Fecundity of the newts Triturus cristatus, T. marmoratus and their natural hybrids in relation to species coexistence

The two larger newt species in Europe, Triturus cristatus and T. marmoratus , have largely exclusive distributions but live in sympatry in a small area in western France, where they also hybridize. Triturus cristatus is increasing in abundance in the area at the expense of T. marmoratus , although this study shows that T. marmoratus has higher relative lifetime reproductive success. Triturus marmoratus lays more eggs than its relative under controlled conditions, and more T. marmoratus eggs in relation to its relative abundance are found in natural breeding sites. The hybrids are even more fecund, but their eggs and embryos have a lower survival rate.
The data are compared for age at maturity and species longevity, and estimates of lifetime reproductive success show that T. marmoratus females are more fecund than T. cristatus females. There are no differences between the species in survival of the embryos. It is therefore not clear why T. cristatus is replacing T. marmoratus , but changes in terrestrial habitats due to human activity might be the explanation.