Integrating fitness components reveals that survival costs outweigh other benefits and costs of group living in two closely related species

Group living can be beneficial when individuals reproduce or survive better in the presence of others, but simultaneously there might be costs due to competition for resources. Positive and negative effects on various fitness components might thus counteract each other, so integration is essential to determine their overall effect. Here, we investigated how an integrated fitness measure (reproductive values; RV) based on six fitness components varied with group size among group members in cooperatively-breeding red-winged and superb fairy-wrens (Malurus elegans and M. cyaneus). Despite life historie differences between the species, patterns of RVs were similar, suggesting that the same behavioural mechanisms are important. Group living reduced RVs for dominant males, but for other group members this was only true in large groups. Decomposition analyses showed that our integrated fitness proxy was most strongly affected by group size effects on survival, which was amplified
through carr...