Sexually dimorphic phrase organization in the song of the indris (Indri indri)

Abstract

Animal acoustic communication often takes the form of complex sequences, composed of multiple distinct acoustic units, which can vary in their degree of stereotypy.Studies of sequence variation may contribute to our understanding of the structural flexibility of primates' songs, which can provide essential ecological and behavioral information about variability at the individual, population, and specific level and provide insights into the mechanisms and drivers responsible for the evolutionary change of communicative traits. Several methods have been used for investigating different levels of structural information and sequence similarity in acoustic displays. We studied intra and interindividual variation in the song structuring of a singing primate, the indri (Indri indri), which inhabits the montane rain forests of Madagascar. Indri groups emit duets and choruses in which they combine long notes, short single units, and phrases consisting of a variable number of units (from two to six) with slightly descending frequency. Males' and females' contributions to the song differ in the temporal and frequency structure of song units and repertoire size. We calculated the similarity of phrase organization across different individual contributions using the Levenshtein distance, a logic distance that expressed the minimum cost to convert a sequence into another and can measure differences between two sequences of data. We then analyzed the degree of similarity within and between individuals and found that: (a) the phrase structure of songs varied between reproductive males and females: female structuring of the song showed a higher number of phrases if compared to males; (b) male contributions to the song were overall more similar to those of other males than were female contributions to the song of other females; (c) male contributions were more stereotyped than female contributions, which showed greater individual flexibility.The picture emerging from phrase combinatorics in the indris is in agreement with previous findings of rhythmic features and song repertoire size of the indris, which also suggested that female songs are potentially less stereotyped than those of males. K E Y W O R D Slanguage evolution, Levenshtein distance, primates, singing, syntax

Born to sing! Song development in a singing primate

“…This could be because, in titi monkeys, males and females sing the same units, while the indris’ repertoire is strongly dimorphic ( Giacoma et al. 2010 ; Zanoli et al. 2020 ).…”

“…2019a ; Clink et al. 2020 ), and an immature vocal apparatus may not be prepared to endure the full adult song, that can reach 110 dB (see Zanoli et al. 2020 ).…”

Linguistic laws of brevity: conformity in Indri indri

“…Besides an introductory sequence (comprising roars and long notes; Pollock 1986;Thalmann et al 1993;Sorrentino et al 2013) indris' songs are made of several discrete vocal units (Gamba et al 2011(Gamba et al , 2016 organized in phrases. Those sequences, also called descending phrases because of a frequency pattern declining along with the phrase (Thalmann et al 1993;Sorrentino et al 2013;Gamba et al 2016), constitute the core part of indris' song, can include from two to six units (Thalmann et al 1993;Giacoma et al 2010;Gamba et al 2016), and their organization is sexually dimorphic (Zanoli et al 2020). Songs can be emitted as duets-when only the members of the reproductive pair sing together-or as choruses when one or more non-reproductive members join the parents when singing.…”

“…b Density plot representing and the variation of the phrase duration in relation to the type of the phrase for both sexes. Plots were generated in R, using the ggplot2 package(Wickham 2016)…”

“…a Descending phrase including two units; b Descending phrase including three units; c Descending phrase including four units; d Descending phrase including five units. Plots were generated in R, using the ggplot2 package(Wickham 2016)…”