Aggression as a personality trait: Mechanisms and outcomes

Behaviour mediates all aspects of an individual‚ÄövÑvºs life and can act as the link between ecology and evolution, influencing population dynamics and eco-evolutionary processes which affect evolutionary pathways. In the past decade, consistent intra-individual differences in behaviour, known as animal personality, have become an increasing focus of attention in behavioural ecology and evolutionary biology. However, despite this increased attention in the field, few studies have considered both the proximate mechanisms and ecological outcomes of animal personality within a single, free-living, unmanipulated population. I addressed this shortcoming in Egernia whitii, a social Australian skink species. I first examined five key personality traits (aggression, boldness, exploration, activity and sociability) in E. whitii. I found that the structural consistency of traits varied in both a temporal and sex-specific manner, and furthermore, the majority of the traits exhibited only moderate to no intra-individual consistency. Aggression was the only trait that exhibited both structural consistency and strong intra-individual consistency over time. Based on this, and previous work in this species, I focused the remainder of this thesis on the personality trait of aggression. I examined the physiological basis of aggression to understand the proximate mechanism(s) that maintains consistent intra-individual variation in aggression and the ultimate outcomes of aggression by examining its ecological role in E. whitii. Hormones and intra-individual differences in metabolism have both been proposed as key proximate physiological mechanisms that organize and maintain correlated suites of behaviour as personality traits. The sex-steroid testosterone in particular facilitates the activation of aggressive behaviour and is important for enabling individuals to mount an aggressive response to conspecific challenges in territorial/mate/offspring defense. Individual E. whitii displayed consistent intra-individual differences in both aggression and baseline circulating plasma testosterone concentrations. However, contrary to the majority of literature that indicates a positive relationship between aggression and testosterone, male E. whitii displayed a negative relationship between circulating plasma testosterone concentrations and aggression at both the baseline level and during the up-regulation of testosterone following an aggressive challenge. An aggressive challenge also resulted in a disruption of intra-individual consistency in circulating plasma testosterone concentrations. Manipulating testosterone concentrations had no corresponding influence on either mean levels of aggression or intra-individual consistency in aggression. While in females there was no association between aggression and aspects of the oxidative stress system (they by-products of the metabolic process), there was a positive association between aggression and antioxidant capacity in males. This suggests that in males, aggression may induce an increased oxidative challenge resulting in an elevation of antioxidant defense. In addition to examining the proximate physiological mechanisms underpinning aggression, I also examined the ecological outcomes of aggression in this species. Competition between individuals is a key component of the agonistic intrasexual interactions that influence resource acquisition, social system dynamics, and ultimately reproductive success. Traits that therefore influence success in competition should be favored in selection. Although aggression in males did not predict the outcome of competition in the laboratory, or the outcome of competition in the field (i.e. territory size, territory overlap, or reproductive output), aggression was important to males (in terms of reproductive output) when habitat quality was limited. This suggests that the importance of aggression is context dependent in male E. whitii. Spacing of E. whitii within the habitat also appears to be determined by habitat quality, with more aggressive individuals found in warmer areas. Additionally, female reproductive effort is influenced by both environmental variation and her aggressive phenotype. These results indicate important ecological outcomes to aggression in this species, with aggression influencing both territory spacing, female reproductive effort, and (in conjunction with environmental variation), male reproductive output, processes which will likely influence population, and potentially evolutionary, dynamics. This thesis represents an integrated, holistic examination of personality in a free-living vertebrate population. It adds to the growing body of work suggesting that the relationship between testosterone and aggression is not straightforward, supports the calls for considering metabolism as a proximate underpinning of personality and indicates that the role that aggression plays in eco-evolutionary dynamics is an exciting area of further research.