Causes and consequences of individual variation in anti-predator traits

Sammanfattning: Predation imposes a potent evolutionary force on prey, moulding the evolution of traits and diversification of species, and can furthermore have powerful ecological consequences. We know that different species vary in how they cope with their natural enemies, and, consequently, a huge range of anti-predator defences are displayed in nature. Yet there is a growing recognition that, if we zoom into a certain species, or even into a single population of animals, there is still a huge and intriguing amount of individual trait variation. In this thesis I investigate what drives individual variation in anti-predator traits, and the implications of this variation for individual animals in two contrasting systems. Specifically, I explore what underlies inter-individual variation in an induced morphological defence in crucian carp and additionally investigate individuality in migratory strategy, risk-taking behaviour and morphology in another common freshwater fish, the roach. I document links between personality-trait variation and anti-predator strategies suggesting that some individuals engage in compensatory strategies to offset some of the costs associated with one trait with the benefits of another. Individuals with a risk-prone personality type (bold individuals) express a greater morphological defence (crucian carp) and are more likely to undertake a winter migration away from predator-rich lakes (roach) as compared to shyer ones. I also show that bold individuals pay a higher predation cost than individuals that adopt a risk-averse behavioural strategy. Furthermore, roach individuals exposed to perceived predation risk (i.e. to a live predator) prior to the migratory period have a higher migration propensity as compared to roach not experiencing experimental predation risk. I also show that variation in migratory timing differs seasonally, and that, at an individual level, survival is strongly linked with the timing of spring but not with autumn migration. Hence, individual variation in the timing of the migratory journey has powerful fitness consequences. Finally, I explore the morphological correlates of migratory behaviour. I show that body morphology is related to migratory life history, as fish from open lakes (with migration opportunity) and migratory individuals from a partially migratory population have more slender, hydrodynamic morphologies as compared to specimens from closed lakes (no migration opportunity) and residents. In summary, my results highlight the powerful role that ecological forces, such as predation, can have in shaping patterns of individual variation in wild populations of animals, and, importantly, that these different behavioural strategies can have profound consequences for survival and thereby individual fitness. Individual variation in survival as a function of phenotype is the raw material upon which natural selection acts, and yet the ecological drivers of evolution remain poorly studied in many wild populations. Working at the individual level in the wild poses significant challenges, but can provide rich insights into the mechanisms of the interplay between ecosystem dynamics and evolutionary processes.