Marine mammals that live together have developed a communication system that allows them to maintain or regain contact with group members over long distances. Individual animals often leave the group during feeding and are able to reunite with their group members by producing and recognizing vocalizations that are specific to their group. Group-specific vocalizations also allow members of a group to maintain group cohesion when more than one group of animals is in the same area. The group-specific vocalizations may act as passwords, allowing members to identify other members in their group. There are only two species of cetaceans that are known to produce group-specific sounds: killer whales and sperm whales.

Killer whales travel in groups (called pods) of up to 30 individuals. They produce discrete calls that are specific to their pod. Photo © Tom Kieckhefer (Go to larger version)

Killer whales live in stable groups called pods. There are two types of killer whale groups found off the Pacific Coast of North America. "Resident" whales live in the same group or pod their entire lives and eat mostly fish. "Transient" killer whales break-off from their birth group to form smaller pods Transients eat marine mammals such as seals and sea lions. The vocalizations of resident whales have been studied in much more detail than transients. Killer whales produce a number of different vocalizations that include whistles, clicks, and pulsed calls. Pulsed calls differ from the narrow tones of whistles and very short clicks because they are a strong burst of sound that cover a large range of frequencies. The roar of sea lions is an example of a pulsed call. (See Individual-specific vocalizations).

Killer whale pulsed calls are further divided into two categories: discrete calls and variable calls. Discrete calls are unique to each pod. They are mostly associated with identifying a pod group, maintaining the pod, and coordinating group members. Members of the pod use discrete calls to keep track of each other's location while they are feeding out of sight of each other. Variable calls are used by many pods and are more associated with social events.

Pictured below are spectrograms of discrete calls from two different sets of resident pods off the coast of British Columbia, Canada. Note how similar the calls in the left column are to each other. They are recorded from animals in the same pod (pod N7). The spectrograms in the right column were recorded from animals of a different pod (pod N8).

Sample spectrograms from two pods of killer whales.

The calls from pod N7 are more tonal than the calls from N8. Often whistles and pulsed sound are seen in the same vocalization. Notice the abrupt change in call frequency and structure with one call. These quick changes are thought to increase the chances of detection and localization by the listening whale.

Sperm whales also produce sounds that are specific to a group. Unlike the group vocalizations that killer whales use to maintain contact during feeding, sperm whales produce most of their group vocalizations between feeding bouts when they are socializing as a group at the surface. They produce rhythmic patterns of clicks, called codas. While each sperm whale may have its own distinct coda pattern (see Individual-specific vocalizations), it has been reported that groups within one geographic area tend to have more similar codas than groups further away. It is thought that similar coda patterns used by one group may help maintain group cohesion after members are done feeding.

References
• Ford, J.K.B. 1991. Vocal traditions among resident killer whales in coastal water off British Columbia. Canadian Journal of Zoology 69:1454-1483.

For more information on how specific marine animals communicate using sound, use the following links:
Marine Mammals	Fish	Invertebrates