Should I stay or should I go? Consistent individual differences in migration tendency

Polymorphisms in life-history strategies and behavioural tendencies are wide-spread in nature and an outstanding question is how they are shaped by natural selection. Partial migration is an intriguing example. In many species, only a subset of individuals undertakes migration while the rest stays back in their natal environment. Studies have revealed that migrants tend to be a non-random subset of the population, differing in morphology (e.g., wings, armament), physiology (e.g., metabolic rate) and behavioural tendencies (e.g. boldness, exploration). We study partial migration in three-spined sticklebacks (Gasterosteus aculeatus), in river-based and land-locked populations in the North of the Netherlands. Their distribution over a variety of habitats, the occurrence of morphological and behavioural variation, their social and parental system, and the many tools available for their study make them ideally suited for investigating the causes and consequences of partial migration. Our first empirical results, based on the comparison of migrants with individuals from land-locked populations, show that there are systematic morphological and behavioural differences between the migratory and non-migratory forms, suggestive for the existence of a migration syndrome. We will discuss these differences in the light of various competing hypotheses and investigate the plausibility and empirical relevance of these hypotheses on the basis of individual-based evolutionary simulations.