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Proceedings of the National Academy of Sciences of the United States of America






e2219345120 (1-7)


Although over 50 y have passed since W. D. Hamilton articulated kin selection and inclusive fitness as evolutionary explanations for altruistic behavior, quantifying inclusive fitness continues to be challenging. Here, using 30 y of data and two alternative methods, we outline an approach to measure lifetime inclusive fitness effects of cooperative polygamy (mate-sharing or cobreeding) in the cooperatively breeding acorn woodpecker Melanerpes formicivorus. For both sexes, the number of offspring (observed direct fitness) declined while the number of young parented by related cobreeders (observed indirect fitness effect) increased with cobreeding coalition size. Combining these two factors, the observed inclusive fitness effect of cobreeding was greater than breeding singly for males, while the pattern for females depended on whether fitness was age-weighted, as females breeding singly accrued greater fitness at younger ages than cobreeding females. Accounting for the fitness birds would have obtained by breeding singly, however, lifetime inclusive fitness effects declined with coalition size for males, but were greater for females breeding as duos compared to breeding singly, due largely to indirect fitness effects of kin. Our analyses provide a road map for, and demonstrate the importance of, quantifying indirect fitness as a powerful evolutionary force contributing to the costs and benefits of social behaviors.


Copyright © 2023 the Authors.

This article is distributed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0).

Data Availability

Article states: "All study data are included in the article and/or SI Appendix."

Original Publication Citation

Koenig, W. D., Barve, S., Haydock, J., Dugdale, H. L., Oli, M. K., & Walters, E. L. (2023) Lifetime inclusive fitness effects of cooperative polygamy in the acorn woodpecker. Proceedings of the National Academy of Sciences of the United States of America 120(19), 1-7, Article e2219345120.


0000-0002-9414-5758 (Walters)

WaltersAppendix01.pdf (248 kB)
Extended Methods, Figures, Tables, Legends, SI

WaltersAppendix02.csv (53 kB)
Dataset S01

WaltersAppendix03.csv (53 kB)
Dataset S02

WaltersAppendix04.csv (17 kB)
Dataset S03


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