Background <p>Metamorphosis is a key event in the life history of many organisms, including amphibians, because it can involve abrupt changes in morphology and habitat. However, metamorphosis can also be bypassed by paedomorphic processes that allow reproduction at the larval stage. The evolution of these alternative developmental processes depends on the payoffs of life in larval and adult environments, as well as the cost of transition. Previous studies on the cost of metamorphosis have focused on the larval stage, which is still subject to the achievement of a minimal size threshold as well as growth to adulthood. Therefore, facultatively paedomorphic species could be valuable models for testing the direct costs of metamorphosis. This is because facultative paedomorphs are sexually dimorphic adults that are capable of metamorphosis.</p> Results <p>By modelling weight loss using an experimental, longitudinal design that manipulated the ecological drivers of metamorphosis in a facultatively paedomorphic amphibian, the palmate newt (<i>Lissotriton helveticus</i>), we found that metamorphosis imposes costs in terms of body weight, whereas temperature has a smaller effect. In contrast, newts that do not metamorphose did not lose weight. The costs were sex-related with females at a disadvantage during metamorphosis. Furthermore, the decrease in food consumption associated with metamorphosis resulted in a loss of body weight.</p> Conclusions <p>Overall, these results emphasise the importance of considering direct costs when studying the evolutionary ecology of metamorphosis. They also show that polymorphic species are suitable models for investigating the drivers of metamorphosis and its loss in micro- and macroevolution.</p>