Animal Cognition

, Volume 22, Issue 6, pp 1115–1128 | Cite as

Acoustic structure of forest elephant rumbles: a test of the ambiguity reduction hypothesis

  • Daniela HedwigEmail author
  • Anahita K. Verahrami
  • Peter H. Wrege
Original Paper


Quantitative assessments of the structure of vocalizations are a fundamental prerequisite to understand a species’ vocal communication system and, more broadly, the selective pressures shaping vocal repertoires. For example, to reduce ambiguity in signal interpretation in the absence of auxiliary visual cues, species in densely vegetated habitats should exhibit more discrete vocal signals than species in open habitats. To test this “ambiguity reduction hypothesis”, we conducted the first quantitative assessment of the rumble vocalizations of the forest elephant. Based on 686 forest elephant rumbles recorded with autonomous acoustic recording units at four sites across Central Africa, we used model-based cluster analyses paired with subsequent evaluation of cluster-discreteness and discriminant function analyses to quantify the structure of rumbles based on 23 source- and filter-related acoustic parameters. Model-based cluster analyses suggest that rumbles can be classified into five to eight types. Similar to previous findings in savannah elephants and contrary to the ambiguity reduction hypothesis, average silhouette coefficients below 0.34 indicated that these rumble types were highly intergraded. However, discriminant function analyses predicted rumble types with at least 75% accuracy whereby the location of the minimum fundamental frequency, middle slope and peak frequency contributed most to separation between types. In line with an increasing number of studies highlighting that a distinction between discrete and graded repertoires may have little biological significance, we propose that ambiguity reduction may take place through the evolution of perceptual and cognitive mechanisms, rather than acting on vocal production.


Discreteness Vocal repertoires Habitat differences Acoustic adaptation Categorical perception Loxodonta cyclotis 



This study was supported by a grant to PHW from the US Fish and Wildlife Service, the Robert G. and Jane V. Engle Foundation, and through a generous gift from Lisa Yang to the Cornell Lab of Ornithology. Research clearance was approved by the Gabon government’s National Center for Scientific Research and Technology, by the Republic of Congo’s Ministry of Forestry, and by the Central African Republic’s Ministry of Education and Water and Forests. Special thanks go to Elizabeth D. Rowland, Andrea Turkalo, Frelcia Bambi, Phael Malonga, Terry Brncic and Herve Londo for superb assistance with data collection and Precious Woods Gabon for critical logistics support.

Supplementary material

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Supplementary material 1 (DOCX 723 kb)
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© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Elephant Listening Project, Center for Conservation Bioacoustics, Cornell Lab of OrnithologyCornell UniversityIthacaUSA

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