Preferred Body Temperature Range Research Articles

Thermal ecology of Northern Water Snakes (Nerodia sipedon) in a northern wetland in central Michigan

Semi-aquatic north-temperate reptiles experience challenging environmental conditions for thermoregulation on both seasonal and daily bases. We studied the thermal biology of north-temperate Northern Water Snakes (Nerodia sipedon) in central Michigan by remote radiotelemetry monitoring of snake body temperature (Tb) using surgically implanted thermally-sensitive radio-transmitters and by measuring operative temperatures (Te) across a range of microhabitats using biophysical models. Our goals included evaluation of thermoregulatory capabilities at a locality that we viewed to be a fine-grained thermal environment, seasonal variations in patterns of thermoregulation, and the effects of sex on thermoregulation. During summer, snakes showed diel Tb cycling apparently using the open Sphagnum mat for late morning warming and shuttling among different microhabitats until early evening when a monotonic decline in Tb ensued and continued through the early morning hours. Snakes attained Tb within their laboratory-determined preferred body temperature range (Tset = 28–33 °C) mostly during late afternoon and with average percentages of Tb values for individual snakes within Tset range when permitted by operative temperatures (Ex) between 68 and 70% of the time depending on method of measurement. Relatively high investment in thermoregulation when thermal conditions were poor occurred only during September but declined thereafter as snakes prepared to overwinter. We did not detect differences in thermoregulation among reproductive females, non-reproductive females, and males. Relative to a population of N. sipedon at a higher latitude in Ontario our snakes showed a relatively high Tset range and thermoregulated more effectively, particularly during the daylight hours. It remains unclear how much inter-population variation in thermoregulation is due to potential adaptations to latitude, to habitat differences, or variations in methodologies.

Effects of change in temperature on the cardiac contractility of broad-snouted caiman (Caiman latirostris) during digestion.

In many reptiles, digestion has been associated with the selection of higher body temperatures, the so-called post-prandial thermophilic response. This study aimed to investigate the excitation-contraction (E-C) coupling in postprandial broad-snouted caimans (Caiman latirostris) in response to acute warming within a preferred body temperature range of crocodiles. Isometric preparations subjected to a temperature transition from 25°C to 30°C were used to investigate myocardial contractility of postprandial caimans, that is, 48 h after the animals ingested a rodent meal corresponding to 15% of body mass. The caiman heart exhibits a negative force-frequency relationship that is independent of the temperature. At 25°C, cardiac muscle was able to maintain a constant force up to 36 bpm, above which it decreased significantly, reaching minimum values at the highest frequency of 84 bpm. Moreover, E-C coupling is predominantly dependent on transsarcolemmal Ca2+ transport denoted by the lack of significant ryanodine effects on force generation. On the contrary, ventricular strips at 30°C were able to sustain the cardiac contractility at higher pacing frequencies (from 12 to 144 bpm) due to an important role of Na+ /Ca2+ exchanger in Ca2+ cycling, as indicated by the decay of the post-rest contraction, and a significant contribution of the sarcoplasmic reticulum above 72 bpm. Our results demonstrated that the myocardium of postprandial caimans exhibits a significant degree of thermal plasticity of E-C coupling during acute warming. Therefore, myocardial contractility can be maximized when postprandial broad-snouted caimans select higher body temperatures (preferred temperature zone) following feeding.

Thermal ecology of the federally endangered blunt-nosed leopard lizard (Gambelia sila).

Recognizing how climate change will impact populations can aid in making decisions about approaches for conservation of endangered species. The blunt-nosed leopard lizard (Gambelia sila) is a federally endangered species that, despite protection, remains in extremely arid, hot areas and may be at risk of extirpation due to climate change. We collected data on the field-active body temperatures, preferred body temperatures and upper thermal tolerance of G. sila. We then described available thermal habitat using biophysical models, which allowed us to (i) describe patterns in lizard body temperatures, microhabitat temperatures and lizard microhabitat use; (ii) quantify the lizards' thermoregulatory accuracy; (iii) calculate the number of hours they are currently thermally restricted in microhabitat use; (iv) project how the number of restricted hours will change in the future as ambient temperatures rise; and (v) assess the importance of giant kangaroo rat burrows and shade-providing shrubs in the current and projected future thermal ecology of G. sila. Lizards maintained fairly consistent daytime body temperatures over the course of the active season, and use of burrows and shrubs increased as the season progressed and ambient temperatures rose. During the hottest part of the year, lizards shuttled among kangaroo rat burrows, shrubs, and open habitat to maintain body temperatures below their upper thermal tolerance, but, occasionally, higher than their preferred body temperature range. Lizards are restricted from staying in the open habitat for 75% of daylight hours and are forced to seek refuge under shrubs or burrows to avoid surpassing their upper thermal threshold. After applying climatic projections of 1 and 2°C increases to 2018 ambient temperatures, G. sila will lose additional hours of activity time that could compound stressors faced by this population, potentially leading to extirpation.

Economic thermoregulatory response explains mismatch between thermal physiology and behaviour in newts.

Temperature is an important factor determining distribution and abundance of organisms. Predicting the impact of warming climate on ectotherm populations requires information about species' thermal requirements, i.e. their so-called 'thermal niche'. The characterization of thermal niche remains a complicated task. We compared the applicability of two indirect approaches, based on reaction norm (aerobic scope curve) and optimality (preferred body temperature) concepts, for indirect estimation of thermal niche while using newts, Ichthyosaura alpestris, as a study system. If the two approaches are linked, then digesting newts should keep their body temperatures close to values maximizing aerobic scope for digestion. After feeding, newts maintained their body temperatures within a narrower range than did hungry individuals. The range of preferred body temperatures was well below the temperature maximizing aerobic scope for digestion. Optimal temperatures for factorial aerobic scope fell within the preferred body temperature range of digesting individuals. We conclude that digesting newts prefer body temperatures that are optimal for the maximum aerobic performance but relative to the maintenance costs. What might be termed the 'economic' thermoregulatory response explains the mismatch between thermal physiology and behaviour in this system.

Behavioral hypothermia of a domesticated lizard under treatment of the hypometabolic agent 3-iodothyronamine.

Ectothermic animals rely on behavioral thermoregulation due to low capacity of heat production and storage. Previously, lizards were shown to achieve ‘fever’ during microbial infection by increasing their preferred body temperature (PBT) behaviorally, thereby attaining a relatively high survival rate. The purpose of this study was to investigate whether domesticated lizards pursued ‘behavioral hypothermia’ induced by a hypometabolic agent 3-iodothyronamine (T1AM). We found that treatment with 8.0 mg/kg T1AM caused a lizard species, the leopard gecko (Eublepharis macularius), to decrease its ventilation and oxygen consumption rates 0.64- and 0.76-fold, respectively, compared to those of the control (P<0.05). The lizards, habituated at an ambient temperature of 30 ± 0.5°C, also showed a significant decrease in the PBT range over a freely accessible thermal gradient between 5°C and 45°C. The upper limit of the PBT in the treated lizards lowered from 31.9°C to 30.6°C, and the lower limit from 29.5°C to 26.3°C (P<0.001). These findings demonstrate that the treated lizards pursued behavioral hypothermia in conjunction with hypoventilation and hypometabolism. Because prior studies reported a similar hypometabolic response in T1AM-injected laboratory mice, the domesticated lizards, as a part of the vertebrate phylogeny, may be a useful laboratory model for biological and pharmacological researches such as drug potency test.

Thermoregulatory strategies in an aquatic ectotherm from thermally-constrained habitats: An evaluation of current approaches

Many ectotherms employ diverse behavioral adjustments to effectively buffer the spatio-temporal variation in environmental temperatures, whereas others remain passive to thermal heterogeneity. Thermoregulatory studies are frequently performed on species living in thermally benign habitats, which complicate understanding of the thermoregulation–thermoconformity continuum. The need for new empirical data from ectotherms exposed to thermally challenging conditions requires the evaluation of available methods for quantifying thermoregulatory strategies. We evaluated the applicability of various thermoregulatory indices using fire salamander larvae, Salamandra salamandra, in two aquatic habitats, a forest pool and well, as examples of disparate thermally-constrained environments. Water temperatures in the well were lower and less variable than in the pool. Thermal conditions prevented larvae from reaching their preferred body temperature range in both water bodies. In contrast to their thermoregulatory abilities examined in a laboratory thermal gradient, field body temperatures only matched the mean and range of operative temperatures, showing thermal passivity of larvae in both habitats. Despite apparent thermoconformity, thermoregulatory indices indicated various strategies from active thermoregulation, to thermoconformity, and even thermal evasion, which revealed their limited applicability under thermally-constrained conditions. Salamander larvae abandoned behavioral thermoregulation despite varying opportunities to increase their body temperature above average water temperatures. Thermoconformity represents a favored strategy in these ectotherms living in more thermally-constrained environments than those examined in previous thermoregulatory studies. To understand thermal ecology and its impact on population dynamics, the quantification of thermoregulatory strategies of ectotherms in thermally-constrained habitats requires the careful choice of an appropriate method to avoid misleading results.

A stringent test of the thermal coadaptation hypothesis in flour beetles

Whole-organism performance depends on body temperature and ectotherms have variable body temperatures. The thermal coadaptation hypothesis posits that thermal reaction norms have coevolved with thermal preference such that organisms attain optimal performance under a narrow range of body temperatures commonly experienced in the wild. Since thermal reaction norms are often similar, researchers interested in the effects of temperature on fitness often use one easily measured thermal reaction norm, such as locomotor performance, and assume it is a good proxy for fitness when testing the thermal coadaptation hypothesis. The extent to which this assumption holds, however, is often untested. In this study, we provide a stringent test of the thermal coadaptation hypothesis in red and in confused flour beetles by comparing the thermal reaction norm for reproductive output to the preferred body temperature range. We also test the assumption that locomotor performance can serve as a proxy for the thermal reaction norm for reproductive output, a more ultimate index of fitness. In both species, we measured the number of eggs laid, righting time, and sprint speed at eight temperatures, as well as the thermal preference in a thermal gradient. The number of eggs laid increased with female sprint speed and with male righting time, and all three performances had similar thermal reaction norms, with 80% of the maximum achieved between 23 and 37°C. Red flour beetles had preferred body temperatures that matched the optimal temperature for performance; confused flour beetles had lower preferred body temperature than the optimal temperature for performance. We found support for the assumption that locomotor performance can serve as a proxy for reproductive output in flour beetles, but we only found evidence for thermal coadaptation in one of the two species.

Thermoregulation during the summer season in the Goode’s horned lizard Phrynosoma goodei (Iguania: Phrynosomatidae) in Sonoran Desert

Reptiles in desert environments depend on habitat thermal quality to regulate their body temperature and perform biological activities. Understanding thermoregulation with respect to habitat thermal quality is critical for accurate predictions of species responses to climate change. We evaluated thermoregulation in Goode’s horned lizard, Phrynosoma goodei, and measured habitat thermal quality at the Reserva de la Biosfera El Pinacate y Gran Desierto de Altar, Sonora, Mexico, during the hottest season of the year. We found that field-active body temperature averaged 38.1 ± 0.38°C, preferred body temperature in laboratory averaged 34.9 ± 0.18°C and preferred body temperature range was 32.5-37.3°C. Operative temperature (i.e. environmental temperature available to the lizards) averaged 43.0 ± 0.07°C, with maximum temperature being near 70°C, and 62.9% of operative temperatures were above preferred body temperature range of P. goodei. Microhabitat thermal quality occupied by the lizards was high in the morning (7:00-10:30) and afternoon (5:50-dusk). We found that despite strong thermal constraints P. goodei was highly accurate and efficient in regulating its body temperature and that it presented a bimodal thermoregulatory pattern, being active in the mornings and in the evenings in order to avoid high mid-day environmental temperatures. Despite its thermoregulatory ability, P. goodei may be vulnerable to climate warming.

Thermoregulation of spotted turtles (Clemmys guttata) in a beaver-flooded bog in southern Ontario, Canada

Body temperature has a major influence on the physiological processes, growth, reproductive output, and overall survival of ectotherms. When a habitat is altered as a result of natural or anthropogenic influences, the available temperatures in the habitat can change, thus affecting an animal's ability to thermoregulate. We studied thermoregulation in response to habitat change in a population of spotted turtles (Clemmys guttata) in Southern Ontario, Canada. Historically, the study site was ditched to draw down water levels to facilitate peat mining, and the resulting drainage ditches were the only habitat containing surface water and turtles were restricted to these drains. Recent colonization of the site by beaver (Castor canadensis) caused increases in water level and water surface area. We followed spotted turtles (N=16) outfitted with radio transmitters and iButtons to estimate body temperatures (Tb) continuously throughout the active season post-flooding. Turtle models outfitted with iButtons (N=50) were deployed in the nine available habitat types to record environmental temperatures (Te). Turtles (N=13) were tested in a thermal gradient under laboratory conditions to determine preferred body temperature range (Tset). The Tset for the population ranged from 20°C to 26°C. In the field, Tb was within the Tset range 28% of the time from March to October, and 67% of the time from July to August. Efficiency of thermoregulation was calculated to be highest in July and August. The habitat type with the highest thermal quality was the shallow flooded zone created by beaver damming, and the habitat with the lowest thermal quality was the drain bottom, the drains being the only aquatic habitat available prior to flooding. This study confirms that beaver flooding provided a wide variety of preferable thermal opportunities for spotted turtles. Further investigation is needed to determine the effects of flooding on spotted turtle thermoregulation during nesting and hibernation.

Thermal ecology of Massasauga Rattlesnakes (Sistrurus catenatus) near their northern range limit

Using populations at their northern range limits, we compared the thermoregulatory behaviour of Massasauga Rattlesnakes ( Sistrurus catenatus (Rafinesque, 1818)) to published results for Ratsnakes ( Elaphe obsoleta (Say in James, 1823)) and Northern Watersnakes ( Nerodia sipedon (L., 1758)) to test the hypothesis that given similar benefits of thermoregulation, costs associated with foraging ecology should shape thermoregulatory behaviour. More than 32 000 body temperature measurements from 34 individual snakes over 4 years were used to quantify thermoregulation by Massasauga Rattlesnakes using standard indices and a new index (%MaxE) that describes how much of the thermoregulatory potential available to a snake is realized. On average, Massasaugas were much cooler (db = 6.9 °C) than their preferred body temperature range (30–33.6 °C) but warmer (de – db = 3.3 °C) than were they not thermoregulating. Massasaugas realized more than half of their environmental potential for effective thermoregulation (%MaxE = 64%). Consistent with there being less conflict between foraging and thermoregulation for ambush predators than active foragers, Massasaugas were more effective thermoregulators than Ratsnakes or Watersnakes during the day. All three species were effective thermoregulators at night, supporting the assumption that species in a cool environment will thermoregulate when there are few competing interests.

The ontogenetic shift in thermoregulatory behaviour of newt larvae: testing the ‘enemy‐free temperatures’ hypothesis

AbstractGrowth and development affect life‐history traits, and consequently organismal fitness. The inevitable increase in body size during ontogeny is associated with changes in both resource use and predation risk, which results in the ontogenetic shift in habitat preferences. In this study, we examined whether the shift in preferred body temperatures (Tps) of newt larvae Triturus alpestris increases the Tps deviation of the most vulnerable stages (after hatching and during metamorphosis) from the Tps range of their main predator, dragonfly larvae Aeshna cyanea– the ‘predator‐free temperatures’ hypothesis. Analyses of thermoregulatory behaviour in the laboratory thermal gradient showed that freshly hatched newt larvae maintained lower water temperatures than larvae in later stages, whose Tps largely fell into the Tps range of dragonfly larvae. With respect to the thermal quality of natural habitat, the anti‐predator effectiveness of the Tps shift decreased during development. Water temperatures in natural habitats were located largely below the preferred body temperature range of both newt and dragonfly larvae, which limits their potential thermal niche partitioning. We conclude that factors other than predator avoidance drive the ontogenetic Tps shift in our model system.

Are Locomotor Performances Coadapted to Preferred Basking Temperature in the Northern Map Turtle (Graptemys geographica)?

We tested a common prediction of the thermal coadaptation hypothesis in the Northern Map Turtle (Graptemys geographica), an aquatic emydid with pronounced aerial basking. We measured the effect of body temperature on two locomotor performances (swimming and righting) to determine optimal temperature of performance (To) for each trait. According to the thermal coadaptation hypothesis, the preferred body temperature range (Tset) of ectotherms should match To of thermally sensitive traits that influence fitness. However, we predicted that preferred basking temperature and locomotor performance of Northern Map Turtles would not be coadapted, given that basking occurs on land and locomotion in water. We also tested for an ontogenetic shift in performance curves. We found that adult Northern Map Turtles have a wide Tset (19–30°C), both hatchlings and adults can achieve near-maximum performance over a wide range of temperatures, and an ontogenetic shift is present for swimming but not for righting. Although To for the two locomotor performances of adult turtles were within Tset, the large range of Tset coupled with the wide breadth in locomotor performance makes falsifying or supporting the coadaptation hypothesis difficult for these traits in Northern Map Turtles. Other metabolic and physiological processes need to be considered to understand more fully thermal coadaptation in aquatic emydids.

The Effect of Reproductive Condition on Thermoregulation in Female Agkistrodon piscivorus Near the Northwestern Range Limit

Females of some temperate-zone snake species appear to exhibit thermophilic behavior during gestation, resulting in differential habitat use between gravid and nongravid individuals. We investigated thermoregulation of 13 female cottonmouths (Agkistrodon piscivorus) near their northwestern range limit in southwestern Missouri during mid-summer. Mean preferred body temperature (23.4 C) and preferred body temperature range (Tset, 20.0-26.5 C) measured in a laboratory thermal gradient were lower than those reported for most snake species. Physical models demonstrated that Tset consistently was available to snakes during the study period, although the frequency of availability varied among habitats. The relatively high thermal quality of the environment (based on Tset availability) allowed snakes to achieve field body temperatures (Tb) closely matching laboratory preferences. Gravid females exploited the thermal environment more efficiently than nongravid females by preferentially occupying the most thermally favorable microhabitats. Accordingly, gravid females consistently maintained higher Tb than nongravid females, demonstrating a functional link between habitat use and thermoregulation. Differences in Tb between gravid and nongravid females were greatest at low ambient temperatures, highlighting their different thermoregulatory strategies. Maintenance of low Tb in nongravid females is consistent with an energy conservation strategy benefiting reproductive investment.

Thermal quality influences habitat selection at multiple spatial scales in milksnakes

:Factors influencing habitat selection may be scale dependent, leading to different selection patterns at different spatial scales. By limiting habitat-selection studies to a single scale, important selection patterns could be missed. Despite this danger, many studies investigate habitat selection at a single scale, often ignoring macro-habitat selection: the selection of a home range within the study area. We investigated macro- and micro-habitat selection in milksnakes. Because of the importance of thermoregulation to ectotherms, we predicted that snakes would select habitats of high thermal quality at both microand macro-habitat scales. In 2003–2004, we located 25 individuals 890 times and characterized the habitat in detail at 279 locations used by milksnakes and at 279 paired random locations. Open habitats (fields, rocky outcrops, marshes) and edges have environmental temperatures that deviate less from the preferred body temperature range of milksnakes and offer characteristics that facilitate thermoregulation compared to forest. At the macro- and micro-habitat scales, milksnakes preferred habitats of high thermal quality: they used fields and rocky outcrops more than forests. Milksnakes also preferred edges at both scales. In addition, milksnakes preferred locations with open canopy and many rocks at the micro-habitat scale. These results support the notion that thermal quality influences habitat use in ectotherms and strengthen the idea that habitat-use studies should be conducted at more than one spatial scale to gain a complete understanding of the factors affecting selection.

THE COST–BENEFIT MODEL OF THERMOREGULATION DOES NOT PREDICT LIZARD THERMOREGULATORY BEHAVIOR

Physiological processes are optimized within a narrow range of body temperatures. Reptiles engage in behavioral thermoregulation to achieve the optimal body temperature range. Interestingly, however, thermoregulatory effort varies over time and across species. The cost–benefit model of thermoregulation of R. B. Huey and M. Slatkin is the only conceptual framework that attempts to explain the observed variation. The model postulates that reptiles should engage in thermoregulation when the costs incurred do not outweigh the benefits. The main cost is the thermal quality of the environment: thermoregulation is more costly in habitats of low thermal quality. We used thermal quality of the habitat, measured by the deviations of operative temperatures from the preferred body temperature range, as an indicator of costs and two quantitative indices of thermoregulation as indices of thermoregulatory investment. Regressions of the indices of thermoregulation on thermal quality of the habitat produced no pattern or ...

THERMAL ECOLOGY OF BLACK RAT SNAKES (ELAPHE OBSOLETA) IN A THERMALLY CHALLENGING ENVIRONMENT

A general model in thermal ecology predicts that ectotherms should stop thermoregulating when the costs outweigh the benefits. Support for this model comes from studies of warm-temperate species, but the extent to which the model can be extrapolated to species living in climatic extremes is unknown, because of the lack of information regarding the thermoregulatory behavior of such species. We tested the applicability of this cost-benefit model using data for black rat snakes (Elaphe obsolete obsoleta) studied at the northern extreme of their range in Ontario. During 1997-1999, we used automated temperature-sensitive radiotelemetry to collect -150000 body temperatures from 53 free- ranging rat snakes. Simultaneously, we used physical models of snakes to measure the environmental operative temperatures available to black rat snakes, and we determined their preferred body temperature range in a laboratory thermal gradient. The mostly forested habitats inhabited by rat snakes in Ontario were more thermally challenging than the habitats of other species studied to date. The preferred body temperature ranges of male, nongravid female, and gravid female black rat snakes were not significantly different (preferred body temperature averaged across all individuals, 28. 1C). However, free-ranging gravid females tended to maintain higher body temperatures in order to thermoregulate more effectively as well as exploit their thermal environment more than males and nongravid females. This difference was most pronounced during the day and prior to egg laying, and constituted the first documentation of such a phenomenon in an oviparous snake. Black rat snakes had indices of thermoregulation effectiveness similar to other species but tended to exploit opportunities for thermoregulation less. Overall, our data provided support that was at best ambiguous for the current cost-benefit thermoregulation model, suggesting that this model may generally be less applicable to species inhabiting climatic extremes. We propose that, for species in extreme climates, the costs associated with thermoconformity may be more important than previously recognized. We identified several problems associated with the index of thermoregulation effectiveness used by previous researchers, and we propose a mathematically simpler alternative that circumvents these problems. We also make rec- ommendations regarding the future use of the various indices of thermoregulation developed in recent years.

THERMAL ECOLOGY AND SEXUAL SIZE DIMORPHISM IN NORTHERN WATER SNAKES,NERODIA SIPEDON

We used more than 326 000 observations of temperature collected by radio telemetry from 38 individuals over three years to investigate thermoregulation and thermal relations of northern water snakes (Nerodia sipedon) near the northern limit of their distribution in Ontario, Canada. We tested hypotheses concerning the effects of feeding, season, sex, and reproductive condition on thermoregulation of individuals. The mean preferred body temperature (PBT) for captive snakes from the study population was 27.1°C, similar to that reported for other populations, and PBT range (defined as the 25th–75th percentiles of selected temperatures) was 25–30°C. When environmental conditions allowed, the mean and range of body temperature (Tb) of free-living snakes were nearly identical to those observed in captivity. The typical daily pattern in snake Tb was an increase in late morning to a plateau temperature in the preferred range, followed by a decrease in the evening with a nightime plateau at approximately the temperature of the water. We calculated indices of thermoregulation to relate Tb data to available ambient temperatures. Despite the northern latitude of the study population, ambient conditions were favorable over much of the activity season and offered the potential for the snakes to maintain Tb within the PBT range almost 24 h per day for much of the season. However, the thermoregulation indices indicated that N. sipedon are only moderate thermoregulators, and often do not exploit opportunities to achieve PBT, particularly during the day. Variation in annual ambient temperatures affected growth rates of the snakes, and was sufficient to affect the age of maturity of females. Feeding did not elicit a thermophilic response under laboratory conditions or when snakes were fed experimentally in the field, suggesting that the benefits of increasing Tb after eating did not outweigh the costs. Reproductive females thermoregulated more carefully than nonreproductive females during the July–August gestation period. Pregnant females may increase thermoregulatory behavior to enhance the rate and efficiency of embryogenesis. Males thermoregulated less than both reproductive and nonreproductive females, particularly later in the activity season when thermoregulation by females increased. We hypothesized that males pursue a survival strategy following the spring mating season and thus remain cool and secretive during the later part of the activity season. This strategy by males clearly contributes to their slowed growth and smaller size relative to females.

Behavioural postures and the rate of body temperature change in wild freshwater crocodiles, Crocodylus johnstoni.

I recorded body temperature and behaviour of eight Crocodylus johnstoni in the wild over a 2-yr period in order to quantify the effect of posturing on body temperature and to provide a mechanistic explanation of how behaviour affects body temperature. Behaviour was categorised according to the proportion of a crocodile's surface area exposed from the water (0% exposed [=diving] to 100% exposed [=basking]). Crocodiles did not simply shuttle between the extremes of 100% exposed and diving but showed an array of intermediate postures. Rates of body temperature change were negative for exposures less than 40% and positive for 60%-100% exposed. This was due to the difference between operative temperature and body temperature, which was negative during diving but increased with the percentage of exposure, up to 25 degrees-30 degrees C during basking. For any particular posture, the rate of body temperature change decreased with increasing mass. Thermal time constants were shortest during diving and longest during basking. A heat-transfer equation predicted the rate of body temperature change well, except that it underestimated the rate of body temperature change during 80% and 100% exposed. Exposing only a small part of their body when in water (20%) slowed heat loss considerably, allowing crocodiles to spend more time in the water while maintaining body temperature within their preferred body temperature range.

Soil temperature, rock selection, and the thermal ecology of the amphisbaenian reptile Blanus cinereus

We studied the thermal ecology of the amphisbaenian reptile Blanus cinereus in central Spain, focusing on the relationship between the available underground thermal environment and the thermal biology of B. cinereus. We specifically examined which soil microhabitats might be used by amphisbaenians, given thermal preferences measured in the laboratory, in relation to daily substrate-temperature cycles. We also examined whether actual rock use is related to thermoregulatory behaviour. Soil-temperature cycles suggest that behavioural selection of low temperatures enables B. cinereus to maximize its activity period and activity area within its preferred body-temperature range. An amphisbaenian moving between the surface and 10 cm depth in sun-exposed soil could maintain its body temperature within the preferred range for 8 consecutive hours. Amphisbaenians selected rocks of intermediate thickness (10-20 cm) as temporary heating stations, but used rocks of different thicknesses at different times of day. In spring, during daylight hours B. cinereus can attain preferred body temperatures in superficial soil layers and maintain them for long periods, presumably at low cost, allowing them to forage where food is most abundant.

Standard metabolic rate and preferred body temperatures in some Australian pythons

Pythons have standard metabolic rates and preferred body temperatures that are lower than those of most other reptiles. This study investigated metabolic rates and preferred body temperatures of seven taxa of Australian pythons. We found that Australian pythons have particularly low metabolic rates when compared with other boid snakes, and that the metabolic rates of the pythons did not change either seasonally or on a daily cycle. Preferred body temperatures do vary seasonally in some species but not in others. Across all species and seasons, the preferred body temperature range was only 4.9˚C. The thermal sensitivity (Q10) of oxygen consumption by pythons conformed to the established range of between 2 and 3. Allometric equations for the pooled python data at each of the experimental temperatures gave an equation exponent of 0.72–0.76, which is similar to previously reported values. By having low preferred body temperatures and low metabolic rates, pythons appear to be able to conserve energy while still maintaining a vigilant ‘sit and wait’ predatory existence. These physiological attributes would allow pythons to maximise the time they can spend ‘sitting and waiting’ in the pursuit of prey.