Paceless life? A meta-analysis of the pace-of-life syndrome hypothesis

The pace-of-life syndrome hypothesis predicts that individual differences in behavior should integrate with morphological, physiological, and life-history traits along a slow to fast pace-of-life continuum. For example, individuals with a slow pace-of-life are expected to exhibit a slower growth rate, delayed reproduction, longer lifespans, have stronger immune responses, and are expected to avoid risky situations relative to fast individuals. If supported, this hypothesis would help resolve ecological and evolutionary questions regarding the origin and maintenance of phenotypic variation. Support for the pace-of-life syndrome hypothesis has, however, been mixed. Here, we conducted a meta-analysis of 42 articles and 179 estimates testing the pace-of-life syndrome hypothesis as it applies to the integration of behaviors with physiological or life-history traits. We found little overall support for the pace-of-life syndrome hypothesis with the mean support estimated as r = 0.06. Support for the pace-of-life syndrome hypothesis was significantly higher in invertebrates (r = 0.23) than vertebrates (r = 0.02) and significantly higher when based on phenotypic (r = 0.10) versus genetic correlations (r = -0.09). We also found that females exhibited correlations between behavior and life-history and physiology that were opposite the predictions of the pace-of-life syndrome hypothesis (r = -0.16) and that these correlations significantly differed from those observed in males (r = 0.01) or males and females pooled (r = 0.12). It was also the case that there was little support for the hypothesis when life-history and physiological traits were independently analyzed (behavior x life-history: r = 0.12; behavior x physiology: r = 0.04). Exploratory post hoc analyses revealed that correlations of behavior with growth rate and hormone levels were more likely to show support for the predictions of the pace-of-life syndrome hypothesis. The lack of overall support found in our analyses suggests that general assertions regarding phenotypic integration due to pace-of-life should be re-evaluated. Significance statement The pace-of-life syndrome hypothesis has been proposed as an overall organizational framework for the integration of behavioral, life-history, and physiological traits. This hypothesis provides potentially profound insights into how and why phenotypic traits might covary and why phenotypic variation may be maintained within populations. Over the last 7 years, this organizational framework has been intensively investigated as it pertains to relationships between behavior and other traits. Here, we conducted an overall analysis of whether the hypothesis was supported. Despite considerable research investment across behavioral ecology, we did not find that available data supported the pace-of-life syndrome hypothesis. This suggests that either the hypothesis has been inappropriately tested or is not generally applicable.