Models of pace-of-life syndromes (POLS): a systematic review

Abstract

Variation in life history (LH) traits along the fast-slow continuum (referred to as pace of life, POL) is thought to result from a trade-off between investments in current versus future reproduction. Originally developed for understanding variation in LH strategies at the among-population level, the POL theory has more recently been applied towards understanding variation in LH traits at the within-population level, and further extended to address the covariance of LH traits with additional behavioural and/or physiological traits, referred to as pace-of-life syndromes (POLS). The article by Réale et al. (Philos T Roy Soc B 365:4051–4063, 2010), which synthesized several earlier reviews and opinions on among-individual covariation between LH, behavioural, and physiological traits, and subsequent research testing POLS in a variety of species, have collectively been cited several hundreds of times—a trend that continues. These works have interdisciplinary impact, informing research in life history biology, behavioural and developmental biology, and the social sciences. In this paper, we review the existing theoretical POLS models that provide adaptive explanations for covariances between LH traits and additional behavioural and/or physiological traits while assuming a trade-off between current and future reproduction. We find that the set of relevant models is small. Moreover, models show that covariances between life history traits and behavioural or physiological traits can arise even in the absence of a current-future reproduction trade-off, implying that observing such covariances does not provide a strong indication regarding the process generating POLS. We discuss lessons learned from existing models of POLS, highlight key gaps in the modelling literature, and provide guidelines for better integration between theory and data.