ABOUT KILLER WHALES
Taxonomy
Class: Mammalia
Order: Cetacea
Family: Delphinidae
Genus: Orcinus
Species: orca
​
Conservation status
IUCN - Red List category
‘Data Deficient’ - criteria being that abundance and/or distribution information are lacking
CITES - Appendix II
Current worldwide population: minimum 50,000
Morphology
The terms orca and killer whale are used interchangeably. Despite the name killer whale, they are not a type of whale, but rather the largest species of dolphin. The term most likely comes from sailors witnessing killer whales hunting whales many times larger than themselves, hence a “killer of whales”. Despite a long and fearsome reputation, killer whales are not considered dangerous to humans, and along with the bottlenose dolphin are one of the most popular cetacean species in the world. The killer whale can be easily identified due to its massive body, tall conspicuous dorsal fin and highly distinctive black and white colouring pattern. Size varies greatly between regions and genders. Body length is approximately 6 to 8.5 m for females, whilst males can reach up to 10 m and weigh a maximum of 10 t. Sexual dimorphism is highly discernable on the dorsal fin, pectoral fins and flukes, which are much more developed in adult males. Reaching up to 2 m tall, the dorsal fin of a male can be twice the height of a female's. The male dorsal fin begins sprouting at sexual maturity. This means that it is not possible to distinguish between sub-adult males and females by solely looking at the dorsal fin. Pigmentation patterns can vary across killer whale populations.
​
​
​
Habitat
The species has a cosmopolite distribution and can be encountered from the ice edges of the Arctic and Antarctic, all the way to warm tropical waters. Killer whales are found in both pelagic and coastal areas, and typically adapt their range to the availability of prey resources. Killer whales appear to be most abundant in the coastal waters of temperate and cold regions where the waters are very productive.
​
Feeding ecology
The killer whale occupies the top trophic position in the food chain. As a species, the killer whale can be considered a generalist predator, feeding on a wide range of prey including fish, squids, marine mammals, sea birds and sea turtles. However, local populations or groups can forage very selectively by adapting hunting and social behaviours. Thereby, at the population or group level, killer whales can display contrasting feeding habits and strong dietary specialisations. The best known example is undoubtedly from the eastern North Pacific Ocean, where the 'resident' killer whales feed on fish prey whereas the transient type of killer whales feed exclusively on marine mammals. Despite occupying the same geographical area, the two types of killer whales do not appear to interact or interbreed.
Illustrations by Frédérique Lucas for NOS
Life history
Lifespan is difficult to assess, and data varies greatly throughout the literature. The average lifespan for males appears to be 30 years, with some reaching 50 to 60 years old. Female life expectancy is about 50 years, but may reach up to 80. Females mature at about 10-12 years old, and give birth to their first calf at about 12-14 years of age. After 16-17 months of gestation, a female gives birth to a calf that weighs about 180 kg and is roughly 2 m long. As a mammal, the female nurses her calf with a highly nutritious fatty milk for up to a year. The calf will then progressively start eating solid food, and will naturally decrease its milk consumption over the course of about two years. A female only gives birth to 4 - 5 offsprings throughout her entire reproductive life. Reproductive senescence, the menopause, occurs in her 40s and marks the end of her reproductive abilities.
​
Neonates are yellowish on what are supposed to be the white body parts, and display foetal folds on their flanks. Calf mortality is very high, reaching up to 50%. The reason for this is thought to be due to the mother inadvertently offloading a high amount of toxins to their calf through their fatty milk. The first calf in particular receives a large concentration of these toxins in relation to their small body size, and survival is rare. The next calf is more likely to survive as the majority of the toxin burden has already been passed to the first calf. As the calf matures, the mortality rate drops.
​
Male killer whales reach sexual maturity at about 15 years of age, at which point the dorsal fin grows considerably. Males are fully grown at 20 years of age, which is also when they reach physical maturity.
Behaviour
Typical behavioural states include travelling, resting, socialising and feeding. When cruising, killer whales adopt a speed of 10 to 13 km/h, usually remaining within the upper 20 m of water. When necessary, killer whales are able to reach brief speed bursts of up to 45 km/h. Because diving is energetically costly, killer whales usually stay less than a minute underwater. However when foraging, they can perform deep dives of up to 200 m and are able to stay underwater for nearly 15 minutes.
​Killer whales are known to display aerial behaviours. These include spy-hopping (lifting the head vertically out of the water), breaching (launching the body out of the water) and tail-lobbing (slapping the water surface with the tail). The reasons for these behaviours remain poorly understood, but they could have social meanings. Spy-hopping enables taking a look at the surroundings above the water surface, and could be triggered by curiosity. Tail-lobbing and breaching may be for communication and other social purposes.
​
Communication & sounds
As social animals that live in groups, killer whales need to communicate with each other to maintain social bonds, and to coordinate feeding behaviours. Acoustics constitute killer whales' primary sense and involve three types of vocalisations. Echolocation clicks are high frequency sounds that are used for finding prey and for navigation. The echoes received from the produced sounds enable the whale to form an image of its surroundings. Whistles are variable, pure tone signals typically associated with social activities, and used for communication over short distances. Pulsed calls act as indicators of affiliation, and are used for communication over long distances. Research has shown that the types of sounds produced by different groups or populations can differ a lot. These differences have been termed dialects, and may be used as a tool for researchers to distinguish among social entities. Communication and sounds used in hunting also vary with target prey. Killer whales that specialise on hunting marine mammals are typically silent when they hunt. This represent an adaptation not to be detected by their acoustically sensitive mammalian prey. In contrast, fish eating killer whales tend to be highly vocal, since their fish prey do not hear the frequencies used by killer whales.
​
​
Sociality
Killer whales live in groups and display a complex social structure. The first and most basic social unit is the matrilineal group (or matriline), which consists of the oldest female, the 'matriarch', and her children and their offsprings. A matriline can span three to four generations, with family members highly bounded, and in some cases, staying together for life. However, group size and dispersion greatly depends on the ecology and feeding habits of the group. For instance, because of hunting requirements, killer whales feeding on seal prey usually adopt a smaller group size which can result in the dispersion of individuals from the natal group. Closely related matrilines tend to temporarily, but regularly, associate for social activities and foraging. Matrilineal groups spending over 50% of their time in association are often referred to as a 'pod'.
Through social learning, killer whales transfer knowledge and ‘traditions’ to the next generations. These can include specialised feeding behaviour, particular foraging and resting areas as well as a unique array of sounds and communication (their dialect).
​
​
Ecotypes
The worldwide killer whale population is made of a mosaic of several ecological forms, or 'ecotypes'. These are distinguished from each other by variations in dietary habits, social structure, morphology, pigmentation patterns, acoustic behaviours and genetics. Killer whales live in highly cohesive matriarchal societies in which feeding and behavioural traits are socially inherited. Because groups tend to associate with related lineages, socially taught feeding strategies tend to be restricted to family members. Groups that rarely or do not associate may gradually develop different foraging strategies for different prey types. Such cultural differentiation may promote social segregation, potentially leading to reproductive isolation. With less interbreeding and gene flow between prey-specialist groups, morphological and pigmentation variations can appear due to a lack of trait homogenisation. Many different ecotypes have been documented around the world, some of which live in different geographical areas, and others that have overlapping ranges but do not associate with each other. It is argued that these ecotypes are on their way to becoming (or, some would argue, already are) distinct species. This has large conservation implications.
​
Threats
Killer whales do not have any natural predators in the ocean, but face many threats from human activities. Prey depletion due to poor fish stock management is a major concern for many populations, leading to reduced survival and reproduction rates. One of the most serious threats to killer whales may be the bioaccumulation of chemical contaminants, known to accumulate in high concentrations in apex predators. These contaminants accumulate in the fatty blubber layer, and can lead to reproductive impairment and immunosuppression, making killer whales more susceptible to disease. This threat is compounded by prey depletion, as in times of food shortage killer whales metabolise their blubber layer, releasing these contaminants into the bloodstream. Noise pollution, including military, oil and seismic activities, as well as live captures, ship collisions and oil spills also put killer whale populations at risk.
​
​