Modification of humpback whale social sound repertoire and vocal source levels with increased noise

Abstract

High background noise is an important obstacle in successful receiver signal detection and perception of an acoustic signal. To overcome this problem, animals modify acoustic signals by increasing the repetition rate, duration, amplitude, or frequency range of the signal. Humpback whales are the most vocal of the baleen species in that they use a wide and varied catalogue of social sounds. More than 36 different sound types (vocal sounds and sounds from energetic surface behaviours) were found during a three year study on migrating humpback whales. During periods of high wind noise, humpback whales modify both their acoustic repertoire as well as vocal signal properties. We found that humpback whale groups gradually switched from primarily vocal to primarily surface-generated communication in increasing wind speeds and background noise levels, but kept both signal types in their repertoire. We also found evidence of the Lombard effect, in that in increased wind-dominated background noise levels, humpback whale groups tended to increase the amplitude of their vocalisations. Determining how whales modify their vocal behaviour in increasing levels of background noise will give us an important insight into how they might cope with increasing levels of anthropogenic noise.