Background
The authors write that "one of the main predictions of bioclimatic envelope models is that populations near the climatic extremes of species' distributions, where thermoregulatory costs are presumably high, are performing sub-optimally and are at greater risk of extinction," citing Thomas et al. (2004). However, they suggest that "if populations are physiologically adapted (genetic variation across generations and/or plastic adjustments) to their respective climates, species may be capable of maintaining high levels of performance throughout their geographic ranges," citing Chowen et al. (2010) and Glanville et al. (2012).

What was done
In a study designed to further test this hypothesis, Smit et al. investigated the effects of air temperature (TA) on body temperature (TB) and the behavior of an arid-zone endotherm, the White-browed Sparrow-Weaver (Plocepasser mahali) at two sites 100 km apart, in the southern Kalahari Desert of South Africa, over two consecutive summer seasons.

What was learned
A key finding of the four researchers' study was the relatively large variation in TB that occurred both within and between conspecific populations, which suggests, in their words, that "an arid-zone passerine responds differently to prevailing weather conditions in two locations over its range, and that it also responds to seasonal changes in weather conditions," which suggests that "a species current range may not be an accurate representation of its climatic tolerance."

What it means
"Taken together with the data of Glanville et al. (2012)," as Smit et al. explain things, this result "suggests that the thermal physiology of endotherms is far more flexible than previously thought, and could potentially contribute to the adaptation of populations under changing climatic conditions," citing Boyles et al. (2011)," so that "when predicting species' responses to climate change, their sensitivity (sensu Williams et al., 2008) should be resolved at the population, rather than species, level."

References
Boyles, J.G., Seebacher, F., Smit, B. and McKechnie, A.E. 2011. Adaptive thermoregulation in endotherms may alter responses to climate change. Integrative and Comparative Biology 51: 676-690.

Chown, S.L., Hoffmann, A.A., Kristensen, T.N., Angilletta, M.J., Stenseth, N.C. and Pertoldi, C. 2010. Adapting to climate change: a perspective from evolutionary physiology. Climate Research 43: 3-15.

Glanville, E.J., Murray, S.A. and Seebacher, F. 2012. Thermal adaptation in endotherms: climate and phylogeny interact to determine population-level responses in a wild rat. Functional Ecology 26: 390-398.

Thomas, C.D., Cameron, A., Green, R.E., Bakkenes, M., Beaumont, L.J., Collingham, Y.C., Barend, F., Erasmus, N., Ferreira de Siqueira, M., Grainger, A., Hannah, L., Hughes, L., Huntley, B., van Jaarsveld, A.S., Midgley, G.F., Miles, L., Ortega-Huerta, M.A., Peterson, A.T., Phillips, O.L. and Williams, S.E. 2004. Extinction risk from climate change. Nature 427: 145-148.

Williams, S.E., Shoo, L.P., Isaac, J.L., Hoffman, A.A. and Langham, G. 2008. Towards an integrated framework for assessing the vulnerability of species to climate change. PLoS Biology 6: 2621-2626.