Call and be counted! Can we reliably estimate the number of callers in the indri's (Indri indri) song?

Abstract

Estimating the number of animals participating in a choral display may contribute reliable information on animal population estimates, particularly when environmental or behavioral factors restrict the possibility of visual surveys. Difficulties in providing a reliable estimate of the number of singers in a chorus are many (e.g., background noise masking, overlap). In this work, we contributed data on the vocal chorusing of the indri lemurs (Indri indri), which emit howling cries, known as songs, uttered by two to five individuals. We examined whether we could estimate the number of emitters in a chorus by screening the fundamental frequency in the spectrograms and the total duration of the songs, and the reliability of those methods when compared to the real chorus size. The spectrographic investigation appears to provide reliable information on the number of animals participating in the chorusing only when this number is limited to two or three singers. We also found that the Acoustic Complexity Index positively correlated with the real chorus size, showing that an automated analysis of the chorus may provide information about the number of singers. We can state that song duration shows a correlation with the number of emitters but also shows a remarkable variation that remains unexplained. The accuracy of the estimates can reflect the high variability in chorus size, which could be affected by group composition, season and context. In future research, a greater focus on analyzing frequency change occurring during these collective vocal displays should improve our ability to detect individuals and allow a finer tuning of the acoustic methods that may serve for monitoring chorusing mammals.

Female indris determine the rhythmic structure of the song and sustain a higher cost when the chorus size increases

“…When we considered songs emitted by 4 singers, our results were inconsistent possibly because of the small sample ( N = 5). Songs with a chorus size exceeding 3 indris are rare (Torti et al 2018) because groups are usually smaller (Bonadonna et al. 2017).…”

Automated bioacoustics: methods in ecology and conservation and their potential for animal welfare monitoring

“…In order to address this issue, we designed a decision tree shown in figure 1 to aid researchers in selecting papers to begin their own investigations into the field. Torti et al [88] implemented a method known as the Acoustic Complexity Index to estimate the number of lemurs (Indri indri) taking part in a choral display in a tropical environment. They found that relatively simple spectrographic analysis was sufficient when identifying up to three singers, but for larger numbers of animals the Acoustic Complexity Index [89] performed well, positively correlating with the number of animals in the environment.…”

“…royalsocietypublishing.org/journal/rsif J. R. Soc. Interface 16: 20190225 methods to determine the number of animals taking part in a choral display [88,90] or in the application of acoustic indices to monitoring biodiversity [90]. This is one of the major challenges bioacoustics faces in terms of its application to animal welfare.…”

Linguistic laws of brevity: conformity in Indri indri

“…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. All the individuals within a family group, aged one year or above, can join the chorus (Torti et al 2018;De Gregorio et al submitted R1).…”