Listening to Hawaiian Spinner Porpoises, Stenella cf. longirostris, with a Three-Dimensional Hydrophone Array
William A. Watkins and William E. Schevill
In 1943 Schevill began his work as Associate in Physical Oceanography at Woods Hole Oceanographic Institute. Schevill originally studied echolocation in German submarines, but later found the institute’s Marine Mammal Bioacoustics Program. Schevill and Watkins met in 1958 when Watkins signed on to Woods Hole as a research assistant in electronics. Watkins eventually joined the biology department in 1972 and later became the Senior Research Specialist until his retirement in 1996.
In 1971 Watkins and Schevill traveled to Kealakekua Bay, HI to study the behavior of a group of resident spinner dolphins (Stenella longirostris). Recordings were made from May 8th to May 13th using a three dimensional array of 4 Atlantic Research LC hydrophones suspended to a depth of approximately 40 meters, allowing for the documentation of depth at source as well as the distances between different locations of sound production. However, only recordings from the last day could be analyzed as the dolphins were originally weary of the device and did not frequent that area of the bay to the same extent as usual. The animals seamed to become accustomed to the system as days passed and their regular appearance in the area may have returned given another two weeks.
Three types of dolphin sounds were recorded in Kealakekua Bay: clicks, pulse-bursts, and squeals. Clicks were characterized as having “directional propagation” such that the animal would have to be positioned in a way that would transmit the sound waves in the direction of the hydrophone. Thus, not every hydrophone picked up the same single series of clicks. Pulse-bursts were frequent and louder than the other dolphin sounds recorded. Watkins and Schevill characterized pulse-bursts as non-directional and were able to document the approximate time of sound reception and location of the source. Certain behavioral displays were associated with increased sound activity, such as leaping, aerial spinning, and high-speed swimming.
Dolphins were observed near the surface during periods of sound production (usually <10 meters), and stayed closer to shore during periods of rest. Calls were returned between dolphins, indicating communication. Pulse-burst sounds were returned within 0.5 seconds, often between at least 2 individuals. Recordings showed that the amount that a sound type was produced varied between individual dolphins. Moreover, spinner dolphins make deliberate choices in sound production.
Watkins and Schevill provide an introduction into the behavior of Hawaiian spinner dolphins including previously unreported aspects of dolphin communication essential to their conservation.
“In Memoriam: William A. Watkins.” Woods Hole Oceanographic Institute. Woods Hole Oceanographic Institute, 8 Oct. 2004. Web. 10 Aug. 2012. <http://www.whoi.edu/main/obituaries/archive?tid=3622>.
Rolfe, W. D.I. “William Edward SCHEVILL: Palaeontologist, Librarian, Cetacean Biologist.” Archives of Natural History 39 (2012): 162-64. Print.
Photo Credit: Figure is from Watkins and Schevill’s article “Listening to Hawaiian Spinner Porpoises” (1974)