Introduction
Mediated learning is crucial in passing knowledge and information from older to newer generations. Humans do it by teaching their children, allowing them to communicate and recognize each other based solely on their sounds. Similarly, animals have a unique way of understanding one another by producing specific sounds for every situation. Dolphins have advanced social cognitive skills, and as social animals, they have a broad vocal repertoire obtained from their surrounding (Oswald et al., 2021).
Dolphins mainly communicate via whistles and burst pulses. The main difference is that whistles are frequency-modulated sounds that have a duration of milliseconds to several seconds, whereas burst pulses are click trains with less than 10 milliseconds intervals (Oswald et al., 2021). Some experienced humans can distinguish between the signature whistles, but burst pulses are not discernible to people. This review paper explores studies published in the last ten years to understand the variance of dolphins’ signature whistles, what influences the signature whistles in groups and individuals, and the change in vocalization behaviors in varying circumstances.
Individual and Group Signature Whistles
Dolphins can produce both individualized signature whistles and stereotyped in-group whistles. Jones et al. (2020) collected data from eight bottlenose dolphins located at the Navy Marine Mammal Program that had lived in the facility for over 21 years. The researchers wanted to assess whether they could identify discernible differences in individualized and in-group whistles. Shared whistles come from everyday interactions, where the dolphins develop group characteristics that they can use to recognize one another if lost. The study revealed that each of the eight dolphins could produce a unique individual signature whistle, and at least five of them had shared in-group signatures (Jones et al., 2020).
The individual signature whistle is critical in identifying each whale in the pool, during training, or in the wilderness. The shared whistle contours allow each dolphin to trace their group when isolated or to gain the group benefits, such as mating. Specifically, the data collected showed that dolphins produced, on average, individual signature whistles at 42.9% and shared whistle contours at 22.4% during isolations (Jones et al., 2020). The stereotyped in-group whistles contained connotations of individual differences, allowing the dolphins to know which one made the sound.
It is crucial to consider what influences the variability in individual dolphin signatures and stereotyped whistles. First, Jones et al. (2020) and Oswald et al. (2021) show that mediated learning allows younger dolphins to acquire vocalizations from their neighbors. It is for this reason that the dolphins had stereotyped in-group whistles. However, these stereotyped whistles vary from one group to another.
La Manna et al. (2022) examined six distinct bottlenose dolphin groups in different sites in the Mediterranean Sea. The findings confirmed previous studies indicating individual groups have homogenous stereotypes that differ by region. Population demography is the strongest contributor to this variability of dolphins’ whistles (La Manna et al., 2022).
The results showed groups with high dissimilarity in number and transient dolphins, leading to the development of widely distinctive signature calls. Therefore, a high number causes the evolution of signature whistles varying in the frequency range, inflection points, duration, and max and end frequencies to improve recognition of each dolphin (La Manna et al., 2022). Environmental features equally influenced the signature whistles because dolphins found near human settlements picked up acoustics they could practice compared to those at the sea bottom.
Patterns, Cues, and Responses to Threat
Data collected from the years of research enabled scientists to identify different types and structures of the signature whistles. Sayigh et al. (2022) used Sarasota Dolphin Whistle Database records to determine frequency modulation patterns and contours that give dolphins distinguishable calls. The distinctiveness in these patterns enables other dolphins to recognize or maintain contact, like humans.
The study focused on changes in pattern over time and the frequency contour, which defines whistle identity. Multi-loop whistles were familiar with upsweep contours from the collected data, followed by single-loop contours. These loops could be similar in shape or differ due to features such as addition, deletion, steps, bi-phonation, and simultaneous pulsed components, among others (Sayigh et al., 2022).
The introduced differences in the contours mean that humans can identify and group various types of signature whistles. The minimum frequency of the contour was 1,781 Hz, while the maximum recorded was 14,766 Hz (Sayigh et al., 2022). The identified unique features of the signature whistles lead to an individualized identity that remains relatively stable, only with subtle changes throughout a dolphin’s life.
Another important aspect of communication is knowing or predicting when one is done so the other can respond. Humans use intonations as a cue to show the break or end of an idea, whereas animals use temporal rules. For instance, squirrel monkeys respond to chuck calls in 0.5 seconds, while Japanese macaques do so within 0.8 seconds (Jones et al., 2022). Dolphins’ signature whistles follow specific rules to indicate that they are done signaling, which prevents interruption or encourages responses from others. Jones et al. (2022) used eight dolphins from a Navy facility in San Diego to study relative amplitude modulation cues for signature whistles.
The results revealed a fade-in and fade-out pattern and amplitude variance in the duration of each signature whistle. The dolphin’s nasal pressure varies as it begins to rise at the beginning of the whistle, reaches a peak during whistling, and begins to fall, signaling the completion of the whistle. The response to the calls is usually within one second, or there is a short overlap, indicating a reliance on other cues described (Jones et al., 2022). This pattern of fade-in and fade-out enables dolphins to communicate effectively during call exchanges.
Understanding the signature whistling pattern dolphins make when they sense danger is essential. Luís et al. (2014) recorded dolphin vocalization in the presence of a boat within a 1,000 m radius and without an operating boat to determine changes in whistling. Vessels present a significant threat to dolphins and should be treated as such. The results indicate that average call rates declined to seven per minute with a ferry boat within the defined range from 22 calls per minute without a boat (Luís et al., 2014). The decrease in the calling rate could imply that dolphins are aware of their prey, thus avoiding attracting attention.
A similar trend occurred with whistling rates as 413 without a boat, 56 with a dolphin-watching vessel, 37 with leisure boats, 33 with ferry boats, and 31 with trawlers around (Luís et al., 2014). The results indicate that the size of the vessel significantly affects dolphins’ vocalization behaviors. Therefore, it is clear that dolphins adjust their acoustic habits by changing vocalization features such as frequency and call rates to mask the presence of a threat.
Conversely, dolphins’ call and whistling rates increase when they encounter danger. Fishing accidents mean that dolphins get caught in the nets with deadly outcomes. Terranova et al. (2022) examined the acoustic behavior of the dolphins caught in deadly fishing activities in the Gulf of Catania. The results show a rise in calling rates, possibly indicating a call for help from the groups.
Specifically, the data indicated 720 vocalizations within 138 minutes of capture, with 436 signature whistles, 233 burst pulses, and 51 non-signature whistles (Terranova et al., 2022). The varying signature whistles suggest the presence of different dolphins who had tried to rescue the captured dolphin. The rise in agitation and whistling frequency signifies the dolphins’ reactions to a death-related experience.
Conclusion
In conclusion, dolphins’ signature whistles allow them to communicate with each other and recognize individual dolphins in a group. The review showed dolphins learn vocalization from neighbors, which influences the in-group whistling contours. Shared contours contain individual connotations that enable others to identify which dolphin communicated in the group. The signature whistles vary based on demographic and environmental characteristics, with larger groups comprising broader variances because of mediated learning. Interactions with humans considerably change the dolphins’ vocalizations because they pick and add certain acoustics to their repertoire.
The studies showed how vocalization behaviors change with threat, as dolphins tend to whistle less, not to alert potential prey. In contrast, their vocalizations, including calls and signature whistles, increase significantly when a dolphin is captured. These acoustic patterns play a crucial role in recognizing and communicating specific contexts, such as risk and identification of individual dolphins in social interactions. Therefore, the studies reviewed in this paper provided crucial insights into dolphin signature whistles’ function, variances, and nature, which is an ongoing discovery process.
References
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Jones, B., Tufano, S., & Ridgway, S. (2022). Signature whistles exhibit a ‘fade-in’ and then ‘fade-out’ pattern of relative amplitude declination. Behavioural Processes, 200. Web.
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Sayigh, L. S., Janik, V. M., Jensen, F. H., Scott, M. D., Tyack, P. L., & Wells, R. S. (2022). The Sarasota Dolphin Whistle Database: A unique long-term resource for understanding dolphin communication. Frontiers in Marine Science, 9. Web.
Terranova, F., Raffa, A., Floridia, S., Monaco, C., & Favaro, L. (2022). Vocal behaviour of a bottlenose dolphin pod during a deadly bycatch event in the Gulf of Catania, Ionian Sea. Journal of Marine Science and Engineering, 10(5), 616. Web.