Among the beautiful, colourful birds inhabiting the Andean mountains of Ecuador, plain-tailed wrens seem rather underwhelming; little drab-looking birds hopping around bamboo thickets looking for the insects that will be their next meal. However, despite their dull appearance, plain-tailed wrens’ songs are anything but plain. Like a stereo recording, you can hear the song coming from two places at once, as it is a perfectly coordinated duet where the male and female rapidly alternate syllables of the tune. The syllables don’t overlap, as each of the singers leaves gaps where their partner interjects with remarkable precision.
A team of scientists from the US and Ecuador, led by Eric Fortune at Johns Hopkins University, investigated how plain-tailed wrens are able to coordinate their amazing duet. From over 150 h of audio recordings, the scientists were able to extract and analyse over 1000 songs from plain-tailed wrens. They discovered that while most of the time pairs of wrens sang together, sometimes both males and females sang by themselves, each singing their part of the song, leaving gaps where their counterpart would normally interject. Because the duration of these gaps was larger and more variable during solo singing than during duets, the authors conclude that wrens do not just follow a fixed pattern when producing their song, but instead rely on auditory cues from their partner to determine the length of the gaps between syllables. Interestingly, the songs of solo males were more variable and infrequent than those of females, who were frequently recorded singing by themselves. Moreover, sometimes males made mistakes during a duet, failing to sing their part of the song. On these occasions, the female would continue singing her part, leaving larger gaps between her syllables until the male joined in again. These observations suggest that female plain-tailed wrens may be the leading duetting partner.
Fortune and his team also examined how the brain of the wrens encoded the song. They captured six birds and recorded the responses of individual neurons in the part of the brain responsible for learning and production of songs (the high vocal centre). The scientists played back the birds’ own duets as well as individual syllables from the male and female singers. The reaction of both the males and females was strongest to the duet, and was larger than the reactions to either the male or female solos or even the sum of the two responses together. However, both males and females exhibited a more pronounced response to the female syllables alone than to the male song. These results demonstrate that the complete song is encoded in both male and female wren brains and, again, suggests that females take the leading role.
The findings from this study might reveal the mechanisms of cooperative behaviour that occurs among other animals. Each partner needs to know the part they play, but they also need to be able to receive cues from their partner in order to know when and how to play their own part. Moreover, they both need to be more tuned in to the leader’s cues for the operation to succeed. It takes two to tango, after all.
