Factors related to the occurrence of hybrids between brown hares Lepus europaeus and mountain hares L. timidus in Sweden

Hybridization occurs among many species, and may have implications for conservation as well as for evolution. Interspecific gene flow between brown hares Lepus europaeus and mountain hares L. timidus has been documented in Sweden and in continental Europe, and has probably to some extent occurred throughout history in sympatric areas. What local factors or ecological relationships that correlate with or trigger hybridization between these species has however been unclear. We studied spatial distribution of hybrids between brown hares and mountain hares in Sweden in relation to characteristics of the sampled localities (hunting grounds). In a sample of 70 brown hares collected from 39 populations in south-central Sweden during 2003-2005, 11 (16%) showed introgressed mtDNA from mountain hares. Among the brown hares from their northern range, i.e. in general the most recent establishments, the corresponding figure was 75% (9/12). The frequency of samples with hybrid ancestry increased significantly with latitude, altitude and hilliness, and were higher (p