Keeping a cool mind: head-body temperature differences in the common wall lizard

The size of a smell: assessment of rival’s relative size from femoral secretions in the common wall lizards, Podarcis muralis (Laurenti, 1768)

Animal communication depends on signals conveying information to a receiver who must perceive and decode them. Signals involved in territoriality are usually complex stimuli that should be correctly interpreted to avoid unnecessary conflicts. Lacertids use both visual and chemical stimuli in modulating their aggressive response against conspecifics and the rival’s size is one of the most important information, affecting the success probability in combat. To assess the actual ability of decoding information about a rival’s size based on its chemical stimulus alone, 60 males of Podarcis muralis were tested for three consecutive days in an arena bearing a mirror (to simulate an equal-sized intruder), and the chemical cues (femoral secretions) from an unknown individual of different size. Significant differences were observed in tongue-flicks number, which grew as the size difference between the focal lizard and the secretion donor decreased. This can be interpreted as the need for the lizard to better evaluate the potential competitor’s characteristics. The size difference also affected the number of bites against the mirror. They increased when the size of the focal lizard was larger than the donor triggering the aggressive response with a higher probability of winning the contest. This confirms that the focal lizard had correctly decoded the information about the opponent’s size by chemical stimulus. Although previous studies have shown that some components of the chemical signals are potentially informative about the signaler’s size, this is the first demonstration that male P. muralis is actually able to decode and use such information.

Close encounters of the three morphs: Does color affect aggression in a polymorphic lizard?

Color polymorphism is genetically controlled, and the process generating and maintaining morphs can affect speciation/extinction rates. Color badges are useful signals in intraspecific communication because they convey information about alternative strategies and can potentially decrease unnecessary conflicts among different color morphs. Competition and aggressive interactions among color morphs can contribute to polymorphism maintenance. This could lead to an uneven spatial distribution of morphs in a population because the local frequency of each morph establishes the intensity of the competition and the fitness of each male. We used a polymorphic lizard, Podarcis muralis, to assess if aggression varies among morphs under two contrasting hypotheses: a heteromorphic versus homomorphic aggression. We used laboratory mirror tests after lizard color manipulation, and we verified the consistency of results with an analysis of the spatial distribution of morphs in a wild population. Both the experiments confirmed that aggression is more common during homomorphic than heteromorphic contests. The adoption of alternative behavioral strategies that minimize risks and costs could facilitate the stable coexistence of the phenotypes and reduce competition. A bias in aggression would advantage rarer morph, which would suffer less harassment by common morphs obtaining a fitness advantage. This process would be negatively-frequency-dependent and would stabilize polymorphism, possibly contributing to sympatric speciation.

Introduction to the special issue-Beyond CTMAX and CTMIN : Advances in studying the thermal limits of reptiles and amphibians

Two themes emerging from the special issue "Beyond CT_MAX and CT_MIN : Advances in Studying the Thermal Limits of Reptiles and Amphibians" are: (1) the need to identify mechanisms that determine the shape of thermal performance curves and (2) how these curves can be best used predictively.

Evidence from Tarentola mauritanica (Gekkota: Phyllodactylidae) helps validate thermography as a tool to infer internal body temperatures of lizards

Infrared (IR) thermal imaging has become an increasingly popular tool to measure body temperature of animals. The high-resolution data it provides with short lag and minimum disturbance makes it an appealing tool when studying reptile thermal ecology. However, due to the common phenomenon of regional heterothermy and surface-to-core temperature gradients, it is essential to select the appropriate body part to measure and provide calibrations to accurately infer internal body temperatures. This work follows from a previous study on lacertid lizards to assess the reliability of thermography-measured body temperatures, from several body locations, as a proxy for internal body temperature in lizards. This study focuses on the Moorish gecko, Tarentola mauritanica, due to its distant phylogenetic relationship and its different ecology and morphology from the previously tested species. A total of 60 adult geckos of both sexes and of a range of sizes were tested in thermal gradients and subjected to a sequence of randomly assorted treatments of heating and cooling. The temperatures of the animals were periodically measured with a thermal camera at six different body parts and, immediately after, the cloacal temperature was then measured with a thermocouple probe. Body parts' temperatures, obtained thermographically, were regressed against cloacal temperature using OLS regression and the pairwise correlations were tested using Spearman coefficients. Relationships among all body parts and between all body parts and the cloaca were strong in all cases (R² > 0.87, Spearman Correlation > 0.95). The observed pattern was very similar to those previously obtained from lacertid lizards. Ultimately, the eye proved to provide the best overall proxy for internal temperature, when accounting for both the slope and intercept of the regression. Hence, this study provides further support for the establishment of the eye as the standard location to infer internal body temperatures of lizards through thermography.

Waitin' on a sunny day: Factors affecting lizard body temperature while hiding from predators

Most animals face predators in their daily life and have evolved antipredator strategies that promote survival while minimizing escaping costs. For example, many animals often hide into refuges when chased. Ectotherms rely on external sources of heat to raise their body temperature, and thermoregulate to keep their body temperature close to the optimal for performance. For many ectotherms living in temperate areas, it can be expected that they pay a cost in terms of heat loss while staying hidden. Indeed, refuges are often more thermally unsuitable than the external environment. Hence, the aim of this study was to assess if and to what extent hiding may result in a decrease of body temperature in a temperate lizard. We used infrared technology to measure the body temperature of a large-sized lizard (Timon lepidus) before individuals escaped from a simulated predation attempt to hide inside a refuge, and after they emerged back from the refuge. We quantified the change of body temperature that lizards experienced while hiding. Results show that while the decrease in body temperature covaried with the time spent hidden, it was also affected by the initial body temperature. Our key finding is that the time spent hidden depends mostly on the temperature inside the refuge. Indeed, lizards hiding in warmer refuges spent more time hidden, likely benefitting from a reduced cooling rate. This suggests that lizards perceive and evaluate the thermal quality of their refuges and integrate this information to react to predation attempts and minimize the potential thermal consequences of hiding.

Dehydration constrains thermoregulation and space use in lizards

Climate change is negatively affecting many species. The increase in mean air temperature is often associated with shifts in distribution, changes in phenology, and local extinctions. Other factors that only partially correlate with air temperature, like water shortage, may also contribute to the negative consequences of climate change. Although the effect of temperature on lizards' ecophysiology is highly studied, many lizards are also at risks of increased water loss and dehydration, which are predicted to increase under climate change. Here we aimed for the first time to explore if lacertid lizards exposed to dehydration thermoregulate less precisely than hydrated lizards and if dehydrated lizards are less active, change the daily pattern of thermoregulation and balance water balance against thermoregulation. We exposed four lizard species with differences in the thermal preference to thermal gradients with or without a source of water. We measured preferred body temperatures, daily pattern of thermoregulation, and the use of space. Dehydration negatively affected thermoregulation in all investigated species. Dehydrated lizards reduced their preferred body temperature and showed a species-specific pattern of hourly change in thermal preference. Furthermore, they more frequently used the colder parts of the gradients and spent more time hidden. Lizards experiencing dehydration may suffer a reduction in survival and fitness because of poor thermoregulation. Similarly, they may spend more time hidden, waiting for more favourable weather conditions. Such inactivity may carry ecological costs especially in those regions that undergo either short or prolonged periods of droughts.

Inter- and intra-population variability of the protein content of femoral gland secretions from a lacertid lizard

Femoral glands of male lizards produce waxy secretions that are involved in inter- and intraspecific chemical communication. The main components of these secretions are proteins and lipids, the latter having been extensively studied and already associated to male quality. On the opposite, the composition and role of proteins are nearly unknown, the only available information coming from few studies on iguanids. These studies got the conclusion that proteins might have a communicative function, notably they could signal individual identity. A generalization of these findings requires the extension of protein analysis to other lizard families, and the primary detection of some patterns of individual variability. Using the common wall lizard Podarcis muralis as a model species, the protein fraction of the femoral pore secretions was investigated to provide the first characterization of this component in a lacertid lizard and to explore its source of variability, as a first step to support the hypothesized communicative role. Samples of proteins from femoral secretions were collected from 6 Italian populations and subjected to 1-dimensional electrophoresis. The binary vector of the band presence/absence was used to define the individual profiles. Protein fraction is found to have a structured pattern, with both an individual and a population component. Although the former supports the potential communicative role of proteins, the latter offers a double interpretation, phylogenetic or environmental, even though the phylogenetic effect seems more likely given the climatic resemblance of the considered sites. Further studies are necessary to shed light on both these issues.

An ecophysiological background for biogeographic patterns of two island lizards?

Distributions of sedentary ectotherms are dependent on temperature and humidity due to their low homeostatic and dispersal abilities. Lizards are strongly conditioned by temperature, but hydric environment may be also important, at least in arid environments. Biotic interactions may also play a role in range patterns, but they are of minor importance in islands where native species monopolize well-delimited niche spaces. On the arid island of São Vicente (Cabo Verde), two endemic lizards display different spatial patterns. While the gecko Tarentola substituta is widely distributed across the island, the skink Chioninia stangeri is restricted to the NE, which is cooler, more humid, and vegetated. We hypothesized that this is due to differences in the fundamental niche, specifically in ecophysiology. We predict that C. stangeri should select for lower temperatures and lose more water by evaporation than T. substituta. We submitted adults of each species to standard experiments to assess preferred body temperatures (T_p) and evaporative water loss (EWL) rates, and examined the variation between species and through time using repeated-measures AN(C)OVAs. Results only partially supported our expectations. Contrary to the prediction, skinks attained higher Tp than geckos but in the long term showed a trend for higher EWL as predicted. Thus, while ecophysiology certainly contributes to functional interpretation of species distributions, it needs to be combined with other evidence such as habitat use and evolutionary history. These findings will be useful to perform mechanistic models to better understand the impact of climate change and habitat disturbance on these endemic species.

Assessing the reliability of thermography to infer internal body temperatures of lizards

For many years lizard thermal ecology studies have relied on the use of contact thermometry to obtain internal body temperature (T_b) of the animals. However, with progressing technology, an interest grew in using new, less invasive methods, such as InfraRed (IR) pyrometry and thermography, to infer T_b of reptiles. Nonetheless few studies have tested the reliability of these new tools. The present study tested the use of IR cameras as a non-invasive tool to infer T_b of lizards, using three differently body-sized lacertid species (Podarcis virescens, Lacerta schreiberi and Timon lepidus). Given the occurrence of regional heterothermy, we pairwise compared thermography readings of six body parts (snout, eye, head, dorsal, hind limb, tail base) to cloacal temperature (measured by a thermometer-associated thermocouple probe) commonly employed to measure T_b in field and lab studies. The results showed moderate to strong correlations (R²=0.84-0.99) between all body parts and cloacal temperature. However, despite the readings on the tail base showed the strongest correlation in all three species, it was the eye where the absolute values and pattern of temperature change most consistently followed the cloacal measurements. Hence, we concluded that the eye would be the body location whose IR camera readings more closely approximate that of the animal's internal environment. Alternatively, other body parts can be used, provided that a careful calibration is carried out. We provide guidelines for future research using thermography to infer Tb of lizards.

Thermal equilibrium and temperature differences among body regions in European plethodontid salamanders

Information on species thermal physiology is extremely important to understand species responses to environmental heterogeneity and changes. Thermography is an emerging technology that allows high resolution and accurate measurement of body temperature, but until now it has not been used to study thermal physiology of amphibians in the wild. Hydromantes terrestrial salamanders are strongly depending on ambient temperature for their activity and gas exchanges, but information on their body temperature is extremely limited. In this study we tested if Hydromantes salamanders are thermoconform, we assessed whether there are temperature differences among body regions, and evaluated the time required to reach the thermal equilibrium. During summers of 2014 and 2015 we analysed 56 salamanders (Hydromantes ambrosii and Hydromantes italicus) using infrared thermocamera. We photographed salamanders at the moment in which we found them and 1, 2, 3, 4, 5 and 15min after having kept them in the hands. Body temperature was equal to air temperature; salamanders attained the equilibrium with air temperature in about 8min, the time required to reach equilibrium was longer in individuals with large body size. We detected small temperature differences between body parts, the head being slightly warmer than the body and the tail (mean difference: 0.05°C). These salamanders quickly reach the equilibrium with the environment, thus microhabitat measurement allows obtaining accurate information on their tolerance limits.

Vascular Patterns in Iguanas and Other Squamates: Blood Vessels and Sites of Thermal Exchange

Squamates use the circulatory system to regulate body and head temperatures during both heating and cooling. The flexibility of this system, which possibly exceeds that of endotherms, offers a number of physiological mechanisms to gain or retain heat (e.g., increase peripheral blood flow and heart rate, cooling the head to prolong basking time for the body) as well as to shed heat (modulate peripheral blood flow, expose sites of thermal exchange). Squamates also have the ability to establish and maintain the same head-to-body temperature differential that birds, crocodilians, and mammals demonstrate, but without a discrete rete or other vascular physiological device. Squamates offer important anatomical and phylogenetic evidence for the inference of the blood vessels of dinosaurs and other extinct archosaurs in that they shed light on the basal diapsid condition. Given this basal positioning, squamates likewise inform and constrain the range of physiological thermoregulatory mechanisms that may have been found in Dinosauria. Unfortunately, the literature on squamate vascular anatomy is limited. Cephalic vascular anatomy of green iguanas (Iguana iguana) was investigated using a differential-contrast, dual-vascular injection (DCDVI) technique and high-resolution X-ray microcomputed tomography (μCT). Blood vessels were digitally segmented to create a surface representation of vascular pathways. Known sites of thermal exchange, consisting of the oral, nasal, and orbital regions, were given special attention due to their role in brain and cephalic thermoregulation. Blood vessels to and from sites of thermal exchange were investigated to detect conserved vascular patterns and to assess their ability to deliver cooled blood to the dural venous sinuses. Arteries within sites of thermal exchange were found to deliver blood directly and through collateral pathways. The venous drainage was found to have multiple pathways that could influence neurosensory tissue temperature, as well as pathways that would bypass neurosensory tissues. The orbital region houses a large venous sinus that receives cooled blood from the nasal region. Blood vessels from the nasal region and orbital sinus show anastomotic connections to the dural sinus system, allowing for the direct modulation of brain temperatures. The generality of the vascular patterns discovered in iguanas were assessed by firsthand comparison with other squamates taxa (e.g., via dissection and osteological study) as well as the literature. Similar to extant archosaurs, iguanas and other squamates have highly vascularized sites of thermal exchange that likely support physiological thermoregulation that "fine tunes" temperatures attained through behavioral thermoregulation.

Thermoregulatory consequences of salt loading in the lizard, Pogona vitticeps

Previous research has demonstrated that dehydration increases the threshold temperature for panting and decreases the thermal preference of lizards. Conversely, it is unknown whether thermoregulatory responses like shuttling and gaping are similarly influenced. Shuttling, as an active behavioural response, is considered one of the most effective thermoregulatory behaviours, while gaping has been proposed to be involved in preventing brain over-heating in lizards. In this study we examined the effect of salt loading, a proxy for increased plasma osmolality, on shuttling and gaping in Pogona vitticeps. Then, we determined the upper and lower escape ambient temperatures (UETa and LETa), the percentage of time spent gaping, the metabolic rate (V̇O2), the evaporative water loss (EWL) during gaping and non-gaping intervals and the evaporative effectiveness (EWL/V̇O2) of gaping. All experiments were performed under isotonic (154 mM) and hypertonic saline injections (625, 1250 or 2500 mM). Only the highest concentration of hypertonic saline altered the UETa and LETa, but this effect appeared to be the result of diminishing the animal's propensity to move, instead of any direct reduction in thermoregulatory set-points. Nevertheless, the percentage of time spent gaping was proportionally reduced according to the saline concentration; V̇O2 was also decreased after salt-loading. Thermographic images revealed lower head than body surface temperatures during gaping; however this difference was inhibited after salt loading. Our data suggest that EWL/V̇O2 is raised during gaping, possibly contributing to an increase in heat transfer away from the lizard, and playing a role in head or brain cooling.