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.