The effects of noise on the vocal behaviour of singing humpback whales (Megaptera novaeangliae)

Abstract

In the last few decades, increasing anthropogenic offshore activities, such as shipping, have resulted in higher levels of noise in the ocean, on both global and local scales. This has resulted in increased concern about the ability of marine animals to maintain effective acoustic communication in a progressively noisier environment. To maintain efficient communication in variable noise levels, several species have developed mechanisms that aim at minimising masking, making it easier for potential receivers to discriminate between signal and noise. These strategies include changing the source levels, the spectral, and the temporal parameters of their sounds. However, several factors may limit animals’ ability to modify their acoustic signals. For example, the anatomy and physiology of the sound production system may bound the possible ranges of source levels, frequency and duration of the acoustic signals that a species can produce. Moreover, social constraints may require the animals to produce signals within a predefined repertoire which is recognised by their conspecifics.Male humpback whales, that use their songs to mediate interactions with other individuals, may be affected by underwater noise. Humpback whale songs are composed of series of stereotyped sounds, called units, and are believed to be a reproductive display directed at other males and/or at females. Underwater noise can mask males’ songs, reducing the probability that they will be detected and correctly decoded by a receiver. Moreover, the songs change with time, however at any given moment, all singers on the same breeding ground sing similar versions, suggesting that the males continuously learn the song from each other. Noise may interfere with this process and negatively affect the reproductive success of the singers.This project investigates whether male humpback whales modify the characteristics of their songs to compensate for variable levels of noise. The thesis first tests for a correlation between different acoustic parameters of the song, such as the source levels and the spectral characteristics of the units. This would suggest that the anatomy of the sound production system could limit the ability of the whales to modify these features independently. Given that any coping mechanism might have evolved in response to variable levels of natural noise, we then assess whether the singers adjust the source levels, the frequency and duration of their sounds to compensate for variable levels of noise generated by environmental and biological sources. Finally, we investigate if the same mechanisms are used to compensate for increasing levels of the most common type of anthropogenic noise, the noise of vessels.Humpback whale songs were recorded using a hydrophone array deployed off the east coast of Australia. In spring, humpback whales travel through this area during their annual migration between breeding and feeding grounds. Here the whales were exposed to variable levels of natural noise, generated by wind and snapping shrimps, and to vessel noise generated by our research vessel that crossed the study area along predefined routes.My study shows that source levels of humpback whale songs are negatively correlated with frequency and decrease by 2.3 dB per octave. Further I demonstrate that humpback whale singers increase their source levels in response to increasing levels of natural noise. Finally, I show that the presence of vessel noise does not elicit a change in the characteristics of the units, including source level. Surprisingly, even when the soundscape is dominated by vessel noise, humpback whale singers keep responding to the underlying natural noise by adjusting their source levels, with no additional response due to the presence of a research boat.The negative correlation between source levels and frequency suggests an anatomical link between these two acoustic features that is consistent with the presence of an air-filled resonator in the sound production system of the whales. By adjusting the source levels of their songs to compensate for variable levels of noise generated by natural sources, the singers show short-term vocal plasticity. Source levels are increased without a corresponding decrease in frequency. Therefore, this vocal plasticity operates within the limits of the sound production system that anatomically links these two parameters, and within the requirements of social conformity that compels the singers to produce unit types whose spectral characteristics fall within the ranges of the shared repertoire. The consistent response to natural noise, even when vessel noise is dominating the soundscape, highlights the importance of considering environmental factors affecting the behaviour of the species when assessing for the effect of human activities on wild animals. This will ensure that the observed behavioural changes result from the anthropogenic disturbance and are not due to a concurrent natural process. This is important to better predict the effect of anthropogenic activities on animals, which will enable us to inform regional and global mitigation procedures, management and conservation plans aimed at ensuring the continued survival of our wildlife.