Production of chirp trains by Indo-Pacific humpback dolphins (Sousa chinensis) in the northern South China Sea

doi:10.21203/rs.3.rs-6832903/v1

Chirps (i.e., whistles shorter than 0.25s) have been reported across various delphinid species but the context of their production and function remains underexplored. In this study, the production of chirps was examined in free-ranging Indo-Pacific humpback dolphins across three locations in the northern South China Sea. The chirps produced by dolphins could be emitted as distinct units (i.e., single chirps) or within chirp trains. These chirp trains were composed of at least three chirps that lasted 69.64 ± 26.01 ms in average and presented inter-chirp intervals of 124.87 ± 79.69 ms. Differences presented between single chirps and individual chirps produced within chirp trains including a longer duration (179.37 ± 45.10 ms) for single chirps as well as frequency and contour differences suggesting the existence of two distinct whistle types (single chirps and chirp trains). Higher chirp train production rates were found in larger groups (p < 0.05) and during socializing than during other activities (p < 0.05), suggesting that these vocalizations likely serve a role in social communication. The presence of young in the group was not linked with higher chirp train production, therefore these signals may be primarily used for inter-adult communication. Furthermore, variations in chirp train production rate were noted across different populations, potentially due to the geographic separation of populations, habitat differences, and differences in social structure, and communication. These findings provide insights into the unique characteristics and potential social functions of chirp trains in Indo-Pacific humpback dolphins, addressing a gap in cetacean acoustic communication.

acoustic signals

call

cetaceans

chirp

communication

Delphinid species produce short duration whistles known as chirps, we studied this vocalization in Indo-Pacific humpback dolphins. Results showed that chirps produced by humpback dolphins were emitted not only as distinct units (e.g., single chirps), but also within chirp trains. Differences in duration, frequency, and structure were observed between single chirps and individual chirps within chirp trains in this study, suggesting they represent distinct vocalization types. Humpback dolphins produced chirp trains predominantly in larger social groups, indicating a potential role of this vocalization in social communication. In addition, variations in chirp train production rates across populations may be attributed to environmental and social structure differences.These findings provide insights into the unique characteristics and potential social functions of chirp trains in Indo-Pacific humpback dolphins, informing our understanding of cetacean acoustic communication.

Vocalizations are widespread among vertebrates, including amphibians, reptiles, and mammals (Chen & Wiens, 2020; Schusterman, 1966). Over the decades, the study of acoustic signals has provided valuable insights into the behaviour, ecology, evolution, and conservation of various species. In mammals, these signals serve as critical tools for monitoring species, understanding interactions among individuals, and assessing environmental impacts (Chen & Wiens, 2020; Ma & Fan, 2023; Schusterman, 1966). For instance, acoustic monitoring protocols have allowed for estimating the influence of artificial structures like wind turbines on bat populations (Behr et al., 2023).

Acoustic signals are especially critical in marine mammal studies due to the challenges of observing animals in aquatic environments. Acoustic signals provide unique information that complements visual observations of marine mammals and is the only source of information when animals are not visible, therefore making it an essential element to study for understanding marine mammals’ behaviour, ecology, and/or communication (Ma & Fan, 2023; Zimmer, 2011). Dolphins, in particular, rely heavily on sound for communication and navigation, and the acoustic behaviour of some species (e.g., bottlenose dolphins, Tursiops truncatus) has been extensively studied. Acoustic studies not only focus on the information which these signals can convey but also analyze the physical characteristics of the sounds dolphins produce. Dolphin vocalizations are generally classified into two main categories based on their function: echolocation signals such as clicks used for navigation and foraging, and communication signals such as whistles used for social communication (Zimmer, 2011). However, there is no strict one-to-one correlation between a sound’s acoustic properties and its function. Describing acoustic signals and investigating their potential function aids in understanding, monitoring, and protecting dolphin populations. For example, whistle characteristics have been used to distinguish ecotypes within a species, thus contributing to ecological and behavioural understanding and conservation (Rege-Colt et al., 2023).

To study whistles systematically, researchers have classified them into distinct categories based on different features including function, duration, or frequency. For instance, signature whistles are characterized by their individual identification function and pops are associated with specific behaviors while moans are characterized by their low-frequency (Jones et al., 2020; Sayigh, 2014). Chirps, a type of whistle that is characterized by a very short duration (<0.30 or 0.25 s), have been documented in species such as spinner dolphins (Stenella longirostris) and bottlenose dolphins (Bazúa-Durán & Au, 2002; Caldwell & Caldwell, 1968; Jones et al., 2020). However, chirps have rarely been the focus of detailed investigation including function investigation and most researchers have classified them as a whistle subcategory (Bazúa-Durán & Au, 2002; Caldwell & Caldwell, 1968), with some suggesting they are an arbitrary category of vocalizations without distinct biological function (Bazúa-Durán & Au, 2002). Nevertheless, no study has investigated the potential function of chirps for dolphins. Understanding the context in which dolphins produce particular vocalizations like chirps and whether they serve specific functions will help in understanding the complex communication of these animals.

Indo-pacific humpback dolphins (Sousa chinensis), hereafter referred to as humpback dolphins, inhabit coastal and estuarine waters of southeast Asia and western Pacific and are listed as “Vulnerable” (VU) on the IUCN Red List of Threatened Species (Ho et al., 2024; Jefferson & Smith, 2016). These dolphins’ acoustic signals have been studied for decades (Van Parijs & Corkeron, 2001b; Wang et al., 2013) including whistles whose characteristics (e.g., frequency) differ among populations (Dong et al., 2019; Van Parijs & Corkeron, 2001b; Wang et al., 2013). Recent findings suggested that some stereotyped whistles produced by humpback dolphins may function as signature whistles, similar to those of bottlenose dolphins (Serres et al., 2024). Nevertheless, studies regarding the function of humpback dolphin vocalizations are still rare if anything at all (Serres et al., 2023, 2024).

Whistles shorter than 0.25s were reported in humpback dolphins but never categorized as such nor investigated further (Dong et al., 2019; Wang et al., 2013). Investigating the production rate, characteristics, and potential functions of particular acoustic signals like chirps could provide new insights into humpback dolphin behaviour and communication. Additionally, understanding whether the production, characteristics, and function of these vocalizations differ across populations or species would increase our knowledge of delphinids’ vocal diversity, plasticity, and their ecological implications. The present study aims to explore the production of chirps by humpback dolphins. Acoustic data collected during encounters with these dolphins in the northern South China Sea were analyzed to examine chirps’ acoustic characteristics and context of production. This research represents the first dedicated effort to study these acoustic signals in humpback dolphins, offering novel insights into this previously overlooked aspect of their acoustic behavior.

Study area and survey effort

Acoustic recording was conducted in three Indo-Pacific humpback dolphin populations in the South China Sea (SCS) between May 2021 and September 2024 including populations inhabiting the coastal waters of the Pearl River Delta (PRD), Zhanjiang (ZJ), and Sanniang Bay (SNB) (Fig 1). Each of these habitats is geographically isolated, with no communication or gene flow between populations (Lin, Chen, et al., 2024; Lin, Karczmarski, et al., 2024; Tang et al., 2021). Systematic boat-based surveys were conducted in each region under comparable sea conditions, with a Beaufort score of less than 3 and swells below 1 meter, ensuring consistency in data collection. Surveys followed pre-defined routes to standardize the effort across all three areas.

Data collection

Surveys were conducted using small, highly maneuverable outboard engine boats approximately 7 meters in length. Each survey team consisted of at least three trained observers who continuously scanned the surrounding area for dolphins using naked eye. Upon sighting dolphins, the boat slowed down and approached them in a parallel way or with a slight angle from the side or the rear to minimize disturbance.

During each encounter, data were recorded including dolphin behavior, estimated group size, presence or absence of young, and environmental conditions such as water depth, temperature, and the number of nearby vessels within 1 kilometer of dolphins. Groups were defined as one or more dolphins observed in close spatial proximity or social association (Liu et al., 2021). Young, including neonates, calves and unspotted juveniles were identified by their small size and predominantly gray skin pigmentation (Piwetz et al., 2021). Behavioral activity states were attributed based on the definitions from previous studies (Karczmarski & Cockcroft, 1999; Serres et al., 2023), with groups displaying unknown behaviors excluded from further analysis. Habitat types were classified into three categories: open water (OW; > 100m from the shore), nearshore (NS; < 100m from the shore), and boat channel (BC; in a harbor or < 100m from a shipping channel).

Once the boat was within approximately 50 meters of the dolphins, ensuring high-quality acoustic recordings, a SoundTrap acoustic recorder (ST300 HF; frequency response: 20–150 kHz ± 3 dB; sensitivity: -172.8 dB re 1 μPa; Ocean Instruments Inc., Dunedin, New Zealand) was deployed. The recorder, weighted with a 5-kg object, was secured with a rope to the front of the boat and positioned at mid-water depth.

Acoustic data analysis

The acoustic data were stored as 16-bit WAV files. Each recording was thoroughly inspected both aurally and visually using Audition CS6 (Adobe Systems Inc., 2013) and Raven Pro Bioacoustics software (version 1.5; Cornell Laboratory of Ornithology, NY). Spectrograms of whistles were generated in Raven Pro with the following settings: fast Fourier transform (FFT) size of 2048, Hann window with 75% overlap, frequency range of 0–50 kHz, and a time series window of 10 seconds.

Whistles were classified into four categories including ‘Inferior’, ‘fair’, ‘good’ and ‘fine’ depending on their quality according to previous studies (Wang et al., 2013; Yuan et al., 2021). Only ‘good’ and ‘fine’ whistles were included in the present study. Chirps were defined as whistles with durations shorter than 0.25 seconds (Richards et al., 1984 in Jones et al., 2020). Only chirps with clearly defined contours and unambiguous start and end points were included in the analysis.

A chirp train (ChT) was defined as a sequence of at least three individual chirps. Inter-chirp intervals (IChIs) were measured as the time between the end of one chirp and the start of the next. Because whistles separated by <0.25s are usually considered to be part of a single whistle unit and most of IChIs (95.24%) within ChTs were shorter than 0.25s, ChTs were considered as distinct whistles in this study (i.e., versus considering individual chirps). To distinguish between single and multiple ChTs, the largest IChI within a chirp train could not exceed the sum of the two preceding IChIs. Chirps that did not belong to ChTs (hereafter: single chirps) and chirps that were produced within ChTs (hereafter: individual chirps) were categorized into one of six tonal types based on contour shape following Bazúa-Durán & Au (2002): U-shaped (U), convex (C), rising (R), descending (D), flat (F), and sine-shaped (S). For each chirp, 10 acoustic parameters were measured using custom MATLAB scripts for frequency-related parameters, and direct measurement from the spectrograms for other parameters (Table 1). To minimize observer bias, blinded methods were use when all acoustic data were recorded and analyzed.

Table 1 Acoustic parameters used to describe Indo-Pacific humpback dolphin chirps

Abbreviation	Acoustic parameter	Description
Dur	Duration (ms)	End time - start time
SF	Start frequency (kHz)	Frequency at start of whistle contour
Ft_0.25	First quartiles frequency (kHz)	Frequency at 0.25 of duration
Ft_0.5	Middle quartiles frequency (kHz)	Frequency at 0.50 of duration
Ft_0.75	Third quartiles frequency (kHz)	Frequency at 0.75 of duration
EF	End frequency (kHz)	Frequency at end of whistle contour
F_min	Minimum frequency (kHz)	Frequency at the lowest point in whistle contour
F_max	Maximum frequency (kHz)	Frequency at the highest point in whistle contour
FR	Frequency range (kHz)	F_max - F_min
F_mean	Mean frequency (kHz)	0.25 (SF + EF +F_min + F_max)

Statistical analysis

The proportion of single chirps and ChTs among the total number of whistles were calculated. The mean, minimum (Min), maximum (Max), and standard deviation (SD) were calculated for each chirp parameter. To investigate the variability of individual chirps across different ChTs and identify acoustic parameters potentially carrying communication-related information, a coefficient of variation (CV) was calculated for each measured parameter using the following formula:

The normality of the data was tested using Kolmogorov-Smirnov and Shapiro-Wilk tests and the homogeneity of variance was tested using F-tests. The potential difference in chirp duration between single chirps and individual chirps was analyzed using an independent t-test. Differences between the five time-series frequency parameters (SF, Ft_0.25, Ft_0.5, Ft_0.75, and EF) of single chirps and individual chirps were analyzed using a Friedman test. The latter analyses were performed using IBM SPSS 19.0 (SPSS Inc., Chicago, IL, USA).

The following statistical analyses were conducted in R 4.4.1 (Hartig, 2018; Lüdecke et al., 2021R Core Team, 2021). To explore the potential function of ChTs for humpback dolphins, two univariate generalized linear models (GLMs) were fitted using the glm function from the lme4 package (Bates et al., 2015). The response variables were (1) the presence/absence of ChTs (binomial family) and (2) the number of ChTs recorded during an encounter (quasipoisson family). Predictor variables included the location (PRD, ZJ, or SNB), group size, presence/absence of young individuals (calves or juveniles), behavioral state (foraging, socializing, milling, traveling, or resting), habitat type (nearshore, open water, or boat channel), and the number of surrounding vessels. The encounter duration was included as an offset to account for increased ChT detection probability during longer encounters. Model diagnostics, including tests for normality and equal variance of residuals, were performed using the DHARMa package (Hartig, 2018). Model selection was guided by Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) values, using the performance package (Lüdecke et al., 2021). Chi-squared tests were used to test the links between the response variables and predictors. When significant effects were detected, post hoc analyses were conducted by re-running the selected model with an appropriate sub-setting. A Bonferroni correction was applied to the post hoc outcomes.

From May 2021 to September 2024, 666 encounters with humpback dolphins were monitored within 123 survey days for a total of 15 058 min (ca. 251 h) of collected acoustic data (Table 2). 7 274 whistles were identified during 235 of these encounters and 2 805 whistles were included in further analysis. Within these whistles, a total of 450 single chirps (16.04% of all whistles) were identified during 124 encounters (52.76% of encounters with whistles) and 1 365 individual chirps belonging to 230 ChTs (8.20% of all whistles) were recorded during 38 encounters (19.29% of encounters with whistles).

Table 2 Summary of the collected acoustic data and extracted Indo-Pacific humpback dolphin signals

Year	2021	2022	2023	2024	Total
Number of encounters	161	218	166	121	666
Acoustic data (min)	3,159	3,605	4,052	4,242	15,058
Number of encounters with whistles	46	62	70	57	235
Number of encounters with ChTs	7	14	9	18	48
Number of encounters with single chirps	23	38	35	28	124
Percentage of encounters with ChTs	15.22%	22.58%	12.86%	31.57%	20.42%
Percentage of encounters with single chirps	50%	61.29%	50%	49.12%	52.76%
Total number of whistles	1272	2784	1697	1521	7274
Number of whistles analyzed	464	845	778	718	2805
Number of ChTs	35	60	47	88	230
Number of single chirps	57	151	131	111	450
Number of individual chirps	224	373	284	484	1365
Percentage of ChTs among analyzed whistles	7.54%	7.10%	6.04%	12.26%	8.20%
Percentage of single chirps among analyzed whistles	12.28%	17.87%	16.84%	15.46%	16.04%

Chirp acoustic parameters

U-shaped individual chirps dominated across all three regions, comprising 59.05% of all recorded individual chirps, followed by rising individual chirps (14.28%), concave-shaped individual chirps (11.50%), and downsweep individual chirps (5.57%) while flat (5.42%) or sine (4.18%) contours were the least common (Fig. 3A). For single chirps, all shapes were represented in relatively similar proportions, including downsweep contour (21.11%), U-shape contour (18.44%), sine contour (17.11%), convex contour (15.33%), flat contour (14.22%), and rising contour (13.78%). Significant differences were found between each time-series frequency parameters in individual chirps (Friedman test, p < 0.01), with the start and end frequencies being the two highest (Table 3). For single chirp, no significant differences were found between time-series frequency parameters (Table 4). In addition, single chirp durations (179.37± 45.10ms, range: 62.60-247.90ms) were significantly higher than that of individual chirps (69.64 ±26.01ms, range: 15.70-205.50ms; t-test, p < 0.01, Fig 3B).

Table 3 Acoustic parameters of individuals chirps in ChTs of Indo-Pacific humpback dolphins

Acoustic parameters	Mean	SD	CV	Min	Max
Dur (ms)	69.64	26.01	37.35	15.70	205.50
IChI (ms)	124.87	79.69	63.82	18.90	1496.00
SF (kHz)	7.12	3.70	52.01	1.05	22.36
Ft_0.25 (kHz)	6.62	3.22	48.75	1.83	18.56
Ft_0.5 (kHz)	6.73	3.16	46.91	2.11	18.00
Ft_0.75 (kHz)	6.99	3.31	47.36	2.14	18.26
EF (kHz)	7.49	3.65	48.70	1.74	21.66
F_min (kHz)	5.76	2.87	49.83	1.05	17.30
F_max (kHz)	8.00	3.80	47.55	2.46	22.36
FR (kHz)	2.24	2.14	95.41	0.00	13.69
F_mean (kHz)	7.09	3.34	47.12	2.13	20.59

Table 4 Acoustic parameters of single chirps of Indo-Pacific humpback dolphins

Acoustic parameters	Mean	SD	CV	Min	Max
Dur (ms)	179.37	45.10	25.15	62.60	247.90
SF (kHz)	7.07	2.72	38.43	2.67	16.03
Ft_0.25 (kHz)	7.06	2.59	36.63	2.67	15.75
Ft_0.5 (kHz)	6.98	2.57	36.86	2.53	16.59
Ft_0.75 (kHz)	6.98	2.54	36.35	2.39	15.47
EF (kHz)	7.20	2.67	37.11	2.53	16.31
F_min (kHz)	5.71	2.23	39.06	1.41	15.25
F_max (kHz)	8.36	2.92	34.98	2.82	17.02
FR (kHz)	2.65	2.10	79.47	0.20	12.94
F_mean (kHz)	7.08	2.41	34.02	2.58	15.80

The average number of individual chirps per ChT was 5.93 ± 3.72 (Fig 3C), and the average IChI was 124.87 ± 79.69 ms (Fig 3B). The contour shape of individual chirps within a ChT could be similar (46.09%) or vary (53.91%, see Fig S1).

ChTs production context

The location, habitat, group size, and behaviour significantly impacted the presence and number of ChTs during encounters with humpback dolphins (Table S1, S2). Dolphins in the Pearl River Delta (PRD) were significantly more likely to produce and produced more ChTs than those in Zhanjiang (ZJ) (Fig4 A, B, Table S1, S2). Significantly more ChTs were produced in open water (OW) compared to boat channels (BC) (Fig4 C, D, Table S3, S4). Socializing was the behavioural activity linked to the highest ChT occurrence and production rate (Fig4 E, F, Table S5, S6). Additionally, larger groups were more likely to emit ChTs (Fig4 G, Table S7). Further, the occurrence of ChTs tended to be lower when young individuals were present (Fig4 H, Table S7).

The present study investigated the production of chirps by humpback dolphins and provided the first description of a vocalization type composed of multiple chirps (i.e., chirp train). The analysis of the context of this signal production revealed the influence of contextual elements including geographic, environmental, and social parameters, providing preliminary information regarding its potential function.

Delphinids’ chirps have typically been classified as whistles characterized by a short duration and have not been further investigated under behavioral contexts (Bazúa-Durán & Au, 2002; Perazio & Kuczaj, 2017). In the present study, ChTs were identified as a specific whistle type characterized by a train-like arrangement of short duration (< 0.25s) whistles with a high prevalence of U-shaped contours. Each ChT was considered as one whistle since most IChIs (95.24%) were lower than 0.25s, duration used to distinguish a whistle including a gap from two individual whistles (Serres et al., 2024). ChTs could be compared to "multi-loop" whistles that also consist of repeated elements (Serres et al., 2024). However, unlike what is observed in such whistles, individual chirps found within ChTs were always separated by an IChI and often presented contour shape variation within a ChT.

The presently described humpback dolphin ChTs do not seem to correspond to any whistle type described in other dolphin species. For instance, the production of chirps within trains has not been described in other species (Bazúa-Durán & Au, 2002; Perazio & Kuczaj, 2017). Additionally, chirps have been described as commonly presenting an upsweep contour (R-type) (Caldwell & Caldwell, 1968; Jones et al., 2020) whereas humpback dolphins primarily produced U-shape individual chirps. These characteristics were observed across all three study areas, suggesting that these acoustic features are widespread within this species. The potential inter-specific differences in chirp production and characteristics can first be explained by differences in the nature of the sounds that different studies refer to when using the term ‘chirp’. Variations in study species, methodologies, and research teams often result in identical sounds being categorized differently (e.g.: "squeaks" and "squeals"), or the same term being applied to distinct sounds across studies (e.g.: " twitters " and " microwhistles ") (Jones et al., 2020; Wood Jr, 1953). The inconsistencies in classification make it difficult to determine whether signals that are compared are the same, even using the same definition. In addition, a broadband vocalization very similar to individual chirps were labelled pulsed calls in other species including belugas and long-finned pilot whales, part of them were emitted in trains as well (Brewer et al., 2023; Vester et al., 2017). Considering that most of the described pulsed calls were tonal in structure or mixed with buzzes, pulsed/noisy elements, making a clear distinction between whistles and pulsed calls is difficult. However, the similar densely Packed Harmonic Structure indicated that individual chirps may be homologous to these pulsed calls, so as ChTs and pulsed call trains (Vester et al., 2017). These observations suggest that individual chirps produced within ChTs may not correspond to chirps described in bottlenose dolphins for instance, but ChTs may be closer to the pulsed calls sequences of belugas.

In line with this hypothesis, individual chirps produced within ChTs differ fundamentally from other chirps, even within the same species. Humpback dolphins’ individual chirps were characterized by higher ending and starting frequency compared to other time-series frequencies, corresponding to the U-shape contour that was most commonly observed. (Dong et al., 2019; Wang et al., 2013). Conversely, no significant differences were found between each time-series frequency parameters and no obvious dominant contour type was observed in single chirps. Moreover, the duration of single chirps was significantly longer than that of individual chirps. These differences highlight the distinctiveness of ChTs among humpback dolphin vocalizations.

Single chirps and ChTs accounted for 16.04% and 8.20% of all recorded whistles, respectively. Similarly, encounters featuring single chirps (52.76% of encounters containing whistles) were more frequent than those including ChTs (20.42%). However, when individual chirps within ChTs were considered separately, their total number (1 365) exceeded that of single chirps (450). It has to be noted that the average whistle duration for other well-studied delphinids such as bottlenose (0.621 s, Bazúa-Durán & Au, 2002) and spinner dolphins (0.493 s, Díaz López, 2011) is higher than that of humpback dolphins (0.365s, Dong et al., 2019; 0.370s, Wang et al., 2013). In the same line, only one individual chirp (0.8%) exceeded 0.2 seconds in duration, whereas 173 single chirps (38.44%) ranged between 0.2 and 0.25 seconds. These findings suggest that the 0.25 seconds threshold may not be appropriate for humpback dolphin chirps, as many whistles classified as chirps under this definition are not particularly short for this species. To address this issue, an adapted definition of chirps for humpback dolphins is recommended. A shorter duration threshold may better capture the unique characteristics of these dolphin vocalizations and improve the accuracy of future studies on this species. Using the ratio of bottlenose and humpback dolphin whistle duration and applying it to chirps could be an option to set a preliminary threshold. Following this method, humpback dolphin chirps would be shorter than 0.150 s (0.621/0.365=1.70 – 0.25/1.70=0.150). Using this adapted threshold, the number of single chirps would be drastically reduced while the number of ChTs would not be affected.

As a subtype of whistles, ChTs were expected to serve a communicative function. Results indicated that humpback dolphins were more likely to emit ChTs within larger groups, during socializing activities, and in the absence of young individuals. These patterns confirm that ChTs are playing a role in inter-individual communication mostly among adults. This observation is in line with the higher chirp production rate of chirps during non-aggressive social contexts in captive dolphins (Perazio & Kuczaj, 2017). Studies investigating the context of chirp emission are rare but the potential function of ChTs can be compared to other vocalization types with similar characteristics. For instance, male bottlenose dolphins produce broad-band, pulsed sounds called "pops," typically emitted in repetitive trains. Pops are crucial in mating contexts as they help males maintain close proximity to consorting females (Sørensen et al., 2024). The structural and contextual particularities of individual chirps suggest that ChTs might also serve a very specific function for humpback dolphins. In addition, the train-like arrangement of ChTs with a high IChI variability may, like within sperm whale codas, encode particular information that facilitates complex social interactions. The types of sperm whale codas are defined according to the rhythm, tempo, and number of clicks, with the inter-click intervals carrying information (Jacobs et al., 2024; Sharma et al., 2024). According to the classification method proposed previously, information can be codified within the acoustic sequences following six different paradigms: repetition, diversity, combination, ordering, overlapping and timing (Pace et al., 2022). Considering the similar structure to codas, ChTs could present a similar structure with IChIs conveying information.

The use of ChTs by humpback dolphins also appears to be influenced by environmental conditions including the location, with significantly more ChTs produced in Pearl River Delta (PRD) compared to Zhanjiang (ZJ). A combination of factors may contribute to dolphins’ acoustic behaviour variation, including differences in habitat characteristics, ambient noise levels, human activities, or social structure (Dong et al., 2019; May-Collado & Wartzok, 2008). Similar to the way environmental and social characteristics may explain differences in humpback dolphin aerial behaviour (Serres et al., 2023), these factors may have played a role in the observed inter-population differences in terms of ChT production. The fact that the ChT production varied depending on the habitat type confirms that environmental variables may play an important role in the observed inter-population differences. Dolphins were more likely to use ChTs in open water areas than in vessel channels or harbors that are areas impacted by intense shipping traffic. Dolphin whistle production and characteristics are modulated in the presence of vessel noise(Van Parijs & Corkeron, 2001a) and ChTs seem to be impacted like other whistle types. Beyond the influence of human activities, the preference for open water areas may also be linked to the larger communication range available in such environments. Habitat structure affects sound transmission and can shape acoustic behavior, potentially driving the evolution of social structures in dolphin populations. For instance, bottlenose dolphins in northern Shark Bay produce more "pop" vocalizations in open areas with larger communication ranges (Sørensen et al., 2024). The higher ChT rate in PRD and in open waters aligns with this pattern, suggesting that ChTs and pops may be influenced by environmental factors in similar ways. This similarity further highlights the potential for broader functional or behavioral parallels between these two vocalizations. Further studies are needed to explore how environmental conditions might impact ChTs and whether environmental changes could have broader implications for humpback dolphin communication, social structure and even population health.

While this study showed that ChTs are produced relatively frequently by humpback dolphins and likely play a role in social interactions, their exact function remains unclear. It is still uncertain whether ChTs convey specific information or serve a more general social purpose. To fully understand the role of ChTs in humpback dolphins, further investigations into their acoustic characteristics and response to environmental factors is needed. Multidisciplinary research involving dolphin tagging studies, playback experiments, or simultaneous acoustic and unmanned aerial vehicle footage will be essential to gain a clearer understanding of the function of ChTs in this species.

ACKNOWLEDGMENTS

We thank Dr. Lin WZ, Chen SL, and Liu BS for their assistant in the fieldwork organization and data collection, and the rest members from the team of the Marine Mammal and Marine Bioacoustics laboratory for supporting this work. This research was financially supported by the National Natural Science Foundation of China ((Awards number 42225604, 42494883, 32150410369), and Science and Technology Talent Innovation Project of Hainan (Grant No. KJRC2023B03). This work was performed under Ethical Statement IDSSE-SYLL-MMMBL-01.

AUTHOR CONTRIBUTION

Hongri Wang: Writing - original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Agathe Serres: Writing - review & editing, Formal analysis, Data collection and curation. Yixi Shi: Data curation. Lijun Dong: review & editing, Songhai Li: Writing - review & editing, Validation, Supervision, Project administration, Funding acquisition, Conceptualization.

Ethic declaration

This work complied with the policies of the People's Republic of China and was performed under the Ethical Statement ‘IDSSE-SYLL-MMMBL-01’. To minimize disturbance, a quiet boat engine was used and turned off or put on neutral once the desired distance from the dolphins was achieved. Even though being in front of the animals may increase the quality of acoustic recordings, the research vessel was never placed in front of dolphins on purpose and instead remained on their side. However, dolphins could turn towards the research vessel by themselves on some occasions. Observers closely monitored the dolphins’ behavior throughout encounters for any signs of disturbance caused by the research vessel. Encounters were concluded either when sufficient data had been collected or when dolphins exhibited avoidance behaviors.

Competing interests

The authors have no conﬂict of interest to declare.

Data availability

The data used for the analysis of ChTs of Indo-Pacific humpback dolphins is available and can be found at https://data.mendeley.com/datasets/pfkrjphyvx/1

Bates, D., Mächler, M., Bolker, B. M., & Walker, S. C. (2015). Fitting linear mixed-effects models using lme4. Journal of Statistical Software, 67(1). https://doi.org/10.18637/jss.v067.i01
Bazúa-Durán, C., & Au, W. W. L. (2002). The whistles of Hawaiian spinner dolphins. The Journal of the Acoustical Society of America, 112(6), 3064–3072. https://doi.org/10.1121/1.1508785
Behr, O., Barré, K., Bontadina, F., Brinkmann, R., Dietz, M., Disca, T., Froidevaux, J. S. P., Ghanem, S., Huemer, S., Hurst, J., Kaminsky, S. K., Kelm, V., Korner-Nievergelt, F., Lauper, M., Lintott, P., Newman, C., Peterson, T., Proksch, J., Roemer, C., … Nagy, M. (2023). Standardised and referenced acoustic monitoring reliably estimates bat fatalities at wind turbines: comments on ‘Limitations of acoustic monitoring at wind turbines to evaluate fatality risk of bats.’ In Mammal Review (Vol. 53, Issue 2, pp. 65–71). John Wiley and Sons Inc. https://doi.org/10.1111/mam.12310
Brewer, A. M., Castellote, M., Van Cise, A. M., Gage, T., & Berdahl, A. M. (2023). Communication in Cook Inlet beluga whales: Describing the vocal repertoire and masking of calls by commercial ship noise. The Journal of the Acoustical Society of America, 154(5), 3487–3505. https://doi.org/10.1121/10.0022516
Caldwell, M. C., & Caldwell, D. K. (1968). Vocalization of Naive Captive Dolphins in Small Groups. Science, 159(3819), 1121–1123. https://doi.org/10.1126/science.159.3819.1121
Chen, Z., & Wiens, J. J. (2020). The origins of acoustic communication in vertebrates. Nature Communications, 11(1). https://doi.org/10.1038/s41467-020-14356-3
Díaz López, B. (2011). Whistle characteristics in free-ranging bottlenose dolphins (Tursiops truncatus) in the Mediterranean Sea: Influence of behaviour. Mammalian Biology, 76(2), 180–189. https://doi.org/10.1016/j.mambio.2010.06.006
Dong, L., Caruso, F., Lin, M., Liu, M., Gong, Z., Dong, J., Cang, S., & Li, S. (2019). Whistles emitted by Indo-Pacific humpback dolphins ( Sousa chinensis ) in Zhanjiang waters, China . The Journal of the Acoustical Society of America, 145(6), 3289–3298. https://doi.org/10.1121/1.5110304
Hartig, F. (2018). DHARMa: residual diagnostics for hierarchical (multi-level/mixed) regression models. R Packag Version 020.
Ho, Y. W., Lin, T. H., Akamatsu, T., & Karczmarski, L. (2024). Fine-scale spatial variability of marine acoustic environment corresponds with habitat utilization of Indo-Pacific humpback dolphins in Hong Kong waters. Ecological Indicators, 158. https://doi.org/10.1016/j.ecolind.2023.111228
Jacobs, E. R., Gero, S., Malinka, C. E., Tønnesen, P. H., Beedholm, K., DeRuiter, S. L., & Madsen, P. T. (2024). The active space of sperm whale codas: inter-click information for intra-unit communication. Journal of Experimental Biology, 227(4). https://doi.org/10.1242/jeb.246442
Jefferson, T. A., & Smith, B. D. (2016). Re-assessment of the Conservation Status of the Indo-Pacific Humpback Dolphin (Sousa chinensis) Using the IUCN Red List Criteria. In Advances in Marine Biology (Vol. 73, pp. 1–26). Academic Press. https://doi.org/10.1016/bs.amb.2015.04.002
Jones, B., Zapetis, M., Samuelson, M. M., & Ridgway, S. (2020). Sounds produced by bottlenose dolphins (Tursiops): a review of the defining characteristics and acoustic criteria of the dolphin vocal repertoire. In Bioacoustics (Vol. 29, Issue 4, pp. 399–440). Taylor and Francis Ltd. https://doi.org/10.1080/09524622.2019.1613265
Karczmarski, L., & Cockcroft, V. G. (1999). Daylight behaviour of humpback dolphins Sousa chinensis in Algoa Bay, South Africa. Zeitschrift Fur Saugetierkunde, 64(1), 19–29.
Lin, W., Chen, S., Liu, B., Zheng, R., Serres, A., Lin, M., Liu, M., & Li, S. (2024). Survival and population size of the Indo-Pacific humpback dolphins off the eastern Leizhou Peninsula. Marine Mammal Science. https://doi.org/10.1111/mms.13156
Lin, W., Karczmarski, L., Chan, S. C. Y., Zheng, R., Ho, Y. W., & Mo, Y. (2024). Population parameters and heterogeneity in survival rates of Indo-Pacific humpback dolphins in a heavily urbanized coastal region of southeast China: implications for conservation. Frontiers in Marine Science, 11. https://doi.org/10.3389/fmars.2024.1252661
Liu, M., Lin, M., Lusseau, D., & Li, S. (2021). Intra-Population Variability in Group Size of Indo-Pacific Humpback Dolphins (Sousa chinensis). Frontiers in Marine Science, 8. https://doi.org/10.3389/fmars.2021.671568
Lüdecke, D., Ben-Shachar, M., Patil, I., Waggoner, P., & Makowski, D. (2021). performance: An R Package for Assessment, Comparison and Testing of Statistical Models. Journal of Open Source Software, 6(60), 3139. https://doi.org/10.21105/joss.03139
Ma, H., & Fan, P. (2023). Application, progress, and future perspective of passive acoustic monitoring in terrestrial mammal research. Biodiversity Science, 31(1). https://doi.org/10.17520/biods.2022374
May-Collado, L. J., & Wartzok, D. (2008). A comparison of bottlenose dolphin whistles in the atlantic ocean: Factors promoting whistle variation. Journal of Mammalogy, 89(5), 1229–1240. https://doi.org/10.1644/07-MAMM-A-310.1
Pace, D. S., Tumino, C., Silvestri, M., Giacomini, G., Pedrazzi, G., Pavan, G., Papale, E., Ceraulo, M., Buscaino, G., & Ardizzone, G. (2022). Bray‐Call Sequences in the Mediterranean Common Bottlenose Dolphin (Tursiops truncatus) Acoustic Repertoire. Biology, 11(3). https://doi.org/10.3390/biology11030367
Perazio, C. E., & Kuczaj, S. A. (2017). Vocalizations produced by bottlenose dolphins (Tursiops truncatus) during mouth actions in aggressive and non-aggressive contexts. International Journal of Comparative Psychology, 30. https://doi.org/10.46867/ijcp.2017.30.00.07
Piwetz, S., Jefferson, T. A., & Würsig, B. (2021). Effects of Coastal Construction on Indo-Pacific Humpback Dolphin (Sousa chinensis) Behavior and Habitat-Use Off Hong Kong. Frontiers in Marine Science, 8. https://doi.org/10.3389/fmars.2021.572535
Rege-Colt, M., Oswald, J. N., De Weerdt, J., Palacios-Alfaro, J. D., Austin, M., Gagne, E., Morán Villatoro, J. M., Sahley, C. T., Alvarado-Guerra, G., & May-Collado, L. J. (2023). Whistle repertoire and structure reflect ecotype distinction of pantropical spotted dolphins in the Eastern Tropical Pacific. Scientific Reports, 13(1). https://doi.org/10.1038/s41598-023-40691-8
Sayigh, L. S. (2014). Cetacean Acoustic Communication. In Biocommunication of Animals (pp. 275–297). Springer Netherlands. https://doi.org/10.1007/978-94-007-7414-8_16
Schusterman, R. J. (1966). Communication between Dolphins in Separate Tanks. Science, 152(3720), 387–387. https://doi.org/10.1126/science.152.3720.387.b
Serres, A., Lin, W., Liu, B., Chen, S., & Li, S. (2023). Context of breaching and tail slapping in Indo-Pacific humpback dolphins in the northern South China Sea. Behavioral Ecology and Sociobiology, 77(6). https://doi.org/10.1007/s00265-023-03337-3
Serres, A., Thomas, J.-H., Dong, L., Chen, S., Liu, B., & Li, S. (2024). Potential signature whistle production by Indo-Pacific humpback dolphins, Sousa chinensis, in the northern South China sea. Animal Behaviour, 218, 149–161. https://doi.org/10.1016/j.anbehav.2024.10.001
Sharma, P., Gero, S., Payne, R., Gruber, D. F., Rus, D., Torralba, A., & Andreas, J. (2024). Contextual and combinatorial structure in sperm whale vocalisations. Nature Communications, 15(1). https://doi.org/10.1038/s41467-024-47221-8
Sørensen, P. M., Connor, R. C., Allen, S. J., Krützen, M., Lebrec, U., Jensen, F. H., & King, S. L. (2024). Communication range predicts dolphin alliance size in a cooperative mating system. Current Biology, 34(20), 4774-4780.e5. https://doi.org/10.1016/j.cub.2024.08.032
Tang, X., Lin, W., Karczmarski, L., Lin, M., Chan, S. C. Y., Liu, M., Xue, T., Yuping, W. U., Zhang, P., & Songhai, L. I. (2021). Photo-identification comparison of four Indo-Pacific humpback dolphin populations off southeast China. Integrative Zoology, 16(4), 586–593. https://doi.org/10.1111/1749-4877.12537
Van Parijs, S. M., & Corkeron, P. J. (2001a). Boat traffic affects the acoustic behaviour of Pacific humpback dolphins, Sousa chinensis. Journal of the Marine Biological Association of the United Kingdom, 81(3), 533–538. https://doi.org/10.1017/S0025315401004180
Van Parijs, S. M., & Corkeron, P. J. (2001b). Vocalizations and behaviour of Pacific humpback dolphins Sousa chinensis. Ethology, 107(8), 701–716. https://doi.org/10.1046/j.1439-0310.2001.00714.x
Vester, H., Hallerberg, S., Timme, M., & Hammerschmidt, K. (2017). Vocal repertoire of long-finned pilot whales ( Globicephala melas ) in northern Norway . The Journal of the Acoustical Society of America, 141(6), 4289–4299. https://doi.org/10.1121/1.4983685
Wang, Z., Fang, L., Shi, W., Wang, K., & Wang, D. (2013). Whistle characteristics of free-ranging Indo-Pacific humpback dolphins ( Sousa chinensis ) in Sanniang Bay, China . The Journal of the Acoustical Society of America, 133(4), 2479–2489. https://doi.org/10.1121/1.4794390
Wood Jr, F. G. (1953). Underwater sound production and concurrent behavior of captive porpoises, Tursiops truncatus and Stenella plagiodon. Bulletin of Marine Science, 3(2), 120–133.
Yuan, J., Wang, Z., Duan, P., Xiao, Y., Zhang, H., Huang, Z., Zhou, R., Wen, H., Wang, K., & Wang, D. (2021). Whistle signal variations among three Indo-Pacific humpback dolphin populations in the South China Sea: a combined effect of the Qiongzhou Strait’s geographical barrier function and local ambient noise? Integrative Zoology, 16(4), 499–511. https://doi.org/10.1111/1749-4877.12531
Zimmer, W. M. X. (2011). Passive acoustic monitoring of cetaceans. Cambridge University Press.

No competing interests reported.

supplementaldata.pdf

Production of chirp trains by Indo-Pacific humpback dolphins (Sousa chinensis) in the northern South China Sea

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Abstract

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SIGNIFICANCE STATEMENT

INTRODUCTION

MATERIAL AND METHODS

RESULTS

DISCUSSION

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References

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Supplementary Files

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