Aggression is a common occurrence in the world of marine mammals. Aggressive displays, or agonistic behavior, are often seen among members of a group or between individuals of different species. Sound can be an important component of agonistic behavior, such as growls as part of a threat display. Animals can listen to these sounds to assess an opponent's willingness to fight or fighting ability. Loud sounds may also be used to startle or defend against a predator. The best descriptions of aggressive displays come from studies of pinnipeds (e.g. elephant seal) and odontocetes (e.g. Atlantic spotted dolphin, bottlenose dolphin, and beluga whale).
Agonistic behaviors are well described for several species of pinnipeds, including elephant seals and fur seals. Males of some species defend territories, others defend females, while still others establish dominance hierarchies. In these, and other types of aggressive interactions, vocalizations play an important role. Both underwater and in-air sounds may be used, but most currently documented sounds include growls, barks, and grunts in air. However, captive leopard seal were found to produce underwater growls and blast sounds in association with aggressive behaviors such as lunging (with mouth open, teeth exposed) and parallel swimming.
Threat displays are also well documented for cetacean, such as in Atlantic spotted and bottlenose dolphins. Aggressive displays include open-mouth threats, rapid head nodding, and head-to-head posturing. These species also produce vocal signals, called burst-pulse sounds during agonistic interactions. Burst-pulsed sounds can have frequency components that extend beyond 100kHz. Some examples of broadband, burst-pulsed sounds produced with aggression have been described as squawks, screams, barks, cracks and pops.
If an interaction escalates, the vocalizations are often accompanied by increasingly aggressive behaviors such as bubble production, jaw claps, body charges, tail slaps, and other actions. Jaw claps are short, very intense, broadband sounds that are made by opening the mouth and clapping the upper and lower jaws together with force. Tail slaps are made by thrusting the tail out of the water and then slapping it down on the water's surface. Both of these actions create very abrupt sounds that have a rapid onset and offset. The following waveform and corresponding spectrogram depicts a series of four jaw claps from a captive beluga whales.
The four claps of sound in the spectrogram line up with the four blue pulses in the waveform directly above. The waveform is aligned with the spectrogram in time on the x axis and shows energy, in relative units, on the y axis.
Below is a series of jaw claps and burst pulsed sounds recorded from an aggressive beluga whale.
Other cetacean species also perform aggressive displays. Male humpback whale, for example, often form competitive groups on the breeding grounds, in which individual males compete with one another for a position next to a female. In these groups, they perform threat displays which include physical posturing and blowing bubble streams, and they also produce a variety of social sounds. These displays provide both visual and acoustic signals of aggression or agitation to other animals within the group.
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