Spinner dolphins are known to be cooperative hunters, executing highly coordinated foraging techniques in groups of 16-28. They must herd prey into patches dense enough to allow each dolphin to meet their energetic needs, and they must do all this at night. Previous studies have shown that the animals have two types of acoustic noises at their disposal: whistles, which are believed to be primarily social sounds, and clicks, which are generally used for echolocation of prey. Visual cues are not very helpful in the dark ocean, so Benoit-Bird and Au set out to learn whether spinner dolphins use acoustic communication to coordinate their foraging trips.
This study was performed at two sites off the coast of Oahu over ten nights in May and June of 2003. The research vessel used active sonar in order to provide a real time picture of prey patches and individual dolphin positions while foraging. When groups of dolphins were observed the boat sometimes stopped and deployed hydrophones to record the sounds produced by the dolphins while foraging.
There are four distinct stages of Spinner dolphin foraging that the dolphins seem to transition between instantly. It is unlikely that the animals are using environmental cues because of how variable each patch of prey can be and how instantaneous the transitions are. For that reason the study focused on sounds that occurred as the dolphins were observed moving through each stage of foraging. The study discovered that almost no whistles were recorded while the animals foraged, ruling out the whistle as an acoustic signaling tool. The only noises recorded during foraging were clicks normally associated with echolocation. The largest numbers of clicks were recorded not while the animals were searching for prey but while the animals were transitioning between foraging stages. The pause between clicks was also too long to be used for locating their prey. The correlation and nature of the clicks suggests that the animals may somehow be using the clicks to coordinate foraging movements. Clicks are directional and high frequency and whistles are not, which might allow the dolphins to keep other animals from eavesdropping on their signals. These properties of clicks also make it difficult for researchers to study echolocation in dolphins.
Prior to this study it was believed that whistles were the main form of communication between Spinner dolphins and clicks were used only for echolocation. It is not clear exactly how the animals use clicks to communicate, but the evidence suggests that it occurs. Further studies will be necessary to understand exactly how the clicks are used, but this paper helped show that there is much more to dolphin communication than we know.
For more information the paper can be found at:
Benoit-Bird, K. J. and Au, W. W. L. 2009. “Phonation behavior of cooperatively foraging spinner dolphins.” Journal of Acoustical Society of America 125 (1): 539-546
Photo Credit: http://www.flickr.com/photos/69024677@N00/309225150 and can also be found on Wikimedia Commons http://commons.wikimedia.org/wiki/File:Spinner_dolphins.jpg.