Rivers constrain female but not male dispersal and genetic structure in brown bears

Understanding landscape barriers to connectivity is essential for studying wildlife population dynamics and developing conservation strategies that promote genetic exchange. Rivers can fragment landscapes and thereby influence genetic metapopulation structure by restricting individual movement and gene flow, yet their impact on brown bear (Ursus arctos) dispersal remains poorly understood. Using a large dataset (N = 519) of SNP-genotypes (96 loci) from fecal samples, we investigated the effects of rivers on sex-specific movement patterns, primarily dispersal and genetic structure in brown bears in northern Sweden. We found that males dispersed over twice as far as females (mean 56.4 km vs. 22.8 km) and crossed rivers significantly more often (42% of male vs. 11% of female dispersals; χ² = 49.33, p < 0.001). Simulated female dispersals in random directions showed higher river crossings (17.7%, t = 90.47, p < 0.001), suggesting philopatry alone cannot explain the low observed crossing rate. Rivers thus acted as semipermeable barriers for females, with genetic structure analysis (DAPC, spatial PCA) showing weak structuring effects for females, but none for males. Our study underscores the need to identify specific crossing sites, evaluate additive effects of other barriers (e.g., roads), and expand research across Sweden to guide connectivity-focused conservation.