Anoles on the Edge: Adapting to the Andes

The Andes Mountains, famous for their extreme landscapes and rich biodiversity, have long fascinated scientists studying how species adapt and diversify. A new study, published as part of the Special Issue on Adaptive Radiation from the Evolutionary Journal of the Linnean Society, explores how anole lizards, living in tropical lowlands to chilly mountaintops, have evolved specialised thermal tolerances to survive across elevations. Understanding these physiological adaptations is crucial—not just for evolutionary biology, but for predicting species’ fates as climates change. This research shines a spotlight on how temperature, a subtle yet powerful force, shapes life in one of Earth’s most complex environments.

The Andes are one of the most biodiverse regions on Earth, in part due to their topography spanning from sea level to 6,000 m+. This steep elevation gradient creates a vast range of temperatures and habitats, driving speciation and adaptive radiation. Scientists have long studied species morphological adaptations, yet less is known about how their physiological traits, like heat and cold tolerances, evolved.

According to Janzen’s hypothesis, tropical species are more specialised to narrow temperature ranges than temperate species. This is because in the tropics the temperature is stable year-round, so organisms evolve to tolerate only a narrow range. As a result, small environmental changes (such as changes in altitude) can shift significant barriers to movement and gene flow. Based on this, researchers predicted that high-elevation species would be more cold-tolerant and less heat-tolerant than lowland species. Tolerance thresholds are the critical thermal minimum (CTmin) and maximum (CTmax) temperatures at which an animal loses locomotor function.

Salazar et al. measured CTmin and CTmax of 14 species of Andean anoles from both clades and found that thermal tolerance varies with elevation. As elevation increased, cold tolerance improved and heat tolerance decreased, supporting the idea of local adaptation to thermal environments. There was no difference between sexes or age of the lizards.

Both Dactyloa and Draconura showed strikingly similar patterns, and this parallel evolution across two independently radiating lineages suggests that strong environmental pressures have shaped thermal physiology similarly, despite different evolutionary histories.

The study used phylogenetic analysis and found that the rate of evolution for heat and cold tolerance was similar, supporting the idea that both limits specialised together across the thermal gradients of the Andes. Unlike some previous studies in ectotherms, where heat tolerance stayed relatively stable across environments, here both cold and heat tolerance shifted significantly with elevation.

Importantly, thermal tolerance breadths—the range between CTmin and CTmax—remained stable across elevation, indicating a fine-tuned balance rather than a broadening of tolerances, supporting Janzen’s hypothesis. Species weren’t generalists surviving a wide range of temperatures, but specialists fine-tuned to their specific environments.

This pattern of thermal specialisation helps explain why the Andes are such a hotspot for biodiversity. Physiological specialisation likely promotes endemism—to be endemic, species are only found in this environment and unable to easily shift to different conditions. An example of this can be seen in New Zealand, where lack of predators and specialised habitats have led to the evolution of the Kākāpō (Strigops habroptilus), a large, flightless parrot, found nowhere else in the world. As a result, even closely related species can diverge rapidly when separated by elevation.

However, this specialisation could be a double-edged sword. Species finely adapted to narrow thermal environments may be more vulnerable to climate change. As temperatures shift, these specialists may struggle to move or adapt quickly enough, highlighting the urgency of studying physiological traits in conservation planning. Understanding physiological limits is therefore critical for predicting climate change impacts.

While this study sheds important light on how Andean anoles adapt to different elevations, it also highlights gaps that need filling. Small sample sizes, limited genetic data and the use of broad-scale climate models rather than fine-scale microclimate data may have influenced the results. Future research needs to dive deeper into local factors like behaviour, habitat use and plasticity to fully understand how these lizards cope with a changing world.

It’s clear that temperature—often an invisible environmental factor—plays a profound role in driving evolutionary change. The parallel evolution of thermal tolerance across two anole clades in the Andes highlights how physiological traits shape biodiversity and reveals important clues for understanding how species might respond to future climate change.

The critical question remains: will species be able to keep pace with the rapid rate at which climate change is affecting their natural environments?