TY - JOUR TI - Experimental evidence that group size generates divergent benefits of cooperative breeding for male and female ostriches AU - Melgar, Julian AU - Schou, Mads F AU - Bonato, Maud AU - Brand, Zanell AU - Engelbrecht, Anel AU - Cloete, Schalk WP AU - Cornwallis, Charlie K A2 - Díaz-Muñoz, Samuel L A2 - Rutz, Christian A2 - Kurvers, Ralf HJM VL - 11 PY - 2022 DA - 2022/10/04 SP - e77170 C1 - eLife 2022;11:e77170 DO - 10.7554/eLife.77170 UR - https://doi.org/10.7554/eLife.77170 AB - Cooperative breeding allows the costs of parental care to be shared, but as groups become larger, such benefits often decline as competition increases and group cohesion breaks down. The counteracting forces of cooperation and competition are predicted to select for an optimal group size, but variation in groups is ubiquitous across cooperative breeding animals. Here, we experimentally test if group sizes vary because of sex differences in the costs and benefits of cooperative breeding in captive ostriches, Struthio camelus, and compare this to the distribution of group sizes in the wild. We established 96 groups with different numbers of males (1 or 3) and females (1, 3, 4, or 6) and manipulated opportunities for cooperation over incubation. There was a clear optimal group size for males (one male with four or more females) that was explained by high costs of competition and negligible benefits of cooperation. Conversely, female reproductive success was maximised across a range of group sizes due to the benefits of cooperation with male and female group members. Reproductive success in intermediate sized groups was low for both males and females due to sexual conflict over the timing of mating and incubation. Our experiments show that sex differences in cooperation and competition can explain group size variation in cooperative breeders. KW - cooperation KW - competition KW - sexual selection KW - sexual conflict KW - bird KW - ostrich JF - eLife SN - 2050-084X PB - eLife Sciences Publications, Ltd ER -