Directionality in the Whistles of Hawaiian Spinner Dolphins (Stenella Longirostris): A Signal Feature to cue Direction of Movement
Marc O. Lammers and Whitlow W. L. Au
published in Marine Mammal Science, April 2003, Published Online August 26 ,2006
Dolphins produce frequency-modulated (FM) whistles that provide a lot of useful information not only for maintaining contact with group members but also for both individual identity and description of physical location. Most of all, whistles promote synchrony and coordination of behavior amongst the group members. It has been concluded that whistles have directionality, meaning that they describe proper orientation and direction of movement of the signaling dolphin, and concentrate sound much better than an omnidirectional source. Furthermore, simple, narrowband, low-frequency whistles have a more omnidirectional character, whereas broadband, higher frequency spinner dolphin whistles radiate with more directivity.
This particular study looked at the whistle characteristics of free-ranging Hawaiian spinner dolphins. Whistles were recorded using a line array of three hydrophones. It was found that the animals that traveled with or towards a hydrophone had a higher harmonic content in their whistles. In order to forage and protect themselves from big sharks, false killer whales, or pygmy killer whales, and many dolphin species travel in pods of 20-100 individuals over large distances. Loss of sight in deep waters requires knowledge of other member’s location and any other changes that they make in their paths. Kelly Benoit Bird would later publish an article (2009) about how the dolphins use sound to maintain group contact and help with coordination and feeding. If listeners are familiar with one another’s usual whistle sounds in certain positions, then a change in that whistle will indicate a change in position. Spinner dolphins can make their whistles more directional with both alterations in frequency and with harmonics. Thus, listeners receive valuable information about the orientation and direction of the movement of the signalers.
There is still a lot that we do not know about the biological significance of whistle directionality. Further information and research is required to understand fully the behaviors and the environment in which these whistles are emitted. Studying the directionality of whistles is important for spinner dolphin management because we can learn more about the impacts of human-made noise on the physiology of dolphins beyond simple noise perception. Does it lead to changes in sensitivity to acoustic signals, permanent behavioral change ( as in change in diving patterns), stranding, tissue damages, or even death? It is important to study these factors in a larger context and study the cumulative effects that fishing vessel noises and other noise pollution may have on spinner dolphins.
Illustration Credit: Illustration done by Peter Madsen http://www.pbs.org/odyssey/odyssey/20030602_log_transcript.html