Wolf-dog hybridization in selected European wolf populations

Human-caused hybridization between genetically distinct species or populations, particularly between domestic animals and their wild counterparts, is a growing concern for biodiversity. Anthropogenic hybridization threatens the integrity and viability of wild species by introducing genetic traits that could reduce fitness and adaptive potential, an issue particularly apparent with wolves in Europe. In response, the Biodiversa+ Wolfness project was initiated in March 2023, focusing on the hybridization between wolves and domestic dogs in Europe. The project underscores the necessity of standardized genetic tools and sampling procedures to accurately identify, estimate, and monitor the prevalence of hybridization in wolf populations. Whereas three previous Wolfness technical reports have tackled a literature review of genetic methods to detect wolf-dog hybridization, the standardization of analytical procedures to reliably detect hybrids, and the sampling considerations to reliably assess hybridization at the population level (D1, D2 and D3 technical reports, respectively; Birkenhain et al. 2025, Stronen et al. 2025a, Ciucci et al. 2025), this document presents the assessment of wolf-dog hybridization at selected European wolf populations, based on available non-invasive and invasive samples collected previously to the Wolfness project. In this report, we provide naïve estimates (i.e. simple sample ratio) of the prevalence of wolf-dog hybrids in Türkiye, Slovenia and Croatia, and the Iberian and Italian Peninsulas.The analyses showed the central importance of using a common definition for wolf-dog hybrids since this can dramatically affect the results and the interpretations across countries and studies. We used the hybrid definition of Stronen et al. (2025a, b) and observed very large differences across populations. For the Iberian population a surveillance monitoring (sensu Ciucci et al. 2025) revealed a hybrid prevalence of 1.8% (95% CI: 0.04 to 3.61%). For the other populations, focused monitoring surveys, intended to monitor the situation over a smaller portion of the range or to evaluate changes through time or in response to management actions, showed very different outcomes: high hybrid prevalence in three regions of the Italian Peninsula (11.9%) that did not find any differences between National Parks and neighboring areas; no hybridization (0%) in a region of Türkiye despite frequent interaction between dogs and wolf packs; varying rates in the Dinaric region (samples from Slovenia and Croatia), with an average of 8.7% but concentrated over a small portion of the range. Overall, these results show that wolf-dog hybridization is highly variable and may be determined by local yet undetermined factors and that evolutionarily distinct populations may have different responses. The wolf-dog hybrid prevalence and the proportion of F1 individuals in the sample (what we call hybridization rate) differs across the populations we assessed and suggests that hybridization is resulting in a low rate of introgression in Iberia. However, recent introgression may appear higher in a portion of Slovenia because the wolf population in the region was established after a lone wolf reached an area outside of the previous wolf range and mated with two dogs in two consecutive years. Although hybridization is known from the Dalmatian region of Croatia and Slovenia, extensive monitoring over many years has indicated that wolf-dog hybridization is otherwise rare in northern Croatia and Slovenia.The comparison of the results for the different populations illustrates the importance of clearly stating the reasons for conducting wolf-dog hybridization surveys (surveillance vs focused monitoring) and highlights the importance of using a common panel of genetic markers and standardized protocols that facilitate comparisons across borders (Harmoinen et al. 2021, Stronen et al. 2022, Birkenhain et al. 2025). The results showed that the panel developed by Harmoinen et al. (2021) can be useful for most European populations; however, the large genetic divergence of Peninsular Italian and Turkish wolves compared to other European wolf populations implies that the panel is less efficient on these populations, and other complementary panels may need to be developed. Similarly, the potential presence of immigrant wolves in any European population implies that hybridization studies should involve groups of reference samples from potential source populations in order to reliably differentiate immigration from hybridization.