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 orcas hunting whales many times larger than themselves, hence a “killer of whales”. Despite a long and fearsome reputation, orcas are not considered dangerous to humans, and along with the bottlenose dolphin are one of the most popular cetacean species in the world. The orca 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 also vary between orca 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. Orcas are found in both pelagic and coastal areas, and typically adapt their range to the availability of prey resources. Orcas appear to be most abundant in the coastal waters of temperate and cold regions where the waters are very productive.

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 usually 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. Living much longer than the ability to have offspring is highly unusual in the natural world, and is a feature known only to occur in two other species: the short-finned pilot whale and humans.

Typically, neonates (new-borns) 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 and approximately 4% of adult orcas die each year, which is relatively low compared to other species.

Males 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

The orca behavioural states that are typically observed include travelling, resting, socialising and feeding. When cruising, orcas adopt a speed of 10 to 13 km/h, usually remaining within 20 m of the surface. When necessary, orcas are able to reach brief speed bursts of up to 45 km/h. Because diving is energetically costly, orcas usually stay less than a minute underwater. However when foraging, orcas can perform deep dives of up to 100-200 m and are able to stay underwater for nearly 15 minutes. 

​Orcas 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 an orca to look at its 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, orcas need to communicate with each other to maintain social bonds, and to coordinate feeding behaviours. Indeed, acoustics constitute an orcas' 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 different orca populations make can vary greatly. These differences have been termed dialects, and are a method for researchers to distinguish which orcas belong to which population. The communication and sounds used in hunting also vary greatly depending on the main food source of the orca. Orca populations that specialise on hunting marine mammals are typically silent when they hunt, as their prey can hear and detect orca vocalisations. In contrast, fish eating orca populations are highly vocal, as the physiology of their prey’s ear cannot detect the frequencies of their sounds. 

Sociality

Highly social, orcas 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 orca, the so-called 'matriarch', her children and their offspring. 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 specific ecology and feeding habits of the group. For instance, because of hunting requirements, orcas feeding on seal prey usually adopt smaller group sizes 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 constitute a pod.

Through social learning, orcas 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). Pods that share some portions of their dialects belong to the same clan. 

Feeding ecology

The orca is an apex predator, and occupies the top trophic position in the food chain. As a species, the orca can be considered to be a generalist predator, feeding on a wide range of prey items 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, sympatric orcas can display contrasting feeding habits and strong dietary specialisations. The most studied example is undoubtedly from the North-eastern Pacific Ocean, where the so-called resident killer whales are fish-feeding, while in contrast, the transient type of orcas feed exclusively on marine mammals. Despite occupying the same geographical area, these two orca types do not interact socially or interbreed.   

Ecotypes

The worldwide orca population is made up 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. Orcas 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 segregation may promote increased 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. There are today many different ecotypes of orca 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. The killer whale, as an example of a “keystone” species vital to a healthy ecosystem, needs to be closely studied in relation to potential threats. Localised extinctions due to, for example, fishery impacts and marine pollution might not have an effect on a single species of killer whale with a large range and diverse diet. However, such local extinctions could constitute the loss of an entire ecologically and genetically distinct species. The division of orcas into ecotypes, with an understanding of their varying ranges and specialist feeding habits, is essential if one is to give due regard to the conservation of marine biodiversity.

Threats

Orcas 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 orca populations, leading to reduced survival and reproduction rates. One of the most serious threats to orcas is the bioaccumulation of POPs (Persistent Organic Pollutants), which are chemical contaminants known to accumulate in high concentrations in apex predators. These contaminants accumulate in the fatty blubber layer in orcas, and can lead to reproductive impairment and immunosuppression, making the orca more susceptible to disease. This threat is compounded by prey depletion, as in times of food shortage the orca will metabolise its 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 orca populations at risk. 

Illustrations by
Frédérique Lucas
for NOS
Illustrations by
Frédérique Lucas
for NOS