Ladegaard
Michael
Ladegaard
Michael
No Thumbnail Available
Search Results
Now showing
1 - 3 of 3
-
PreprintNarrow acoustic field of view drives frequency scaling in toothed whale biosonar( 2018-10) Jensen, Frants H. ; Johnson, Mark P. ; Ladegaard, Michael ; Wisniewska, Danuta M. ; Madsen, Peter T.Toothed whales are apex predators varying in size from 40-kg porpoises to 50-ton sperm whales that all forage by emitting high-amplitude ultrasonic clicks and listening for weak returning echoes [1, 2]. The sensory field of view of these echolocating animals depends on the characteristics of the biosonar signals and the morphology of the sound generator, yet it is poorly understood how these biophysical relationships have shaped evolution of biosonar parameters as toothed whales adapted to different foraging niches. Here we test how biosonar output, frequency, and directivity vary with body size to understand the co-evolution of biosonar signals and sound-generating structures. We show that the radiated power increases twice as steeply with body mass (P ∝ M1.47±0.25) than expected from typical scaling laws of call intensity [3], indicating hyperallometric investment into sound production structures. This is likely driven by a strong selective pressure for long-range biosonar in larger oceanic or deep-diving species to search efficiently for patchy prey. We find that biosonar frequency scales inversely with body size (F∝ M-0.19±0.03), resulting in remarkably stable biosonar beamwidth that is independent of body size. We discuss how frequency scaling in toothed whales cannot be explained by the three main hypotheses for inverse scaling of frequency in animal communication [3-5]. We propose that a narrow acoustic field of view, analogous to the fovea of many visual predators, is the primary evolutionary driver of biosonar frequency in toothed whales, serving as a spatial filter to reduce clutter levels and facilitate long-range prey detection.
-
ArticleLatencies of click-evoked auditory responses in a harbor porpoise exceed the time interval between subsequent echolocation clicks(Acoustical Society of America, 2023-02-07) Beedholm, Kristian ; Ladegaard, Michael ; Madsen, Peter T. ; Tyack, Peter L.Most auditory evoked potential (AEP) studies in echolocating toothed whales measure neural responses to outgoing clicks and returning echoes using short-latency auditory brainstem responses (ABRs) arising a few ms after acoustic stimuli. However, little is known about longer-latency cortical AEPs despite their relevance for understanding echo processing and auditory stream segregation. Here, we used a non-invasive AEP setup with low click repetition rates on a trained harbor porpoise to test the long-standing hypothesis that echo information from distant targets is completely processed before the next click is emitted. We reject this hypothesis by finding reliable click-related AEP peaks with latencies of 90 and 160 ms, which are longer than 99% of click intervals used by echolocating porpoises, demonstrating that some higher-order echo processing continues well after the next click emission even during slow clicking. We propose that some of the echo information, such as range to evasive prey, is used to guide vocal-motor responses within 50–100 ms, but that information used for discrimination and auditory scene analysis is processed more slowly, integrating information over many click-echo pairs. We conclude by showing theoretically that the identified long-latency AEPs may enable hearing sensitivity measurements at frequencies ten times lower than current ABR methods.
-
ArticleAmazon river dolphins (Inia geoffrensis) use a high-frequency short-range biosonar(The Company of Biologists, 2015-10-07) Ladegaard, Michael ; Jensen, Frants H. ; de Freitas, Mafalda ; da Silva, Vera Maria F. ; Madsen, Peter T.Toothed whales produce echolocation clicks with source parameters related to body size; however, it may be equally important to consider the influence of habitat, as suggested by studies on echolocating bats. A few toothed whale species have fully adapted to river systems, where sonar operation is likely to result in higher clutter and reverberation levels than those experienced by most toothed whales at sea because of the shallow water and dense vegetation. To test the hypothesis that habitat shapes the evolution of toothed whale biosonar parameters by promoting simpler auditory scenes to interpret in acoustically complex habitats, echolocation clicks of wild Amazon river dolphins were recorded using a vertical seven-hydrophone array. We identified 404 on-axis biosonar clicks having a mean SLpp of 190.3±6.1 dB re. 1 µPa, mean SLEFD of 132.1±6.0 dB re. 1 µPa2s, mean Fc of 101.2±10.5 kHz, mean BWRMS of 29.3±4.3 kHz and mean ICI of 35.1±17.9 ms. Piston fit modelling resulted in an estimated half-power beamwidth of 10.2 deg (95% CI: 9.6–10.5 deg) and directivity index of 25.2 dB (95% CI: 24.9–25.7 dB). These results support the hypothesis that river-dwelling toothed whales operate their biosonars at lower amplitude and higher sampling rates than similar-sized marine species without sacrificing high directivity, in order to provide high update rates in acoustically complex habitats and simplify auditory scenes through reduced clutter and reverberation levels. We conclude that habitat, along with body size, is an important evolutionary driver of source parameters in toothed whale biosonars.