Chestnut-crowned babblers show affinity for calls of removed group members: a dual playback without expectancy violation

Abstract

Cooperative breeding typically evolves within discrete, stable groups of individuals, in which group members derive direct and/or indirect fitness benefits from cooperative behaviour. In such systems, strong selection on group discrimination should emerge. Despite this prediction, relatively few studies have investigated the mechanism of group discrimination in cooperative vertebrates, and the results of many may be confounded by ‘expectancy violation’, since test individuals from which stimuli were derived remained in the group during experimentation. Here, we used a novel experimental protocol that eliminates this confounding effect in a test of group discrimination in cooperatively breeding chestnut-crowned babblers, Pomatostomus ruficeps , using long-distance contact calls. Long-distance contact calls were found to be individually specific, but to lack obvious group level signatures. Using dual playbacks of removed group and nongroup members, we found that these calls allow effective discrimination: groups unanimously approached the speaker broadcasting calls of group members and concomitantly increased their contact call rates. Together, our results suggest that group discrimination emerges from individual recognition. Additionally, the affiliative behaviour of group members towards playbacks of removed members contrasts with the aggressive responses towards nongroup members found in all other studies. One explanation for these differences stems from our elimination of expectancy violation, but further studies are required to verify this.