Bottlenose dolphin (Tursiops truncatus) vocal modifications in response to spectrally pink background noise

Abstract

The increase of oceanic shipping is a global predicament. The resulting proliferation of underwater noise levels is a serious concern for marine mammal welfare, as it has the potential to interfere with the communicative signals of bottlenose dolphins (Tursiops truncatus). The Lombard effect and other noise-induced vocal modifications may be employed to compensate for reduced signal-to-noise ratios. This study aimed to determine which vocal modifications dolphins use during experimentally controlled background noise conditions. Three dolphins (ages 30–52) participated in behavioral hearing tests using an adaptive up-down staircase, go/no-go procedure with 15 or 40 kHz tones. Tones decreased by 3 dB increments if a dolphin responded to the tone with a conditioned whistle, and increased by 3 dB if they did not. Dolphins performed this task during ambient noise (control) conditions, as well as three elevated bandpass noise (experimental) conditions: 0.6–5 kHz (115 dB re 1 μPa) and 0.6–10 kHz (115 and 125 dB re 1 μPa). The acoustic parameters of the dolphins’ response whistles and victory squeals, such as duration, frequency, amplitude, and response latency, were analyzed and compared between control and noise conditions. These data provide a complement to field studies of odontocete noise-induced vocal modifications in the wild.