Hybridization between a Euro-Siberian (Vipera berus) and a Para-Mediterranean viper (V. aspis) at their contact zone in western France

International audience; Western European vipers are well-defined species with parapatric distributions that reflect contrasting thermal niches and climatic adaptations. Contact zones are usually narrow, coincide with steep ecological gradients and are associated with clear habitat segregation. Natural hybridization has been demonstrated between several species but has not been detected in others. The cold-adapted adder (V. berus, subgenus Pelias) is not known to hybridize with the warm-adapted aspic viper (V. aspis subgenus Vipera). For over 12 years, we have monitored sympatric populations of V. berus and V. aspis in western France where the two species exhibit very similar life cycles. We tested for possible hybridization because individuals with intermediate morphological traits have been reported in the past and were recently detected in the study population. Our results demonstrate that hybridization actually occurs and is directional since it involves females V. aspis viper and male V. berus in all analyzed cases. We discuss our results in the frame of previous findings on contact zones to evaluate in which conditions hybridization may occur.