Climate-driven impacts on species vary widely in both direction and magnitude on a global scale (Williams and Jackson, 2007). For most ectothermic species, environmental changes can adversely affect their performance, as nearly all aspects of their life history are closely influenced by temperature conditions in their habitats (Angilletta et al., 2002; Ibargüengoytía et al., 2010; Kubisch et al., 2016; Zhu et al., 2025). One method for quantifying ectothermic physiological responses is to examine their thermal performance curves, which describe how body temperature affects their performance or fitness (Huey and Stevenson, 1979; Huey and Kingsolver, 1989; Kingsolver et al., 2013). In brief, an organism’s thermal performance increases gradually from a critical thermal minimum (CT_min) to an optimal temperature (T_opt) and then declines rapidly as body temperature approaches the critical thermal maximum (CT_max) (Angilletta et al., 2002). These thermal traits are traditionally measured in terrestrial ectotherms, revealing geographic patterns of local adaptation and responses to climate variations (Chown and Terblanche, 2007; Kingsolver et al., 2013). For instance, species from higher latitudes typically exhibit greater thermal breadth (the difference between CT_max and CT_min) and lower CT_min values compared to tropical species (Clusella-Trullas et al., 2011; Kingsolver et al., 2013; Sunday et al., 2011).

Several studies have integrated physiological thermal tolerance metrics with environmental data, using correaltive or mechanistic modelling approaches, to predict population declines and local extinctions of ectotherms across latitudinal and elevational gradients (Chen et al., 2011; García-Robledo et al., 2016; Sinervo et al., 2010). Based on these approaches, tropical ectothermic species are often considered at greater risk of extinction than their temperate counterparts because they tend to operate close to their physiological thermal optima and are assumed to have limited physiological and behavioural plasticity (Deutsch et al., 2008; Huey et al., 2009; Morley et al., 2019; Polato et al., 2018). Despite their widespread use, these approaches can yield inaccurate or incomplete predictions, particularly in tropical systems, for three main reasons. First, they commonly rely on ambient air temperature as a proxy for the organismal habitat temperature, overlooking the fact that ectotherms can experience substantially different thermal conditions within their microhabitats (Pincebourde and Suppo, 2016; Potter et al., 2013). Second, tropical environments, are characterised by strong spatial and temporal thermal heterogeneity at fine scales, which can directly influence the thermal physiology and thermoregulatory opportunities of small ectotherms (Pincebourde and Suppo, 2016). Third, much of the empirical support underlying predictions of tropical ectotherm vulnerability, especially in lizards, comes from low- and mid-altitude tropical ecosystems, such as tropical rainforests, resulting in a strong elevational bias (Brusch et al., 2016; Huey et al., 2009; Muñoz et al., 2014, 2022). Consequently, the thermal responses of high-altitude tropical mountain species remain poorly understood, limiting our ability to accurately predict their vulnerability to climate change.

The rapid uplift of the Andes created complex and diverse ecosystems with distinct climate conditions that vary by altitude (Esquerré et al., 2019). High-elevation tropical Andean ecosystems, for example, exhibit steep altitudinal gradients with marked macroclimate variations, which interact with abiotic and biotic factors to create a mosaic of thermal microclimates (Arroyo and Cavieres, 2013; Woods et al., 2015). When first formed, these heterogeneous and fragmented Andean ecosystems provided new habitats for some groups to explosively diversify (Drummond et al., 2012; Hutter et al., 2013; Losos, 2009). This is true for Stenocercus lizards, which inhabit various South American landscapes, including dry and humid tropical forests, montane forests, and paramo, at elevations between 0 and 4000 m asl (Torres-Carvajal, 2000).

Two studies have examined aspects of the thermal biology of high-altitude tropical montane Stenocercus species. Andrango et al. (2016) conducted the first study to model local extinction risks for the Ecuadorian tropical Andean species S. guentheri. By integrating temperatures recorded with physical models placed in various microhabitats (i.e. operative temperature), the species’ preferred body temperature, and air temperatures from weather stations at georeferenced points within the species’ distribution, they predicted that 26.7% of this species’ populations will be extinct by 2050. In the only other study, Guerra-Correa et al. (2020) reported that tropical Andean Stenocercus lizards exploit diverse thermal microenvironments, allowing them shelter and remain active without experiencing thermal stress or reduced performance. Their findings emphasise the importance of microhabitat structure for these species and highlighted the need to investigate additional thermal adaptations, many of which remain poorly understood.

In this study, we aim to examine how vegetation structure and composition combined with local environmental conditions influence key thermophysiological traits of multiple Stenocercus populations from the Ecuadorian Andes, and how these interactions translate into differences in thermal vulnerability. We hypothesise that the interplay between pronounced daily temperature fluctuations, characteristic of high-altitude tropical montane ecosystems, and local vegetation assemblages plays a central role in shaping these lizards’ thermal preferences and tolerances. Specifically, we predict that lizards inhabiting areas with greater daily temperature variability and more complex vegetation structure will exhibit broader thermal tolerance ranges and more flexible thermal preferences than those in areas with lower temperature variability and less vegetative landscapes. This assumption is grounded in the climatic variability hypothesis and theories of phenotypic plasticity, which states that ectotherms inhabiting environments with large diurnal temperature fluctuations and structurally complex microhabitats promote broader thermal physiological responses, especially when coupled with behavioural thermoregulation (Janzen, 1967; Angilletta, 2009; Muñoz and Bodensteiner, 2019). To test these hypotheses, we assessed three thermophysiological traits including the preferred body temperature (T_pref), critical thermal minimum (CT_min) and maximum (CT_max) across six Stenocercus species inhabiting three high-altitude ecosystems. These physiological data were then integrated with climate records and spatially distributed operative temperature models to quantify thermal vulnerability using established indices, including thermal safety margins (TSM) and warming tolerance (WT) under present and future warming scenarios.

Across the studied tropical Andean ecosystems, pronounced differences in daily temperature variability and microhabitat thermal conditions were closely associated with variation in lizard thermal preferences, tolerances, and vulnerability, supporting our hypothesis that local climate and vegetation interactions shape thermophysiological traits of high-altitude Stenocercus populations. For instance, lizards inhabiting ecosystems characterised by high diurnal temperature conditions, such as Inter-Andean ecosystems, exhibit the highest CT_max and T_pref values. In contrast, paramo lizards show the lowest CT_min values due to the cold conditions they are exposed in their microenvironments. Furthermore, microhabitat analyses revealed that structurally complex vegetation in tropical montane ecosystems created diverse thermal mosaics, allowing lizards to behaviourally thermoregulate by exploiting sun-exposed or shaded areas. Despite this buffering capacity, thermal safety margins and warming tolerance varied markedly among populations. Inter-Andean populations were the most vulnerable to warming due to frequent exposure to temperatures near or above their critical thermal maximum, whereas paramo populations maintained positive safety margins and warming tolerance, indicating greater resilience to overheating. Overall, these results demonstrate that daily temperature variability and vegetation-driven microclimatic heterogeneity jointly influence thermal flexibility and vulnerability in high-altitude tropical montane lizards.

Various physical mechanisms in the Tropical Andes, such as the El Niño-Southern Oscillation and the presence of trade winds to the north, drive temporal variability that shapes the region’s climate (Martínez et al., 2011; Young et al., 2007). This climatic variability, combined with the complex topography of the Andes, has created a diverse array of climatic niches, promoting physiological specialisation in response to differing thermal and rainfall regimes (Salazar et al., 2024; Sheldon, 2019). For example, macroclimate conditions in high-altitude tropical Andean ecosystems show marked daily temperature fluctuations (Figure 4), which we propose directly influence the thermal exposure of Stenocercus lizards at the microhabitat scale. Additionally, the uplift of the Andes led to the formation of isolated land areas, each hosting distinct ecosystems with representative vegetation assemblages.

One such ecosystem is the inter-Andean valleys, where precipitation is limited due to the rain shadow effect caused by the surrounding high cordilleras (Neill, 1999). These arid valleys are dominated by xerophytic plant species from the families Cactaceae and Agavaceae, along with small shrubs and trees from the Euphorbiaceae and Fabaceae families (Neill, 1999; Werner, 2009). This distinctive vegetation composition results in open microhabitats with reduce shaded areas, thereby exposing lizards to intense solar radiation and elevated temperatures, especially during midday hours. Consequently, S. guentheri and S. chota populations inhabiting inter-Andean ecosystems exhibit higher T_pref and CT_max values compared to those living in evergreen and paramo ecosystems.

In contrast, evergreen ecosystems, now reduced to highly localised and increasingly fragmented remnants, are under severe threat from burning, grazing, and the relentless expansion of the agricultural frontier (Ministerio del Ambiente del Ecuador, 2013). Despite their vulnerability, these ecosystems provide a markedly different thermal environment for lizards. Characterised by a low and open canopy, evergreen ecosystems support a rich assemblage of epiphytes, particularly from the Orchidaceae and Bromeliaceae families (Ministerio del Ambiente del Ecuador, 2013). This complex vertical structure contributes to a cooler and more thermally buffered microclimate, offering lizards access to shaded refuges and reduced exposure to extreme heat. As a result, lizard populations in these ecosystems (S. ornatus and S. festae) tended to exhibit lower thermal preferences and heat tolerances than those in the more exposed inter-Andean valleys.

Finally, paramo ecosystems, situated at even higher elevations, present a unique suite of environmental conditions. These ecosystems are shaped by intense daily temperature fluctuations, strong winds, high ultraviolet radiation, frequent precipitation, and low atmospheric pressure (Ruiz et al., 2011; Young et al., 2011; Cuesta et al., 2012; CEPF, 2021). The vegetation in these landscapes consists of dense shrub-herbaceous assemblages dominated by sclerophyllous shrubs such as Chuquiraga and Diplostephium, cushion-forming plants like Xenophyllum and Azorella, and low-growing grasses of the genus Calamagrostis (Cuesta et al., 2012; Romoleroux et al., 2023). These plant forms create microhabitats that are both thermally challenging and spatially heterogeneous. Lizards inhabiting paramo ecosystems (S. guentheri, S. angel, and S. cadlei) are therefore adapted to withstand cold temperatures and tend to prefer lower body temperatures than their counterparts in drier, sun-exposed inter-Andean environments. Additionally, the unique structural and climatic features of paramo ecosystems contribute to shaping distinct physiological tolerances, highlighting the critical role of habitat-specific environmental pressures in driving thermal adaptation in high-altitude tropical Andean Stenocercus lizards.

According to the thermal adaptation hypothesis, the minimum and maximum temperatures of an organism’s microenvironment should align with its thermal limits, thereby minimising the physiological costs associated with maintaining thermal tolerances (Angilleta, 2009; Janzen, 1967; Kaspari et al., 2015). This framework may help explain the observed variation in thermophysiological traits among high-altitude tropical montane Stenocercus populations, which differ according to the ecosystem types they inhabit. However, considering the relatively recent diversification of this lizard group in the northern Andes (Torres-Carvajal, 2007b), further research is needed to determine whether their physiological traits result from short-term phenotypic plasticity, longer-term thermal acclimation, or genetic adaptation, as documented in other taxa (Bujan et al., 2020; Martin et al., 2019; Sørensen et al., 2016). The isolated land formations created by the uplift of the Andes may have further promoted the divergence of thermophysiological traits within this lizard clade by facilitating climate niche specialisation at the microhabitat scale (Dangles, 2023; Flantua and Hooghiemstra, 2018; Hazzi et al., 2018). Yet, this degree of specialisation may come at a cost, increasing these lizards’ vulnerability to ongoing climate warming. The extent to which Stenocercus lizards can buffer against this threat likely depends on the availability of suitable microhabitats that reduce overheating risk, as well as behavioural strategies that allow them to effectively regulate their body temperature.

Daylight ambient temperatures in tropical montane microhabitats begin to rise around 0800 hours and decreased by 1700 hours, defining the activity period of Stenocercus lizards. However, microenvironmental temperature conditions within and beyond this activity window vary across tropical Andean ecosystems, exposing each lizard population to differing levels of vulnerability (Pincebourde and Suppo, 2016). Simple vulnerability indices, such as thermal safety margins (TSM) and warming tolerance (WT), are widely used to evaluate species vulnerability to rising temperatures (Deutsch et al., 2008). These indices quantify how close a species’ thermal limits and optimal body temperatures are to projected warming scenarios, offering insights into their capacity to cope with rising temperatures (Clusella-Trullas et al., 2021). This framework is based on the premise that temperature sensitivity of physiological performance plays a critical role in determining species distributions and survival (Sunday et al., 2012). Consequently, these indices serve as valuable tools for comparing vulnerability across different species or populations.

Although S. chota and S. guentheri inhabit inter-Andean ecosystems, their microhabitats presented contrasting thermal characteristics, leading to opposing thermal vulnerability outcomes. The S. guentheri population from Jerusalem RPPF experienced the most severe thermal stress of all evaluated populations. On the one hand, they suffered from cold physiological stress during some hours of nighttime, as their CT_min is higher than the lowest temperatures available in their microhabitats. On the other hand, daytime temperatures in their microhabitats exceeded their average CT_max, being greater at midday when temperatures are more than 10°C higher compared to the average CT_max. As a result, they exhibited negative thermal safety margins and minimal warming tolerance, which may increase mortality rates and decrease population growth (Kearney, 2013; Munch and Salinas, 2009). Conversely, S. chota showed no evidence of thermal stress from either cold or heat based on the thermal conditions in their microhabitats. However, their narrow thermal safety margin indicates that they inhabit environments near their physiological optimum (Deutsch et al., 2008). Therefore, warming is likely to cause these lizards to retreat to cool refuges limiting their activity and constraining other functions such as foraging and reproduction (Sinervo et al., 2010).

Lizard species inhabiting evergreen ecosystems, such as S. festae and S. ornatus, exhibited average CT_min values below the minimum temperatures of their microhabitats. However, midday temperatures in their micro-environments approached or exceeded their average CT_max. Despite this, these populations maintained favourable thermal safety margins, suggesting positive physiological responses to cope with high temperatures in their microhabitats. Interestingly, S. festae exhibited negative warming tolerance under current temperature conditions and across the two projected warming scenarios, suggesting an inability to tolerate even slightly higher temperatures than those currently experienced (Anderson et al., 2022). Conversely, S. ornatus showed no physiological limitations under any of the three assessed warming scenarios, indicating a greater capacity to cope with rising temperatures. Although evergreen ecosystems typically offer suitable thermal microhabitats that lizards to avoid overheating, ongoing habitat fragmentation is diminishing these refuges and increasing exposure to high temperatures. This phenomenon is evident at El Gullan SS, where large portions of the surrounding landscape have been converted into pine plantations, grazing areas, and agricultural lands. Such extensive alteration is suggested to intensify thermal stress for S. festae populations. Therefore, urgent action is needed to halt the continued degradation of these ecosystems and to preserve the thermal habitats essential for the survival of this species.

Stenocercus species from paramo ecosystems, including S. angel, S. guentheri, and S. cadlei, inhabit the coldest environments among all analysed populations. The extremely low nighttime temperature in their microhabitats fall below their average CT_min for substantial periods, increasing their dependence on thermal refuges to avoid extreme cold and enhancing their cold tolerance (Pintanel et al., 2019; Salazar et al., 2024). In contrast, the maximum temperatures in their microhabitats remained below their average CT_max, suggesting they are unlikely to experience physiological stress from heat exposure. Additionally, thermal safety margins indicate that these lizards possess sufficient physiological resilience to avoid overheating. This is further supported by warming tolerance calculations, showing that even under the highest projected warming scenario of +2°C, paramo lizards will not be physiologically constrained. These findings align with other studies, which suggest that lizards inhabiting thermally variable environments exhibit greater physiological tolerance to warming, making them less vulnerable to rising temperatures compared to populations in more thermally stable conditions (Lara-Resendiz et al., 2019; Yuan et al., 2018).

In the context of climate change, terrestrial ectotherms must rapidly respond to novel thermal environments without prolonged exposure to suboptimal temperatures that could impair their performance. By either adapting physiologically, adjusting their behaviour or employing both strategies, organisms can mitigate potential impacts of climate change on their populations (Angilletta et al., 2002; Muñoz et al., 2014). Behavioural thermoregulation is an effective short-term strategy to minimise exposure to extreme climate conditions (Angilletta et al., 2002). Additionally, studies on tropical anole species suggest that behavioural thermoregulation can shield organisms from selective pressures on upper physiological thermal limits, while cold tolerance remains under stronger selection (Muñoz et al., 2014). Thus, examining how organisms interact with the available microthermal habitats can help elucidate patterns of physiological divergence in tropical montane species as well as support predictions of vulnerability to climate change (Angilleta, 2009; Huey et al., 2003).

In conclusion, the microscale environmental conditions experienced by high-altitude tropical Andean Stenocercus populations have shaped specialised thermophysiological profiles tailored to the thermal variability in their habitats. However, thermophysiological responses vary among populations depending on their exposure to environmental changes. Particular attention should be given to populations such as S. guentheri (inter-Andean) and S. festae, which face challenges due to either reduced performance under high temperature conditions in their habitats or limited physiological tolerances to warming. For these populations, thermoregulatory behaviours may be the primary mechanism to cope with rising temperatures, given the constraints ectotherm tropical Andean species face in shifting their elevational ranges (Forero-Medina et al., 2010). Long-term monitoring of these populations is important for understanding their adaptative strategies in behaviour and physiology, as well as the implications for their ecological niches. Moreover, such monitoring could inform conservation policies that advocate for the protection of high-elevation climate refugia. While it is evident that global warming poses significant risks to populations like S. festae and S. guentheri, other Stenocercus populations, such as those inhabiting paramo ecosystems may not be as adversely affected by rising temperatures as reported for other cold-adapted lizard species (Doan et al., 2022). However, further research into the mechanisms of cold tolerance in these lizards is essential to better understand how these traits might evolve or constraint adaptation to warming scenarios. Finally, exploring the adaptive significance of thermal developmental phenotypic plasticity (an area currently unexplored in Stenocercus lizards) could enhance predictions of vulnerability in the face of climate change. This mechanism, which has been suggested to have organisational effects that potentially constrain phenotypic development later in life (While et al., 2018), may hold key insights into the resilience of tropical Andean species in a warming world.