An organism’s life history strategy is shaped by traits that have a natural hierarchy. The organism’s phenotype determines how the individual acquires resources [71] through morphological, physiological, and behavioural adaptations. Development describes the allocation of those resources to competing processes: supporting somatic maintenance vs. supporting the germline vs.development in either [72]. The consequences of these allocations are life history traits . These traits are characterised by probability distributions of survival and fertility. As survival and fertility govern the number of individuals in populations, life history traits govern not just an organism’s fitness but also itsdemography - population size, structure, and dynamics [1]. Traits at all hierarchical levels covary with, and feed-back on one another, and are subject to constraints imposed by evolutionary history, physical laws, bauplan, and the environment [73]. Natural selection acts to favour life history strategies that maximise the organism’s fitness in its environment, given those constraints [1].

Trait currencies and their units vary considerably among life history studies. Moreover, these units do not necessarily correspond with trait types or hierarchies. Physiological traits often measure energyexpenditure (e.g., metabolic rate). Behavioural traits typically quantify energetic expenditure (e.g., foraging efficiency) or space usage (e.g. , home range size). However, space units are also common among morphological traits (e.g., specific root length). Rates of energetic allocation are used to describe resources given to developmental processes (e.g ., change in body size over time), thus combining energy and time currencies. Time is commonly used to measure life history traits, whether as durations (e.g ., mean life expectancy), rates (e.g ., number of offspring per year), or temporal probabilities linked to survival, development, and reproduction (e.g. , probability of reaching maturity at a given age). Life history traits are often converted to expectations of life history for populations or species, which is especially necessary where events occur only once. For example, an individual only dies once, but the aggregate deaths in a population yield the distribution of survival for the population’s average phenotype, from which we can estimate e.g.mean life expectancy. Selection on phenotypic, developmental, or life history traits can quantify the trait’s contribution to fitness (e.g ., selection pressure). Shaded areas on the figure are indicative, but not exhaustive, of common currencies used at each level and their overlap.