Large-brained birds display lower extra-pair paternity.

Extra-pair paternity (EPP) benefits to improve the reproductive success via extra-pair fertilizations without the costs of parental care in males and through improved offspring quality with additional food and parental care in females among species of birds. Variations in the EPP appear to link to behavioral and ecological factors and sexual selection. According to the "relationship intelligence hypothesis", the cognitive abilities of the birds play an important role in maintaining long-term relationships. Here, we undertook the first comparative test of the relationships between extra-pair paternity and brain size, testis size, and life histories among 315 species of birds using phylogenetically controlled comparative analyses and path analysis. After controlling for the effects of shared ancestry and body mass, the frequency of EPP was negatively correlated with relative brain size, but positively with testis size across species of birds. However, the frequency of EPP was not linked to life-history traits (e.g. incubation period, fledging period, clutch size, egg mass, and longevity). Our findings suggest that large-brained birds associated with enhanced cognitive abilities are more inclined to maintain long-term stable relationships with their mates and to mutualism with them than to increase the frequency of EPP.