Methods for measuring mammalian personalities: In which animals and how accurately can we quantify it?

Abstract

<jats:title>Abstract</jats:title> <jats:p>The study of personality, and individual differences in behaviour has experienced a steady rise in popularity in the past years. In this review and meta-analysis, we aim to introduce the concept of personality and related phenomena. A behavioural trait should meet two basic conditions to be considered a personality trait – it should be consistent (1) in time and (2) across contexts. In mammals, the two most common orders in personality studies are primates and rodents. We therefore introduce different approaches to personality testing in these two orders. Primate personality studies are based on psychology studies and often rely on the observer’s ratings. Rodent personality studies originate in the studies of physiology and use an experimental approach. We present a more detailed overview of methodological issues of repeatability as a statistical tool for measuring consistency across time. The classic methods of computing repeatability do not consider habituation and other trends which may become confounding factors and lead to underestimation of repeatability. We also discuss consistency across contexts and different understandings of the context definition. We illustrate the variability of personality studies in mammals with a meta-analysis of repeatability estimates. We found that repeatability of behaviour depends on the methodology of behavioural testing and statistical analyses used, but also the number of test repetitions and differences between the focal behaviours. Repeatability decreased with more repetitions and the tests of aggressiveness and exploratory behaviour yielded lower repeatability estimates than the tests of activity.</jats:p>

Inter-individual differences in laboratory rats as revealed by three behavioural tasks

“…Particularly songbirds and rats have been studied the most in this regard 5 . Even though inter-individual behavioural differences are more pronounced in wild and wild-derived animals 1 , 6 , 7 , some differences may also be found between laboratory animals 1 . In laboratory animals, however, the focus is on the differences in personality traits between various strains 8 .…”

“…Activity may be measured in any experimental setup and it is quite easily assessed by simply measuring the path the animal travels (for references see Table S1 ). The exploration—avoidance axis represents the reaction to a new (not necessarily risky) situation 4 and in rodents, it is usually assessed in experiments such as Open field test (empty arena), Hole board test (arena with holes in the ground), or in more complex novel environment designs (for references see 6 or Table S1 ). The third most commonly tested axis is boldness, which corresponds to the reaction of the individual to a risky situation 4 .…”

“…Individual scores on these axes should comprehensively describe animal behaviour in any experimental or real-life situation. Nevertheless, most studies do not test the full spectrum of animal behaviour, but rather focus on traits corresponding to only one or two axes (see a meta-analysis 6 or a review 2 ; see also Table S1 in Supplementary materials for experiments commonly used for extraction of personality axes). Correlation of any two axes is referred to as a behavioural syndrome, which is manifested when the behaviour of an animal in one context is dependent on its behaviour in another situation, possibly reflecting limited behavioural plasticity 16 , 17 .…”

“…Although it is assumed that inter-individual differences in behaviour are relatively stable in time, it does not mean the behaviour of an individual animal cannot change 6 , 18 . Nevertheless, relative differences among individuals should remain consistently stable throughout different contexts 19 – 21 .…”

“…When testing for behavioural consistency, two types of repeatability estimates can be distinguished. The first one, agreement repeatability (R A ), expresses the reproducibility of the absolute scores the of behaviour of the animals tested, therefore it does not account for changes of behaviour in time 6 , 22 , 23 . In general, repeatability estimates vary from 0 (no repeatability) to 1 (maximum repeatability), with the average R A estimate of personality traits of 0.37 1 .…”