Increased global temperatures have opened previously inhospitable habitats, such as at higher elevations. However, the reduction of oxygen partial pressure with increase in elevation represents an important physiological constraint that may limit colonization of such habitats, even if the thermal niche is appropriate. To test the mechanisms underlying the response to ecologically relevant levels of hypoxia, we performed a translocation experiment with the common wall lizard (Podarcis muralis), a widespread European lizard amenable to establishing populations outside its natural range. We investigated the impacts of hypoxia on the oxygen physiology and reproductive output of gravid common wall lizards and the subsequent development and morphology of their offspring. Lowland females transplanted to high elevations increased their haematocrit and haemoglobin concentration within days and maintained routine metabolism compared with lizards kept at native elevations. However, transplanted lizards suffered from increased reactive oxygen metabolite production near the oviposition date, suggesting a cost of reproduction at high elevation. Transplanted females and females native to different elevations did not differ in reproductive output (clutch size, egg mass, relative clutch mass or embryonic stage at oviposition) or in post-oviposition body condition. Developing embryos reduced heart rates and prolonged incubation times at high elevations within the native range and at extreme high elevations beyond the current range, but this reduced oxygen availability did not affect metabolic rate, hatching success or hatchling size. These results suggest that this opportunistic colonizer is capable of successfully responding to novel environmental constraints in these important life-history stages.

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.

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Ectothermic animals living at high elevation often face interacting challenges, including temperature extremes, intense radiation, and hypoxia. While high-elevation specialists have developed strategies to withstand these constraints, the factors preventing downslope migration are not always well understood. As mean temperatures continue to rise and climate patterns become more extreme, such translocation may be a viable conservation strategy for some populations or species, yet the effects of novel conditions, such as relative hyperoxia, have not been well characterised. Our study examines the effect of downslope translocation on ectothermic thermal physiology and performance in Pyrenean rock lizards (Iberolacerta bonnali) from high elevation (2254 m above sea level). Specifically, we tested whether models of organismal performance developed from low-elevation species facing oxygen restriction (e.g., hierarchical mechanisms of thermal limitation hypothesis) can be applied to the opposite scenario, when high-elevation organisms face hyperoxia. Lizards were split into two treatment groups: one group was maintained at a high elevation (2877 m ASL) and the other group was transplanted to low elevation (432 m ASL). In support of hyperoxia representing a constraint, we found that lizards transplanted to the novel oxygen environment of low elevation exhibited decreased thermal preferences and that the thermal performance curve for sprint speed shifted, resulting in lower performance at high body temperatures. While the effects of hypoxia on thermal physiology are well-explored, few studies have examined the effects of hyperoxia in an ecological context. Our study suggests that high-elevation specialists may be hindered in such novel oxygen environments and thus constrained in their capacity for downslope migration.