CURRENCIES OF LIFE HISTORY TRAITS
We identify four key ways to quantify life history data (Box 2). These ways are often linked to hierarchies of biological organisation and the researcher’s own perspective. These four life history currenciesdefine how an organism acquires and allocates resources [43], and the consequences for its vital rates. Evolutionary ecologists typically see the world through an adaptive lens and might study selection strength by measuring how schedules of life history traits affect fitness. In contrast, behavioural ecologists perceive evolution of life history in light of an organism’s use of energy or space in acquiring resources, whereas developmental biologists focus on energy allocation to ontogenetic cellular and organic processes. From a demographer’s perspective, life histories consist of events measured in time or frequency, as these events capture or directly affect survival and/or reproduction over age or stage. However, even within these broad perspectives and currencies, the units of life history trait measurement often vary. For example, demographic measures can be confused by mixing rates, durations, ages, frequencies, and times-to-events. Currencies can also be confused by measuring traits in one currency as proxies for processes in another. Examples include using morphological or behavioural sexual displays as proxies for allocation of energy to reproduction [44], or body size as an allometric proxy for survival, reproduction, metabolic rate, and use of space [3,5,45,46]). Past comparative analyses have often been performed on rather heterogeneous life history trait currencies, including the very first one published by Stearns 40 years ago. The mixture of currencies confounds the interpretation of the structuring axes of life history variation across the Tree of Life. As such, we urge to standardise the currency in future analyses to enable cross-study, cross-taxonomic fair comparisons.