Abstract
Hybrid zones offer valuable insights into evolutionary processes. In species with bimodal reproduction, hybridization can produce transitional phenotypes that may pinpoint the evolutionary pathways connecting distinct reproductive modes. The fire salamander (Salamandra salamandra) is one of the two amphibians known to display bimodal viviparity, with populations exhibiting larviparity-birth of free-living aquatic larvae-across most of its distribution, while pueriparity-birth of fully developed terrestrial metamorphs-is geographically restricted. These strategies differ in maternal investment, ontogeny, and offspring phenotype. Here, we investigate variation in reproductive modes and associated offspring body condition across a hybrid zone between two lineages, S. s. gallaica and S. s. bernardezi, that differ in reproductive (larviparous and pueriparous, respectively) and morphological traits (i.e., body size, head shape, and dorsal coloration). We aimed to characterize the spatial distribution of reproductive modes across the contact zone and explore potential fitness advantages of 'pure' and intermediate reproductive modes using differences in offspring body condition as an indirect proxy. We found a gradual and continuous shift in reproductive modes across the contact zone, with "pure" pueriparous mode in the north and larviparous in the south, and mixed reproductive modes in central populations, where hybrids appear to be viable and fertile. Offspring body condition varied significantly between reproductive modes, with offspring from mixed-strategy females not only being viable but also exhibiting the highest body condition. This may result from reduced intra-brood competition for matrotrophic resources, or alternatively, from increased sibling competition that favors offspring with higher performance. This system offers a unique opportunity to examine transitional viviparous forms in a natural context, providing an excellent model for understanding parity evolution, life-history trade-offs, and the evolutionary significance of reproductive shifts in vertebrates.