Seal vocalization mechanisms
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Seal Vocalization Mechanisms: Sound Production and Modulation
Airborne and Underwater Sound Production in Seals
Seals are capable of producing vocalizations both above and below the water surface, utilizing a variety of sound types. Studies on gray seals show that their vocalizations can be grouped into three main categories based on acoustic characteristics: tonal sounds (produced both underwater and at the surface), and two types of pulsed sounds with different bandwidths, durations, and repetition rates. These differences suggest that seals use several independent mechanisms for sound production, which are influenced by the physical properties of air and water as sound propagation media. The mechanisms for generating air- and water-borne sounds differ significantly, but seals can efficiently use their vocal tracts in both environments, adapting their vocal output to the medium in which they are communicating Nowak2015Nowak2020.
Role of the Vocal Tract and Articulatory Movements
The vocal tract plays a crucial role in shaping the acoustic features of seal vocalizations. In harbor seals, for example, the position and movement of the jaw are strongly correlated with changes in formant frequencies, particularly the first formant (F1). This is similar to how humans use jaw and other articulatory gestures to modulate vocal sounds. The study of a trained harbor seal demonstrated that jaw opening angle directly affects the resonance structure of vocalizations, indicating that seals possess a degree of articulatory control over their vocal output .
Resonance and Source-Filter Interactions
The source-filter theory explains that vocalizations are shaped by both the sound source (typically the larynx) and the filtering effects of the vocal tract. In Weddell seals, resonance within the vocal tract amplifies certain frequencies, but the primary determinant of call amplitude is the sound generated at the larynx. Resonance plays a secondary, yet measurable, role in enhancing specific frequency components of underwater calls, with evidence of resonance frequencies corresponding to the physical dimensions of the trachea and larynx .
Vocal Plasticity and Learning in Seals
Seals exhibit notable vocal plasticity, adjusting their vocalizations in response to environmental factors such as ambient noise. Harbor seal pups, for instance, can lower the fundamental frequency (F0) of their calls when exposed to noise, a rare ability among mammals. This adjustment is specific to the noise condition and suggests active modulation of vocal effort. In some cases, seals also show changes in call amplitude and spectral tilt, similar to the Lombard effect observed in human speech .
Vocal learning is another key mechanism in seal vocalization. Both harbor and gray seal pups can copy specific acoustic stimuli, with accuracy improving over time. This learning ability is used to achieve conformity within groups and may contribute to the development of adult vocal repertoires, facilitating social interactions and possibly mate choice . Additionally, seal pups demonstrate rhythmic vocal interactions, with evidence of antisynchronous coordination, indicating complex timing mechanisms in their vocal behavior .
Adaptation to Ambient Noise
Seals can adjust certain vocal parameters in response to changes in ambient noise. For example, spotted seals increase the amplitude of their growl vocalizations as ambient noise levels rise, although other call types and parameters such as duration and frequency remain unaffected. This selective adjustment suggests that seals use specific strategies to maintain communication effectiveness in noisy environments .
Diversity of Vocalizations and Seasonal Patterns
Seals produce a wide range of vocalizations, including tonal calls, roars, knocks, moans, and even ultrasonic sounds. Weddell seals, for example, emit ultrasonic vocalizations with fundamental frequencies reaching up to 50 kHz, especially during winter darkness. The functional significance of these high-frequency calls is not yet fully understood, but their presence highlights the diversity and complexity of seal vocal mechanisms . In spotted seals, vocal activity peaks during the breeding season, with stereotypic underwater calls associated with reproductive behaviors .
Conclusion
Seal vocalization mechanisms are highly adaptable and complex, involving multiple independent sound production strategies, significant articulatory control, and the ability to learn and modify calls in response to environmental and social factors. These mechanisms allow seals to communicate effectively in both air and water, adjust to varying noise conditions, and develop diverse vocal repertoires that support social and reproductive behaviors Nowak2015Nowak2020Goncharova2024+7 MORE.
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