No Clean Slates at Metamorphosis: Do Environmental Legacies Shape Evolutionary Outcomes in Amphibians?

Developmental environments shape trait expression in ways that often persist into, or only become evident during, later stages of an organism's life. These "carry-over" effects are well documented across taxa, even in species that undergo dramatic remodeling during metamorphosis. Amphibians, a textbook model of metamorphic life histories, enjoy a rich literature documenting carry-over effects. Despite this wealth of information, key questions remain about how carry-over effects influence amphibian health and evolution and how such effects, themselves, evolve. Here, we provide the first broad synthesis of amphibian carry-over effects, drawing on over 170 empirical studies to assess how diverse early-life environments affect post-metamorphic traits across taxa, developmental stages, and experimental contexts. This synthesis reveals strong biases towards studying life-history traits as carry-over effects and conducting studies under laboratory conditions. In contrast, far less is known about how early-life environmental variation shapes physiological, behavioral, and morphological carry-over effects, or how such effects impact performance under field conditions. These gaps are critical given mounting evidence that early-life environments can impose "hidden" costs later in life that are not captured by size-based metrics, and that interacting stressors in nature can result in unexpected health outcomes. Building on these studies, we highlight key unanswered questions regarding the mechanisms underlying carry-over effects and how those mechanisms may shape evolutionary potential or constraint. Our overarching goal is to synthesize existing knowledge and motivate new research on this influential yet incompletely understood driver of adult trait and fitness variation in a taxonomic group increasingly challenged by environmental change.