Thermal-metabolic phenotypes of the lizard Podarcis muralis differ across elevation, but converge in high elevation hypoxia

In response to a warming climate, many montane species are shifting upslope to track the emergence of preferred temperatures. Characterizing patterns of variation in metabolic, physiological, and thermal traits along an elevational gradient, and the plastic potential of these traits, is necessary to understand current and future responses to abiotic constraints at high elevations, including limited oxygen availability. We performed a transplant experiment with the upslope-colonizing common wall lizard (Podarcis muralis) in which we measured nine aspects of thermal physiology and aerobic capacity in lizards from replicate low- (400 m above sea level [ASL]) and high-elevation (1700 m ASL) populations. We first measured traits at their elevation of origin and then transplanted half of each group to extreme high elevation (2900 m ASL; above the current elevational range limit of this species), where oxygen availability is reduced by ∼25% relative to sea level. After three weeks of acclimation, we again measured these traits in both the transplanted and control groups. The multivariate thermal-metabolic phenotypes of lizards originating from different elevations differed clearly when measured at the elevation of origin. For example, high-elevation lizards are more heat tolerant than low-elevation counterparts (countergradient variation). Yet, these phenotypes converged after exposure to reduced oxygen availability at extreme high elevation, suggesting limited plastic responses under this novel constraint. Our results suggest that high-elevation populations are well-suited to their oxygen environments, but that plasticity in the thermal-metabolic phenotype does not pre-adapt these populations to colonize more hypoxic environments at higher elevations.

Low annual temperature likely prevents the Holarctic amphipod Gammarus lacustris from invading Lake Baikal

Species with effective thermal adaptation mechanisms allowing them to thrive within a wide temperature range can benefit from climatic changes as they can displace highly specialized species. Here, we studied the adaptive capabilities of the Baikal endemic amphipods Eulimnogammarus verrucosus (Gerstfeld, 1858) and Eulimnogammarus cyaneus (Dybowsky, 1874) compared to the potential Holarctic Baikal invader Gammarus lacustris Sars, 1863 at the cellular level including the energy metabolism and the antioxidant system. All species were long-term exposed to a range of temperatures (1.5 °C to mimic winter conditions and the three species-specific preferred temperatures (i.e., 6 °C for E. verrucosus, 12 °C for E. cyaneus and 15 °C for G. lacustris). At 1.5 °C, we found species-specific metabolic alterations (i.e., significantly reduced ATP content and lactate dehydrogenase activity) indicating limitations on the activity level in the Holarctic G. lacustris. Although the two Baikal endemic amphipod species largely differ in thermal tolerance, no such limitations were found at 1.5 °C. However, the cold-stenothermal Baikal endemic E. verrucosus showed changes indicating a higher involvement of anaerobic metabolism at 12 °C and 15 °C, while the metabolic responses of the more eurythermal Baikal endemic E. cyaneus may support aerobic metabolism and an active lifestyle at all exposure temperatures. Rising temperatures in summer may provide a competitive advantage for G. lacustris compared to the Baikal species but the inactive lifestyle in the cold is likely preventing G. lacustris from establishing a stable population in Lake Baikal.

Metabolic performance and thermal preference of Westslope Cutthroat Trout Oncorhynchus clarkii lewisi and non-native trout across an ecologically relevant range of temperatures

The physiology and behaviour of fish are strongly affected by ambient water temperature. Physiological traits related to metabolism, such as aerobic scope (AS), can be measured across temperature gradients and the resulting performance curve reflects the thermal niche that fish can occupy. We measured AS of Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi) at 5, 10, 15, 20, and 22°C and compared temperature preference (Tpref) of the species to non-native Brook Trout, Brown Trout, and Rainbow Trout. Intermittent-flow respirometry experiments demonstrated that metabolic performance of Westslope Cutthroat Trout was optimal at ~15 °C and decreased substantially beyond this temperature, until lethal temperatures at ~25 °C. Adjusted preferred temperatures across species (Tpref) were comparatively high, ranging from 17.8-19.9 °C, with the highest Tpref observed for Westslope Cutthroat Trout. Results suggest that although Westslope Cutthroat Trout is considered a cold-water species, they do not prefer or perform as well in cold water (≤ 10°C), thus, can occupy a warmer thermal niche than previously thought. The metabolic performance curve (AS) can be used to develop species‐specific thermal criteria to delineate important thermal habitats and guide conservation and recovery actions for Westslope Cutthroat Trout.

Hydrogen sulfide exposure reduces thermal set point in zebrafish

Behavioural flexibility allows ectotherms to exploit the environment to govern their metabolic physiology, including in response to environmental stress. Hydrogen sulfide (H 2 S) is a widespread environmental toxin that can lethally inhibit metabolism. However, H 2 S can also alter behaviour and physiology, including a hypothesized induction of hibernation-like states characterized by downward shifts of the innate thermal set point (anapyrexia). Support for this hypothesis has proved controversial because it is difficult to isolate active and passive components of thermoregulation, especially in animals with high resting metabolic heat production. Here, we directly test this hypothesis by leveraging the natural behavioural thermoregulatory drive of fish to move between environments of different temperatures in accordance with their current physiological state and thermal preference. We observed a decrease in adult zebrafish ( Danio rerio ) preferred body temperature with exposure to 0.02% H 2 S, which we interpret as a shift in the thermal set point. Individuals exhibited consistent differences in shuttling behaviour and preferred temperatures, which were reduced by a constant temperature magnitude during H 2 S exposure. Seeking lower temperatures alleviated H 2 S-induced metabolic stress, as measured by reduced rates of aquatic surface respiration. Our findings highlight the interactions between individual variation and sublethal impacts of environmental toxins on behaviour.

Hydrogen sulfide exposure reduces thermal set point in zebrafish

Behavioural flexibility allows ectotherms to exploit the environment to govern their metabolic physiology, including in response to environmental stress. Hydrogen sulfide (H2S) is a widespread environmental toxin that can lethally inhibit metabolism. However, H2S can also alter behaviour and physiology, including a hypothesised induction of hibernation-like states characterised by downward shifts of the innate thermal setpoint (anapyrexia). Support for this hypothesis has proved controversial because it is difficult to isolate active and passive components of thermoregulation, especially in animals with high resting metabolic heat production. Here, we directly test this hypothesis by leveraging the natural behavioural thermoregulatory drive of fish to move between environments of different temperatures in accordance with their current physiological state and thermal preference. We observed a decrease in adult zebrafish (Danio rerio) preferred body temperature with exposure to 0.02% H2S, which we interpret as a shift in thermal setpoint. Individuals exhibited consistent differences in shuttling behaviour and preferred temperatures, which were reduced by a constant temperature magnitude during H2S exposure. Seeking lower temperatures alleviated H2S-induced metabolic stress, as measured by reduced rates of aquatic surface respiration rate. Our findings highlight the interactions between individual variation and sublethal impacts of environmental toxins on behaviour.

Thermal preferences and effects of temperature on fitness parameters of an endemic Argentinean tarantula (Grammostola vachoni)

Mygalomorphs depend on thermal microhabitats for successful behavior, but their thermal biology is still poorly known. Grammostola vachoni Schiapelli and Gerschman, 1961 is an endemic tarantula from Argentina and it is listed as vulnerable in the IUCN Red List. However, little is known about its biology; therefore, we attempted to explore the thermal biology of juveniles and adult females of G. vachoni under laboratory conditions. We characterized the preferred temperatures, evaluated the relationship between temperature and locomotor performance, and calculated the thermal sensitivity. Individuals showed a peak temperature preference and spent 40% of the total time at 25–29 °C; we did not find any significant differences in temperature preference between juveniles and females. We found that locomotor performance is strongly affected by high temperatures. Different sprint speeds of juveniles and females were found at 5, 35, and 40 °C. The highest thermal sensitivity was recorded in the lowest temperature range and thermal sensitivity was lowest in the highest temperature ranges. Our results are relevant in the context of climate change, because thermal parameters constitute a useful tool to explore some possible effects of this change on body temperature and thus the physiological performance and vulnerability of ectotherms like G. vachoni.

What geckos are – an ecological-biogeographic perspective

Geckos are a hyper-diverse, ancient, and globally distributed group. They have diverged early from other squamates and thus can be expected to differ from them along multiple ecological, life history, and biogeographic axes. I review a wide range of gecko traits, comparing them to those of other lizard taxa, to identify the unique, and unifying, attributes of geckos among lizards, based on comprehensive databases of lizard distributions and biological attributes. Few traits completely separate geckos from other lizard taxa, yet they differ to a large degree along many axes: they are more restricted to low latitudes and altitudes, are especially diverse on islands, but relatively scarce in America. They are small lizards, that lay small, fixed clutch sizes, for which they compensate only partially by laying frequently. Because they mature at relatively similar ages and have similar lifespans to other lizards, geckos produce fewer offspring over a year, and over their lifetimes, perhaps implying that they enjoy higher survival rates. While being the only large lizard clade of predominantly nocturnal lizards a large proportion of species is active by day. Gecko body temperatures and preferred temperatures are lower than those of other lizards –even when they are compared to lizards with similar activity times. Worryingly, most geckos have small ranges that often reside completely outside of protected areas – much more frequently than in other reptile and vertebrate taxa.