Killer Whale (Orcinus orca)
Social Organization
Image courtesy of British Killer Whale Wild Adoption Program
Social Spacing:
Orca pods reside in home ranges because their habitat is too large to defend, and their food source is constantly moving. Resident pods tend to travel within specific home ranges while transient groups' ranges are unpredictable (Similä, 1996). Resident pods were observed to remain within a range of about 800 km (500 mi.) of the coastline, while traveling in direct routes- moving generally from headland to headland along the coast. Transients, however, may spend twice as much time traveling as their movements tend to be circuitous, often following the contours of the shoreline, transient whales have been sighted within a 1,450-km (900-mi.) range (Hammers, 2003). Transients form smaller pods and roam over a larger area while residents form larger pods and have smaller home ranges (Cetacea, 2004).
Social Structure:
The social structure of these animals consists of a highly stable, hierarchical set of relationships based on the matriline (Norris, S., 2002). Male whales remain with mothers until their mothers die; females eventually mate and form their own matriline (Norris, S., 2002). Resident societies are organized largely on the basis of maternal genealogy (Ford, 1991).
The smallest and most fundamental social units in
a resident pod are maternal
groups, which compromises
of individuals related by unilineal descent from a single living female.
(Ford, 1991). Matriline
groups may contain up to four generations of animals, although only
2-3 are typical (Ford, 1991). One or more
maternal groups may
travel together in a subpod. Whales in a subpod are likely to be
closely related; a subpod contains mothers and daughters, and probably
sisters
and cousins (SeaWorld
Inc., 2004).
The next larger group is the subpod, which contains one or more matrilineal groups that always travel together and are very likely closely related. Dispersal of individuals from their natal subpod appears to be extremely rare or absent: no whale has been seen to leave one subpod and join another permanently. The usual societal group is the pod, which is made up of one or more subpods (Ford, 1991).
Image courtesy of National Marine Fisheries Service
Killer whales live in cohesive long-term social units called pods which consist of small numbers of related matrilineal groups that band together (Brault, 1993; Norris, S., 2002), as well as travel and forage together (Norris, S., 2002). Typical killer whale pods contain mature females, their young and variable proportions of males and post-reproductive females (Brault, 1993; Ford 1998). Females and juveniles generally remain in the center of the pod, while adult males swim at the wings (Ford, 1998). The size of a resident pod varies from as few as five to as many as fifty individuals, while transient whales often travel alone or in groups of two to seven individuals (Ford, 1991). Offshore whales seem to travel in larger groups of 30 to 60 individuals and are seldom seen in coastal waters (Hammers, 2003; Similä, 1996). Pod sizes may, however, change with different geographic locations; off Alaska and Antarctica, groups of more than 100 animals have been seen (SeaWorld Inc., 2004). Sometimes smaller pods may join to form groups of 50 or more individuals (up to 500 in some cases) which are sometimes referred to as herds or aggregations (SeaWorld Inc., 2004).
A clan is a social level above the pod. Clans are made up of pods in an area with similar dialects and are thought to be related. These pods may have developed from one ancestral pod that grew and fragmented over time.
Finally, the top level of the killer whale social structure is a community. A community is composed of several pods that have been seen to travel together. Pods from one community have not been observed traveling with those of another, even if their ranges overlap (SeaWorld Inc., 2004).
Among the pods that prey on fish, both males and females apparently remain within their natal pods well into sexual maturity. (Hoelzel, 1994). Killer whales are long lived animals, and a new pod is only thought to be formed by the gradual splitting away of female units after a matriarch’s death (Guinet, 1991). The process of group splitting, leading to the formation of new pods, is accompanied by divergence of the vocal repertoire of the original pod (Ford, 1991). Although, within pods there are sometimes groups of individuals whose members associate more with each other than with other members of the pod, this suggests that pods might sometimes split; though no individual in a resident pod has been seen to switch from one pod to another (Animal Planet, 2002; Brault, 1993; Norris, S., 2002) because resident pods exhibit long-term stability, with no immigration or emigration of members (Saulitis, 2000).
Socializing Behavior:
Guinet (1991) noted that killer whales were observed stranding voluntarily, individually or in association, when “playing” along the beach; this behavior is defined as ‘social beaching play.’ Social beaching play consisted of a series of intentional strands performed by whales; adult males rarely participated in social beaching play, and were never observed to strand themselves completely on the beach, which could be due to the greater size compared to females which may lead to a greater risk. The possible functions of social beaching play for adult female killer whales include socialization and practice to refine hunting skills, or it may be a determinant of dominance. For juveniles, this play is considered important for motor training and improvement of social skills.
Image courtesy of Cabrillo High School Aquarium
The stable social structure within killer whale populations creates the potential to form populations with distinct social and behavioral characteristics (Similä, 1996). This resource division leads to differences in social and ranging behavior, and genetic differentiation has been described between sympatric populations of these two behavioral strategists. Some level of reproductive isolation may also occur. (Hoelzel, 1994). Killer whales are highly cultural creatures and may stand alongside- and perhaps in some ways ahead of- chimpanzees as the exemplar of a nonhuman animal whose life and evolution is shaped by cultural processes (Norris, S., 2002). Culture, as well as stable and distinctive vocal patterns, or dialects, is generated and maintained by social learning through imitation or teaching, and thus is passed from generation to generation by cultural, rather than genetic, transmission. (Norris, S., 2002; Ford, 1991)
Transients vocalize less frequently, often change direction abruptly when swimming, and remain underwater for up to 15 minutes at a time, whereas residents vocalize frequently, keep to predictable routes and rarely stay under for more than four minutes at a time (Cetacea, 2004).