Plural breeding among unrelated females and other insights on complex social structure in the cooperatively breeding Variegated Fairywren

Abstract

ABSTRACT Cooperatively breeding species vary widely in degree of social complexity, and disentangling relationships among group members can reveal the costs and benefits of cooperation. Here, we describe the social system of the cooperatively breeding Variegated Fairywren (Malurus lamberti) and explore how social complexity and group dynamics may affect cooperation and conflict. We used a combination of field-based population monitoring and detailed social association observations to determine group membership annually across four breeding seasons (2014–2017) and used a ddRAD-seq genotyping method to determine genetic relationships within social groups. Social groups ranged in size from two to eight individuals and nearly half of all social groups had multiple adult individuals of both sexes. Approximately half of all groups exhibited plural breeding, in which multiple females within the same social group nested individually in the same territory. Genetic relationships were diverse among social groups due to high rates of extra-pair paternity and immigration, with most groups containing close relatives and non-relatives of each sex. Notably, although related females were often present within a social group, co-breeding females in the same social group were never closely related to each other. Given the extensive variation in relatedness among group members, cooperation in the Variegated Fairywren is likely maintained by a combination of direct and indirect fitness benefits.