Contrasted thermal preferences translate into divergences in habitat use and realized performance in two sympatric snakes

Linking physiology and climate to infer species distributions in Australian skinks

Climate has a key impact on animal physiology, which in turn can have a profound influence on geographic distributions. Yet, the mechanisms linking climate, physiology and distribution are not fully resolved. Using an integrative framework, we tested the predictions of the climatic variability hypothesis (CVH), which states that species with broader distributions have broader physiological tolerance than range-restricted species, in a group of Lampropholis skinks (8 species, 196 individuals) along a latitudinal gradient in eastern Australia. We investigated several physiological aspects including metabolism, water balance, thermal physiology, thermoregulatory behaviour and ecological performance. Additionally, to test whether organismal information (e.g. behaviour and physiology) can enhance distribution models, hence providing evidence that physiology and climate interact to shape range sizes, we tested whether species distribution models incorporating physiology better predict the range sizes than models using solely climatic layers. In agreement with the CVH, our results confirm that widespread species can tolerate and perform better at broader temperature ranges than range-restricted species. We also found differences in field body temperatures, but not thermal preference, between widespread and range-restricted species. However, metabolism and water balance did not correlate with range size. Biophysical modelling revealed that the incorporation of physiological and behavioural data improves predictions of Lampropholis distributions compared with models based solely on macroclimatic inputs, but mainly for range-restricted species. By integrating several aspects of the physiology and niche modelling of a group of ectothermic animals, our study provides evidence that physiology correlates with species distributions. Physiological responses to climate are central in establishing geographic ranges of skinks, and the incorporation of processes occurring at local scales (e.g. behaviour) can improve species distribution models.

Thermal Preferences of Cowpea Seed Beetles (Callosobruchus maculatus): Effects of Sex and Nuptial Gift Transfers

Simple Summary

The thermal environment is crucial for organismal functioning, and many cold-blooded organisms, including insects, behaviorally regulate their body temperature. Why do insects inhabit given thermal conditions? We propose that access to water affects thermal preference and that insects with poor access to water inhabit colder environments, which reduces evaporation and preserves water. We studied the seed beetle Callosobruchus maculatus, which, as adults, do not drink or eat; however, males provide their mates with sperm, as well as nuptial gifts, including nutrients and water sources. We compared preferred temperatures between males and females that had access to mates or remained unmated and measured the sizes of the transferred gifts. We found that females preferred higher temperatures than males, but these preferences did not change due to mating or the transfer of larger or smaller gifts. It appears that males and females receive and lose certain amounts of water during mating, but they do not alter their thermal preferences according to the amount of water they receive or lose.

Abstract

The thermal environment influences insect performance, but the factors affecting insect thermal preferences are rarely studied. We studied Callosobruchus maculatus seed beetles and hypothesized that thermal preferences are influenced by water balance, with individuals with limited water reserves preferring cooler habitats to reduce evaporative water loss. Adult C. maculatus, in their flightless morph, do not consume food or water, but a copulating male provides a female with a nuptial gift of ejaculate containing nutrients and water. We hypothesized that gift recipients would prefer warmer habitats than gift donors and that both sexes would plastically adjust their thermal preferences according to the size of the transferred gift. We measured the thermal preference in each sex in individuals that were mated once or were unmated. In the mated group, we measured the sizes of the nuptial gifts and calculated proportional body mass changes in each mate during copulation. Supporting the role of water balance in thermal preference, females preferred warmer habitats than males. Nevertheless, thermal preferences in either sex were not affected by mating status or gift size. It is likely that high rates of mating and gift transfers in C. maculatus living under natural conditions promoted the evolution of constitutive sex-dependent thermal preferences.

Short-term captivity does not affect immediate voluntary thermal maximum of a neotropical pitviper: Implications for behavioral thermoregulation

Ectotherms depend on temperature to maintain their physiological functions and through behavioral changes, they can avoid overheating in their habitats. The voluntary thermal maximum (VT_Max ) represents the maximum temperature tolerated by individuals before actively moving to a colder place. However, if and how VT_Max might change after capture and in captivity remains understudied. We investigate if measurements taken in captivity are a good proxy for thermal tolerance of wild individuals. As thermal history has been shown to affect behavioral response and physiological parameters, herein we hypothesized that VT_Max of the neotropical viper Bothrops pauloensis varies throughout the captivity period. We measured the VT_Max of individuals immediately after capture and in three trials during a short-term period in captivity. Measurements were done by recording their body temperature at which they exited a heating box experimental setup. In contrast to our hypothesis, the VT_Max was not significantly affected by time in captivity but there was interindividual variation. There were also no significant differences between field and captivity measurements, in spite of the small effect size. Our results indicate that the VT_Max of this snake population is not affected by a short-term captivity period. Furthermore, an invariant VT_Max might indicate low phenotypic plasticity, as individuals do not appear to adjust their tolerance to short-term exposure to higher temperatures and potential vulnerability to threats such as global warming. We expect that our results can contribute to understanding the effect of captivity on thermal tolerance in neotropical squamates, allowing for insights into their thermal physiology and ecology.

Independent influence of thermoregulatory cost on the lower and upper set-points of a heliothermic lizard

Studies on ectothermic vertebrates generally lead to average indicators of thermal preferences measured in the laboratory, which do not say about responses to natural environmental change and may not inform about individual variation and its triggering mechanisms. We studied whether and how changes in costs of thermoregulation influence the preferred temperature (T_p) of individual lizards and their energetic investment in thermoregulation by exposing specimens to three treatments of increasing costs, recording body temperature (T_b) and distance walked (energetic investment). Moderate costs induced an investment trade-off between energy-investment in thermoregulation and T_b, and highlighted individual variation that decreased with higher costs. Lower average T_p's were observed in the high costs trials as a result of the decreased values of the lower and upper voluntary T_b's. As costs increased, lizards walked a shorter overall distance and accepted lower T_b's, but lizards still engaged in costly thermoregulation. Individual variation resulted in two main thermoregulatory patterns that in certain ecological contexts may have an important role in decision-making and adjusting to temperatures that are far from optimal temperatures for performance and physiological processes.

Tree lizard (Urosaurus ornatus) growth decreases with population density, but increases with habitat quality

Habitat selection models can explain spatial patterns in the relative abundance of animals in different habitats based on the assumption that fitness declines as density in a habitat increases. Ectotherms, such as lizards, may not follow predictions of density-dependent habitat selection models because temperature, which is unaffected by density, strongly influences their habitat selection. If competition for limited resources decreases fitness, then crowding should cause a decrease in body size and growth rates. We used skeletochronology and body size data from tree lizards (Urosaurus ornatus) at six sites that each spanned two habitats varying in quality to test the hypothesis that habitat selection is density dependent because growth is limited by competition for resources and by habitat quality. First, we tested that the maximum body size of lizards decreased with higher densities in a habitat by comparing growth between sites. Second, we tested whether body size and growth were higher in the habitat with more resources by controlling for density in a habitat and comparing growth between habitats in different sites. We found evidence of density-dependent growth in females, but not in males. Females in more crowded sites reached a smaller maximum size. Females in the higher quality habitat also grew larger than females in the lower quality habitat after controlling for differences in density between the habitats. Therefore, we found partial support for our hypothesis that competition for resources limits growth and causes density-dependent habitat selection.

Thermoregulatory performance and habitat selection of the eastern box turtle (Terrapene carolina carolina)

Environmental conditions may affect individual physiological processes that influence short-term performance and ultimately growth, survival and reproduction. As such, habitats selected by animals must provide suitable and adequate resources. Ectothermic species are highly dependent on climatic conditions and ambient temperatures that dictate body temperature regulation and in turn physiological processes. We investigated the thermoregulatory performance, habitat selection, and movements of an ectothermic vertebrate, the Eastern box turtle (Terrapene carolina carolina) to assess the importance of thermoregulatory physiology in habitat selection. We evaluated the relationship between habitat selection and thermoregulatory performance in Southwest Ohio over two active seasons from May until October. We found that T. carolina selected shaded habitats, including evergreen and deciduous forests, as well as herbaceous grasslands, conformed to the ambient temperatures throughout the active season, although these habitats had temperatures below those expected based on thermal optima of box turtles. Further, we found that movement was not correlated with internal body temperature. Our study shows that thermal conditions are not paramount in habitat selection of box turtles, but that cooler temperatures do not have an effect on the extent of their locomotion.

Thermal ecology of three coexistent desert lizards: Implications for habitat divergence and thermal vulnerability

How ectotherms exploit thermal resources has important implications for their habitat utilization and thermal vulnerability to climate warming. To address this issue, we investigated thermal relations of three sympatric lizard species (Eremias argus, Eremias multiocellata, and Phrynocephalus przewalskii) in the desert steppe of Inner Mongolia, China. We determined the thermoregulatory behavior, body temperature (T _b), operative temperature (T _e), selected body temperature (T _sel), and critical thermal maximum (CT_max) of adult lizards. Based on these physiological parameters, we quantified the accuracy and effectiveness of thermoregulation as well as thermal-safety margin for these species. The three species were accurate and effective thermoregulators. The P. przewalskii preferred open habitats, and had a higher T _b than the two Eremias lizards, which preferred shade habitats and shuttled more frequently between the shade and sun. This indicated that the three sympatric lizards have different thermoregulatory behavior and thermal physiology, which might facilitate their coexistence in the desert steppe ecosystem. In addition, the P. przewalskii had higher T _sel and CT_max, and a wider thermal-safety margin than the two Eremias lizards, suggesting that the two Eremias lizards would be more vulnerable to climate warming than P. przewalskii.

Differential fitness in field and forest explains density-independent habitat selection by gartersnakes

The ideal free distribution concept predicts that organisms will distribute themselves between habitats in a density-dependent manner so that individuals, on average, achieve the same fitness in each habitat. In ectotherms, environmental temperature has a strong impact on fitness, but temperature is not depletable and thus not density dependent. Can density-dependent habitat selection occur in ectotherms when habitats differ in thermal quality? We used an observational study of habitat selection by small snakes in field and forest, followed by manipulative habitat selection and fitness experiments with common gartersnakes in enclosures in field and forest to test this hypothesis. Snakes were much more abundant in the field, the habitat with superior thermal quality, than in the forest. Gartersnakes in our controlled experiment only used the forest habitat when snake density was highest and when food was more abundant in the forest; habitat selection was largely density independent, although there was weak evidence of density dependence. No female gartersnake gave birth in the forest enclosures, whereas half of the females gave birth in the field enclosures. Growth rates of females were higher in field than in forest enclosures. Overall, our data indicate that temperature appears to be the most important factor driving the habitat selection of gartersnakes, likely because temperature was more limiting than food in our study system. Snakes, or at least temperate snakes, may naturally exist at population densities low enough that they do not exhibit density-dependent habitat selection.

Isolating weather effects from seasonal activity patterns of a temperate North American Colubrid

Forecasting the effects of climate change on threatened ecosystems and species will require an understanding of how weather influences processes that drive population dynamics. We have evaluated weather effects on activity patterns of western ratsnakes, a widespread predator of birds and small mammals in eastern North America. From 2010-2013 we radio-tracked 53 ratsnakes in the fragmented region of central Missouri. We relocated each snake 4× per week and used movement frequency as an index of activity. We used generalized linear mixed models within an information-theoretic approach to evaluate temporal and weather variables as potential predictors of snake activity. While snakes were generally sedentary, activity showed a linear response to relative humidity and a quadratic response to air temperature, peaking near 30 °C. Seasonal activity patterns differed between sexes and among years, but snakes were generally least active in mid-summer, regardless of weather. Our findings provide strong evidence that air temperature and relative humidity differentially affect activity patterns of an important predator and are the mechanism explaining increased nest predation rates with warmer temperatures.

Microclimate preferences correlate with contrasted evaporative water loss in parapatric vipers at their contact zone

Terrestrial ectotherms predominantly use behavioural means to thermoregulate and thereby optimize performances. However, thermoregulation can impart physiological challenges to other critical processes such as water balance by increasing evaporative water loss (EWL). Like thermoregulation, water balance is influenced by both external factors (e.g., microhabitat and environmental constraints) and endogenous traits (e.g., evaporative water loss rates, dehydration tolerance). Although thermoregulation and water balance are tightly linked, the role of water balance is often overlooked when evaluating species climatic adaptation and response to global warming. We studied two congeneric viperid species (the Aspic Viper, Vipera aspis (L., 1758), and the Common Viper, Vipera berus (L., 1758)) with contrasted climatic affinities (south European versus boreal, respectively). These parapatric species are syntopic in narrow contact zones where microhabitat partitioning has been reported. We compared total EWL and cutaneous evaporative water loss (CEWL) of the two species and monitored the thermal and hydric conditions of the microhabitats used in syntopic populations. We found that the boreal V. berus has greater EWL, both total and cutaneous. Accordingly, this species selected more humid microhabitats throughout the year. Humidity appears to be an important determinant of habitat selection, and therefore, V. berus is likely vulnerable to changing precipitation at the southern limit of its distribution.

Thermal games in crayfish depend on establishment of social hierarchies

An unequal resource distribution is commonly seen in dominance hierarchies, in which the individual with the higher status is more successful in obtaining the resource. One possible resource is preferred temperature. When situations allow, ectotherms regulate their body temperature by behaviourally selecting different environmental conditions, achieving, when possible, a preferred temperature. Using a shuttlebox, the preferred temperature for Procambarus clarkii was determined to be 23.9°C with upper and lower voluntary escape temperatures of 25.9 and 21.8°C, respectively. If this preferred temperature zone (21.8–25.9°C) was valued as a resource, given the choice between a preferred temperature and a non-preferred temperature, crayfish should compete over the preferred temperature, with the dominant individual of dyadic pairs achieving the preferred temperature more often than the subordinate. Using a dual-choice experimental tank, competition over a binary temperature choice between rank-established paired crayfish was determined under both warm and cold challenge conditions (warm vs preferred temperature and cold vs preferred temperature, respectively). In naive pairings, similar levels of competition over the preferred temperature occurred in both warm and cold challenge trials, as predicted by game theory. In established pairings, however, dominant crayfish gained significantly greater access to preferred temperature in both warm and cold challenge conditions. These results demonstrate that crayfish engage in a cost–benefit assessment during their initial agonistic contests over temperature, but as hierarchies mature, these thermal games are decided by the dominant animal gaining primary access to the temperature resource.

Trophic niche overlap in two syntopic colubrid snakes (Hierophis viridiflavus and Zamenis longissimus) with contrasted lifestyles

In many organisms, including snakes, trophic niche partitioning is an important mechanism promoting species coexistence. In ectotherms, feeding strategies are also influenced by lifestyle and thermoregulatory requirements: active foragers tend to maintain high body temperatures, expend more energy, and thus necessitate higher energy income. We studied diet composition and trophic niche overlap in two south European snakes (Hierophis viridiflavus and Zamenis longissimus) in the northern part of their range. The two species exhibit contrasted thermal adaptations, one being highly mobile and thermophilic (H. viridiflavus) and the other being elusive with low thermal needs (Z. longissimus). We analyzed feeding rate (proportion of snakes with indication of a recent meal) and examined more than 300 food items (fecal pellets and stomach contents) in 147 Z. longissimus and 167 H. viridiflavus. There was noticeable overlap in diet (overlap of Z. longissimus on H. viridiflavus = 0.62; overlap of H. viridiflavus on Z. longissimus = 0.80), but the similarity analyses showed some divergence in diet composition. Dietary spectrum was wider in H. viridiflavus, which fed on various mammals, birds, reptiles, and arthropods whereas Z. longissimus was more specialized on mammals and birds. The more generalist nature of H. viridiflavus was consistent with its higher energy requirements. In contrast to our expectation, feeding rate was apparently higher in Z. longissimus than in H. viridiflavus, but this could be an artifact of a longer transit time in Z. longissimus, given its lower mean body temperature. These results allow a better understanding of the ability to coexist in snakes belonging to temperate climate colubrid communities.