Aarhus University Seal / Aarhus Universitets segl

Michael Ladegaard

Echolocation in small cetaceans: PhD dissertation

Research output: Book/anthology/dissertation/reportPh.D. thesisResearch

Many animals exploit sound for navigational purposes, but only some produce signals dedicated to probe their environment actively through echolocation. Only in bats and toothed whales has echolocation evolved to serve as a primary sense informing not only navigation, but also foraging on highly agile prey under conditions of poor lighting. This sixth sense has been intensively studied especially in laboratory settings using stationary animals, but in recent decades an increasing knowledge has formed on more natural aspects reflecting the critical evolutionary advantage of biosonar, namely identifying, tracking, and capturing prey.
I have sought to expand on this knowledge by studying poorly understood aspects of toothed whale echolocation. This has taken me to the Amazon rain forest, where I have studied the use of echolocation in Amazon river dolphins to show that this species uses a directional, short-range biosonar operated at very high update rates compared to marine toothed whales (Chapter 2). I further studied how these river dolphins adjust their biosonar as they close in on and intercept prey in the first study to measure on-axis source parameters on wild toothed whales during prey capture (Chapter 3). In a more controlled experimental setup using a highly trained bottlenose dolphin, I have also investigated a hitherto rarely studied mode of echolocation in toothed whales involving discrete click packets that was previously only reported for stationary animals echolocating targets at fixed ranges. In that study I show how a freely swimming dolphin also produces click packets as it seeks out and approaches a target starting from ranges up to several hundred metres until about 120 m from the target, after which usual biosonar adjustments are used (Chapter 4). In other experiments on trained harbour porpoises, I have focused on how animals cope with clutter during prey interception, on potential jamming effects caused by conspecific echolocation, and on how the biosonar is adjusted in response (Chapter 1). In a different study we demonstrate that porpoises strongly decrease biosonar output levels as they approach a target, but with no adjustment to the target strength, which makes us conclude that returning echo levels are adjusted to match the animal's dynamic hearing range, without excluding target size information from the perceived echo levels (Chapter 5). Furthermore, I have been involved in a review on the evolutionary aspects behind the acoustic field of view in toothed whales (Chapter 6), and in a field study on the ontogeny and development of echolocation in sperm whale calves (Chapter 7). It is my hope that this dissertation will help shed more light in the dark world of echolocation in toothed whales, the adaptations to biosonar operation in various contexts, and the biosonar adjustments that toothed whales employ when closing in on their echolocation targets.
Original languageEnglish
Place of publicationAarhus University
Number of pages179
Publication statusPublished - 20 Jun 2017

    Research areas

  • Beam pattern, biosonar, click packet, directivity, dolphin, echolocation, gain control, odontocete, porpoise, river dolphin, source parameter, sperm whale, toothed whale

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ID: 114598028