The causes and consequences of heterospecific aggression in brook stickleback (Culaea inconstans)

Intrinsic agonistic behaviour between heterospecifics may cause niche shifts that promote diversifying selection and phenotypic divergence between coexisting species that mimics divergence driven by exploitative competition over resources. Agonistic behaviour may also evolve over defendable resources in sympatry. Several lakes with coexisting Culaea inconstans and Pungitius pungitius and solitary C. inconstans populations occur in northern Ontario. In an observational study, C. inconstans in sympatry were found to be more aggressive towards heterospecifics than in allopatry, and all C. inconstans populations were more aggressive than sympatric P. pungitius. In an experimental laboratory study, allopatric and sympatric C. inconstans reared in a common environment exhibited heritable variation in the agonistic traits that distinguished allopatric from sympatric adults and juveniles. This is the first study that indicates C. inconstans may have aggressively forced P. pungitius into a different niche, and that C. inconstans has evolved increased aggression when coexisting with P. pungitius.