Singing in a noisy ocean: vocal plasticity in male humpback whales

ABSTRACT The songs produced by male humpback whales are believed to be a reproductive display shared by all singers within the same population. Ocean noise can interfere with the transmission of acoustic signals such as humpback whale songs. However, humpback whales evolved in an environment characterised by variable levels of noise generated by natural sources. This study investigates whether singing males compensate for natural noise by changing the characteristics of their sounds. Songs were recorded off eastern Australia during periods of time when the soundscape was dominated by natural noise. Source level, peak frequency and duration were measured for 2,318 song units from 19 singers. Source levels were positively correlated with noise levels, while there was no correlation between the peak frequency or duration of the units and noise levels. Our study shows that male humpback whales increase the source level of their units in response to increasing natural noise, i.e. they have a Lombard response, but they do not modify their spectral or temporal characteristics. This suggests that the need to adhere to the shared repertoire prevents changes to distinctive features of song units, i.e. frequency and duration, however, vocal plasticity allows adjusting source levels to the environmental conditions.