Female indris determine the rhythmic structure of the song and sustain a higher cost when the chorus size increases

Among the behavioral traits shared by some nonhuman primate species and humans there is sing- ing. Unfortunately, our understanding of animals’ rhythmic abilities is still in its infancy. Indris are the only lemurs who sing and live in monogamous pairs, usually forming a group with their off- spring. All adult members of a group usually participate in choruses that are emitted regularly and play a role in advertising territorial occupancy and intergroup spacing. Males and females emit phrases that have similar frequency ranges but may differ in their temporal structure. We exam- ined whether the individuals’ contribution to the song may change according to chorus size, the total duration of the song or the duration of the individual contribution using the inter-onset inter- vals within a phrase and between phrases. We found that the rhythmic structure of indri’s songs depends on factors that are different for males and females. We showed that females have signifi- cantly higher variation in the rhythm of their contribution to the song and that, changes according to chorus size. Our findings indicate that female indris sustain a higher cost of singing than males when the number of singers increases. These results suggest that cross-species investigations will be crucial to understanding the evolutionary frame in which such sexually dimorphic traits occurred.