Tissue samples
We obtained tissue samples from 278 previously-collected specimens of female and male common vole, Microtus arvalis (Pallas 1778) from both Western Europe (France, Great Britain and Spain) and Eastern Europe (Czech Republic, Hungary, Poland, Russia, Serbia and Ukraine) (Fig. 1A, Table S1). Specimens were collected between 1995 – 2015 (voles from 1 site were caught simultaneously) and consisted of appendages (legs, toes, ears, tail tips and muscle tissues) preserved in 96-99% ethanol at 4-15 °C. Sex was not documented for all specimens, therefore, we could not account for sex in the model.