Heat and water loss versus shelter: a dilemma in thermoregulatory decision making for a retreat-dwelling nocturnal gecko

Ecological responses of squamate reptiles to nocturnal warming

Nocturnal temperatures are increasing at a pace exceeding diurnal temperatures in most parts of the world. The role of warmer nocturnal temperatures in animal ecology has received scant attention and most studies focus on diurnal or daily descriptors of thermal environments' temporal trends. Yet, available evidence from plant and insect studies suggests that organisms can exhibit contrasting physiological responses to diurnal and nocturnal warming. Limiting studies to diurnal trends can thus result in incomplete and misleading interpretations of the ability of species to cope with global warming. Although they are expected to be impacted by warmer nocturnal temperatures, insufficient data are available regarding the night-time ecology of vertebrate ectotherms. Here, we illustrate the complex effects of nocturnal warming on squamate reptiles, a keystone group of vertebrate ectotherms. Our review includes discussion of diurnal and nocturnal ectotherms, but we mainly focus on diurnal species for which nocturnal warming affects a period dedicated to physiological recovery, and thus may perturb activity patterns and energy balance. We first summarise the physical consequences of nocturnal warming on habitats used by squamate reptiles. Second, we describe how such changes can alter the energy balance of diurnal species. We illustrate this with empirical data from the asp viper (Vipera aspis) and common wall lizard (Podarcis muralis), two diurnal species found throughout western Europe. Third, we make use of a mechanistic approach based on an energy-balance model to draw general conclusions about the effects of nocturnal temperatures. Fourth, we examine how warmer nights may affect squamates over their lifetime, with potential consequences on individual fitness and population dynamics. We review quantitative evidence for such lifetime effects using recent data derived from a range of studies on the European common lizard (Zootoca vivipara). Finally, we consider the broader eco-evolutionary ramifications of nocturnal warming and highlight several research questions that require future attention. Our work emphasises the importance of considering the joint influence of diurnal and nocturnal warming on the responses of vertebrate ectotherms to climate warming.

Cool shade and not-so-cool shade: How habitat loss may accelerate thermal stress under current and future climate

Worldwide habitat loss, land-use changes, and climate change threaten biodiversity, and we urgently need models that predict the combined impacts of these threats on organisms. Current models, however, overlook microhabitat diversity within landscapes and so do not accurately inform conservation efforts, particularly for ectotherms. Here, we built and field-parameterized a model to examine the effects of habitat loss and climate change on activity and microhabitat selection by a diurnal desert lizard. Our model predicted that lizards in rock-free areas would reduce summer activity levels (e.g. foraging, basking) and that future warming will gradually decrease summer activity in rocky areas, as even large rocks become thermally stressful. Warmer winters will enable more activity but will require bushes and small rocks as shade retreats. Hence, microhabitats that may seem unimportant today will become important under climate change. Modelling frameworks should consider the microhabitat requirements of organisms to improve conservation outcomes.

Heat-avoidance behavior associates with thermal sensitivity rather than tolerance in aphid assemblages

How to predict animals' heat-avoidance behaviors is critical since behavior stands the first line for animals dealing with frequent heat events under ongoing climate warming. However, the discrepancy between the scarcity of research on heat-avoidance behaviors and the commonness of eco-physiological data for thermal tolerance and for thermal sensitivity such as the temperature-dependent survival time makes it difficult to link physiological thermal traits to heat-avoidance behavior. Aphids usually suck plant sap on a fixed site on the host plants at moderate temperatures, but they will leave and seek cooler feeding sites under stressful temperatures. Here we take the cereal aphid assemblages comprising different species with various development stages as a model system. We tested the hypotheses that heat tolerance (critical thermal maximum, CT_max) or heat sensitivity (temperature-dependent declining rate of survival time, similarly hereinafter) would associate with the temperature at which aphid activate heat-avoidance behavior. Specifically, we hypothesized the aphids with less heat tolerance or greater heat sensitivity would take a lower heat risk by leaving the host plant earlier. By mimicking the linear increase in ambient temperature during the daytime, we measured the CT_max and the heat-avoidance temperature (HAT, at which aphids leave the host plant to find cooler places) to understand their heat tolerance and heat-avoidance behavior. Then, we tested the survival time of aphids at different temperatures and calculated the slope of survival time declining with temperature to assess their heat sensitivity (HS). Finally, we examined the relationships between CT_max and HAT and between HS and HAT to understand if the heat-avoidance behavior associates with heat tolerance or with heat sensitivity. The results showed that HS and HAT had a strong correlation, with more heat sensitive individuals displayed lower HAT. By contrast, CT_max and HAT had a weak correlation. Our results thus provide evidence that heat sensitivity is a more reliable indicator than thermal tolerance linking with the heat-avoidance behavior in the aphid assemblages. Most existing studies use the indexes related to thermal tolerance to predict warming impacts. Our findings highlight the urgency to incorporate thermal sensitivity when predicting animal responses to climate change.

Extreme tolerance for nocturnal emergence at low body temperatures in a high-latitude lizard: implications for future climate warming

High-latitude lizards live in environments where ambient air temperature at night is frequently below retreat temperatures, which likely has implications for nocturnal emergence and activity. However, patterns of lizard activity at night under current temperate climates are poorly understood, a situation that limits our understanding of potential effects of climate change. We investigated patterns of nocturnal emergence and activity in the cold-adapted, viviparous gecko (Woodworthia 'Otago/Southland'). We measured operative environmental temperature (T _e) available to geckos that emerged at night and simultaneously assessed nighttime emergence activity using time-lapse trail cameras. Also, we assessed field body temperature (T _b) of emerged geckos of various life history groups at night using thermography to understand how current weather conditions affect field T _b of emerged geckos. Our results show that T_e , nocturnal emergence activity and field-active T _b increased with nighttime air temperature. Nocturnal emergence was highest in spring and summer but also occurred in autumn and (unexpectedly) in winter. Geckos were active over a broad range of T _b down to 1.4°C (a new record low for lizards) and on rock surfaces typically warmer than air temperature or T _b. We conclude that this nocturnal, high-latitude lizard from the temperate zone is capable of activity at low winter temperatures, but that current climate limits emergence and activity at least in autumn and winter. Activity levels for cool-temperate reptiles will probably increase initially as climates warm, but the consequences of increased nocturnal activity under climate change will probably depend on how climate change affects predator populations as well as the focal species' biology.

A nocturnally foraging gecko of the high-latitude alpine zone: Extreme tolerance of cold nights, with cryptic basking by day

Lizards that inhabit high-latitude alpine zones are exposed to extreme temperatures and long winters and most are diurnal heliotherms. Yet some poorly known nocturnal species exist in such locations, including several viviparous geckos from New Zealand. We studied the orange-spotted gecko (Mokopirirakau 'Roy's Peak'), a cryptic, nocturnal and viviparous lizard known only from the alpine zone (1150-1800 m a.s.l.) in the South Island (~44°S). Our field study investigated (1) the influence of female reproductive condition and sex on daytime body temperatures, including relationships with microhabitat rock temperatures, (2) the influence of temperature and other weather conditions on gecko emergence by night and day, and (3) the thermal microclimates available year-round to orange-spotted geckos. Building a better understanding of these lizards aids in species conservation efforts, for example in developing monitoring programmes, and provides insights into the evolution of thermal mechanisms in cold environments. Reproductive females maintained higher daytime body temperatures than non-reproductive females and males, suggesting pregnancy-related thermophily. On summer days, all reproductive groups reached similar body temperatures to New Zealand geckos from lower elevations, suggesting similar thermal preferences. Using trail cameras, we obtained evidence of geckos openly basking during the day (previously undocumented for this species) when temperatures of exposed lizard models (=T_exp) were 3.2-39.3 °C. We also observed emergence at night at low T_exp (-0.8-14.6 °C), when some T_bs were probably 0-6 °C. Diurnal activity increased as T_exp rose to peak at ~30 °C before dropping again at higher temperatures, whereas nocturnal activity unexpectedly decreased with increasing T_exp. Our study provides evidence of diurnal activity in a 'nocturnal' gecko that may be essential to squamate viviparity at high-latitude, high-elevation sites. It also suggests remarkable capacity for locomotor activity at extremely low T_b.

Thermal heterogeneity of selected retreats in cool-temperate viviparous lizards suggests a potential benefit of future climate warming

Rocky retreats are limited and geologically constrained resources for rock-dwelling nocturnal lizards. Such lizards should seek retreats that offer thermoregulatory benefits without the risk of overheating during the day, and that protect from predation. For cold-adapted species where air temperature is frequently lower than optimum temperature for performance, factors influencing retreat-site selection and whether future warmer conditions will force superficial rock slabs to be abandoned on hot days remain poorly known. Here, we predicted that retreats selected by a nocturnally foraging, cool-temperate gecko from southern New Zealand would be thermally heterogeneous and that future warmer temperature will force lizards to abandon daytime retreats on hot days. We sampled loose rock slabs (potential retreats) in a tussock-grassland site in all seasons. We measured seasonal rock temperature profiles and field body temperature (T_b) of captured geckos using thermography and quantified the physical characteristics of each potential retreat. We found that both physical characteristics and rock temperatures determine choice of retreats. Field T_b of lizards positively correlated with retreat and air temperatures. Also, retreat temperatures, including those of the substrate below the rock slabs, showed complex heterogeneity enabling lizards to choose microsites within retreats to achieve preferred body temperatures intermittently. Observed seasonal shifts in characteristics of occupied rocks imply that lizards choose retreats to maximise warmth in spring, minimise risk of overheating (remain below voluntary thermal maximum, VT_max) in summer and avoid freezing over winter. Our study demonstrates the importance of microclimatic conditions in influencing retreat-site selection. Climate warming might lead to seasonal changes in use of rock slabs and possibly be beneficial initially, but longer-term implications need to be examined.