Guess Who: Delineating Post-Metamorphic Amphibian Life Stages and Sexes Using a Standardised Visible Approach

Among animals, adulthood is synonymous with sexual maturity and often defined by the capacity of an individual to store and/or use functional gametes. Yet, under field conditions, where evidence of gametes cannot always be easily ascertained, researchers rely on other cues of maturity, such as size and sex-based traits, which also enable for the differentiation of the sexes. In this study, we used a multi-season data set to help define adulthood for the green and golden bell frog, Randoidea [Litoria] aurea males and females that improves upon past methods using externally visible attributes. Our decision-making approach is nuanced as it considers both the presence and level of expression of visibly detectable sex attributes, namely nuptial pad colouration and gravidity level, with expression levels of these traits also used to define minimum size thresholds of maturity for each sex independently. This protocol allows individuals with high expressions of sex-based traits of maturity to be assigned as adults even if they are below minimum size thresholds, thus accounting for precocious development, and for individuals above these thresholds to be assigned as adults regardless of trait expression, thus accounting for temporal variability in these traits within the adult life stage that are related to hormone cycles. We believe this decision-making pipeline can be applied to other amphibians by adjusting trait thresholds to species- or population-specific specifications.