Generation Time: A Reliable Metric to Measure Life‐History Variation among Mammalian Populations

Oli and Dobson proposed that the ratio between the magnitude and the onset of reproduction (F/α ratio) allows one to predict the relative importance of vital rates on population growth rate in mammalian populations and provides a reliable measure of the ranking of mammalian species on the slow‐fast continuum of life‐history tactics. We show that the choice of the ratioF/α is arbitrary and is not grounded in demographic theory. We estimate the position on the slow‐fast continuum using the first axis of a principal components analysis of all life‐history variables studied by Oli and Dobson and show that most individual vital rates perform as well as theF/α ratio. Finally, we find, in agreement with previous studies, that the age of first reproduction is a reliable predictor of the ranking of mammalian populations along the slow‐fast continuum and that both body mass and phylogeny markedly influence the generation time of mammalian species. We conclude that arbitrary ratios such asF/α correlate with life‐history types in mammals simply because life‐history variables are highly correlated in response to allometric, phylogenetic, and environmental influences. We suggest that generation time is a reliable metric to measure life‐history variation among mammalian populations and should be preferred to any arbitrary combination between vital rates.