Susan E. Parks - email@example.com
Cornell Bioacoustics Research Program
159 Sapsucker Woods Rd.
Ithaca, NY 14850
Philip K. Hamilton & Scott D. Kraus
(New England Aquarium, Central Wharf, Boston, MA 02110)
Peter L. Tyack
(Woods Hole Oceanographic Institution, Woods Hole MA 02543)
Popular version of paper 4aAB12
Presented Thursday morning, May 27, 2004
147th ASA Meeting, New York, NY
|The North Atlantic right whale (Figure 1) is one of the most endangered species of whale in the world. Reduced to low numbers by whaling before the 19th century, this population of whales has failed to show signs of recovery, despite protection from whaling for over 65 years. Relatively little is known of how right whale use sounds in their daily life from potentially locating prey to contacting other whales. This paper describes a particularly striking sound produced by male right whales which may play a role in right whale mating. This sound is referred to as a 'gunshot' sound because it is similar to the sound of a rifle being fired|
The Gunshot is a brief, intense sound. In the Bay of Fundy there is typically an initial sound, presumably traveling in a straight path from the whale to the recording hydrophone, followed by a clear echo (Figure 2). The time delay between the initial sound and the echo corresponds to the expected time for the sound to travel twice the depth of the water suggesting that the recorded echoes were reflections off the bottom. Acoustically, Gunshot sounds are similar to sounds made from a whale slapping its flipper or tail on the water, but are noticeably louder when recorded at the same distance from the whale.
(Please click on spectrogram to hear an example "gunshot" sound)
Who, When, Where?
Lone males vs. Social Groups
The Gunshot sound has been recorded from male right whales in two very different situations in the Bay of Fundy during the summer months. The sound was recorded from lone males showing stereotyped repetitive surfacing behavior nine times and from males engaged in rowdy social groups, referred to as 'surface active groups' on 49 separate occasions. These surface active groups consist of anywhere from 2-20+ whales and often involve multiple males competing to get close to a single female who is producing calls that attract more whales. To date, there have been no observations of females making the Gunshot sound.
The precise mechanism by which right whales make the Gunshot sound is unknown but our observations indicate that the whales produce the sound internally. Even though the sound is similar to slapping sounds made by whales slapping the water, close observations of whales indicate that the sound is produced without hitting the water's surface with a flipper or fluke. The entire body of the whale producing the Gunshot sound has been seen to vibrate or jiggle at the time of sound production. Whales produce the most intense Gunshot sounds immediately after a head-lift. The information on the timing of the multiple pulses of the sound, the intensity of the sound, and the production of the sound at or near the water's surface may provide clues to the mechanism of sound production. Gunshot sounds have only been reported in balaenid whales (including the bowhead whale and other species of right whale). There may be unique anatomical features of this group that could explain the production of this distinctive sound.
There are several potential functions for a such loud sound. The three primary hypotheses include using the sound to stun prey, for echolocation to navigate or locate food and other whales, or to communicate with other whales in social situations. The prey of right whales in the Bay of Fundy consists primarily of large concentrations of copepods, a tiny shrimp-like organism, and it is unlikely that the right whales would need to use sound to stun or disorient their relatively slow-moving prey. The production of Gunshot sounds by stationary whales at the surface that are not traveling or feeding and in surface active groups where the whales are in close body contact with other whales and the female is already making sounds, make it unlikely that echolocation for finding food or other whales is a primary function of the sound. The remaining hypothesis, that the Gunshot sound functions for communication with other right whales, seems the most plausible.
Only adult male right whales have been documented producing this sound in the Bay of Fundy. This has led to the hypothesis that the sound serves a reproductive function. Use of sound to mediate reproduction (both to repel competitors and attract mates) is common in many species, particularly in the marine environment where visibility is limited. The gunshot sound may represent a distinct sound used by males in mating situations.
Given the current observations of Gunshot sound production in North Atlantic right whales in two very different behavioral contexts it seems even more likely that Gunshot sounds serve multiple functions. The use of the sound as an aggressive threat signal appears to be strongly supported by the production of Gunshot sounds in surface active groups with multiple males competing for access to a single female. The role of the sound in reproductive advertisement is supported by the displays of lone adult males on nine different occasions. The similarity in the patterns of behavior and sound production between different individuals suggest that this display is commonly used by mature males. The proposed function of this display to attract females is a hypothesis, because it is currently unknown what the outcome of these male displays might be, and no observations of females joining displaying males have been made. Females may be able to assess individual males within a surface active group by characteristics of the Gunshot sounds that they produce. It seems likely that the potential function of this sound encompasses both of the two proposed functions, with Gunshots functioning as direct agonistic threats between males while allowing for assessment of individual male quality by females.