Killer whale

Identification

With its stocky body (broad in girth relative to length) and powerful musculature, the killer whale (Orcinus orca) is amongst the most robust of all the cetaceans. It's head is rounded and tapers to a conical snout with a blunt, indistinct beak. The dorsal fin is conspicuous and situated in the middle of the back. In adult males, the fin is erect or tilted forwards and can be 1.8 m or more tall. It is triangular in shape, with a height of two or more times the length of the base. Females and juveniles have a more modest dorsal fin that is generally moderately falcate and less than 1 m tall. The flippers are large, broad, rounded and paddle-shaped, and are particularly obvious when killer whales breach or spy hop. The tail flukes are concave on the rear margin, often have pointed tips, and are separated by a deep, median notch.

The killer whale's colouration is a striking combination of black and white, with sharp demarcation between light and dark zones: black is dominant on the back and flanks, and white on the belly, and most animals have a light grey saddle marking just behind the dorsal fin. There is a large, elliptical white patch on the side of the head just above and behind the eye (the eye patch). The entire chin and throat are white, and the white continues posteriorly along the ventral mid-line, narrowing as it passes the flippers. Behind the navel, this white stripe branches into three, one extending onto each flank and the other continuing along the ventral mid-line past the anus. In males, the central branch is long and narrow, while in females it is broad and rounded. Small areas of black mark the urogenital and mammary slits.

Pods may travel in tight formation or be spread across more than 1 km, often breathing and diving in one co-ordinated movement (and generally with an adult male at each end).This powerful whale can travel at speeds of up to 55 km per hour (30 knots) when speed swimming. Two forms of killer whale have been identified: "transient" and "resident", differing both behaviourally and physiologically in their foraging behaviour, habitat use, group size and age/sex class structure, dorsal fin shapes, pigmentation, vocal dialects and mitochondrial DNA.

Distribution & Habitat

Killer whales are found in all the oceans of the world but are most prevalent in colder waters at high latitudes. They generally prefer deep water, but may be found in shallow bays, inland seas and estuaries. Throughout their range, the distribution of animals is patchy in all areas.

No regular, long migrations are known, but killer whales may move locally according to ice cover in high latitudes and according to food availability in high latitudes and elsewhere. Killer whale occurrence and movements have been associated with the movements of prey species off Canada, Norway and Iceland, Puget Sound, Washington, and the British Isles.

Off the west coast of North America, transient killer whales form small pods and roam over a large area. Residents tend to live in larger pods and have smaller home ranges. Offshore animals also form large pods, and move along the continental shelf.

Natural History & Ecology

The killer whale is the largest member of the dolphin family. Adults range from 5.5 to 9.8 m long. Males reach an average length of 7.3 m (maximum 9.8 m) and at least 8 tonnes in weight. Females grow on average to 6.2 m (maximum 7.0 m) and 4 tonnes in weight, and are smaller and less robust than males in form.

The reproductive biology of this species is not well understood. Females become sexually active at an age of at least 7 years, probably older. Males are thought to be fertile by the age of 10 to 12. Mating occurs throughout the year, but in the Northern Hemisphere mainly from September to January with a peak from October to December. Most calves are born between October and March and gestation lasts for 13-16 months. The period of calf dependency is prolonged in this species, calves remaining with their mothers for as long as 10 years.

The killer whale is a versatile predator with an extremely varied and opportunistic diet. Killers are known to eat a range of prey items, from squid, fish and birds, to turtles, seals and other cetaceans, including mysticete whales, sperm whales and small cetaceans; the diet varying between regions, amongst age and sex classes, and from year to year.

Several studies have suggested that transient killer whales feed primarily on marine mammals, and resident whales mainly on fish. Although main food items include fishes and cephalopods, larger animals may be more likely to take other marine mammals than smaller individuals.

Killers often feed co-operatively when hunting, especially when pursuing marine mammalian prey, but not always when hunting fishes and other small prey. When feeding, whales commonly lunge at the surface, dive for 30 seconds to 4 minutes and swim in various directions. Resident killer whales use echolocation to detect fish, although transients appear largely silent when foraging, presumably to minimise detection by their mammalian prey. Food sharing has been observed.

Social Behaviour

Killer whales live in extended family groups called pods and studies of these pods have revealed a highly sophisticated social hierarchy. The existence of gregarious groups of killer whales has been known for many years. A pod of killer whales off south-eastern Australia, for example, were individually known to whalers for almost 100 years and acted as participants with whalers in symbiotic herding and killing of humpback and right whales. It is only fairly recently, however, that the cohesiveness of these pods and their recurrence in certain places over long periods of time have been demonstrated by photo-identification and acoustic studies.

Group size ranges from 1 to 100 individuals, although groups typically number from 5 to 20 animals. Occasionally, several pods join up to form a temporary 'superpod'.

A pod of killer whales typically contains adult males, females and juveniles, and calves (a pod being defined as a coherent group with stable membership, several of which may join up occasionally to form larger communal aggregations). Pods therefore seem to be mostly family or breeding units, although these whales have a social system (seemingly unique among cetaceans) in which both sexes remain in their natal group throughout adulthood. Mating patterns are unknown, but sexual activity commonly occurs when different pods meet. Thus the community (a number of pods) appears to be the basic reproductive unit.

Killer whales may have a matriarchal social system, given the longer life span of females and the close association between mothers and sons. Pods may disperse at times for several hours, for example, the mature males forming one sub-group and the females and immature males another, travelling in the same direction, but up to 4 miles apart.

Courtship and copulatory behaviour in killer whales has been observed and often appears similar to general play behaviour. Individuals may appear to chase one another, with a great deal of physical contact, and fluke slaps may be directed towards other whales. Mating killer whales have been observed to align their genital areas in several different copulatory positions which may be held for as long as 30 seconds.

Killer whales tend to be inquisitive and approachable, and are often active on the surface: lobtailing, flipper-slapping and spyhopping. They frequently breach, and surface activities such as jumping, slapping and wave riding are all common. Play is most evident amongst young whales and may sometimes be sexually orientated.

Rubbing against a hard surface or against other whales is a frequently observed activity and serves as a comfort movement and as a means to remove dead skin. Some rock rubbing areas may be important socially.