Unpredictable animals: individual differences in intraindividual variability (IIV)

When an individual is repeatedly observed or tested in the same context, it does not always express the same behaviour. Intraindividual variability (IIV) refers to the short-term, unpredictable, reversible variation in behaviour that often occurs in this situation. Although individual differences in IIV have been well documented in humans, this topic has been virtually ignored by researchers studying other animals. Here, we review evidence from humans and animals that IIV can vary in important ways across individuals (e.g. as a function of age or prior experience) and that individual differences in IIV may be related to differences in performance. However, most statistical models currently used to study individual differences in behaviour in animals rely on the assumption that IIV does not vary across individuals. Using ‘boldness’ data for hermit crabs, Pagurus bernhardus, and Wardʹs damselfish, Pomacentrus wardi, we show how to measure IIV when behaviour systematically changes over a series of observations (e.g. as a result of habituation), and how to avoid the adverse effects of censored data on estimates of IIV. After controlling for systematic changes in behaviour over time, we observed strong, significant individual differences in IIV in both species. That is, some individuals were much more predictable in the same situation than were others. We conclude by discussing proximate and ultimate factors that might have contributed to interindividual variation in IIV in these species, and the implications of our findings for methods currently used to study individual differences in behaviour in animals.