The pace-of-life syndrome revisited: the role of ecological conditions and natural history on the slow-fast continuum

The pace-of-life syndrome (i.e., POLS) hypothesis posits that behavioral and physiological traits mediate the trade-off between current and future reproduction. This hypothesis predicts that life history, behavioral, and physiological traits will covary under clearly defined conditions. Empirical tests are equivocal and suggest that the conditions necessary for the POLS to emerge are not always met. We nuance and expand the POLS hypothesis to consider alternative relationships among behavior, physiology, and life history. These relationships will vary with the nature of predation risk, the challenges posed by resource acquisition, and the energy management strategies of organisms. We also discuss how the plastic response of behavior, physiology, and life history to changes in ecological conditions and variation in resource acquisition among individuals determine our ability to detect a fast-slow pace of life in the first place or associations among these traits. Future empirical studies will provide most insights on the coevolution among behavior, physiology, and life history by investigating these traits both at the genetic and phenotypic levels in varying types of predation regimes and levels of resource abundance. We revisit the pace-of-life syndrome hypothesis, suggesting that behaviors involving a risk of death or injury should coevolve with higher metabolic rates, higher fecundity, faster growth, and heightened mortality rates. Empirical support for this hypothesis is mixed. We show how relaxing some of the assumptions underlying the pace-of-life syndrome hypothesis allows us to consider alternative relationships among behavior, physiology, and life history, and why we fail to meet the predictions posed by the pace-of-life syndrome hypothesis in some populations. Our discussion emphasizes the need to re-integrate the role of the species' natural history, ecological conditions, and phenotypic plasticity in shaping relationships among behavior, physiology, and life history.