The Indris Have Got Rhythm! Timing and Pitch Variation of a Primate Song Examined between Sexes and Age Classes

Abstract

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.

Keywords: gender differences; lemurs; musical abilities; pitch pattern recognition; singing primates.

Figure 1

Spectrogram of the indris' song. In this particular song recorded in the Maromizaha Forest, a reproductive pair is singing with a male offspring (Group 1 MZ).

Figure 2

Schematic representation of a spectrogram (A) describing acoustic parameter collection on the isolated pitch of a song. Letters A and B mark different singers, letters SP mark the starting points of a unit (1, 2, 3…) in the song. The color bars indicate the starting and final points of the units given by two different indris (e.g., blue for a male; red for a female). Duration of the units is reflected in the schematized Praat textgrid as an interval of the same color, where solid colors indicate non-overlapping parts and striped patterns indicate co-sung portions. Duration of the IOIs of a descending phrase is marked by solid green bars. In the spectrum (B) of the third unit (in a descending phrase of four units), the green dotted line marks the frequency corresponding to the upper limit of the second quartile of energy in the spectrum (Q50). The sound spectrum displays sound pressure level (Spl) on the x-axis, frequency on the vertical axis.

Figure 3

Schematic representation of a spectrogram showing the pitch contour of a portion of the song of three different indris. Red lines mark the starting point of each unit, which were entered in the Granger causality test. Black lines and arrows indicate the dyad and the direction of the test, for which we reported F statistics (F_stat) and p-values (p) as examples.

Figure 4

Bar plot of the average percentage of synchronized songs in the indris. Capped lines represent negative Standard Deviation. Each bar indicates the direction of the Granger causality test for each type of dyad (AF, adult females; AM, adult males, NA, non-adults).

Figure 5

Bar plot of the average IOI duration between DP types in the sexes (males in blue, females in magenta). Capped lines represent ± Standard Deviation.

Figure 6

Density plots obtained in R (MASS package) for male descending phrases DP2s (A), female DP2s (B), male DP3s (D), female DP3s (E), male DP4s (G), and female DP4s (H). The bar plots show the average frequency corresponding to the upper limit of the second quartile of energy in the spectrum (Q50) for males and females, for DP2s (C) DP3s (F), and DP4s (I). Units within a DP are indicated by different colors. Capped lines represent ± Standard Deviation.