The Wonder Years: A Demographic Approach to Revisit the Age-at-Size Conundrum in Amphibians

Characterizing age-structure patterns in amphibian populations is crucial to unravel the drivers of their demographic dynamics and implement biologically informed conservation management. Consequently, accurately assessing the age of individuals in wild populations is of utmost importance in the face of the current amphibian crisis. However, age estimation in amphibians has so far remained elusive due to the difficulty of lifetime tracking, especially during the terrestrial juvenile stage, and to the uncertainty associated with alternative methods like skeletochronology. Here, we illustrate the usefulness of long-term monitoring programs based on capture-mark-recapture to address age estimation from body size data. Specifically, we combined repeated body size measurements of marked individuals of unknown age throughout their adult life with growth records of individuals marked as postmetamorphic juveniles and recaptured years later as sexually mature adults, focusing on 10 amphibian species in central Spain. Growth models fitted to mark-recapture data provided robust estimates of size-at-age, supported by data on individuals of known age for five of the study species. For three additional species, known age records disproved mark-recapture models, but rendered enough data to build alternative growth models. The inferred consensus size estimates allowed discerning first- and second-year age classes in some species, contributing to assess the age at maturity of individuals and population-specific patterns of recruitment. Consequently, our demographic approach provided multi-evidence information to address age-at-size estimation in amphibians, thus demonstrating the value of long-term mark-recapture programs to fill pervasive gaps in our knowledge about amphibian demographic dynamics.