Population-dependent conflict between individual sociability and aggressiveness

In some species, populations with the highest average aggressiveness also have the lowest sociability, suggesting a conflict between the expression of these behaviours. While this negative relationship has been found between populations, whether the same relationship can be found within a population, where the most aggressive individuals are also the least social ones, has not been tested in vertebrates. In the present study, we found a negative correlation between aggressiveness and sociability in only one of two populations of juvenile threespine sticklebacks, Gasterosteus aculeatus, reared in a common environment. Nevertheless, highly aggressive individuals that were also highly social were absent in both populations. Our results suggest that specific constraints or selection pressures resulted in the absence of this behaviour combination in both populations. Our findings also suggest that population-specific selection pressures may have resulted in the negative correlation present in only one population, although an effect of genetic drift or population-specific maternal effects are also possible. Our results do not support the hypothesis that this correlation was due to genetic constraints.