Some small insectivorous bats live at the edge of their physiological limits. Bergmann’s rule implies that larger animals occur in colder environments. I showed that bat size is related to elevation, which confirms Bergmann’s 175-year-old prediction. A Bayesian multilevel model showed an increase of 0.11 mm in forearm length per 100 m, meaning that bats are likley to become smaller with an increasing temperature. I use this to spatially estimate habitat loss. Image by Alexander Herr.