Cognitive variation in threespined stickleback (Gasterosteus aculeatus)

Cognition encompasses important mechanisms with which animals are able to adjust their behavior in response to environmental cues. These cognitive processes play a clear role in many fitness-related behaviors such as foraging, predator avoidance, and courtship. Thus, how these processes have evolved are of key scientific interest. Historically, research on the evolution of cognitive traits has largely focused on variation between species. However, particularly in the last couple of decades, there has been increasing interest in examining variation in cognition within a species. These studies no longer look at intraspecific variation as noise, but see it as being potentially adaptive and therefore impacting evolutionary trajectories. Yet, while many of these studies seemingly demonstrate the benefits of different cognitive traits, this has inevitably led to questions about why intraspecific variation is maintained. How costly are these traits? Are there trade-offs that maintain variation? The aim of the research in this dissertation is to study the mechanisms that drive and maintain intraspecific variation in cognition in threespined sticklebacks. This is done from four different angles. First, I tested the hypothesis that learning is part of an overall suite of correlated traits related to how an individual copes with changes in the environment, and that trade-offs between early learning and responsiveness to changes in learning outcomes may maintain cognitive variation. I found that individuals that showed a higher cortisol stress response and that were more reactive to a predatory threat were slower to learn a novel discrimination task, but not necessarily faster to respond when learning conditions changed. Second, I tested the hypothesis that sticklebacks from populations inhabiting different environments are primed to learn different cue associations faster within novel learning conditions. When individuals from two separate populations were trained on either a color vs. spatial discrimination task, the two populations excelled on different tasks: fish from a river habitat performed significantly better on the side version than they did on the color version, while the opposite was observed in fish from a pond habitat. Third, I explored the underlying causes of why some individuals are more responsive than others when there is a change in learning outcomes (i.e., differences in behavioral flexibility) by asking whether individual differences in reversal learning performance were more strongly associated with variation in boldness, neophobia and/or inhibitory control. I found that early performance on reversal learning trials was associated with all three behavioral traits, while time to criterion during reversal learning was independent of the other behaviors. Finally, I took advantage of the radiation of sticklebacks to ask whether behaviors predicted to facilitate adaptation to new environments (i.e., neophilia and inhibitory control) have evolved as stickleback have repeatedly colonized freshwater environments . I found heritable population-level variation in both behaviors, suggesting that increased flexibility has evolved during the stickleback radiation. Altogether these studies highlight the wide range of both intra- and inter-population cognitive variation that can be found in threespined stickleback and further elucidate how trait correlations and ecological differences may drive the maintenance of this variation