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

Valeria Torti,Carlo Comazzi,Chiara De Gregorio,Marco Gamba,Jonah Ratsimbazafy,Daria Valente,Cristina Giacoma,Longondraza Miaretsoa,David Reby

doi:10.1371/journal.pone.0201664

Valeria Torti, Carlo Comazzi + Show 7 more

Open Access

https://doi.org/10.1371/journal.pone.0201664

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Journal: PloS one	Publication Date: Aug 3, 2018
Citations: 17	License type: CC BY 4.0

Affiliation: University of Turin, Institut Pasteur de Madagascar

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.

Highlights

Species living in social groups may communicate at long distance transmitting information through the use of loud vocal signals
At the light of the previous research, we examined whether we could estimate the number of indris emitting in a chorus using screening of the spectrograms and the reliability of this methods when compared to the real chorus size
In 74 recordings, more than 50% took part to the song, and in 19 songs we observed 50% or less of the group participating to the chorus

Summary

Introduction

Species living in social groups may communicate at long distance transmitting information through the use of loud vocal signals (e.g., carnivores [1]; primates [2,3]). The production of these signals is not limited to a single emitter but can involve the participation of several callers, resulting in a chorus display, that may or may not include all the animals in a group [4]. We do have limited knowledge of the ability of conspecifics in extracting. Reliability of spectral methods to count animals in choruses. Through the Project BIRD (ACP SandT Program, Contract FED/2009/217077). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

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