Hormonal and behavioural responses to simulated territorial intrusion in the cooperatively breeding white-browed sparrow weaver, Plocepasser mahali

Abstract

Abstract. Behavioural investigations of stable groups of white-browed sparrow weavers indicate that they not only bred cooperatively, but also defended territories from other groups. Territory defence involved regular patrolling of boundaries plus a characteristic chorus vocalization that was given by all group members in concert. Song was performed exclusively by the breeding male and only during the breeding season. Territorial choruses were given by all members of the group at all seasons and could also be initiated by all members, although the breeding male and female initiated more choruses than helpers. Simulated territorial intrusions (achieved by placing a caged group within a territory and playing back tape-recorded choruses through a speaker placed alongside) resulted in dramatic increases in territorial aggression by all group members. Plasma levels of testosterone were not affected by the intrusions in breeding birds or helpers. However, plasma levels of luteinizing hormone were significantly elevated in the breeding females after challenge. These data are consistent with other observations in this species that territorial aggression is independent of circulating testosterone, but is correlated with luteinizing hormone. Corticosterone levels were higher in controls than in birds exposed to simulated territorial intrusions with the exception of breeding females. Although the significance of this latter result is unclear, it does seem likely that 'psychological castration' of non-breeders is not mediated through elevated circulating levels of glucocorticosteroids.