Data from: Heat tolerance is more variable than cold tolerance across species of Iberian lizards after controlling for intraspecific variation

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 if
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 variance homoscedasticity 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. 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 null hypothesis of equal variance 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.