Life Histories, Axes of Variation in

Abstract

The diversity of traits across species is organized around main axes of variation in life history. Among them, the slow–fast continuum first described by Stearns (1983) is the most frequently analyzed. After presenting the history of this slow–fast continuum, we perform an updated review of its empirical support from analyses of vertebrates. We show that conflicting results and interpretation reported in previous studies can be attributed to various problems, including the lack of consideration of dimension as the most crucial issue. When these problems are solved, in particular by doing a principal component analysis using only traits measured in units of time, the slow–fast continuum is the main axis of variation in life history across vertebrate species. Species range from a fast end with short developmental time, early reproduction, frequent reproductive attempts, and short lifespan to a slow end with long developmental time, delayed reproduction, spaced out reproductive attempts, and long lifespan. All these traits have similar loadings on the slow–fast continuum, supporting the concept of physiological time. We then discuss the possible existence of other axes of life history variation and identify some lines for future research.