Population-level personalities in zebrafish: Aggression-boldness across but not within populations

Many animals exhibit personality types, for example, with some individuals being consistently both more aggressive and more willing to explore novel situations than others. Often those behavioral syndromes are shaped by pleiotropic genetic and physiological mechanisms or joint selective pressures that constrain variation within and between 2 seemingly different types of behavior. Here, we used wild zebrafish to ask whether the behavioral syndromes typically measured within populations also occur across populations, as might be expected for syndromes created by forces such as a hormone with multiple effects. We found major differences across populations in both aggression and boldness. Zebrafish captured in the wild from running streams in Northern India were both bold and aggressive, approaching predators and conspecifics more frequently than did zebrafish from slower moving irrigation canals or a large, still, lake near Kolkata, India. We did not find any sites that were bold, but not aggressive, or aggressive, but not bold, suggesting that these behavioral combinations are not physiologically possible or that they are eliminated by population-level selection. Within populations, however, we found evidence for an aggression-boldness syndrome within only 1 of 5 measured populations (from an irrigation canal). Zebrafish also became markedly more aggressive after 3 months in a laboratory environment. These results offer a natural example of population-level personality types in wild organisms and show that population-level patterns and environmental plasticity may be as important as within-population constraints. Our work also confirms that wild zebrafish offer promising insight into higher order phenomena such as clade diversification and selection.