Alpine newts normally develop as larvae in a pond, metamorphose, and spend part of the year on land. In some cold, high-altitude or otherwise harsh sites, particularly in the Balkans and parts of the Alps, populations are facultatively paedomorphic: individuals reach sexual maturity while retaining external gills and remain aquatic year-round.
The likely driver is a cost comparison. Where the terrestrial growing season is very short and the aquatic habitat is permanent, delaying or skipping the transition to land buys extra growth and breeding opportunity. The trait is facultative rather than fixed, and paedomorphic and metamorphosed individuals can occur in the same water body.
The species is a caution about introductions as well as a curiosity. Alpine newts introduced to Britain and elsewhere are of conservation concern, partly because moved amphibians can carry pathogens including the salamander-killing fungus Batrachochytrium salamandrivorans, to which this species is susceptible.