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Comparative cold-hardiness capacities of South American lizards in the genus Liolaemus
The ability to survive freezing temperatures is of paramount importance to organisms living at high elevations. Many ectothermic vertebrates have traits that allow them to cope with extreme cold, such as the ability to supercool and/or tolerate freezing of extracellular fluids. I studied several aspects of cold-hardiness of six lizard species in the genus Liolaemus (Iguania: Liolaemidae) living along a 2525-m elevation gradient (1550-4075 m) in northwestern Argentina. Specifically, I tested for supercooling, freeze tolerance, and the temperature of ice inoculation. I hypothesized that high-elevation (>3000 m) species would have greater capacities to supercool and tolerate freezing, while being more susceptible to ice inoculation, relative to their low-elevation congeners. Both conventional statistical analyses and those using independent contrasts with phylogenetic regressions revealed no correlation between supercooling capacity and elevation. Because of this, and the fact that all six species were able to supercool, this ability may be the ancestral condition for the lineage. This may have enabled independent invasions of cooler climates by members of this diverse genus. However, independent adaptation appears to explain freeze tolerance, as larger individuals were more susceptible to ice formation and high-elevation lizards survived longer during freezing events than their low-elevation congeners. Body size may partially explain freeze-tolerance capacity, as larger lizards have greater thermal inertia, which allows them to retain heat longer. Also, large-bodied Liolaemus generally occur at higher elevations. The cold-hardiness capacities characterized in this study also were correlated with seasonal low temperatures experienced in each environment. These results may explain the success of Liolaemus, which has numerous independent origins of species living in cold climates. Future studies should consider cold hardiness in a more holistic context of the biology of these lizards (alongside behavior, morphology, and biochemistry) and include comparisons among more species, including those from high latitudes, to further test cold hardiness as an adaptation to cold environments.