Clicks, whistles and pulses: Passive and active signal use in dolphin communication
Introduction
Dolphin communication is both complex and contextual [1]. Dolphins communicate using both vocal and non-vocal signals including visual, tactile, kinesthetic, and chemoreceptive [2] including cross-modal abilities [3]. Because of the highly developed acoustic sense in dolphins, researchers have emphasized the recording and analysis of vocalizations [4]. Dolphin sound production and reception are highly directional in both frequency and intensity [5]. Dolphins produce two sounds simultaneously producing clicks on the right side and whistles on the left side [6]. Dolphins are also capable of internal sound pointing by reshaping their sound focusing organ (the melon) and parameters including frequency, intensity, and duration can all be modulated independently, providing opportunity for detailed encoding of information [7]. The directional nature of dolphin sound adds a complication to many studies and requires triangulation involving multiple hydrophones or separation of individuals during recording.
Dolphin sounds are divided into three primary categories: Whistles, clicks, and burst pulsed sounds. All sounds can be used socially while echolocation is thought to be primarily for navigation and hunting. Spectrally distinct sound types include 1) Whistles – primarily social communication including frequency-modulated whistles, amplitude modulated whistles and whistle squawks, 2) Clicks – navigation and orientation including echolocation click trains, and buzzes and 3) Burst pulsed sounds – primarily social sounds including squawks, barks and pops. Recently discovered synchronized vocalizations include squawks, screams, and brays, and show prosodic elements to dolphin communication. Dolphins also make in-air vocalizations including chuffs, raspberries, and non-vocal impulse sounds including jaw-claps, tail cavitation, tail slaps, and bubbles [8].
Dolphins are both predator (on fish, squid) and prey (sharks, orcas) and use both passive and active sonar. Hearing sensitivity is excellent and many species produce signals with a bi-modal frequency bands around 40–50 kHz and 130–140 kHz [9]. But when do dolphins listen passively and when do they actively search? Passive acoustic vigilance and “eavesdropping” on neighbors allows for information sharing from one individual to another without cost, while active vigilance via echolocation is used for final targeting of prey or clarification of information already passively detected. However, the costs of active vigilance may have a cost as it does for other species [10].
The need for stealth is best exemplified by the use of crypticity by killer whales (Orcinus orca). Resident killer whales in the Pacific Northwest eat fish and use regularly spaced clicks when hunting their prey, primarily fish that do not hear high frequencies. Transient killer whales, that eat small dolphins and porpoise who hear high frequencies, use irregularly spaced click patterns that are hidden in the background noise [11].
Section snippets
Measurement techniques and information content
Historically, whistles are the most studied dolphin vocalization because of their ease in measurement. The majority of literature on whistles report qualitative comparative visual assessments of frequency contours made by human judges while quantitative techniques (Discriminant Function Analysis, Principal Component Analysis) have been occasionally attempted [12]. Neural networks (NN) have been used to quantify whistles, although NN analysis requires a priori data to train a computer [13].
Conclusions
Scientists face the daunting task of trying to understand and decode animal signals by applying previous research techniques used with other taxa or human subjects. Human perceptual abilities and human-biases continue to challenge our research techniques when assessing other species. This paper discussed multiple examples of mistakes (and corrections) made in dolphin research over decades, including constraining animals to assess their natural abilities, misunderstanding sensory systems,
Acknowledgments
Thanks to G Harp, D Vakoch, C. Maccone, and J Elliot for thoughtful discussions on the subject.
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