Orcinus orca

DEFINITION & DIAGNOSIS

Orcinus orca comes from the Delphinadae family (oceanic dolphins) and are the largest members of the dolphin family. The largest male orcas can grow up to 10 metres while for females, upto 8.5 metres. They have proportionately bigger dorsal fins that other big delphinids ranging between 1/10 th to 1/5 th of their body length. Orcinus orca can be easily identified by their colouration- black and white. It has also been seen that their skull is larger and holds the largest brain compared to all the other dolphins except Pseudorca crassidens (false killer whale).

TAXONOMIC HISTORY

Species: Orcinus orca

Genus Orcinus, Family Delphinidae, Class Mammalia, Phylum Chordata, Kingdom Anamelia.

Synonyms: Orca ater, Orca capensis, Orcinus glacialis, Delphius gladiator, Orcinus Nannus, Orca recipinna, Delphinus orca

FEATURES

Killer whales are predominantly black with a white midsection mammals with a blunt head that has no distinct beak. Females usually grow to be 7m and males 8.2m. They also have large flippers which, in adult males, can measure up to 20% of their body length but in females and young males they will only achieve up to 11-13% of their body length. They also have a dorsal fin which in males can reach up to 1.8 m in length but in females it only reaches 0.9m. This difference in dorsal fin length can be useful in determining the sex of  O.orca. The white midsection runs across the whole lower jaw but tightens between the flippers. This white area can be seen as more yellowish incertain oceans, mainly in the Antarctic, and more predominantly in adolescents.

ANATOMY AND PHYSIOLOGY 

A killer whale’s skin is very smooth and the outer layer continuously sheds and behind the dorsal fin and back, there is a grey- white patch known as a ‘saddle patch’ (Figure.1). O.Orca have dorsal fins and have paddle shaped pectoral fins which help control directional movement. The skeleton of a killer whale is robust and long and has a skull, backbone and a bone structure of the pectoral flippers. In general it was seen that O.Orca’s facial anatomy was slightly different to a typical delphinid structure of an asymmetrical nasal sac but some structures were smaller compared to several other species. Their teeth are conical shaped and curved inward and backward (Figure 2). The temporal fossa is very large, showing that there is a strong temporal muscle helping with closure of the jaw. 

Meuth examined that the amino acid sequence of myoglobin of O.Orca had similarities to Globicephala than to other delphinids and phocoenids with myoglobin. 

The reniculi of the kidney of the killer whale was found to be in groups of four which are connected. Differences have been seen with Hyperoodon based on the venous return in the kidney and the O.Orca have no peripheral venous complex whilst Hyperoodon does.

REPRODUCTIVE BEHAVIOUR

The breeding cycles range over many months and vary depending on where the species are found. For example, in the northeast Atlantic, mating takes place between late autumn to midwinter. The approximate annual birth is between 4 to 5% and the annual pregnancy rates are roughly around 13.7 to 39.2%, and the growth spurt of an adolescent male killer whale varies within a range of 5.5 to 6.1 m, this is also the the time in which they reach sexual maturity. This was confirmed after comparing and examining two different male adolescents. The individual with 656-cm and testes masses of 3,632 g (R) and 2,270 g (L) was not sexually mature, but the individual that was 724-cm with 11,400 g (R) and 12,200 g (L) testes was sexually mature. A further examination of 57 mature males found in the Antarctic showed us that the average testis width is 22 cm and length is 55 cm. The average testis mass was calculated at 10,000 g with a maximum mass of 23,100 g. Prior to this peak, the growth curves of males are similar to that of females. 

The length for a female killer whale to become sexually mature ranges between 4.6 to 5.4 m. This length varies depending on whether the individual is found in the northeastern Atlantic or the Antarctic. If they are found in the Atlantic they become sexually mature around 4.6m and if they are found in the Antarctic they become sexually mature around 5.4m. The ovaries size range from 10 to 12 cm by 5 to 7 cm. The maximum size of foetuses varies geographically. The largest found in the North Pacific was 274cm,  the one in North Atlantic is 255cm and 250 cm for the Antarctic. The smallest foetuses recorded are 228 cm for the North Pacific, 183cm for the North Atlantic, and 227 cm for the Southern Hemisphere . Calves are usually dependent for at least 2 years and weaning takes place when a calf grows to 4.3 m in length with lactation lasting for around 12 months . The sex ratios at birth on average looks like it is 1:1, however the ratio of males to females has been reported as 1.34:1 for the Marion Islands  and 0.83:1 for the northeast Pacific.

ECOLOGY

O.orca are carnivores and also opportunistic feeders so their diets change seasonally and

based on the region they’re in. They mainly consume fish but it has been found that they can also prey on seabirds and other marine mammals such as minke whale, squid and pinnipeds. The estimated daily food intake is thought to be around 4% of their body weight. Predation for killer whales can also determine their migration such as in the Atlantic it is dependent on the migration of herring. Although O.Orca predate on many different species’, the only predator for orcinus orca is humans. They are mainly hunted for oil and meat or killed as they are competition for fishermen. In Japan and Norway the fresh meat of killer whales is eaten and the old meat is usually used for fertilisers or for bait. To figure out the age of O.orca the teeth can be sectioned and the dentine or cementum layers can be counted but this can be hard to determine due to the presence of accessory layers as well. The estimated lifespan of killer whales is thought to be 25 years but could be as long as 35 to 40 years. Killer whales aren’t subject to many diseases but the main one they face is infection in the pulp cavity due to the wearing down of teeth. If the infection penetrates through the pulp cavity it can cause a jaw abscess. In captive killer whales the main killers are pneumonia, bacterial infections, systemic mycosis and mediastinal abscess.

Written by Jeevana Thavarajah

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