Rebecca Dunlop – r.dunlop@uq.edu.au
Michael Noad
School of Veterinary Science
University of Queensland, Cetacean Ecology and Acoustics Lab
Gatton, QLD 4343, Australia
Douglas Cato
Institute of Marine Science
University of Sydney
Sydney, NSW, Australia
Popular version of paper 4pAB5
Presented Thursday afternoon, June 6, 2013
ICA 2013 Montreal
Trying to communicate using sound in a noisy environment can be challenging. Humans tend to increase the volume and pitch of their speech when it becomes noisier, many animals do the same. To attract attention when it’s noisy, humans sometimes also use other sounds like clapping, or visual signals like waving. These visual signals are especially useful if the receiver is far away from the sender or speaker. This study set out to determine how humpback whales cope when it’s noisy.
Humpback whales are very vocal. Male humpback whales ‘sing’ and many studies on humpback whales have focused on their song. All humpback whales also produce what are known as ‘social sounds’, which are other vocal sounds that are not ‘sung’ but ‘said’. They can produce many different types of vocal sound, from very low ‘grumbles’ up to very high frequency ‘bird-like chirps.' It is likely that theses sounds are used for communication within their group, for example a mother might call to her calf, as well as to signal other groups in the area. Humpbacks also make sounds when they are at the surface by slapping the water with their tail or pectoral fins, or breaching (where they leap out of the water and slam their body on the surface). No one is sure why they make these surface sounds, but it is highly likely they are also used for communication between and within groups of whales. Therefore humpback whales can communicate using two types of sound; vocal sounds and surface-generated sounds.
Humpback whales, like all marine mammals, are exposed to many sources of noise; background shipping, biological noise from snapping shrimp or other animals in the sea, noise from wind and waves and surf. Wind-dependent noise is generated by surface waves breaking. Previous work found that when noise in the sea increased due to increased wind speeds, these whales changed how they communicate. They tended to use more surface-generated sounds and less vocal sounds. They also were found to increase the volume, or source level, of their vocal sounds. So, they used two different strategies to cope with increasing background noise.
The whole point of increasing the source level of vocal sounds in noise is to make sure the receiver (the one being spoken to) can hear what the sender (or speaker) is saying. This becomes harder the further away the receiver is from the sender. So, receivers far away from the speaker will have more difficulty hearing the speaker in higher noise than receivers close by. However, when the sea noise produced by the wind was very high, we suspect that humpback whales had difficulty in producing vocal sounds at a high enough level to successfully communicate to receivers that were far away. One solution could be to use another signal that is louder and/or able to gain the attention of the far away receiver. We found that the peak (loudest part of the sound) of surface sounds tended to be louder than the loudest part of the vocal sounds. Using louder sounds that attract attention may help when the speaker is trying to communicate to receivers that are far away in noisy conditions and this may explain why humpback whales use two strategies to cope with increases in background noise.