Behavioural ecology: promiscuous fathers sire young that recognize true family.

Most theories of kin selection assume that animals are able to distinguish relatives from non-relatives. This is especially difficult in situations where mixed parentage precludes that relatedness is recognised by familiarity. Recent work shows that, within the same brood, young bluegill sunfish that are fathered by cuckolders--but not those sired by parental males--pick out their relatives using self-referent phenotype matching and not familiarity.