| Summary: | Summary As apex predators, deep-diving toothed whales play an important ecological role in the vast mesopelagic habitats of the oceans. These whales use echolocation to catch prey in complete darkness at depth. In my PhD, I have explored how deep-diving toothed whales operate and adjust their biosonars to successfully exploit the rich mesopelagic foraging niche. In Chapter 1 I provide background and review the results of my PhD in a selected broader context. In Chapter 2 I present the first insight on the foraging scene of an echolocating sperm whale, showing that this whale encountered numerous organisms, but only targeted a few of these. Similar to beaked whales, this sperm whale mainly performed prey captures at depths of low prey densities, presumably to reduce the sensory complexity and mitigate to confusion effect of large prey aggregations. Additionally, I demonstrate that the powerful sound emission of sperm whales enable long-range echolocation. In Chapter 3 we study how three male sperm whales adapt their powerful, long-range biosonar system during benthic feeding. We show a reduction of click output levels likely to reduce reverberation and an increase of biosonar sampling rates in accordance with the shorter extent of their foraging scene. In Chapter 4 I explore the ontogeny of sperm whale calves, showing that yearling calves dive to 300 m and beyond while clicking with inter-click intervals similar to adult sperm whales. The largest calf reached depths where adult sperm whales forage, while it echolocated for extended periods and emitted two buzzes, indicating that biosonar mediated foraging develops within the first year of life for this species. In Chapter 5 I demonstrate how short-finned pilot whales adjust their output levels and sensory sampling rates according to the distance to their prey. During deep daytime dives, these whales furthermore perform marked adjustments of their output levels when targeting highly mobile calorific prey. This prey type additionally requires longer buzzes with ...
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