The two-tiered life history model: from interrogating assumptions to refining concepts and hypotheses

The two-tiered life history (LH) model proposes that different sources of extrinsic mortality (EM) have opposing effects that regulate development toward both slower and faster LH traits. Although the 53 commentators generally endorsed the two-tiered model and empirical conclusions of the target article, the devil is in the details. Some commentators challenged model assumptions (e.g., the mechanistic basis of the two-tiered model; whether the model is genetically confounded; the relative importance of child versus adult mortality). Other commentators proposed extensions/modifications of model concepts and hypotheses (e.g., incorporating density-dependent regulation; use of formal models to generate and test hypotheses; connection to the internal predictive adaptive response [PAR] model). In this reply, we review and address these challenges and proposed extensions/modifications. We hope that this iterative process advances our understanding of the complexity of EM, its opposing tiers, and their dualistic effects-both hierarchical and countervailing-on variation in human life histories.