Fauna of Scotland
Only six amphibians and four land reptiles are native to Scotland. The amphibians include three species of newt: the great crested, of which fewer than 1,000 individuals survive; the smooth, and the palmate. The other amphibians are the common toad, the natterjack toad, found in only four locations in the south-west, and the common frog. A single alien amphibian is known in Scotland, the Alpine newt, a recent escapee confined to the Edinburgh area.
Metamorphosis
In newts, there is no true metamorphosis because newt larvae already feed as predators and continue doing so as adults. Newts' gills are never covered by a gill sac and will be resorbed only just before the animal leaves the water. Just as in tadpoles, their lungs are functional early, but newts use them less frequently than tadpoles. Newts often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase, prolactin is the required hormone, and for adaptation to the land phase, thyroxin. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.