A crucial, common feature of speech and music is that they show non-random structures over time. It is an open question which of the other species share rhythmic abilities with humans, but in most cases the lack of knowledge about their behavioural displays prevents further studies. Indris are the only lemurs who sing. They produce loud howling cries that can be heard at several kilometres, in which all members of a group usually sing. We tested whether overlapping and turn-taking during the songs followed a precise pattern by analysing the temporal structure of the individuals' contribution to the song. We found that both dominants (males and females) and non-dominants influenced the onset timing one another. We have found that the dominant male and the dominant female in a group overlapped each other more frequently than they did with the non-dominants. We then focused on the temporal and frequency structure of particular phrases occurring during the song. Our results show that males and females have dimorphic inter-onset intervals during the phrases. Moreover, median frequencies of the unit emitted in the phrases also differ between the sexes, with males showing higher frequencies when compared to females. We have not found an effect of age on the temporal and spectral structure of the phrases. These results indicate that singing in indris has a high behavioural flexibility and varies according to social and individual factors. The flexible spectral structure of the phrases given during the song may underlie perceptual abilities that are currently unknown in other nonhuman primates, such as the ability to recognize particular pitch patterns.

An increase in ocean noise levels could interfere with acoustic communication of marine mammals. In this study we explored the effects of anthropogenic and natural noise on the acoustic properties of a dolphin communication signal, the whistle. A towed array with four elements was used to record environmental background noise and whistles of short-beaked common-, Atlantic spotted- and striped-dolphins in the Canaries archipelago. Four frequency parameters were measured from each whistle, while Sound Pressure Levels (SPL) of the background noise were measured at the central frequencies of seven one-third octave bands, from 5 to 20 kHz. Results show that dolphins increase the whistles’ frequency parameters with lower variability in the presence of anthropogenic noise, and increase the end frequency of their whistles when confronted with increasing natural noise. This study provides the first evidence that the synergy among SPLs has a role in shaping the whistles' structure of these three species, with respect to both natural and anthropogenic noise.

Contextual variation in the loud calls of strepsirhine primates is poorly understood. To understand whether songs given by indris in different contexts represent acoustically distinct variants and have the potential to elicit context-specific behaviours in conspecific listeners, we investigated the acoustic variability of these songs and the distance travelled by vocalizers after their emissions. Songs of 41 individuals were recorded from 16 indri groups in four different forest sites in eastern Madagascar. We collected a total of 270 duets and choruses arising during territorial defence, advertisement and cohesion. We demonstrated that the structure of indri songs conveyed context-specific information through their overall duration, but shared the sequential pattern of harsh units (roars) followed by long notes and, finally, descending phrases. Analysing in detail individual contributions to advertisement songs and cohesion songs, we found that the acoustic structure of units could be classified correctly with a high degree of reliability (96.23% of long notes, 80.16% of the descending phrases, 72.54% of roars). Future investigations using playback stimuli could explore the relationship between acoustic features and the information transmitted by the song.