Life histories are not just fast or slow.

Life history studies support the fast–slow continuum as the dominant but not unique axis structuring life history variation. Other important axes of life history variation associated with development and reproductive tactics exist, and exploration of further axes, clusters, and boundaries of life history variation is needed. Existing life history analyses are venturing far from theory and could benefit from a stronger focus on hypothesis testing rather than exploration. We make recommendations to identify the structuring axes of life history variation through data choices and analytical methods of dimensionality reduction with recourse to a comprehensive model of life history. Life history strategies, which combine schedules of survival, development, and reproduction, shape how natural selection acts on species' heritable traits and organismal fitness. Comparative analyses have historically ranked life histories along a fast–slow continuum, describing a negative association between time allocation to reproduction and development versus survival. However, higher-quality, more representative data and analyses have revealed that life history variation cannot be fully accounted for by this single continuum. Moreover, studies often do not test predictions from existing theories and instead operate as exploratory exercises. To move forward, we offer three recommendations for future investigations: standardizing life history traits, overcoming taxonomic siloes, and using theory to move from describing to understanding life history variation across the Tree of Life. [ABSTRACT FROM AUTHOR]