Improving individual identification in captive Eastern grey wolves (Canis lupus lycaon) using the time course of howl amplitudes

Abstract

Many bioacoustic studies have been able to identify individual mammals from variations in the fundamental frequency (F₀) of their vocalizations. Other characteristics of vocalization which encode individuality, such as amplitude, are less frequently used because of problems with background noise and recording fidelity over distance. In this paper, we investigate whether the inclusion of amplitude variables improves the accuracy of individual howl identification in captive Eastern grey wolves (Canis lupus lycaon). We also explore whether the use of a bespoke code to extract the howl features, combined with histogram-derived principal component analysis (PCA) values, can improve current individual wolf howl identification accuracies. From a total of 89 solo howls from six captive individuals, where distances between wolf and observer were short, we achieved 95.5% (+9.0% improvement) individual identification accuracy of captive wolves using discriminant function analysis (DFA) to classify simple scalar variables of F₀ and normalized amplitudes. Moreover, this accuracy was increased by 100% when using histogram-derived PCA values of F₀ and amplitudes of the first harmonic. We suggest that individual identification accuracy can be improved by including amplitude changes for species where F₀ has only been included so far. Using DFA on PCA values of both F₀ and amplitude could optimize vocal identification in a range of mammal bioacoustic studies.

Keywords::

• acoustic amplitude
• Canis lupus lycaon
• howl
• individuality
• vocal recognition
• wolf

This article is related to:

Identifying individual wild Eastern grey wolves (Canis lupus lycaon) using fundamental frequency and amplitude of howls

Acknowledgements

The authors would like to thank Nottingham Trent University for providing funds for the equipment, and also the Borror Laboratory of Bioacoustics for providing the recording, Joan Tilley for making the original recordings and to Monty Sloan of Wolf Park for additional information on the wolves.