The behavior and broadband acoustic signaling of Hawaiʻian spinner dolphins (Stenella longirostris)

Abstract

This dissertation explores the design of dolphin social acoustic signals by addressing the question: how have the signals used by these animals been adapted to the physical, sensory and social environments they inhabit? To obtain answers to this question, the behavior and acoustic signals of spinner dolphins (Stenella longirostris) and spotted dolphins (Stenella frontalis) were examined. A rhythmic pattern of behavior is described for a population of spinner dolphins resident off the island of O'ahu. The population is tied to both trophic and physiographical resources associated with the island's coastline, where groups fuse and fragment in predicable patterns over the course of a daily cycle of activity and rest. Acoustic signals are thought to playa critical role in the coordination of animals within and between groups. A broadband analysis of the whistles and burst pulses of these two species reveals that they span a much broader range of frequencies than typically discussed in the literature. A considerable amount of energy is found in the higher frequency harmonics of whistles, as well as in the ultrasonic bands of burst pulses. Many burst pulses in fact have energy exclusively at ultrasonic frequencies and are thus inaudible to human hearing. Most if not all of the frequency hearing sensitivity typical for dolphins appears to be exploited in the design of their social acoustic signals. Evidence is also presented on the directionality of whistles together with a discussion on how the directional transmission of harmonics may provide important cues to listening animals about the orientation and direction of movement of a signaler. The "mixed-directionality" of dolphin whistles is proposed as a signal design feature that may be very important for the coordination of individuals within groups. Finally, a discussion is presented on how these findings fit together to provide a more complete picture of the nature of dolphin social acoustic signals. Their functional design is considered along with the features that make them well suited for echolocation and communication underwater. A roadmap is provided for future investigators who wish to further explore the design of dolphin social acoustic signals.