All animals face a complex environment full of obstacles that they must overcome in order to survive and reproduce. How an individual responds to its environment is essential to overcoming such obstacles in order to maximize fitness. In my thesis, I focused on the roles of persistence and aggression in achieving fitness-relevant goals. Persistence is continuing in a course of action in spite of difficulty or resistance, and aggression is any instance where an individual uses physical, and potentially damaging, force against a conspecific. I used fruit flies (Drosophila melanogaster) as a model system to examine the ways in which males use persistence and aggression to attain fitness-relevant goals such as defending resources, gaining access to females, and mating. I first examined how a male’s age affected his persistence in courting recently mated females, who are generally unreceptive, and found that older males were more persistent than younger males (Chapter 2). Next, I showed that males of different ages differed in their courtship persistence in the presence of a competitor, and that males were able to subtly, but directly, interfere with one another’s courtship attempts (Chapter 3). I then demonstrated how males were able to use aggression in a mate guarding context to reduce the likelihood that a competitor male mated with their recent mate (Chapter 4), and as a form of resource defense to defend a desirable food patch in the presence of a potential mate (Chapter 5). Finally, I considered male persistence in the pursuit of unreceptive females as a form of male sexual aggression towards recently mated and sexually immature females (Chapters 5 and 6). Overall, my thesis work demonstrates how complex, and sometimes intertwined, the roles of persistence, aggression, and sexual coercion can be even within a ‘simple’ model organism, such as the fruit fly. / Dissertation / Doctor of Philosophy (PhD) / All animals face obstacles from their environment that they must respond to in order to survive and reproduce. In this thesis, I focused on how males use aggressive and persistent behaviours to overcome environmental obstacles. Aggression is the use of physical force against another individual, and persistence is when an individual continues in a course of action despite difficulties. I used fruit flies as a model to examine how males fight with one another over access to food and potential mates. I also studied how males persistently harass females in order to mate with them. In many experiments, I found that males use a combination of aggressive and persistent behaviours to achieve a goal. Overall, my results show that even in a simple species like fruit flies, individuals can use complex combinations of behaviours to achieve a variety of goals.

A rich diversity of social behaviours exists in the animal kingdom, and these behaviours have evolved to perform a variety of adaptive functions. Social behaviours show variation both among and within species, however the mechanisms that give rise to this variation are not well understood. Using fruit flies (Drosophila melanogaster), my goal was to uncover the genetic and behavioural mechanisms that underpin natural variation in two different social behaviours: sociability and sexual aggression. First, I showed that sociability, which is the tendency of animals to engage in friendly activities together, is influenced by indirect genetic effects (IGEs), and that encounters among individuals drive these effects (Chapter 2). I then showed that sociability and social plasticity have low-moderate heritability (Chapter 3), and sociability is not correlated between the sexes or with activity. I then generated lineages of flies with high and low sociability using artificial selection (Chapter 4). The evolved lineages had significantly diverged sociability which was not associated with fitness measures or nearest-neighbor distances, but was negatively correlated with intrasexual aggression (Chapter 4). Finally, in sexual aggression, which I quantified as male forced copulation rate, I showed that evolved differences and differences due to social plasticity were both associated with the differential expression of many genes, but only a few of these genes were significant in both (Chapter 5). I also showed that these sets of genes are enriched in neuropeptide hormone and serotonin gene ontology categories, and that 4 of 7 chosen genes were validated for their effects on sexual aggression. Overall, this thesis sheds light on the complex mechanisms that underlie variation in these social behaviours, and it paves the way for future research to further elucidate some of these mechanisms, especially on the genetic basis of sociability using the evolved lineages I generated. / Thesis / Doctor of Philosophy (PhD) / Individual animals tend to vary in many traits including social behaviours. Using fruit flies, my goal was to understand what causes individuals to vary in two social behaviours: sociability and sexual aggression. I found that highly sociable flies tended to influence other flies to become more sociable due to a change in how much these flies interacted. I also found that individual differences in sociability are moderately heritable, and the genetic variation contributing to this is different between the sexes. Also, less sociable flies tended to be more aggressive than highly sociable flies. Finally, for sexual aggression, I showed that variation in a male’s success in forcibly mating with a female was associated with changes in the expression of hundreds of genes, but these changes were mostly unique for evolved versus environmentally induced variation. Future work will similarly look to identify genes involved with individual differences in sociability.