High rates of hybridization between European polecats and domestic ferrets across the Iberian Peninsula

Hybridization between wild species and their domestic relatives is increasingly recognized as a conservation threat, particularly when driven by human activities. In Europe, the European polecat (Mustela putorius) hybridizes with domestic ferrets (Mustela putorius furo), but this phenomenon has been only studied in detail in Great Britain. We evaluated the extent of hybridization between polecats and ferrets across the Iberian Peninsula, where ferrets frequently escape from hunters. A total of 342 individuals (246 polecats and 96 ferrets) were genotyped using 12 microsatellite markers, and introgression was inferred through Bayesian clustering analyses. The two subspecies were genetically differentiated (FST = 0.146), with polecats showing higher genetic diversity. Nevertheless, introgression was widespread: 35% of polecats and 15.6% of ferrets displayed > 3% admixture, including one F1 hybrid and several backcrosses. Introgressed polecats occurred in all Iberian genetic units, with higher levels in the North- and South-Iberian units than in País Vasco and Girona. Fourteen individuals (4%) were misidentified based on phenotype, underscoring the limitations of morphological criteria to detect hybrids. These findings highlight the need to integrate genetic tools into conservation and management programs. The widespread introgression detected raises concerns for the long-term conservation of M. putorius, as hybridization may erode locally adapted genotypes and reduce evolutionary potential. We recommend implementing genetic screening protocols in areas of intense hunting activity and regulating ferret use. This study provides the first genetic evidence of ferret–polecat hybridization in the Iberian Peninsula and a basis for conserving this little-known mammalian carnivore.