Implications of target signal choice in passive acoustic monitoring: an example of age‐ and sex‐dependent vocal repertoire use in African forest elephants (Loxodonta cyclotis)

• Published in: Remote sensing in ecology and conservation 2024-08, Vol.10 (4), p.435-447
• Main Authors: Swider, Colin R., Hedwig, Daniela, Wrege, Peter H., Parks, Susan E.
• Format: Article
• Language: English
• Subjects: Acoustic tracking; Acoustics; African forest elephants; Age; Algorithms; Animal populations; Animal species; Audio data; Auditory communication; Behavior; call type; Cryptic species; Data collection; Demographics; Demography; detection; Elephants; endangered species; Females; Gorillas; Loxodonta cyclotis; National parks; passive acoustic monitoring; Population studies; Remote monitoring; Remote sensing; repertoire; Sampling; Sex; Signal detection; Vocalization behavior
• ISSN: 2056-3485; 2056-3485
• DOI: 10.1002/rse2.380

Passive acoustic monitoring (PAM) is an effective remote sensing approach for sampling acoustically active animal species and is particularly useful for elusive, visually cryptic species inhabiting remote or inaccessible habitats. Key advantages of PAM are large spatial coverage and continuous, long‐term monitoring. In most cases, a signal detection algorithm is utilized to locate sounds of interest within long sequences of audio data. It is important to understand the demographic/contextual usage of call types when choosing a particular signal to use for detection. Sampling biases may result if sampling is restricted to subsets of the population, for example, when detectable vocalizations are produced only by a certain demographic class. Using the African forest elephant repertoire as a case study, we test for differences in call type usage among different age‐sex classes. We identified disproportionate usage by age‐sex class of four call types—roars, trumpets, rumbles, and combination calls. This differential usage of signals by demographic class has implications for the use of particular call types in PAM for this species. Our results highlight that forest elephant PAM studies that have used rumbles as target signals may have under‐sampled adult males. The addition of other call types to PAM frameworks may be useful to leverage additional population demographic information from these surveys. Our research exemplifies how an examination of a species' acoustic behavior can be used to better contextualize the data and results from PAM and to strengthen the resulting inference. Passive acoustic monitoring (PAM) is an effective remote sensing approach for sampling elusive and visually cryptic species that are acoustically active but difficult to study using other methods. However, the vocal behavior of such species is often not well understood, limiting the inference that can be reliably drawn from PAM. We explore the implications of demographic patterns in vocal behavior and repertoire use for PAM, using a case study of the critically endangered African forest elephant (Loxodonta cyclotis) in the Congo basin. We highlight how the age and sex specificity of potential PAM target signals will determine the subset of the population that is sampled, and we suggest that this aspect of PAM should receive more attention in future studies, regardless of species.