Killer Whale (Orcinus orca)
Social Relationships
Image courtesy of National Marine Fisheries Service
As with most species, a social hierarchy exists within a group of killer whales. The killer whale social hierarchy is matriarchal or female-dominant. The animals establish dominance and communicate their social order by slapping their tails against the water, head-butting, jaw-snapping, biting, raking (tooth-scratching), and other vigorous postures and gestures (SeaWorld Inc., 2004)
Interactions between resident and transient groups have never been observed. In fact, they appear to avoid each other (Ford, 1991). The transients are so different in seasonal distribution, social structure, behavior, and mitochondrial and nuclear DNA from fish-eating residents, many scientists believe they could be a distinct species altogether (Animal Planet 2002; Hoelzel, 1994; Saulitis, 2000)
Killer whales in a pod appear to establish strong social bonds. Behavior studies suggest that certain animals prefer associating with one another (Brault, 1993). Studies of the dialects and movement patterns of killer whale pods suggest great constancy of pod composition (Hammers, 2003). Pods often join and intermingle for social activities (Anderson, 2003). Very distinct dialects are developed between adjacent pods. While pods meet and intermix briefly, they completely separate again, and few cases of long-term intermixing appear to occur (Hammers, 2003). Discrete calls generally serve as signals for maintaining contact within the pod, and the use of repertoires of pod-specific calls enhances this function by conveying group identity and affiliation (Ford, 1991). Pods that share the majority of calls in their repertoire and form distinct acoustic subgroups are referred to as a subclan (Ford, 1991).
Image courtesy of SeaWorld
Mating:
Females begin breeding at about age 14-15 years of age and only breed for about 25 years, stopping at about the age of 40 (35-45 years). Males mature at 12-14 years old and reach full size about 6 years after sexual maturity (Anderson, 2003; Brault, 1993). Adult whales give birth to a single baby (twins rarely occur) about every 4-6 years, only producing 4-6 surviving offspring in their life (Anderson, 2003; Brault, 1993). Gestation lasts for 16-17 months (Anderson, 2003). Calves nurse for about a year and are associated with their mothers for much of their juvenile period (Anderson, 2003; Brault, 1993). At birth, the calf measures 8 feet long and weighs 400 pounds (Animal Planet, 2002). Almost half of all calves die within their first year (Anderson, 2003)
Killer whales mate year round and it is believed that they have sex for pleasure (Animal Planet, 2002). There is no mating within the matrilineal group to prevent inbreeding. This is done through the recognition of dialects (Anderson, 2003). Killer whales tend to mate with individuals from other pods who sound very different from themselves (Norris, S., 2002). This occurs when pods encounter each other (Brault, 1993). The social organization of killer whales is more typical of serial polygeny (promiscuity) and could lead to a less structured pattern of genetic variation at the population level (Hoelzel, 1994).
Mating System:
Promiscuity occurs in 60% of all mammals, including the killer whale (Hoelzel, 1994). Overlap promiscuity occurs in orca pods when two pods meet each other in overlapping home ranges. Orcas are promiscuous because they cannot mate within the pod, in order to prevent inbreeding, and they must mate when they come across another pod to ensure reproductive success and to help prevent sperm competition.
Image courtesy of Harvard-Westlake Science Department