperatures. More broadly, our results suggest that wide performance breadth could be an adaptation to thermal heterogeneity in space and time for a species with low thermal inertia.     

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.     

Size evolution in island lizards

Aim  The island rule, small animal gigantism and large animal dwarfism on islands, is a topic of much recent debate. While size evolution of insular lizards has been widely studied, whether or not they follow the island rule has never been investigated. I examined whether lizards show patterns consistent with the island rule.
Location  Islands worldwide.
Methods  I used literature data on the sizes of island–mainland population pairs in 59 species of lizards, spanning the entire size range of the group, and tested whether small insular lizards are larger than their mainland conspecifics and large insular lizards are smaller. I examined the influence of island area, island isolation, and dietary preferences on lizard size evolution.
Results  Using mean snout–vent length as an index of body size, I found that small lizards on islands become smaller than their mainland conspecifics, while large ones become larger still, opposite to predictions of the island rule. This was especially strong in carnivorous lizards; omnivorous and herbivorous species showed a pattern consistent with the island rule but this result was not statistically significant. No trends consistent with the island rule were found when maximum snout–vent length was used. Island area had, at best, a weak effect on body size. Using maximum snout–vent length as an index of body size resulted in most lizard populations appearing to be dwarfed on islands, but no such pattern was revealed when mean snout–vent length was used as a size index.
Main conclusions  I suggest that lizard body size is mostly influenced by resource availability, with large size allowing some lizard populations to exploit resources that are unavailable on the mainland. Lizards do not follow the island rule. Maximum snout–vent length may be biased by sampling effort, which should be taken into account when one uses this size index.     

The influence of temperature,sexual condition,and season on the metabolic rate of the lizardPsammodromus hispanicus

Summary Male and femalePsammodromus hispanicus from southern Europe were acclimated to four seasonal conditions of photoperiod and night time temperature. During the dark period, the lizards' body temperatures fell to ambient air temperature but during the light period the lizards were allowed to thermoregulate behaviourally and at such times the lizards' mean body temperature varied from 29.0°C to 32.6°C. The resting metabolic rate of these lizards was measured in 5°C steps from 5°C to 30°C or 35°C. Sexual condition had little effect on resting metabolic rate, but at low temperatures lizards acclimated to winter or spring seasonal conditions had lower resting metabolic rates than those acclimated to summer or autumn conditions. At temperatures above 20°C seasonal acclimation had no effect on resting metabolic rate. It is considered that the reduction in low temperature metabolic rate in spring and winter is induced by low night time temperatures and serves to conserve energy during those seasons when lizards must spend long periods at low temperature without being able to feed.     

Thermal biology and locomotor performance of the Andean lizard Liolaemus fitzgeraldi (Liolaemidae) in Argentina

Ectotherms thermoregulate to maintain their body temperature within the optimal range needed for performing vital functions. The effect of climate change on lizards has been studied as regards the sensitivity of locomotor performance to environmental temperatures. We studied thermoregulatory efficiency and locomotor performance for Liolaemus fitzgeraldi in the Central Andes of Argentina. We determined body temperature, micro-environmental temperatures and operative temperatures in the field. In the laboratory, we measured preferred temperatures and calculated the index of thermoregulatory efficiency. We estimated the thermal sensitivity of locomotion by measuring sprint speed (initial velocity and long sprint) and endurance at five different body temperatures. Body temperature was not associated with either micro-environmental temperature, nor did it show differences with preferred temperatures. Thermoregulatory efficiency was moderate (0.61). Initial velocity and long sprint trials showed differences at different temperatures; however, endurance did not. Moreover, the optimal temperatures for the performance trials showed no significant differences among themselves. We conclude that Liolaemus fitzgeraldi has thermal sensitivity in locomotor performance with respect to body temperature and that it is an eurythermic lizard that experiences a large variation in body temperature and that has thermal flexibility in the cold.     

Total lactate dehydrogenase activity of tail muscle is not cold-adapted in nocturnal lizards from cool-temperate habitats

The dependence of metabolic processes on temperature constrains the behavior, physiology and ecology of many ectothermic animals. The evolution of nocturnality in lizards, especially in temperate regions, requires adaptations for activity at low temperatures when optimal body temperatures are unlikely to be obtained. We examined whether nocturnal lizards have cold-adapted lactate dehydrogenase (LDH). LDH was chosen as a representative metabolic enzyme. We measured LDH activity of tail muscle in six lizard species (n = 123: three nocturnal, two diurnal and one crepuscular) between 5 and 35 °C and found no differences in LDH-specific activity or thermal sensitivity among the species. Similarly, the specific activity and thermal sensitivity of LDH were similar between skinks and geckos. Similar enzyme activities among nocturnal and diurnal lizards indicate that there is no selection of temperature specific LDH enzyme activity at any temperature. As many nocturnal lizards actively thermoregulate during the day, LDH may be adapted for a broad range of temperatures rather than adapted specifically for the low temperatures encountered when the animals are active. The total activity of LDH in tropical and temperate lizards is not cold-adapted. More data are required on biochemical adaptations and whole animal thermal preferences before trends can be established.     

Thermal regimes during incubation do not affect mean selected temperatures of hatchling lizards (Bassiana duperreyi, Scincidae)

Incubation temperatures profoundly affect many phenotypic traits of squamate reptiles, and mean selected body temperatures of such animals also are plastic in response to environmental factors. Plausibly, then, incubation temperatures might affect hatchling thermoregulation, either via adaptation (i.e., populations that historically experience different nest conditions, also will diverge in hatchling thermoregulatory behaviour) or phenotypic plasticity (incubation temperatures directly modify hatchling behaviours). We tested this hypothesis with a montane scincid lizard (Bassiana duperreyi), using thermal-imaging methods to quantify temperatures (of both head and body) selected by hatchling lizards. The young lizards kept their heads cooler than their bodies, but mean selected temperatures did not differ among hatchlings from three populations with differing thermal regimes in natural nests, nor were they affected by thermal conditions during incubation. The conservatism of mean selected temperatures stands in strong contrast to the lability of many other phenotypic traits in response to incubation temperatures in this species.     

Life-history traits of two Mediterranean lizard populations: a possible example of countergradient covariation

The trade-off between clutch and offspring size, which is a central topic in life-history research, is shaped by natural selection to maximize the number of surviving offspring, but it also depends on the resources available for reproduction. Conspecific populations living in different environments may differ in adult body size, clutch mass, clutch size, offspring size, and/or post-natal growth rates, due either to phenotypic plasticity or to local adaptation. Here, we compare these traits and their relationships between two populations of the lizard Psammodromus algirus separated by a 600-m altitudinal gradient. We used a common garden design to control incubation temperature and food availability, with two different feeding treatments. Females were larger at the high-elevation site. Although SVL-adjusted clutch mass did not differ between populations, high-elevation females laid more but smaller eggs than low-elevation ones. Hatchlings were larger at lower elevation. Our common garden experiment revealed that low-elevation hatchlings grew faster than high-elevation hatchlings under both feeding treatments. However, higher food availability at higher altitude allows high-elevation lizards to grow faster and attain larger adult sizes, especially in the case of females. The two key adaptations of low-elevation lizards, large eggs and hatchlings and the ability to grow rapidly after hatching, are likely to enhance survival in low-productivity Mediterranean lowlands. Our data support the hypothesis that the reproductive strategies of these populations provide an example of countergradient variation, because the genotypes that encode for fast growth and large body size occurred in low food availability habitats where juveniles grew slowly and attained small adult sizes.     

Post-feeding thermophily in lizards (Lialis burtonis Gray,Pygopodidae): Laboratory studies can provide misleading results

Many ectothermic vertebrates raise their preferred body temperature after feeding, likely expediting digestion. However, most studies documenting this phenomenon have relied upon laboratory thermal gradients, which grossly oversimplify an animal's environment. We explored the validity of thermal gradient methodology by investigating post-prandial thermophily in an Australian pygopodid lizard (Lialis burtonis Gray). Mean body temperatures did not differ between fed and unfed lizards in field enclosures. Feeding influenced body temperature in a thermal gradient, but in opposite directions depending on details of the methodology. When we introduced L. burtonis into the gradient at the warmer end, fed lizards had higher body temperatures than unfed conspecifics. However, the opposite was true when lizards were introduced at the cooler end. These contrasting results indicate that lizards with food in their stomachs did not seek out higher temperatures, but instead were more sedentary than unfed lizards. Our study highlights the need for caution in interpreting thermal gradient results unaccompanied by field data, and it demonstrates how minor changes in equipment design or procedures can significantly alter conclusions from laboratory studies.     

Size constraints and the evolution of display complexity: why do large lizards have simple displays?

Social, environmental, and perceptual factors have been suggested to account for the evolution of visual signal diversity in lizards. Previous investigations have inferred that signal complexity may also be related to body size. In this study, we use three complementary comparative analyses to investigate whether body size has influenced macro-evolutionary trends in display modifier repertoire size for 110 species of iguanian lizards. We found evidence that signal complexity, as measured by repertoire size, is negatively associated with body size. However, this relationship was not strictly linear. Rather, body size seems to impose a threshold on signal evolution. Specifically, the evolution of large repertoire size appears to be less likely above a particular size threshold, which results in large-bodied lizards having a significantly lower probability of evolving elaborate displays. This relationship may reflect the influence of body size on resource use and the emergent social dynamics it promotes. Large lizards tend to be herbivorous and typically do not defend foraging patches. Consistent with this hypothesis is the previously reported finding of a similar size threshold dividing herbivorous from insectivorous lizards. We suggest to fully understand the evolutionary processes acting on communicative systems, it is important that we identify both the selective forces involved and the nature of their influence. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76 , 145–161.     

Allometry of reproduction in two species of gekkonid lizards (Gehyra): effects of body size miniaturization on clutch and egg sizes

In squamate reptiles there is an allometric pattern for small-bodied females to have smaller clutches and proportionally larger eggs than large-bodied females, and this pattern occurs both among and within species. The allometric patterns in two species of the gecko Gehyra were studied to see how evolutionary reductions in adult body size affect fecundity and offspring size among species, and how these changes affect allometric relationships within species. Gehyra dubia has two eggs per clutch (the typical clutch size for gekkonid lizards), whereas the smallerbodied G. variegata has a single egg per clutch. Within both species, egg size increased with female body size. The data are consistent with at least two mechanistic hypotheses: (1) that the width of the pelvis constrains egg size; and (2) in species with invariant clutch sizes, larger females can only allocate additional energy towards egg size and not number. More direct tests of these hypotheses are warranted. Miniaturization of body sizes in Gehyra is correlated with a clutch size reduction of 50% (from two to one), and a large (1.7-fold) compensatory increase in relative egg mass. However, the small-bodied G. variegata (one egg per clutch) had a lower relative clutch mass than did G. dubia. These findings have implications for understanding the influence of evolutionary reductions in body size on reproductive traits, and for allometric trends in squamate reptiles in general.     

Influences of sex,ontogeny and body size on the thermal ecology of Liolaemus lutzae (Squamata,Liolaemidae) in a restinga remnant in southeastern Brazil

Variations in body temperature (T_b) of lizards can be partially explained by intrinsic factors such as sex, ontogeny and body size. Liolaemus lutzae is a lizard species restricted to restingas in the Brazilian coast in the state of Rio de Janeiro. Herein, we studied sexual dimorphism and influences of sex, ontogeny, and body size to the T_b of L. lutzae. Adult males were larger than adult females, probably due to both intersexual selection and intra-sexual selection. There was intersexual difference in lizards' T_b (males hotter than females), but T_b did not differ after factored out for the effects of body size. The mean T_b of juvenile lizards was higher than that of adults after factored out for the effect of body mass. It is possible that adults may have excluded juveniles from microhabitats with better thermal regimes. Also, this might have occurred due to requirements of juveniles to maintain high growth rates. Forage searching for prey by juveniles also exposes them to high environmental temperatures. Juveniles also may have higher T_b than co-specific adults (relative to body mass) to favor prey capture. In absolute values, adult lizards tended to use microhabitats with lower temperatures than that used by juveniles, possibly to avoid risks of overheating and death. Body temperature and snout–vent length were positively related, as well as body temperature and body mass, presumably caused by the thermal inertia of the bodies (trend of a body to resist to changes in its temperature). Intrinsic factors such as sex, ontogeny and body size can affect the thermal ecology of L. lutzae, despite coastal habitat features to which they are exposed also influences the body temperature of active lizards in restinga habitats.