Summary 1.In reptiles, growth is subject to proximate environmental influences, such as food availability and temperature, that may be crucial during the early stages of postnatal development. Mediterranean regions, with their severe summer drought, offer an excellent opportunity to examine the effects of environmental variations in precipitation and productivity on the timing of reproduction and growth rates of lizards. 2. In this study, we compared the incubation time, size at hatching, growth rates and changes in juvenile body condition of two nearby populations of the lizard Psammodromus algirus separated by 600-700 m altitude in central Spain. We combined a reciprocal incubation experiment at 27 and 30 ° C with a reciprocal transplant experiment to distinguish between environmental and population-specific sources of geographical variation. 3. At both temperatures, eggs from the high-elevation site hatched sooner. Several important phenotypic traits of juveniles were primarily determined by the growing environment: the high-elevation site provided more food and allowed hatchlings to grow faster and reach larger size. 4. Environmental effects overrode familial ones, as shown by the larger growth rate of half-sibs released at the high-elevation site. However, both the size and the growth rate of juveniles were also influenced by their mother's site of origin, which means that population differences may reflect a genetic differentiation and/or different maternal effects. 5. Low precipitation and associated food scarcity were seemingly the main factors constraining the growth rates of juveniles at the lowland site.

Research addressing the effects of global warming on the distribution and persistence of species generally assumes that population variation in thermal tolerance is spatially constant or overridden by interspecific variation. Typically, this rationale is implicit in sourcing one critical thermal maximum (CTmax) population estimate per species to model spatiotemporal cross‐taxa variation in heat tolerance. Theory suggests that such an approach could result in biased or imprecise estimates and forecasts of impact from climate warming, but limited empirical evidence in support of those expectations exists. We experimentally quantify the magnitude of intraspecific variation in CTmax among lizard populations, and the extent to which incorporating such variability can alter estimates of climate impact through a biophysical model. To do so, we measured CTmax from 59 populations of 15 Iberian lizard species (304 individuals). The overall median CTmax across all individuals from all species was 42.8°C and ranged from 40.5 to 48.3°C, with species medians decreasing through xeric, climate‐generalist and mesic taxa. We found strong statistical support for intraspecific differentiation in CTmax by up to a median of 3°C among populations. We show that annual restricted activity (operative temperature > CTmax) over the Iberian distribution of our study species differs by a median of >80 hr per 25‐km2 grid cell based on different population‐level CTmax estimates. This discrepancy leads to predictions of spatial variation in annual restricted activity to change by more than 20 days for six of the study species. Considering that during restriction periods, reptiles should be unable to feed and reproduce, current projections of climate‐change impacts on the fitness of ectotherm fauna could be under‐ or over‐estimated depending on which population is chosen to represent the physiological spectra of the species in question. Mapping heat tolerance over the full geographical ranges of single species is thus critical to address cross‐taxa patterns and drivers of heat tolerance in a biologically comprehensive way.

Aim In an effort to disentangle the ecological processes that confine ectotherms to alpine environments, we studied the thermoregulatory and microhabitat selection behaviours of the rock lizard Iberolacerta cyreni, which is endemic to some mountains of central Spain, and of the wall lizard Podarcis muralis, which is a potential competitor of rock lizards.Location We chose three areas in the Sierra de Guadarrama (central Spain) that differed in their thermal quality [mean deviation of environmental operative temperatures from the lizards' preferred thermal range (PTR)] and refuge availability: a pine forest (1770 m a.s.l.) in which P. muralis was the only species found, and two mixed shrub and rock sites (1770 and 1900 m a.s.l.) where both species were present.Methods In the field we collected data on refuge availability, sun exposure, body temperature (T b ) and operative temperature (T e ). Thus, we estimated the thermal habitat quality of the areas sampled and the thermoregulation accuracy and effectiveness of both species. ResultsThe pine forest had the lowest thermal quality and refuge availability. The lower-elevation shrub site offered the best thermal quality, but refuges were much scarcer than at the higher-elevation site. Both species thermoregulated accurately, because mean deviations of body temperature (T b ) from PTR were considerably smaller than those of T e . Podarcis muralis had higher T b values than did I. cyreni, which had similar T b values at both shrub sites, whereas P. muralis had lower T b values at higher elevation. Overall, the thermoregulatory effectiveness (extent to which T b values are closer to the PTR than are T e values) of both species was similar, but whereas I. cyreni thermoregulated more efficiently at higher elevation, the opposite was true for P. muralis. At the lower-elevation shrub site, I. cyreni remained closer to refuges than did P. muralis. Main conclusionsOur results suggest that the pine forest belt might prevent the expansion of rock lizards towards lower elevations as a result of its low thermal quality and scarcity of refuges, that the thermoregulatory effectiveness of rock lizards in alpine environments depends more on refuge availability than on thermal habitat quality, and that competition with wall lizards is unlikely to explain either the distribution or the thermoregulatory effectiveness of rock lizards.

Local adaptation and range restrictions in alpine environments are central topics in biogeographic research with important implications for predicting impacts of global climate change on organisms. Temperature is strongly coupled to elevation and greatly affects life history traits of oviparous reptiles in mountain environments. Thus, species may encounter barriers for expanding their ranges if they are unable to adapt to the changing thermal conditions encountered along elevational gradients. We sought to determine whether thermal requirements for embryonic development provide a plausible explanation for elevational range limits of two species of lacertid lizards that have complementary elevational ranges in a Mediterranean mountain range (Psammodromus algirus is found at elevations below 1,600 m and Iberolacerta cyreni is found at elevations above 1,600 m). We combined experimental incubation of eggs in the laboratory with modelled estimates of nest temperature in the field. In both species, increasing temperature accelerated development and produced earlier hatching dates. The species associated with warmer environments (P. algirus) experienced an excessive hatching delay under the lowest incubation temperature. Moreover, newborns from eggs incubated at low temperatures showed poor body condition and very slow rates of postnatal growth. In contrast, eggs of the strictly alpine species I. cyreni exhibited shorter incubation periods than P. algirus that allowed hatching before the end of the active season even under low incubation temperatures. This was countered by lower reproductive success at higher temperatures, due to lower hatching rates and higher incidence of abnormal phenotypes. Elevational range limits of both species coincided well with threshold temperatures for deleterious effects on embryonic development. We suggest that incubation temperature is a major ecophysiological factor determining the elevational range limits of these oviparous lizards with predictable consequences for mountain distributions under future warmer climates.

Intraspecific variation in morphology has often been related to fitness differences through its effects on performance. In lizards, variation in hind limb length can be shaped by natural selection for increased locomotor performance, sexual selection on the number or size of femoral pores involved in chemical signalling, or both. Here, we analyse the selective forces involved in sexual dimorphism and differences in hind limb length between two populations of Psammodromus algirus living at different elevation. Males were more robust and had longer hind limbs and limb segments than females, and low-elevation lizards had longer limbs than high-elevation lizards. However, differences in locomotor performance were small and non-significant, making natural selection for faster runs an unlikely explanation for the observed pattern. On the other hand, males had more femoral pores than females, and lizards had more pores at lower elevation, although the difference was significant only for males (which invest more in chemical signalling). In males, the number of pores, which remains constant along a lizard's life, was not correlated with hind limb length. However, femur length was positively correlated with mean pore size, allowing low-elevation males to have larger than expected pores, which could increase the effectiveness with which they spread their signals in a dry and warm habitat where chemicals become volatile rapidly. Also, saturation of the sexual coloration of the head was higher for low-elevation males, suggesting that sexual selection pressures may be more intense. Overall, our results indicate that sexual selection plays a significant role in shaping intraspecific variation in hind limb length.

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The widespread observation that heat tolerance is less variable than cold tolerance (‘cold‐tolerance asymmetry’) leads to the prediction that species exposed to temperatures near their thermal maxima should have reduced evolutionary potential for adapting to climate warming. However, the prediction is largely supported by species‐level global studies based on single estimates of both physiological metrics per taxon. We ask whether cold‐tolerance asymmetry holds for Iberian lizards after accounting for intraspecific variation in critical thermal maxima (CTmax) and minima (CTmin). To do so, we quantified CTmax and CTmin for 58 populations of 15 Iberian lizard species (299 individuals). Then, we randomly selected one population from each study species (population sample = 15 CTmax and CTmin values), tested for differences between the variance of both thermal metrics across species, and repeated the test for thousands of population samples as if we had undertaken the same study thousands of times, each time sampling one different population per species (as implemented in global studies). The ratio of variances in CTmax to CTmin across species varied up to 16‐fold depending on the populations chosen. Variance ratios show how much CTmax departs from the cross‐species mean compared to CTmin, with a unitary ratio indicating equal variance of both thermal limits. Sampling one population per species was six times more likely to result in the observation of greater CTmax variance (‘heat‐tolerance asymmetry’) than cold‐tolerance asymmetry. The probability of obtaining the data (given the null hypothesis of equal variance being true) was twice as likely for cases of cold‐tolerance asymmetry than for the opposite scenario. Range‐wide, population‐level studies that quantify heat and cold tolerance of individual species are urgently needed to ascertain the global prevalence of cold‐tolerance asymmetry. While broad latitudinal clines of cold tolerance have been strongly supported, heat tolerance might respond to smaller‐scale climatic and habitat factors hence go unnoticed in global studies. Studies investigating physiological responses to climate change should incorporate the extent to which thermal traits are characteristic of individuals, populations and/or species. A free Plain Language Summary can be found within the Supporting Information of this article.