Mapping the correlations and gaps in studies of complex life histories

For species with complex life histories, phenotypic correlations between life-history stages constrain both ecological and evolutionary trajectories. Studies that seek to understand correlations across life history differ greatly in their experimental approach: some follow individuals ("individual longitudinal"), while others follow cohorts ("cohort longitudinal"). Cohort longitudinal studies risk confounding results through Simpson's Paradox, where correlations observed at the cohort-level do not match that of the individual-level. Individual longitudinal studies are laborious in comparison, but provide a more reliable test of correlations across life-history stages. Our understanding of the prevalence, strength and direction of phenotypic correlations depends on the approaches that we use, but the relative representation of different approaches is unknown. Using marine invertebrates as a model group, we used a formal, systematic literature map to screen 17,000+ papers studying complex life histories, and characterized the study type (i.e. cohort longitudinal, individual longitudinal or single stage), as well as other factors. For 3,315 experiments from 1,716 articles, most (67.4%) focused on a single stage, and just 1.7% used an individual longitudinal approach. While life-history stages have been studied extensively, we suggest that the field prioritize individual longitudinal studies to understand the phenotypic correlations among stages.