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J Anim Ecol. 2013 Jan;82(1):39-54. doi: 10.1111/1365-2656.12013. Epub 2012 Nov 21.

Quantifying individual variation in behaviour: mixed-effect modelling approaches.

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Evolutionary Ecology of Variation Group, Max Planck Institute for Ornithology, Eberhard-Gwinner-Stra├če, 82319 Seewiesen (Starnberg), Germany.


Growing interest in proximate and ultimate causes and consequences of between- and within-individual variation in labile components of the phenotype - such as behaviour or physiology - characterizes current research in evolutionary ecology. The study of individual variation requires tools for quantification and decomposition of phenotypic variation into between- and within-individual components. This is essential as variance components differ in their ecological and evolutionary implications. We provide an overview of how mixed-effect models can be used to partition variation in, and correlations among, phenotypic attributes into between- and within-individual variance components. Optimal sampling schemes to accurately estimate (with sufficient power) a wide range of repeatabilities and key (co)variance components, such as between- and within-individual correlations, are detailed. Mixed-effect models enable the usage of unambiguous terminology for patterns of biological variation that currently lack a formal statistical definition (e.g. 'animal personality' or 'behavioural syndromes'), and facilitate cross-fertilisation between disciplines such as behavioural ecology, ecological physiology and quantitative genetics.

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