Adaptive individual differences : the evolution of animal personalities

Individual differences in behaviour are a ubiquitous phenomenon within animal populations. Great tits (Parus major), for example, differ in the speed with which they explore a novel object (Verbeek et al., 1994), three-spined sticklebacks (Gasterosteus aculeatus) differ in their aggressiveness towards territorial intruders (Huntingford, 1976), and mice (Mus musculus) and rats (Rattus norvegicus) differ in how quickly they solve a maze problem (Benus et al., 1987). Quantitative behavioural differences such as these play an important role in this thesis, but in themselves are hardly surprising. After all, for most quantitative traits, phenotypic differences are to be expected, for example due to noise in the development of the phenotype or stochastic state differences among individuals (e.g., in energy reserves). At first sight, these differences do not seem to call for a deeper explanation. Two basic observations suggest, however, that there may be more to behavioural variation than meets the eye (Sih et al., 2004b). First, behavioural differences are often stable for some period of time. A great tit that explores a novel object faster than a conspecific will tend to be faster also several weeks later (Verbeek et al., 1994). Similarly, the rank order in the level of aggression that individuals show towards territorial intruders tends to remain stable throughout the breeding cycle in sticklebacks (Huntingford, 1976). Second, behavioural differences often extend to a range of different situations and contexts (sensu Sih et al., 2004a) in a systematic way. The faster a great tit approaches a novel object, for example, the faster it explores a novel environment, the more aggressively it behaves towards conspecifics, the quicker it forms rigid foraging habits and the more willing it is to take risks (Groothuis & Carere, 2005). Similarly, the more aggressively a stickleback behaves towards a conspecific territorial intruder during the breeding cycle, the more aggressive it is also towards a heterospecific intruder and the bolder it is when approaching a predator at nonbreeding times (Huntingford, 1976). Such findings are surprising. If behavioural differences were solely due to random factors such as noise in the development of the phenotype, such differences should be uncorrelated, both through ontogeny and across different situations and contexts. The above findings, however, indicate that behavioural differences are often much more structured. Behavioural differences that are maintained through time and across contexts are termed personalities in humans (Pervin & John, 1999) and, in analogy to this, the term animal personalities (also: coping styles, Koolhaas et al., 1999; temperament, Reale et al., 2007; behavioural syndromes, Sih & Bell, 2008) has been adopted in the literature (Gosling, 2001). Throughout this thesis I will follow this usage, using the term animal personalities to refer to behavioural differences that are (i) stable through part of the ontogeny of individuals and (ii) correlated across a range of situations and contexts.