We present the first comprehensive study of the only known lowland population of the common adder (Vipera berus) in Slovenia, with the aim of gaining insight into its sex and age structure, colour polymorphism, microhabitat selection, population density, and taxonomic affiliation, and to derive conservation guidelines. In 2024, we conducted field surveys along three road sections; individuals were identified by head (pileus) scalation, colour morph was determined, body mass measured, body length obtained from calibrated photographs, the number of rows of subocular scales counted, and population density estimated using the spatially explicit capture-recapture (SECR) method. The results reveal expected seasonal differences in activity between sexes, an adult-biased sample, pronounced sexual dimorphism in body mass and colour polymorphism with sex-specific patterns, and non-random microhabitat selection. Morphological characters, the historical connectivity of floodplain forests along the Mura and Drava, and the close proximity of a Balkan subspecies population in southwestern Hungary support the hypothesis that this population belongs to the Balkan subspecies (Vipera berus bosniensis), which nonetheless requires genetic confirmation. Given its unique habitat context and melanism frequency in Slovenia and unresolved taxonomic status, the population warrants protection. To ensure effective conservation, we recommend maintaining mosaic and edge habitats, seasonally adjusting major management interventions, mapping and safeguarding hibernacula, and implementing long-term, standardised monitoring.
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