Animal ‘personality’, defined as repeatable inter-individual differences in behaviour, is a concept in biology that faces intense controversy. Critics argue that the field is riddled with terminological and methodological inconsistencies and lacks a sound theoretical framework. Nevertheless, experimental biologists are increasingly studying individual differences in physiology and relating these to differences in behaviour, which can lead to fascinating insights. We encourage this trend, and in this Commentary we highlight some of the benefits of estimating variation in (and covariation among) phenotypic traits at the inter- and intra-individual levels. We focus on behaviour while drawing parallels with physiological and performance-related traits. First, we outline some of the confusion surrounding the terminology used to describe repeatable inter-individual differences in behaviour. Second, we argue that acknowledging individual behavioural differences can help researchers avoid sampling and experimental bias, increase explanatory power and, ultimately, understand how selection acts on physiological traits. Third, we summarize the latest methods to collect, analyse and present data on individual trait variation. We note that, while measuring the repeatability of phenotypic traits is informative in its own right, it is only the first step towards understanding how natural selection and genetic architecture shape intra-specific variation in complex, labile traits. Thus, understanding how and why behavioural traits evolve requires linking repeatable inter-individual behavioural differences with core aspects of physiology (e.g. neurophysiology, endocrinology, energy metabolism) and evolutionary biology (e.g. selection gradients, heritability).
The authors declare no competing or financial interests.
D.G.R., V.C. and S.A.B. wrote the paper. V.C. wrote the R script to analyse data from Norin et al. (2016).
D.G.R. and S.A.B. were supported by postdoctoral fellowships from the Fonds de Recherche du Québec - Nature et Technologies.
Tommy Norin generously provided the data for the examples presented in the paper (available at http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12503/abstract). These data (in long format) and the associated R scripts are available on Figshare at: http://dx.doi.org/10.6084/m9.figshare.3464216.
Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.146712.supplemental
- Received July 26, 2016.
- Accepted October 7, 2016.
- © 2016. Published by The Company of Biologists Ltd