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 behavioral displays prevents further studies. Indris are the only lemurs who sing. They produce loud howling cries that can be heard at several kilometers, 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 behavioral 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 relatively unknown in other non-human primates, such as the ability to recognize particular pitch patterns.
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.
In this study, we analysed the patterns of variation of the European treefrog’s advertisement call at four levels of organization: within individuals, within populations, among populations of the same species, and among different species of the same clade. At the within‐individual level, call acoustic properties are distinguished into static and dynamic properties. At the within‐population level, two sources of call variation were analysed: temperature and body size. Temperature affects both temporal and spectral properties of the call. Body size mostly affects spectral properties. At the between‐population level, calls do not show significant differences with respect to temporal properties, but they do differ in two spectral (and stereotyped) properties: the fundamental frequency and the difference in amplitude between dominant and fundamental frequencies. Finally, at the between‐species level, call differences are much more conspicuous: they involve both spectral and temporal and both static and dynamic properties. At all four levels, body size is associated with call variation, explaining 11% of the differences among populations of the same species and 73% of the differences among species of the same clade. On the basis of these results, we hypothesize that patterns of variation of call acoustic properties, their constraints, and their biological functions are intrinsically associated. We discuss the role that such an association might play in the evolution of acoustic signals.
In some primate species, males and females within a social group emit loud calls in a coordinated manner or chorus. Indri indri emits a very conspicuous loud call that elicits the loud calls of neighboring groups. Previous investigations have hypothesized that the main functions of the indri chorus are related to territorial announcement, intergroup avoidance, and group cohesion. We investigated sex differences in indri song. We recorded and analysed songs given by 10 different groups over 160 d. Overall singing duration did not vary between the sexes. However, males emitted significantly fewer but longer notes. Adult males and females of each group participated in the song with sex-specific repertoires. Females had a song repertoire of 8 note types; males shared all of their 6 notes with females. Apart from the initial roars, in all note types shared by both sexes, male notes were significantly longer than female ones, whereas variations in frequency parameters differed according to the note type. These findings suggest that indri song may provide cues to conspecifics, such as group size and sex composition, which could influence interactions between groups.
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