Low hybridization between an Euro-Siberian (Vipera berus) and a Mediterranean (V. aspis) viper at their contact zone in Western France

Parapatric distributions have attracted considerable interest and several mechanisms can be considered to explain this pattern. Western European vipers are well-defined species with parapatric distributions that reflect contrasted thermal niches and climatic adaptations. Previous studies in narrow contact zones often suggested a clear habitat segregation and hybridization has been demonstrated in more related species species but not in others notably more phylogenetically distant. The cold adapted adder ( V. berus , subgenus Pelias ) is not known to hybridize with the warm adapted aspic viper ( V. aspis subgenus Vipera ). We have monitored for over 12 years sympatric populations of V. berus and V. aspis in Western France where the two species show very similar life cycles. We wanted to test for possible hybridization since individuals with intermediate morphological traits have been reported in the past and were recently detected in the study population. Our results demonstrated that hybridization actually occurs and is directional since it always involves females V. aspis viper and male V. berus . We discuss our results in the frame of previous findings on contact zones to evaluate in which condition hybridization may occur.