Female ground tits prefer relatives as extra-pair partners: driven by kin-selection?

Area of Science:

• * Behavioral Ecology
• * Evolutionary Biology
• * Ornithology

Background:

• * Socially monogamous female birds frequently engage in extra-pair mating, with genetic benefits proposed as a primary driver.
• * Hypotheses include inbreeding avoidance, outbreeding avoidance, and kin selection, each predicting different mating preferences based on relatedness.
• * The ground tit (Parus humilis), a facultative cooperative breeder, serves as a model system to test these hypotheses.

Purpose of the Study:

• * To investigate the genetic relatedness preferences of female ground tits when selecting extra-pair mating partners.
• * To test the predictions of inbreeding avoidance, outbreeding avoidance, and kin-selection hypotheses in a wild bird population.
• * To understand the evolutionary drivers behind female extra-pair mating strategies in socially monogamous species.

Main Methods:

• * Analysis of social pair formation relatedness in ground tits.
• * Assessment of female extra-pair mating decisions in relation to relatedness with social mates and potential extra-pair partners.
• * Examination of relatedness patterns between females and their extra-pair sires (both within and outside the social group).

Main Results:

• * Female ground tits showed no avoidance of closely related social mates but actively preferred extra-pair partners to whom they were more related.
• * Females selected extra-pair sires (within or outside the group) with higher relatedness than expected by chance.
• * Extra-pair mating occurred at an intermediate inbreeding level without apparent fitness reduction, supporting the kin-selection hypothesis.

Conclusions:

• * The findings strongly support the kin-selection hypothesis, indicating that female ground tits preferentially engage in extra-pair mating with relatives.
• * This preference suggests a potential inclusive fitness benefit for females, challenging previous explanations focused solely on inbreeding avoidance.
• * The study provides a novel perspective on the evolution of extra-pair mating in birds and highlights the role of kin selection in cooperative breeding systems.