The orca is the world’s largest predator of warm-blooded animals (2), and the only cetacean to routinely hunt marine mammals (5), including seals, sea lions, dolphins and sea otters (3) (5) (7). The orca will even kill large whales (2) (3) (5) (10), and also hunts fish (including sharks and rays), squid, octopuses, sea turtles and seabirds, such as penguins (1) (2) (3) (5) (6) (7). Despite this varied diet, different orca populations often specialise on particular prey. For example, some of the best studied orcas occur in the eastern North Pacific, where three distinct forms have been identified: ‘resident’ orcas feed on fish, particularly salmon; ‘transients’ hunt marine mammals and occasionally seabirds; and ‘offshores’ appear to feed on sharks. These forms also differ in appearance and behaviour, and do not interact (5) (7) (11) (12).
An intelligent and versatile predator, the orca shows a wide range of different hunting techniques. Some orcas work together to ‘herd’ fish, before stunning the prey with strikes from the tail flukes (5) (6) (7) (13), or join together in coordinated attacks on large whales (2) (5). Others chase prey such as tuna or dolphins for long periods, to exhaust them (7), while seals or seabirds resting on ice may be dislodged into the water by rushing to the surface to create a wave (2) (5). Some orcas in Argentina have even learnt to intentionally strand themselves on beaches to reach seals and sea lions on the shore (1) (2) (5) (6) (14), a behaviour which appears to be taught to the young (14). In many areas, orcas have also learnt to follow fishing fleets to feed on discarded fish, or to take prey directly from longlines (7). The orca itself has no natural predators other than humans (5).
The orca is a social species, communicating using a variety of screams, clicks and whistles, as well as through physical behaviours such as breaching, slapping the flippers or tail, and ‘spyhopping’ (bringing the head out of the water). Click-like sounds are also used for echolocation (2) (5). Orca groups usually consist of up to 40 or 50 individuals, although larger numbers may gather when several groups temporarily join together (2) (5).
The group structure of the orca can be quite complex. For example, in ‘resident’ orcas, the basic social unit is a matriline, consisting of a mature female, her adult offspring, and her daughters’ offspring, with individuals in this group having very strong bonds and staying in the group for life, although they mate outside of it (2) (3) (5) (6) (7). Groups of related matrilines which are frequently seen together and share a unique call repertoire are known as a pod, while above this are ‘clans’, which include a number of pods with similar vocal ‘dialects’; these pods are probably distantly related, but may not form a particular social unit. Finally, ‘communities’ are made up of pods which regularly associate but may not necessarily be closely related (2) (5) (7). The social organisation of ‘transient’ orcas is less well known; the basic social unit is still a matriline, but offspring may disperse from the group they were born into, and so groups tend to be smaller and more dynamic than in resident orcas (3) (5) (7). Transient orcas also tend to share a related call repertoire, rather than showing group-specific ‘dialects’, and are also generally silent while foraging (5).
Male orcas reach sexual maturity at around 15 years, but do not become physically mature until about 21, while females reach sexual maturity in their early teens (2) (5) (7). Breeding can occur at any time of year, although in the northern hemisphere births usually peak between October and March (2) (3) (5) (7). The female orca gives birth to a single calf after a gestation period of around 15 to 18 months, and the calf is weaned at 1 to 2 years old (2) (5) (6). Although potentially living for up to 90 years (2) (5) (7), a female orca only produces a calf once every 3 to 8 years, up to the age of about 40 (2) (5) (6) (7). After reaching 40, the female ceases to reproduce, but may instead take on a role as a ‘grandmother’, passing on important experience to younger relatives (6) (15).