New publication: Acoustic behavior of endangered Hawaiian false killer whales

Written by MMRP alumna Brijonnay Madrigal

We are excited to share a new publication in Royal Society Open Science, which describes the acoustic behavior of false killer whales from the endangered Main Hawaiian Islands insular population. This research encompasses one chapter of former MMRP Ph.D. student and current Scripps postdoctoral researcher, Dr. Brijonnay Madrigal's dissertation.

Authors: Brijonnay Madrigal, William Gough, Jens Currie, Lars Bejder, Augusta Hollers, Robin Baird, Aran Mooney, and Aude Pacini

Figure 1. False killer whale with a CATS tag attached. Credit: Grace Olson (Pacific Whale Foundation)

How do scientists better understand the communication of individual animals when we don’t know who is speaking and what they are doing underwater? For cetaceans, passive acoustic monitoring is an important approach to study these animals that spend most of their time underwater and rely on acoustic signaling to communicate. The use of acoustic biologging tags has enabled the collection of high-resolution data to study acoustic behavior of top predators. The Main Hawaiian Islands (MHI) insular population of false killer whales (Pseudorca crassidens) is the most endangered toothed whale population in Hawaiʻi under the Endangered Species Act. Despite ongoing management efforts to address threats, the population has continued to decline to a current population size of less than 150 individuals. Therefore, it is crucial to understand the behavior of this population to better inform conservation measures critical for the protection of this species. Our understanding of the social context of individual false killer whales has generally been limited, until now.

Study Objectives

The goal of this study was to use data recorded from non-invasive archival , suction-cup tags (DTAG/CATS), to describe the acoustic behavior of MHI insular false killer whales to better understand the behavioral context of social signals (Figure 1). Our objectives were to (1) classify and characterize the repertoire of individual false killer whales; (2) describe nonlinear features observed in calls that enhance communication between individuals; and (3) analyze the relationship between social sound production and diving behavior. These findings can help us evaluate social context on a small scale and provide foundational information to determine the potential function of these signals.

KEY FINDINGS

False killer whales produce diverse call types, like humans have diverse vocabularies

Our results show that MHI insular false killer whales have a more diverse repertoire than previously described including 52 pulsed call types. Some call types are shared between individuals, and some are unique to individuals (Figure 2). Calls were identified as possible focal call (PFC) because we lacked localization capabilities with CATS tags to confirm the source and relied on signal-to-noise ratio (SNR) to identify tagged animal calls. We identified predominate PFC types that were repeated or favored by specific individuals. For example, animal HIPc332 was the only animal to produce call type MHI52 and HIPc706 produced call type MHI1 more than any of the other tagged animals.

Figure 2. Predominant possible focal call (PFC) types produced by each tagged animal (DTAG–HIPc332; CATS–HIPc706, HIPc265, HIPc805). Panels (a–d) depict the proportion of calls for each of the most common calls. The panels on the right depict spectrograms (DTAG: 240 kHz sampling rate, FFT size 2048, Hann window, 50% overlap; CATS: 96 kHz sampling rate, FFT size 1024, Hann window, 50% overlap) for each corresponding call type. Percentage of total calls and sample size (n) is included for each call type.

Can you say two things at once?

Odontocetes have two blowholes so they have the ability to produce two sound types (e.g. clicks and call) simultaneously which is called biphonation (audio example below, Figure 3). Biphonation rates were high (78% of PFC), similar to killer whale biphonation rates. Although clicks are commonly used for echolocation to navigate and locate prey, the clicks produced by these animals occur with calls at the same time in distinct patterns, so they likely function in communication and encode additional information for individuals. Based on evidence from killer whales, biphonation may function in individual discrimination when subgroups of false killer whales are spatially distant.

Audio recording of calls produced by a tagged false killer whale

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Figure 3. A spectrogram of three predominate possible focal call (PFC) types produced by one tagged false killer whale containing biphonation (call and clicks are produced simultaneously), as indicated by the arrows in the first panel.

Speed up and listen

Call rates and production of predominate call types varied by individual across dive states (surface, descent, ascent, bottom phase) (Figure 4). We also observed that call rates decreased as swim speed increased. This could be due to energy limitations of calling while swimming faster, or could be a by-product of the recording where higher speeds induce more flow noise at the tag hydrophones, which may decrease the SNR of signals, reducing the likelihood they are identified as being produced by the animal. Alternatively, when animals are swimming faster, they may decrease calling rates in order to listen for calling conspecifics in the area as they are commonly observed increasing high speed travel before joining another distant individual or subgroup (R.W.B., personal observations, 2025).

Figure 4. Possible focal call (PFC) production of each animal across the deployment. Panel (a) shows an illustration depicting the dive states including surface (dive state 0), descent (Dive State 1), ascent (Dive State 2) and bottom phase (Dive State 3). Surface includes waters from 0 to 2 m. Bottom phases were designated as dives less than 100 m. Call proportions for each CATS tagged animal are depicted in panel (b) across all dive states (0—light blue, 1—medium blue, 2—dark blue, 3—pink). Individual HIPc332 was excluded since DTAG kinematic data was unavailable.

This study provides invaluable insights into this species’ social behavior and by intercepting the coconut wireless of Hawaiʻi false killer whales using tag technology, our findings can inform management strategies necessary to advance conservation efforts of this top predator to the Hawaiʻi ecosystem.

If you want to read about this work in more detail, you can find the article here. If you’d like to contact the lead author with questions, feel free to reach out to Brijonnay at brijonnay.madrigal@gmail.com