Biogeographia – The Journal of Integrative Biogeography

Dark colour aberrations in reptiles are rarely mapped and poorly understood. We document the first axanthism-like Aesculapian snake (Zamenis longissimus) from Lombardy, Italy, and merge this record with 44 earlier reports from the literature and iNaturalist. An ensemble ecological niche model calibrated with three uncorrelated WorldClim variables (i.e., mean annual temperature, mean diurnal temperature range and annual precipitation) achieved good performance (AUC = 0.84 ± 0.04; TSS = 0.58 ± 0.08). Mean diurnal temperature range emerged as the strongest predictor, with suitability increasing in warmer regions that experience smaller day–night thermal amplitudes. Predicted hotspots for the dark morph include the Atlantic coasts of Spain and France, the Tyrrhenian margin from Liguria into Piedmont, the eastern Adriatic littoral, and Black Sea coastlines of Russia, Georgia and Turkish Thrace. These areas largely coincide with published and citizen-science records, lending confidence to model outputs despite the sparse dataset. Anyway, the genetic basis and fitness effects of this colour remain untested; directed surveys and molecular work in the predicted zones are now needed. Our map offers a concise guide for future sampling.