The killer whale (Orcinus orca) belongs to the family Delphinidae and is the only extant species of the genus Orcinus.
Unlike other species of dolphins, killer whales have a rather robust body. Their length and the black coloration make them one of the most easily recognizable cetaceans. They show a very clear sexual dimorphism. Not only because of their length but also in their size and shape of their fins. Adult males have a very tall triangular-shaped dorsal fin, which can measure up to 1,8 meters from the base to the tip, whereas females and juveniles have a much shorter and falcated dorsal fin. Therefore, it is more difficult to distinguish females form juvenile males, because the dorsal fin of the latter only starts growing until they are 10 years old. The oval flippers are paddle-shaped, bigger in males than in females. They can be up to 2 meters long and 1 meter width. All fins have are essential for locomotion, as for example the caudal fin, which is responsible for the propulsion of the animal, but are not limited to this function. Killer whales have developed a very complex corporal language, which they use not only in their social relationships but also in the organisation of the group and in their hunting techniques, avoiding the use of vocalizations, that could alert their prey. The rounded head which lacks a differentiated beak, has a big melon and shows a protruded upper jaw.
The species shows a unique coloration. The body is mainly black, except for some white or grey areas, such as the ventral patch, which extends from the throat to the genital area, where it forms two more thinner lines extending back and upwards to the flanks. This patch has the shape of a trident, typical of the species. On the dorsal part of the body, and just behind the dorsal fin there is a whitish patch called saddle patch. This almost symmetrical mark extends on both sides of the body and it varies in the different killer whale populations around the world. In addition it is also used in photo-identification studies, because it is different for every individual. On the head, behind the eye, there is a white area called the post-ocular patch. This marked contrast between the black and white areas is an evolutive adaptation of the killer whales that helps them in their hunting dives. Under the water, the white areas vanish fading the shape of the body and making the prey get confused. This white areas vary depending on the killer whale type and the geographic distribution.
Killer whales have between 40 and 48 homodont conic teeth measuring between 10 and 12 centimetres of height and 3,5 centimetres in diameter. They are slightly leaned backwards and inwards. Teeth are equally distributed along the jaw, fitting the upper ones between the lower ones when the animal closes his mouth.
Adult males measure up to 9,5 meters and weight up to 8 tones, whereas females measure slightly more than 6 meters weighting up to 5 tones. Calves measure between 1,8 and 2,5 metres at birth and weight up to 180 kg.
At present there is only one killer whale species accepted, although 4 different main groups with different geographical distribution, can be distinguished. Some authors consider that the taxonomy of the species should be reviewed, because morphologic, ecologic and genetic evidences suggest that a certain speciation rate may be happening within the species.
- Type A: Is the best-studied, and mostly sighted killer whale community. It is formed by three different groups with distinctive behaviour, morphology and distribution.
- Resident killer whales: They are found in the Nort-West Pacific waters of Alaska and Vancouver (Canada). Their saddle patch is completely gray, sometimes showing an open black line, but never extending forward than the middle point of the dorsal fin basis. The post-ocular patch is oval-shaped, medium-sized and parallel to the body longitudinal axis. They have a very strong social structure, usually composed by 6 yo 60 individuals that spend their whole life within the same familiar group. Resident killer whales are highly endogamic. They only mate with other animals of the same clan or with killer whales from other groups with shared ancestors. The exclusively feed on fishes, mainly salmon..
- Transient killer whales: The best known and the most studied are found in the North-West Pacific, but they can also be found all over the planet. The bigger whales are found in this group, measuring females up to 7,5 metres an males more than 9 metres. Dorsal fins are triangular-shaped and more pointed that the dorsal fin of the resident killer whales. Saddle patch is always closed and never has a black line dividing it. Their social structure is not as stable as in the resident killer whales, because males usually leave the group when they reach sexual maturity. Then they travel alone or in groups composed exclusively be males, searching other killer whales to mate. They have a smaller variety of less complex vocalizations. They only produce sounds during 5% of their time. Their diet is very varied and besides fishes it also includes sea lions, seals, and the antarctic minke whale (Balaenoptera bonaerensis). This diet has transformed their hunting techniques, showing very sophisticated methods, such as the intentional stranding that they show in Punta Norte (Península Valdés, Argentina) in order to catch sea lions and sea elephants calves.
- Offshore killer whales: The were discovered in 1997. The prefer deep water areas not closer than 15 km to the coast, although they sometimes may approach to the coast. They may be observed around the Vancouver island and other islands off Canada. Their dorsal fin has a curved tip, and their saddle patch is sometimes very similar to the saddle patch of the resident killer whales. Their size is rather small. Their groups are formed by 20 to 70 individuals and their diet includes fishes and marine mammals, although, according to recent studies, it would be mainly composed by sharks, especially the genus Somniosus.
Resident and transient killer whales share some geographical areas, but they tend to avoid each other. Some killer whales have been seen traveling at high speed through areas occupied by other groups, trying to avoid conflicts. According to DNA analysis conducted from skin samples, both groups genetically separated 10,000 years ago.
- Type B:They exclusively inhabit antarctic and subantarctic waters. Some authors consider them as a different species Orcinus glacialis. They are smaller than the type A killer whales. Their dorsal coloration is also different. Their dorsal coloration is different, the cape is black and thin and its lower parts, on the flanks, are dark grey. Post-ocular patches are bigger and oriented horizontally. They show a yellowish coloration on their body, more obvious on the white areas, and caused by the diatoms living on them. Some authors believe they can perform quick migrations to warmer waters in order to clean their body. Some Type B killer whales have been observed swimming fast in front of the coast of Patagonia heading north to Brazil and returning to Antarctic waters afterwards. They usually form groups from 150 to 200 animals. They mainly feed on fishes, although their diet may also include marine mammals and cephalopods. Their hunting strategies are highly elaborated as when they make seals fall from ice blocks into the water. First they take the ice block to deeper waters and then they swim towards it but pass below the surface, thus creating a wave that will cause the block to bend and the seal to fall into the water.
- Type C: These killer whales are only distributed in antarctic and subantarctic waters. Some authors call them Orcinus nanus, because they are the smaller type of killer whales, measuring 1 to 3 meter less than the other types. Like in Orcinus glacialis they also show a greyish coloration. Their post-ocular patches are much smaller and inclined upwards in a 45°. They mainly feed on fishes, mainly the cod.
Some recent studies on the gene of the cytochrome of the mitochondrial DNA of types B and C killer whales, suggest that this genetic change that took place 150.000 years ago and that separated both types is contributing to their divergent evolution. This may lead to the separation of both types in two different species.
- Type D: This type was first described from a mass stranding event that took place in New Zealand in 1955, and from observations made since 2004. These killer whales are easy to distinguish thanks to their tiny post-occular patches compared to their huge melon. They show a circumpolar distribution in antarctic waters between 40° and 60° S. The familiar groups are rather big, formed by 9 – 35 individuals. Their diet is still unknown, but some authors suggest they will mainly feed on fish, because some killer whales have been spotted eating cod and approaching long-line vessels. This is the less studied type of killer whale.
Nowadays there is only one species of killer whale accepted by the international scientific community, the Orcinus orca, but scientists admit that there are different types as a result of a long evolutionary process. These types differ in geographic distribution, morphology, diet, culture, etc.
DISTRIBUTION AND MIGRATORY PATTERNS
Killer whales show are one of the species of cetaceans with a bigger geographic distribution, because they are distributed in all oceans. They also are found in shallow bays, enclosed seas and estuaries. They have also been spotted in rivers. It is a cosmopolitan species, not limited by environmental factors such as temperature, water depth or a specialized diet. Although they can be found in very different habitats, they prefer tempered and cold coastal waters with high primary production. The higher killer whales densities are found in the North Western American coast, in Iceland and Norway. They are also found in the Arctic, where they inhabit free-ice waters, and in the Antarctic ocean, associated to pack ice. In the southwestern Atlantic they inhabit temperate and cold waters, from Tierra del Fuego to the south east of Brazil, where sightings are rather sporadic. They are usually seen off the coast of Argentina. In the southeastern Pacific killer whales have been reported from Chile to Ecuador, especially in the Galapagos Archipelago.
In the Mediterranean sea and the waters surrounding the Iberian peninsula, there are different groups with different distributions.
Killer whales of the Gulf of Cadiz: Although they are not abundant, killer whales can also be found in the Mediterranean sea, however this population is mainly restricted to the golf of Cadiz. There resides 1 clan formed by 1 subpod and 2 stable familiar groups.
- The first one is a subpod (which means a familiar group with common matrilineal ancestors in it) and it is formed by three familiar groups. Their members spend most of their time in the centre of the Strait, although they can be seen cloe to Barbate during spring. They mainly feed on bluefin tuna (Thunnus thynnus).
- The second group inhabits waters off Barbate, where they were first spotted in 2001. Their diet is still unknown.
- The las groups has only been found in waters off Barbate and close to the Bay of Algeciras. They feed on different species of fish.
The major part of information comes from the first group, which interacts with the bluefin tuna fisheries. Although these groups are not well known, 42 individuals have been identified so far, 39 of which have been repeatedly spotted until 2008. There have only been found 3 dead animals since 1999. There might still be some unidentified individuals, but scientist think that the population does not exceed the 50 individuals.
- Killer whales of the Cantabrian coast: There are only some opportunistic sightings in this area and there is no information about their abundance or distribution. Some scientific projects conducted in the area suggest that, although killer whales are permanently spotted, the population is very small and their distribution is fragmented. Studies carried out by Hammond and Locker (1987) concluded that it was possible to observe killer whales in the Atlantic waters north of the Iberian Peninsula before 1985.
- Killer whales in the Mediterranean: There are some sporadic sightings documented in Catalan, Balearic and Italian waters, as well in the Ligurian sea. However no familiar cohesion have been found between the individuals. The species is considered as vagrant, because sightings are geographically dispersed throughout the basin. Some studies concluded that the species was more abundant in the Northwestern Mediterranean Sea.
HABITAT AND FEEDING
Killer whales are top predators, with a very varied diet which includes different species of marine mammals, fishes, squids, birds, sea turtles and sharks. They have developed a diversity of hunting strategies depending on prey and region. There have been documented more than 40 marine mammals species in the killer whale’s diet, including great whales, sperm whales, porpoises, dolphins, belugas, sea lions, seals and sea otters. The trophic habitats varies depending on prey availability which varies seasonally.
REPRODUCTION AND LIFE HISTORY
Killer whales have one of the longest gestation periods amongst the cetacean. Their reproductive cycles vary depending on the population and the geographical area. In the northwestern Pacific calving occurs throughout the year with peaks in october and march, whereas in the northwestern Atlantic, calving occurs between late autumn and mid winter. In Patagonia, calves are observed in two periods, between february and spring, and between november end december.
Females reach sexual maturity at the age of 12 to 14 years, when they will give birth to their first calf, whereas males reach sexual maturity at 15. At that point their dorsal fin grows quickly acquiring a tall and triangular shape typical of males. However males only reach social maturity at the age of 21 – 25. Gestation lasts between 12 and 18 months, and they give birth to a single calf, as many marine mammals species do. There are some birth of twins have been documented. Mortality rate amongst newborns during their first 6 months of life is 43%. Nursing lasts 2 years and size at weaning is around 4,3 metres.
Calving interval ranges between 5 to 10 years. Their reproductive life last for 25 years, during which they can give birth up to 5 calves, followed by a post-reproductive period 10 to 30 years long. Females life expectancy is estimated to be between 50 and 80 years, whereas usually lives around 30 years with maximum longevity of 50 to 60 years.
ECOLOGY, BEHAVIOUR AND IDENTIFICATION
Many groups have a varied diet, whereas other groups are specialized in a specific type of prey. This variety of diets has led to the development of many different types of complex behaviours, such as the “arreo”, or the “carousel feeding”, where the whales herd the prey in a compact ball. Other have developed extreme predator behaviours, such as the intentional stranding in Patagonia and the Crozet Islands. The main goal of this behaviour is to capture sea lion and sea elephant pups. The variety of hunting techniques and their adaptability are a consequence of their familiar cohesion and the transmission of knowledge from one generation to the next. In fact, some authors believe that killer whales are one the few animal species in which familiar culture could be considered. The number of individuals of a group or its cohesion depend on the killer whale type, the geographical distribution and its feeding. For example, resident groups with a fish diet will live in big groups, because they will be required to cooperate during hunting, whereas transient groups that feed on marine mammals live in smaller groups, because the cooperation degree is smaller.
Killer whales of the Strait of Gibraltar have developed different ways to obtain their food: the traditional hun method is the passive acoustic, they remain silent listening to the bluefin tuna shoal approach. Once they have detected the fishes, they start pursuing the fishes. This behaviour can last for 30 minutes during which killer whales alternate themselves in order to round up and drive their prey to exhaustion. This technique is also known as “hunting by exhaustion” and requires the killer whale a great amount of energy.
There are alternative methods to obtain food, as for example hunt the fishes trapped in the “almadraba”. The almadraba consists in the setting of nets in a maze that leads to a central pool, where bluefin tunas get trapped. Killer whales have been observed to herd the fishes towards these traps in order to catch them more easily.
Another technique is called the “wait”. Between june and september a traditional fishing method known as “la piedra”, consisting in throwing fishing line with bait tided up to a stone into the sea, is used in order to fish bluefin tuna. The stone will drawn the bait to the desired depth. When a tuna fish get trapped, the fisherman start pulling it off the water. The characteristic noise produced by the reel warns the killer whale, which will get the fish at the surface before the fisherman can pull it off the water. This hunting method is not energetically expensive for the killer whale.
According to different studies, killer whales have a matrilineal social structure, dominated by the older female in the group which is responsible for transmitting its experiences and abilities to the next generation. Resident killer whales maintain a very strong familiar structure, which is composed by the dominant female, its descendants and their descendants, including up to 4 generations in the same familiar group. This familiar unit will remain together during all their lives. Social structure has 4 clearly separated levels, being the pod the basic unit. The next level is the subpod, formed by different familiar groups (1-3) which shared a common ancestor in the recent past. Two or more subpods that share dialect and complex hunting strategies may join forming a clan, which may be the result of a expanding and division process of an ancestral subpod. The higher level of organisation in resident killer whales is the community, or group of clans which join to travel together, but don’t have any genealogical relationship. On the other side, transient killer whales also form matrilineal groups, but they are not so stable. Males may temporarily or permanently leave the group when they reach sexual maturity.
Aspects such as diet and survival are transmitted from one generation to the next. Additionally every group of killer whales have its own traditions, which are not shared with other groups. Even dialects are different amongst groups. For example, the Johnstone Strait, which separates Vancouver Island from the continent, is the center of the distribution area of a group of 200 killer whales divided in 16 different groups, which join there in summer. This group is the northern resident community. One of their preferred activities is to rub their bodies on the smooth pebbles of some shallow water beaches. The purpose of this activity, whether it is a form to get ride of external parasites or a social habit, is still unknown. The southern resident community of killer whales in that area never rub their bodies on the pebbles, but they are more prone in performing acrobatic aerial displays.
However the most impressive factor of the species is the development of a communication mechanism which can be considered as a language. In the 1970s scientist discovered that every group has its own call repertoire considering call type, pattern and number of clicks. Every dialect is a unique sign of acoustic identity that calves learn from their mothers and other members of the group. They also learn how to identify and recognize the other dialects used by other groups, because they prefer to socialize with their close relatives. However males search other groups to breed, with the aim of avoiding consanguinity. The similar the dialects are, the closer are the groups.
Killer whales have so remarkable characteristics, that they are one of the cetacean species more easily recognisable.
- Significantly large body size: killer whales are the biggest dolphins, measuring up to 9 metres.
- Rounded head with poorly differentiated beak: they have a rather big melon, and their beak is not differentiated.
- Coloration: is very characteristic, mostly black with white patches in the ventral and post-ocular areas.
- Genital area with a W-shaped white patch.
- Dorsal fin: females and juveniles have falcate dorsal fins whereas adult males have tall triangular dorsal fins.
- Big and oval-shaped flippers.
- Flukes are black on their dorsal side and greyish white on their ventral side.
INDIVIDUAL IDENTIFICATION CHARACTERISTICS
In addition to the above mentioned characteristic, killer whales also have marks or scars that will allow us to indentify every individual:
Ligh-coloured saddle patch behind the dorsal fin: Is unique for every animal as finger prints are unique for humans. Its shape and coloration pattern may help on the individual identification.
Scars and marks: When a scar or mark is selected as main identification factor it should be considered that this scar or mark can disappear. Thus is better to focus on older marks.
Shape and scars on the dorsal fin: It refers to the scars, notches and nicks on the posterior edge of the dorsal fin. Their distribution, number, size and shape are unique for every individual.
CONSERVATION STATUS AND MAIN THREATS
Many aboriginal cultures from different parts of the planet, such as the Nazca (Perú), the Yámanas and the Onas (Tierra del Fuego, Argentina) the Kwakiutl (British Columbia, Canada) the Haida (northwestern Canada), the Ainus (Japan), and many others, sea killer whales with a mix of fear and fascination. Many of them thought killer whales even had some kind of divine power and they even make offerings in order to get their favour. There are many legends and tales involving killer whales in these cultures. The admiration and respect that this ancient different cultures showed towards killer whales, disappeared in newer cultures, which describe them as the most dangerous predator of the sea. Later on, between the 1950s and the 1970s it became one of the targets of the whaling industry. Along 35 years Japanese, Russian and Norwegian whalers killed around 5,000 killer whales. In many regions such as Japan, Norway, Alaska and Morocco, they were considered responsible of the decrease of the fisheries. For that reason, governments sponsored killing groups.
Killer whales also are one of the most wanted cetaceans by the dolphinarium industry which started around 1962 in the Pacific coast of the USA, expanding, first north to Canada, and afterwards, throughout the world. Nowadays, Spain is the country where more dolphins live in captivity. Although capturing killer whales with the aim of exhibit them in public displays is forbidden, there still are some coastal fisheries that try to kill killer whales, because they see these animals as direct competitors.
In the Strait of Gibraltar, killer whales maintain an intrinsic relationship with human fishing activities. Although this relationship can be seen as positive for killer whales, it has negative consequences for the species. The survival rate of calves has decreased drastically. Since 2007 neither one of the three born calves survived their first year. This population of killer whales mainly feed on bluefin tuna which population is being depleted by aggressive fishing methods such as the almadraba. This fishing technique catches the bluefin tunas that enter the Mediterranean to spawn. For that reason adult tunas cannot spawn in the Mediterranean sea. The few individuals that survive are also catch on their way back to the Atlantic. For this reason, this fishing method is neither selective nor sustainable and is depleting the population of bluefin tuna, which is threatened with extinction. Mediterranean killer whale population may not be able to adapt so quickly to a diet change, thus the decrease of the bluefin tuna is also affecting the relationship between killer whales and fishermen. Killer whales may have been killed in Moroccan waters in order to ensure fishing resources. In July 2004, two killer whales where killed and 6 more in september 2005. Killer whale population may disappear in a short period of time if indiscriminate killing persists. This results conclude that the population will show a regression and will decrease in 20% during the next 5 generations. Killer whale of the Mediterranean are catalogued as a species of special interest, which doesn’t confer any kind of protection or conservation measure.
Other threats include direct effects caused by oil spills, which are responsible of the death of many animals, and the high levels of toxicity. Killer whales are located at the apex of the food chain, being therefore more affected by pollution through bioaccumulation of organochlorine and other chemical compounds.
One of the biggest problems is their live capture in order to be exhibited in dolphinariums, where they are forced to take part in shows. As stated above, killer whales are animals with very strong social bonds. Kidnaping them and taking them away from their families, may cause, in many occasions, the death of the animal. If the individual survives capture, it will be condemned to live in a very small environment, which causes changes in behaviour, trauma and the decrease of the animals life expectancy in about 10 years.
In addition to all these factors, aspects such as the intense maritime traffic, the use of sonar and the marine constructions can seriously affect the survival of many killer whale population throughout the world. Although there is not a global population estimate, the Pacific population is estimated around 11.000 individuals and the Norwegian around 5.000. The biggest populations however are found in the Antarctica, with around 75.000 individuals.
Killer whales are protected by different international conventions such as the Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), the Appendixes I and II of the Convention on Migratory Species (CMS), and is listed by the IUCN (International Union for the Conservation of Nature) under the status of data deficient.
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