The Lombard effect in humpback whales

Abstract

The Lombard reflex is an increase in the subject's vocal levels in response to increased noise levels. While it has been demonstrated in humans and a small number of mammals and birds including some whales, it has not been demonstrated in humpback whales. During their southward migration off eastern Australia humpback whales were tracked visually from an elevated land station. An array of calibrated hydrophone buoys was used to simultaneously track vocalizing whales acoustically and to measure ambient noise. Two hundred and ninety two social vocalizations were recorded and analysed from 15 passing groups of whales when there was no detectable boat noise or singing whales in the area. Vocalization source levels increased significantly by a mean of 0.75dB per 1dB increase in background noise (broadband 40Hz – 2kHz). Unlike most previous Lombard studies, however, the vocal level increased even though the background noise was much lower than the vocal level. Thus the whales maintained a signal excess of approximately 75dB which suggests that these social vocalizations may function as signals over distances of several kilometres.