Facultative paedomorphosis is a widespread polyphenism in newts and salamanders, but it is declining due to the introduction of non-native species. Conversely, caudates have often been introduced outside their range and could be good models to improve our understanding of the factors that facilitate the expression of metamorphosis versus paedomorphosis. In particular, the alpine newt is a common amphibian species that has been introduced into many countries and whose many native paedomorphic populations have become extinct. By surveying ponds where metamorphic alpine newts were introduced in France over eight years, this study aimed to show what we can learn from such introduced populations by highlighting key life history traits of both phenotypes and a co-existing native species, the palmate newt, that can also express paedomorphosis. Although paedomorphosis is rare outside of the main European hotspots, paedomorphs were repeatedly found in the introduced population, including in a pond where they were dispersing. Interestingly, the relative frequencies of paedomorphs in the adult population of both species were low and match those of the paedomorphs of the native palmate newts over the years. This resulted in a complex size-structured newt community. Taken together, these results allow the discussion of alternative scenarios for the origin of aliens and possible reasons for the presence of paedomorphs in such populations. Ultimately, they call for reporting such developmental variants in the introduced populations of newts and salamanders and tracing the origin of the introductions to infer on genetic and environmental causes.
