Isochronous singing in 3 crested gibbon species (Nomascus spp.)

Abstract

The search for common characteristics between the musical abilities of humans and other animal species is still taking its first steps. One of the most promising aspects from a comparative point of view is the analysis of rhythmic components, which are crucial features of human communicative performance but also well-identifiable patterns in the vocal displays of other species. Therefore, the study of rhythm is becoming essential to understand the mechanisms of singing behavior and the evolution of human communication. Recent findings provided evidence that particular rhythmic structures occur in human music and some singing animal species, such as birds and rock hyraxes, but only 2 species of nonhuman primates have been investigated so far (Indri indri and Hylobates lar). Therefore, our study aims to consistently broaden the list of species studied regarding the presence of rhythmic categories. We investigated the temporal organization in the singing of 3 species of crested gibbons (Nomascus gabriellae, Nomascus leucogenys, and Nomascus siki) and found that the most prominent rhythmic category was isochrony. Moreover, we found slight variation in songs' tempo among species, with N. gabriellae and N. siki singing with a temporal pattern involving a gradually increasing tempo (a musical accelerando), and N. leucogenys with a more regular pattern. Here, we show how the prominence of a peak at the isochrony establishes itself as a shared characteristic in the small apes considered so far.

Keywords: accelerando; isochrony; music; rhythm; singing primates; song; tempo.

Figure 1

Spectrogram of a song by a group of Nomascus gabriellae. The fundamental frequency (f0) of each individual contribution is highlighted with a different colors: the male in blue and the 2 females in orange and yellow, respectively of N. gabriellae, highlighting the contributions of the 3 animals to the chorus with different colors.

Figure 2

T _k and r_k distribution for the 3 Nomascus spp. (A, F, and K) Nomascus spp. (B, G, and L) Probability density functions of t_k for each species; (C, H, and M) Ternary plots for the 3 species, each axis represents 1 t_k, and each area within the graph represents a rhythm resulting from 3 consecutive t_k. (D, I, and N) Probability density functions of rhythm ratios (r_k), a shift of central peak to the right of the 1:1 ratio may be diagnostic of an accelerando. (E, J, and O) Bar plot showing the average adjusted r_k occurrence for on-integer (dark colors) and off-integer (light colors) ratio ranges. Photo credits: C. Mancassola.