It is all relative: population estimates enhance kin recognition in the guppy

Kin recognition plays a fundamental role in social evolution, enabling active inbreeding avoidance, nepotism, and promoting cooperative social organization. Many organisms recognize kin based on phenotypic similarity - a process called phenotype matching - by comparing information associated with their own phenotype against the phenotypes of conspecifics. However, recent theory demonstrates that to accurately judge phenotypic similarity (and hence, relatedness), individuals require estimates of the populations distribution of phenotypes as a "frame of reference." Here, I use the Trinidadian guppy (Poecilia reticulata) to provide the first empirical test of this population estimation theory. I varied the phenotypic distributions of the groups in which focal individuals developed and found that, as adults, their patterns of inbreeding avoidance and nepotistic intrasexual competition differed as predicted by population estimation theory. Individuals reared with conspecifics more similar to themselves treated novel conspecifics as less closely related, suggesting a shifted population estimate. Individuals reared with more phenotypically variable conspecifics exhibited less extreme kin discrimination, suggesting a broader population estimate. These results provide experimental evidence that population estimates inform phenotype matching, and are acquired plastically through social experience. By calibrating phenotype matching to the population distribution of phenotypes, population estimation enhances kin recognition, increasing opportunities for the evolution of inbreeding avoidance and nepotism.