Abstract
Individual repeatability (R), defined as the proportion of observed variance attributable to among‐individual differences, is a widely used summary statistic in evolutionarily motivated studies of morphology, life history, physiology and, especially, behaviour. Although statistical methods to estimate R are well known and widely available, there is a growing tendency for researchers to interpret R in ways that are subtly, but importantly, different. Some view R as a property of a dataset and a statistic to be interpreted agnostically with respect to mechanism. Others wish to isolate the contributions of ‘intrinsic’ and/or ‘permanent’ individual differences, and draw a distinction between true (intrinsic) and pseudo‐repeatability arising from uncontrolled extrinsic effects. This latter view proposes a narrower, more mechanistic interpretation, than the traditional concept of repeatability, but perhaps one that allows stronger evolutionary inference as a consequence (provided analytical pitfalls are successfully avoided). Neither perspective is incorrect, but if we are to avoid confusion and fruitless debate, there is a need for researchers to recognise this dichotomy, and to ensure clarity in relation to how, and why, a particular estimate of R is appropriate in any case.
Highlights
ACKNOWLEDGMENTS Ideas presented here benefited from discussion with Ryan Earley and Mark Briffa at a workshop funded by a BBSRC US partnering award (BB/M025799/1)
I am grateful to Erik Postma, Tom Houslay, Petri Niemala, Anne Charmantier, and three anonymous reviewers for valuable comments on an earlier draft of this manuscript
A priori dismissing one of these perspectives provides obvious potential for confusion and fruitless debate and is unlikely to advance our understanding of both the causes and the consequences of individual variation
Summary
While preferred terminology varies among researchers (and according to whether the phenomenon is univariate or multivariate), animal personality is usually defined as the presence of behavioural differences among-individuals that are repeatable across time and/or context This definition has led empiricists to design studies that target multiple behavioural observations per individual subject, allowing statistical separation of among-individual from withinindividual variance. Interpreting repeatability estimates of personality, with its effect size often standardised to a repeatability This approach, though not without its critics in behavioural ecology (Beekman and Jordan 2017) has highlighted the fundamental evolutionary importance of (behavioural) variation at multiple hierarchical levels (e.g., within- and among-individuals; Dingemanse 2017). Thereby I hope to (i) help researchers draw appropriate understanding from published results, and (ii) encourage authors to be more explicit in what they want R to represent and why
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