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Source levels of humpback whales decrease with frequency suggesting an air-filled resonator is used in sound

Elisa Girola1, Michael J Noad1, Rebecca A Dunlop1

  • 1Cetacean Ecology and Acoustics Laboratory, School of Veterinary Science, University of Queensland, Gatton, Queensland 4343, Australia.

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Humpback whale songs show source levels vary by unit type and decrease with frequency. This suggests an air-filled resonator influences sound production in these marine mammals.

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Area of Science:

  • Bioacoustics
  • Marine Mammal Communication
  • Animal Acoustics

Background:

  • Acoustic signal transmission distance is influenced by source level and frequency.
  • Biomechanical linkages can constrain independent modification of sound characteristics in some species.
  • Humpback whale (Megaptera novaeangliae) songs are complex acoustic signals crucial for communication.

Purpose of the Study:

  • To investigate variability in source levels and their relationship with frequency in humpback whale songs.
  • To determine if biomechanical constraints affect sound production in humpback whales.
  • To understand how sound characteristics influence acoustic transmission in humpback whales.

Main Methods:

  • Acoustic recordings of humpback whale songs were collected off eastern Australia using a fixed hydrophone array.
  • Singing whales were acoustically tracked to correlate songs with individuals.
  • An empirical, frequency-dependent model estimated transmission loss.
  • Source levels and peak frequencies of 2408 song units from 19 singers were measured.

Main Results:

  • Source levels ranged from 138 to 187 dB re 1 μPa at 1 m (RMS).
  • Peak frequency varied between 52 and 3877 Hz.
  • Unit type accounted for significant source level variability (up to 17 dB difference in means).
  • Source levels were negatively correlated with peak frequency, decreasing by 2.3 dB per octave.

Conclusions:

  • Humpback whale song source levels exhibit significant variability, influenced by unit type.
  • A negative correlation exists between source level and frequency in humpback whale songs.
  • This relationship supports the hypothesis of an air-filled resonator in the whale's sound production system.