Minke whales are not likely to hear ultrasound

Houser et al. (1) made a breakthrough by measuring auditory evoked potentials (AEP) from Minke whales, resembling responses from other mammals. Surprisingly, they report AEP responses to sound pulses with peak frequencies up to 90 kHz, several octaves above both the frequency content of Minke whale vocalizations and their predicted upper frequency hearing limits (2). Here we propose an alternative interpretation of their data, namely that the Minke whales’ auditory system responded to energy in the stimuli occurring below 20 kHz and lack ultrasonic sensitivity.

This we base on the reported response amplitudes to their two stimuli at 64 kHz (and similarly 45). At that frequency, two short 5-cycle stimuli were tested: An untapered one, without any signal envelope, producing a broadband stimulus, and a higher amplitude one, tapered with a Tukey window (3), resulting in a more narrowband stimulation. Fig. 1 shows that both these signals, when transmitted through the transducer (4), have considerable power below ultrasonic frequencies. The AEP response to the untapered stimulus was about twice that of the tapered one. This is paradoxical if Minke whales indeed have ultrasonic hearing, since the peak exposure level at 64 kHz in the spectrum of the untapered signal was 11 dB lower than for the tapered stimulus. In contrast, the exposure level at 17 kHz for the untapered signal is at least 20 dB higher than that of the tapered signal. Thus, the AEP response increased when the signal energy below 20 kHz increased, while the opposite happened at ultrasonic frequencies. Thus, the most parsimonious interpretation is that Houser et al. recorded AEP responses to frequency components below 20 kHz, and not to energy at ultrasonic frequencies, and we conclude that noise mitigation efforts directed towards baleen whales should remain on low frequencies, and not on ultrasonic frequencies.