Thys, Bert

Abstract: Within animal populations, competition is often most intense among individuals of the same sex since such individuals require the same limited resources to maximize reproductive success. Although competition between females is widespread \u2013 including in species with traditional sex-roles \u2013 the functional significance of, and nature of selection on, individual differences in female competitive trait expression remains poorly understood. Crucial in this regard is that many traits with a potential role in female same-sex competition (e.g. same-sex aggression, plumage traits in birds) are typically repeatedly expressed across an individual\u2019s life (i.e. so-called labile traits) and can therefore (co)vary both among and within individuals. Acknowledging this multilevel and multivariate nature of phenotypic variation in labile traits is paramount since it is the (co)variation occurring on the among-individual level that represents the raw material for selection to act on. In this thesis, using free-living great tits (Parus major) and repeated measurement designs on the same individuals within and across years, I firmly integrated female competitive trait expression into a multilevel and multivariate framework with other behavioural traits (i.e. antipredator boldness and exploratory behaviour). Subsequently, revolving around the notion of life history trade-offs, I aimed at identifying the short-term and lifetime fitness consequences associated with individual differences in female behavioural phenotypes. In the case of behaviour, among-individual variation in average trait expression across repeated observations is referred to as animal personality and when two or more behaviours covary on the among-individual level they form a so-called behavioural syndrome. Using repeated behavioural observations on the same females both within and across years (i.e. across short and longer time scales), I demonstrated that females differed in their average level of same-sex aggression (during simulated territorial intrusion), antipredator boldness (as manifested by hissing behaviour) and exploratory behaviour (in a novel environment) (Chapters 1-3). Moreover, female aggression formed a behavioural syndrome with exploratory behaviour (Chapter 1), but not hissing behaviour (Chapter 3), indicating that female aggression can covary with other ecologically relevant behaviours on the among-individual level. At the same time, substantial amounts of the total phenotypic variation in all studied behaviours, both within and across years, occurred on the within-individual level (i.e. behavioural plasticity). Using a behavioural reaction norm approach, I demonstrated that a part of the within-year plasticity in hissing behaviour was explained by the reproductive value (i.e. age) of the offspring (Chapter 2). Individual plasticity in female aggression was instead related to female age, where aggression decreased within females across their lifespan. Moreover, females differed in their degree of age-related plasticity (i.e. an individual-by-age interaction), whereby on average more aggressive females showed a steeper age-related decline in aggression compared to less aggressive ones. As a result, females became more similar in their aggression with age and the amount of among-individual variation varied across age-classes (Chapter 3). I also found evidence for age-related plasticity in the size of a (annually regrown) melanin-based plumage patch, the black breast stripe, which increased within females with age. Interestingly, the relationship between this plumage patch and female aggression also varied across age-classes, suggesting an age-dependent signalling function of this plumage trait in the context of female competition (Chapter 5). Together these results highlight that explicitly adopting a multivariate and lifetime perspective is a crucial step towards identifying proximate causes and ultimate consequences of individual differences in female behavioural trait expression. With regard to proximate causes, molecular genetic studies can provide essential information on the specific genetic architecture underlying individual differences in (behavioural) phenotypes. Recent progress in this field enables to quantify variation in genes in (non-model) free-living animal species, thereby vastly improving our insights into the evolution of individual differences in (behavioural) phenotypes in the wild. In some model species, allelic variation in the gene coding for the serotonin transporter (SERT) has been found to influence the expression of anxiety- and aggression-related behaviour. Yet, using a free-living bird species, I revealed no strong support for associations between genomic sequence variation in the exonic (i.e. coding) regions of the SERT gene and individual differences in female aggression, even when assessing potential age-dependent effects of SERT genotype on aggression (i.e. no genotype-by-age interaction; Chapter 4). SERT genotype was also not strongly associated with variation in hissing behaviour, although I identified two genetic polymorphisms of particular interest for future research. These findings illustrate that the role of the SERT gene in the expression of aggression- and antipredator-related behaviours in the wild remains equivocal, emphasizing the need for replicated studies across a wide variety of free-living species. From an ultimate perspective, understanding how individual differences in behaviours are maintained within populations remains one of the central challenges within evolutionary ecology. Life history theory predicts that individuals differ in how they resolve the trade-off between current reproduction and future survival and that behaviours mediate this trade-off at the individual level. Specifically, adaptive individual differences in behaviour are predicted to result from divergent life history strategies with more aggressive and bold individuals investing heavily into current reproduction but dying early in life. In contrast to this prediction, I revealed a lack of associations between individual differences in behaviours (female aggression, hissing behaviour) and both survival to the next year and female longevity, indicating the overall absence of survival selection on these behavioural traits across the four-year study period (Chapter 3). On the other hand, individual differences in behaviour were related to the investment into current reproduction, yet not necessarily in the same direction or via the same mechanisms across behavioural traits and/or years (Chapter 6). Specifically, using path analyses on two years of data, I demonstrated that female provisioning rate played a role in linking female aggression with fledgling mass in one year, but not the other. Hissing behaviour, instead, directly affected fledgling mass in both years, although via mechanisms that remain to be identified. Importantly, these patterns would have remained undetected if I had failed to apply a multivariate and path-analytical approach. In a subsequent analysis, I revealed that annual offspring recruitment selection also varied across behavioural traits and years. Together, my results therefore appear to suggest that heterogeneous selection across years, instead of life history trade-offs between current reproduction and future survival, is a likely mechanism maintaining the observed individual differences in female behavioural phenotypes. Overall, my thesis highlights the importance of acknowledging the multilevel and multivariate nature of phenotypic variation in behavioural (including competitive) trait expression across the lifetime of females. Moreover, integrative and multivariate approaches were proven particularly useful in identifying whether and how selection acted on individual differences in female behavioural phenotypes in the wild. Although revolving around female behavioural (including competitive) trait expression, the results in this thesis also directly contribute to a more fundamental understanding of the expression of labile traits in general

Integrating female competition within a multivariate and life history framework

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