Vertebrates that have adapted to endure greater swings in seasonal temperatures tend to have greater elevational range sizes, the theory goes, but a new study analyzing more than 16,500 terrestrial vertebrates suggests this pattern is reversed in species adapted for high daily temperature fluctuations; instead, these species have smaller elevational range sizes, the study suggests. This finding -- which focuses on daily temperature changes over those that happen over a season -- could help scientists estimate how animals with different evolutionary histories will navigate climate change, a key issue in biology today. According to the Climatic Variability Hypothesis, species that experience greater seasonal climatic fluctuations are likely to be more physiologically flexible -- and thus likely to be found across wider ranges of elevations. Meanwhile, little attention has been paid to the effects of daily changes in temperature on species' elevation ranges and sizes. Therefore Wei-Ping Chan and colleagues used a database of 16,592 species of vertebrae at 180 different elevation points, inputting the data into a model with nine variables, including daily temperature variations. While their analysis confirms that adaption to greater seasonal temperature does correlate with greater elevational ranges, the opposite pattern was found in species adapted to higher fluctuations in daily temperatures; the latter were associated with smaller elevational distributions. Thus, the authors say, specialist species (who inhabit narrower ranges) are favored in settings where daily fluctuations are dominant, while generalists are favored where seasonal fluctuations are the main climate influence. These findings, which represent a novel hypothesis on the relationship between climatic variability and species range size, have implications for understanding biological responses to climate change -- and the forecast does not bode well for "thermal specialists" in the long-term, the authors say. In a related Perspective, Timothy M. Perez et al. discuss the challenges that thermal specialists may face, noting that projected, long-term increases in seasonal temperature variability, coupled with decreases in daily temperature variability due to faster nighttime versus daytime warming, will select against these species.