Demographic measures of an individual's "pace of life": fecundity rate, lifespan, generation time, or a composite variable?

Comparative analyses have demonstrated the existence of a "pace-of-life" (POL) continuum of life-history strategies, from fast-reproducing short-lived species to slow-reproducing long-lived species. This idea has been extended to the concept of a "pace-of-life syndrome" (POLS), an axis of phenotypic covariation among individuals within species, concerning morphological, physiological, behavioral and life-history traits. Several life-history metrics can be used to place species in the fast-slow continuum; here, we asked whether individual variation in POL can also be studied using similar life-history measures. We therefore translated measures commonly used in demographic studies into individual-level estimates. We studied fecundity rate, generation time, lifespan, age at first reproduction, fecundity at first reproduction, and principal component scores integrating these different metrics. Using simulations, we show how demographic stochasticity and individual variation in resources affect the ability to predict an individual's POL using these individual-level parameters. We found that their accuracy depends on how environmental stochasticity varies with the species' position on the fast-slow continuum and with the amount of (co) variation in life-history traits caused by individual differences in resources. These results highlight the importance of studying the sources of life-history covariation to determine whether POL explains the covariation between morphological, physiological, and behavioral traits within species. Our simulations also show that quantifying not only among-individual but also among-population patterns of life-history covariation helps in interpreting demographic estimates in the study of POLSs within species. Significance statement It has been demonstrated that there is a continuum of life-history strategies, from fast-reproducing short-lived species to slow-reproducing long-lived species. This pattern of variation in the tempo of life-history strategies has been named the pace-of-life continuum. Recently, it has been suggested that within a population, variation in pace of life explains differences between individuals in their morphological, behavioral, and physiological traits. This paper provides guidelines on how to quantify the pace of life of individuals using demographic approaches that have been developed to study the pace of life of species.