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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.