Introduction
Orcas (scientific name Orcinus orca) fall under the order Cetacea, suborder Odontoceti, and they are the largest members of the family Delphinidae, the same group of dolphins and porpoises. They also became known as killer whales when Spanish sailors witnessed their attacks on larger whales and started calling them "whale killers", a name that gradually translated into "killer whale". Orcas They can be found in all of the world's ocean, from Arctic and Antarctic regions to tropical waters. Individual orcas belong to regional ecological groups, called ecotypes, that have distinct range and behaviors. Scientists recognize at least 10 ecotypes around the world.
In the Pacific region, four different populations can be found, and each one differ from the others in prey preference, vocal activity, behavior, morphology, and genetics. The Resident killer whales mainly prey on fish, live in stable matrilineal units, called pods, that join together to socialize on a regular basis. They are further subdivided into Northern Residents (found mostly off the Alaska coast) and the Southerns Residents (found in the inland passage in Washington and British Columbia). The Transient killer whales are found from Southern California to Alaska and they manly feed on other mammals, such as whales and seals. The Offshore killer whales are the most elusive and less well known, they are found well away from the coast, and they feed primarily on sharks, squid and other pelagic fish.
DIET AND EATING HABITS
Killer whales have a very diverse diet, which depends on the area where they live, but at the same type each ecotype is extremely specialized on a particular prey (SeaWorld Parks & Entertainment). They feed on a large array of preys, such as bony fish, sharks, rays, seals, sea otters, penguins, sea turtles, dugongs, and even larger whales, such as grey whale and humpback whale calves.
Food Preferences
Killer whales are top predators in all marine environments and each ecotype is adapted to eat specific preys based on their habitat.
In the Antarctic:
- Type A whales eat minke whales and southern elephant seals.
- Large Type B whales eat seals and minke whales.
- Small Type B whales eat fishes and penguins.
- Type C whales eat mostly Antarctic toothfish.
- Type D whales are more elusive, but they have been observed eating Patagonian toothfish.
In the Pacific region:
- Transient whales primarily eat seals, sea lions, walruses, and other whales.
- Resident whales primarily feed on Chinook salmon and they are so specialized on this prey that the endangerment of this salmon has become a limiting factor for their survival. They also feed on a particular species of squid (Gonatopsis borealis), and other species of fish such as the rockfish, the Pacific halibut and the Pacific herring.
- Offshore whales mainly eat other fishes such as sculpin, Pacific halibut, and Pacific sleeper shark.
In the North Atlantic:
- Type 1 whales are specialized in seals and small fishes such as herrings and mackerel.
- Type 2 whales mainly feed on other cetaceans such as dolphins and baleen whales.
Hunting Techniques
Killer whales hunts cooperatively in pods for food using a wide array of techniques, which depends on their prey. In the Pacific region, killer whales purse a fish alone or in a small group and they share it with the family. When hunting a school of fish, they encircle it and herd it before attacking. In Norwegian waters, killer whales use a technique called "carousel-feeding" in which they herd small fishes in a ball close to the surface and then they stun the fish with the fluke and eat it. To hunt a large baleen whale, they attack from different angles until they tire it out. Transient whales usually "sneak attack" marine mammals or they drive and trap small groups in bays before attacking. In the Antarctic, individuals slide out onto floating ice to hunt penguins, and in the same way they are also able to slide up onto beaches to attack elephant seals. Another way they attack seals on big ice floes is by swimming next to each other towards the ice and then diving below at the last moment, to create a large wave that washes the seal into the water.
Feeding Apparatus
Teeth
Most odontocetes share a particular anatomical feature that is characteristic of their predatory way of life: prehensile teeth (Fontaine, 2007). This type of dentition is an example of homodont dentition, a type of dentition that makes mastication ineffective. Prey is indeed swallowed as a whole and teeth are instead used to grasp and shred the prey into smaller pieces. A killer whale's teeth are about 7.6 cm long and 2.5 cm in diameter, and each side of each jaw has between 10 and 14 teeth, for a total of 40-56 (SeaWorld Parks & Entertainment).
Stomach
Whales in general have a multicompartmental stomach, similar to the the one of ruminants, with whom they share a common ancestor (Fontaine, 2007). The first part is an extension of the esophagus, does not contain any digestive glands, and it just serves to accumulate food and get it ready for the action of gastric acids. The second part is where hydrochloric acid and pepsinogen are released to break down the food. The third part connects the stomach to the intestine where bile and pancreatic juices further break down the food. The nutrients are reabsorbed and the rest is excreted out of the body.
THERMOREGULATION
Killer whales are warm-blooded animals, like all mammals, and therefore they are adapted to control the rate of heat loss in relation to changes in metabolic activity and environment temperature (Parry, 1949). Some of the adaptations are the blubber and the rete mirabile.
The Blubber
Marine mammals in general are equipped with a unique type of adipose tissues, called blubber, which is not just an insulator, but it actively controls the heat leaving the body, preserving the body temperature. The blubber is composed of epidermis, dermis, and hypodermal tissue. The epidermis is the most superficial layer and its thickness varies along the body. The dermis consists of connective tissue fibers and it is very dense at the base of the epidermis and less dense closer to the hypodermis. The hypodermis is composed predominantly of fat cells and it is responsible for the thickness of the blubber as a whole. The blubber is highly vascularized and this helps regulating the heat lost to the environment. The blubber has also the function to store energy, so that the animal can stay long periods without food, and and to keep the animals afloat, since blubber is less dense than water.
The Rete Mirabile
The rete mirabile is a counter-current heat exchanger, which is composed of an intertwined network of veins and arteries (Storey, 2011). The cold blood that returns from the body extremities in the veins runs next to the warm blood going towards the extremities in the arteries. This system allows heat to flow from arteries to veins (conserving the heat), and to cool the arterial blood going towards colder regions of the body. These retes are found in different parts of the body, such as flukes and reproductive organs.
SENSORY SYTEM
Orcas have very well-developed senses. Hearing is an important sense since sound travels faster in water than in air. Orcas use high-frequency sounds, both for communication and echolocation. Vision is another important sense and orcas are specialized to see both in and out of water. The sense of touch is also well developed, especially in the areas around blowhole, eyes and mouth. Olfactory lobes in the brain and olfactory nerves are absent like in other toothed whales, showing that killer whales don't have a sense of smell. Little is still known about their sense of taste.
Echolocation
Echolocation is the ability of organisms to locate and identify objects by emitting high-frequency sound waves and listening for echoes. Killer whales use echolocation to navigate, communicate, and prey, especially because of the environment they live in, which is usually dark and unclear (Peng, n.d.). Killer whales do not have vocal cords in the larynx (Cranford, 2011). They produce sounds by passing air through a structure in the head called phonic lips, which is analogous to the human nasal cavity but that act similarly to human vocal cords. As air passes through, the phonic lips are sucked together, causing the tissue to vibrate. The direction of these vibrations can be controlled with great precision. The vibrations pass through the tissue of the head to the melon, which acts as an acoustical lens and focus the sound waves into a beam that is projected forward into the water. The sound will bounce off objects and their echoes will reflect back to the whale. The air will enter then the vestibular sac, and from here it can be either recycled or passed out through the blowhole. The lower jaw then receives the echoes and will send it to the middle ear bones, the inner ear and finally to the brain, which will interpret the echoes.
Killer whales are able to adapt their vocal behavior to the prey they are hunting. For example, transients that feed on marine mammals, which have sensitive underwater hearing, vocalize less during hunting compared to residents that feed on fish, which have poor hearing abilities.
Southern resident killer whales may even be able to tell the difference between their favorite prey, the Chinook Salmon, and the Coho Salmon, based on the salmon's swim bladder (Au et al., 2010). A swim bladder is an organ filled with air that is used by fish to maintain their depth, and it's the perfect target for echolocation. During an experiment, researchers placed similarly-sized and anesthetized Chinook, Coho, and Sockeye salmon in a tank, and then simulated killer whale clicks. The measurements of the returning echoes showed that each species has a different structure that is probably recognizable by foraging killer whales.
Sound types - clicks, whistles, and pulsed calls
The diverse vocal repertoire of orcas includes clicks, whistles, and pulsed calls (Bergler, 2019). Clicks are brief pulses of sound, usually given in series. Orca clicks are quite variable in structure. The duration ranges from 0.1 to 25 ms, and the repetition rates from a few to over 300/s. Frequency can be either narrow or broadband and ranging as high as 30 kHz. Some clicks are composed of pairs of pulses, with interpulse intervals of 1.3 to 2 ms. Clicks are mostly used for navigation and localization. Whistles are narrow band tones, characterized by a continuous waveform, with no or few harmonic components. Frequency ranges between 1.5 and 18 kHz and duration from 50 ms to 12 s. Whistles can extend into the ultrasonic range (with frequency reaching up to 75kHz in some Northeast Atlantic populations but not in the Northwest Pacific). Whistles are mostly used in close-range social interactions. Pulsed calls are the most common and studied vocalization. They typically show sudden and patterned shifts in frequency, based on the pulse repetition rate, which is between 250 and 2000 Hz.
Whistles and pulsed calls can be produced at the same moment and odontocetes in general are able to do that because there are two sites of sound production that are controlled independently (Kremers, 2016). They are composed of two identical sound producing structures that consist of fatty dorsal bursae within a pair of phonic lips, one in the left nasal passage and one in the right nasal passage. A recent study suggested that the two brain hemispheres, which sleep independently, may also allow odontocetes to coordinate prey capture and communication by simultaneously emitting clicks and social sounds.
Vision
Cetaceans have a well developed visual system, adapted to both aquatic and aerial media, essential for several functions such as predator avoidance and prey capture, navigation, communication, social interactions, recognition of other members of the same species, and coordination of group movements (Mass, 2013). Killer whales are powerful predators, able to hunt even large preys such as baleen whales. They actively use underwater vision but also aerial vision, and in particular this ability is observed when hunting for seals. Killer whales lift their heads above the water surface to see above the water, and they are then able to grab seals right off the ice. Their good vision has been observed both in oceanaria and in the wild. However, anatomical and physiological data are still limited.
Retinal structure is the feature of visual anatomy that can help us to understand many aspects of a cetacean's vision (Mass, 2013). In particular, the distribution of ganglion cells across the retinal surface, especially the area of high ganglion cell density, is important for fine visual discrimination, and therefore, it determines visual acuity (the ability to resolve spatial details). Retinal topography has never been studied in the killer whales. However, the study of certain features and the dimensions of the eye has helped estimating the killer whale visual acuity.
In the killer whale's retina, ganglion cells are very large compared to terrestrial mammals, and they have a low cell density (Mass, 2013). The ganglion cell diameter ranges between 8-100 µm, with the majority within a range of 20-40 µm. The topographic distribution shows two spots of high density (unlike terrestrial mammals that have only one): one is located in the temporal and one in the nasal quadrant, 20 mm from the optic disk, connected by a horizontal line. The presence of two areas of high density is because cetaceans can see well both above and under water. In the temporal area, mean peak cell density is 334 cell/mm^2, and in the nasal area is 288 cells/mm^2. These high-density areas predict a resolution of 9.6' in water and 12.6' air, values that are within range of visual acuities of many terrestrial mammals.
Other remarkable features of a cetacean'e eye are a thick sclera, a thickened cornea, and a highly developed vascular network, all structures taking part in protecting the eye from mechanical damage and underwater cooling (Mass, 2009). Also, in cetaceans the eyeball is not as spherical as in terrestrial mammals, but its anterior part is flattened and the eyecup is of almost hemispherical shape. In terrestrial mammals, the convex outer surface of the cornea is the major refractive element of the eye, since it separates media with different refractive indices (the corneal refractive index is ~1.35 and the air refractive index is ~1). Since water refractive index is very similar to the cornea one, in cetaceans light refraction and focusing of an image are almost entirely performed by the lens. This is the reason why lens are of spherical shape in cetaceans, and they are similar to fish, since in both cases the eye is adjusted to optical properties of water.
In terrestrial mammals, accommodation (refraction adjustment to the distance to the object) is performed through the change in shape of the lens due to contraction and relaxation of ciliary muscles (Mass, 2009). In cetaceans, ciliary muscles are poorly developed or absent, suggesting that accommodation occurs via another mechanism, called axial displacement of the lens due to changes in intraocular pressure, which can change because of contraction of the massive retractor muscle that produces axial displacement of the eye in the orbit.
In cetaceans, iris and pupil are also adapted to underwater vision (Mass, 2009). The pupil has developed to react in a wide range of illuminations (from well illuminated surface to low illumination in the depth) and to have a wide range of sizes. The pupil have an unusual shape. The upper part of the iris has a protuberance, called operculum. In low light conditions, the operculum is contracted (raised), so the pupil has a round shape, like in other mammals. In medium light conditions, however, the operculum advances downward, and the pupil turns into a U-shaped slit. In high light conditions, the operculum advances so far that the slit becomes closed.
Another adaptation of the cetacean eye to low underwater illumination is a well developed tapetum, the reflective layer that lies behind the retinal pigment epithelium (Mass, 2009). In cetaceans, the tapetum is made of extracellular collagen fibrils, which allow significant light reflection back to the retina, thus increasing visual sensitivity in dim light conditions.
NERVOUS SYSTEM
The brain of a few cetacean species have been studied relatively extensively, particularly the bottlenose dolphin one, especially due to its popularity in captivity (Orca Nation). However, there is still little information on the brain of the largest Delphinid species, the orca, despite the fact that orcas, like dolphins, show complex and unusual social, communicative, and cognitive capacities. Some examples are learning-based cooperative foraging strategies, cultural variation and transmission, and mirror self-recognition.
The Orca Brain
Orcas are extremely social and intelligent animals and the main reason is a very complex brain, which is not only the second largest brain in the animal kingdom, after Sperm Whales, but it is also very differently structured from ours, in order to make orcas the most successful marine creature.
There are four main features that distinguish an orca brain from a human one, and they are all correlated with cognition (Orca Nation). The first one is cortical thickness. This part of the brain is associated with memory, language, thought, and consciousness. Cetaceans in general have a very thick cerebral cortex, but not as thick as humans and primates; however, cetaceans' cortices are structured in a different way. The second important feature regards gyrification, which is the amount of wrinkling and folding in the cortex. Cortical folds allow cells to be closer to each other so that the transmission of neural impulses between cells is faster and requires less energy. As a consequence, brains with more wrinkles and folds are more adept at handling data and processing it faster. In general, cetaceans' brain is heavily folded. As a comparison, the gyrencephaly index GI for humans is 2.2, whereas the GI for orcas is 5.70, making them the most gyrified brain in the world. The third important features are a highly developed set of brain lobes, called paralimbic system, compared to land mammals (humans included), which is associated to spatial memory and navigation, and a highly developed amygdala, associated with emotional learning. Finally, orcas have the most elaborated insular cortex, or insula, in the world. The insula is involved in consciousness and plays an important role in functions linked to emotions such as empathy, compassion, self-awareness, and interpersonal experience. Complex emotions in orcas were indeed observed in many occasions by scientists and trainers.
Sleep Patterns
Killer whales are conscious breathers, unlike humans that are unconscious breathers. What it means is that they can never fall completely asleep because of their need to breathe out of water (Kachar et. al, 2018). Their sleep pattern is called unihemispheric, which means "one-sided sleep". They shut down one hemisphere of the brain and the opposite eye controlled by that hemisphere, while the other side of the brain and one eye remain vigilant. If they fell completely asleep, they would suffocate and drown. Killer whale calves, on the contrary, are able to spend the first months of their lives without sleeping (Lyamin et al., 2005). The reasons behind this behavior are numerous. It can be a way of staying safe from possible predators while they are still young, or a way of staying warm through constant activity while they develop their own blubber.
Killer whales can be found at the surface while sleeping. During sleep, they form tight circles and synchronize their movement. In this way they can, for example, avoid collision with boats. They can sleep from five to eight hours at a time.
REPRODUCTION
Killer whales are sexually dimorphic, which means that males and females are shaped differently. Adult males are larger and they can be more than 9 m long (Perrin et al., 2002). They are recognizable by their upright dorsal fins, up to 2 m tall, which is the largest of any whale. Adult females are slightly smaller, with a dorsal fin about 70 cm tall. The shape of the dorsal fin varies between individuals and populations, as a consequence of both damage and genetic influences.
Females become sexually mature in their early teens and can live up to 100 years (Perrin et al., 2002). Males mature a little later and die a little younger. A female reproduce once every 3-4 years and the gestation period lasts about 15-17 months. They give birth to about 5 calves, but not all them survive to maturity. After the age of 40, the female gains the social status of "babysitter" and teacher to young whales.
Female Anatomy
Female cetaceans have two ovaries, a uterus, a vagina, and a placenta during gestation, like other mammals (Reproductive system). One difference is that one ovary is larger than the other one and it is used more often. The uterus is composed of two cavities instead of one, like other mammals, and the fetus develop in one of those. The vagina is composed of several folds in order to prevent water from entering and to limit sperm advancement. Since pregnancy requires a lot of energy, a female is able in this way to have some control over reproduction by changing the angle of penetration and therefore avoiding fertilization. Thus, a female is able to choose a suitable male that will maximize her offspring fitness. Another difference from terrestrial mammals is the absence of menstruation. Cetaceans instead reabsorb the blood in the wall of the endometrium if the female does not get impregnated.
Male Anatomy
In male cetaceans, the reproductive system is entirely internal, an adaptation that improve their hydrodynamics when swimming (Reproductive system). The penis emerges from the genital slit during copulation, originating from two fused structures that are attached to the pelvic bones. Since cetaceans don't have hind legs, the pelvic bones are only vestigial and used for penile erection. The testicles are located in the abdominal cavity, where the environment is too warm for sperm production. For this reason, testicles are covered in blood vessels, called rete mirabile, which direct the blood toward the dorsal fin for cooling. Due to the presence of folds and twists in the vagina that limit sperm advancement, a male with a penis adapted to the complex structure has higher chances to reproduce.
RESPIRATION
Orcas are mammals and therefore they need to breathe air. Despite they have a similar respiratory system to the one of land mammals, their system is adapted to underwater living.
How Orcas Breathe
Orcas, like all toothed whales, have one blowhole (which is the equivalent of our nostrils) on the top of their head, through which they inhale air. The second nasal duct has evolved over time to be used for echolocation. Breathing takes place at the surface when the blowhole is clear of water (Cozzi et al., 2017). The blowhole is not always open, like nostrils in terrestrial mammals, but orcas have to consciously open the nasal plug in order to let air in (this is why cetaceans in general are called conscious breathers). Gas is exchanged very quickly and a full breath lasts less than a second in order to allow fully oxygenation of blood in the pulmonary alveoli. This fast respiratory act allows for 75-90% of total lung volume exchange in 0.3 s, compared with, for example, 27% in 0.5 s of a galloping horse. The larynx connects the blowhole with the almost horizontal trachea and is separated from the pharynx in order to prevent water from food to enter the respiratory tract. The trachea is short and wide, both characteristics that increase capacity and speed of the airflow. Also, respiratory muscles, such as the diaphragm, are required a powerful effort in order to facilitate this fast ventilation. Finally, lungs are not as large as one would expect and they do not carry much oxygen. The reason is that orcas dive to significant depths and therefore, if the lungs were large, they would impair their capability of diving.
DIVING ADAPTATIONS
Cetaceans have adapted to marine life in a long process starting in the early Eocene and their degree of body transformation is still unsurpassed in the realm of marine mammals. Orcas share the same sleek, streamline bodies as other dolphins have. This body shape helps them to move through water easily and to swim really fast. Orcas can reach 30 miles per hour and they are one of the fastest mammals alive today. Despite their need to come to surface regularly to breathe, orcas are able to reach depths over 100m and they only dive for less than a minute up to five minutes (The Ocean Portal team Reviewed by Nick Pyenson, 2020).
An Adapted Skeleton
The skeletal adaptations to a marine environment were probably the first to be observed and studied (Cozzi, 2010). The most evident are: telescopic modification of the skull in order to create space for the melon and to improve hydrodynamics; a larger and more flexible rib cage with a minimal sternum; the transformation of the thoracic limbs into flippers and the disappearance of the hind limbs in addition to the changes of the vertebral column; the absence of the sacrum and the presence of hemal arches between the lumbar and the causal sector.
In the last years, further investigations of the cetacean skeleton were performed and new findings are revealing more adaptations to a life in the water (Cozzi, 2010). It was already known that some bones in cetaceans have a different density compared to the corresponding bones in terrestrial mammals. Tympanic bullae, for example, are particularly heavy in cetaceans. What was unknown until recently is that in a cetacean fetus calcium salts heavily deposit prematurely in the tympanic bullae and the reason may be the need of the newborn animal to readily be able to receive and translate acoustic signals from the environment, a necessity for orientation in the water.
Withstanding Pressure Change and Preventing "The Bends"
One the biggest physiological challenge for a diving mammal is adapting to the change in pressure, which results in mechanical distortion and tissue compression, especially in gas-filled body spaces, such as middle ear cavity, air sinuses, and lungs (How do deep-diving sea creatures withstand huge pressure changes, 2006). Cetaceans have developed a middle ear cavity lined with extensive venous plexus that becomes engorged at depths, thus reducing the air space and preventing tissue distortion and disruption. Cetaceans also posses larger Eustachian tubes that communicate with the tympanic ear cavity and the pterygoid sinuses of the head. These air sinuses are highly vascularized and help equilibration of air pressure. Lastly, lungs are susceptible to compression damage as well. In cetaceans, peripheral airways are reinforced, allowing lungs to collapse when animals is diving and therefore forcing air away from alveoli, where gas exchange occurs. This is important because it prevents the absorption of nitrogen into the blood and therefore the development of high blood nitrogen levels, which may cause a narcotic effect other than leading to bubble formation (a condition known as "the bends") during the ascent.
Saving Oxygen
Orcas are able to slow down their heartbeat and to stop the blood flow to certain parts of the body such as kidneys and liver when diving and hunting (Secrets of the deepest diving whales). Their body is also specially adapted to store oxygen in blood and muscles, rather than keeping it in their lungs like humans do. This is possible because of the high levels of myoglobin in blood and hemoglobin in muscles, two particular proteins that store oxygen. Orcas (and other marine mammals like seals and whales) are able to keep more myoglobin in their muscles due to special "non-stick" abilities that allow them to pack great amounts of oxygen without clogging them up (The Ocean Portal team Reviewed by Nick Pyenson, 2020). Proteins at high concentrations tend to stick together. However, myoglobin in deep-diving mammals has evolved to be positively charged (Gill, 2013). Because of this property, proteins repel each other, thus allowing animals to pack high concentrations of these proteins, and consequently storing more oxygen. The shape of their body also contributes to save oxygen by allowing them to glide instead of actively swimming at times, therefore conserving more oxygen (Secrets of the deepest diving whales).
THE EXCRETORY SYSTEM
Osmoregulation
Osmoregulation is the process to maintain internal water and electrolyte concentration within a narrow range. Because marine mammals live in an aquatic medium different from the internal environment, they are required to actively maintain it within a certain range. Water and electrolytes can enter the animal via ingestion of food and water (performed water), and compared to terrestrial mammals, they consume more water since their food is rich in water. Water can be also derived from metabolic processes (metabolic water). Marine mammals like cetaceans are usually able to get all the water they need from food, but in case of osmotic stress, they can drink seawater, only if they are able to produce a highly concentrated urine. However, even if they can, they do not usually do it and prioritize ways to instead reduce water loss (Encyclopedia of Marine Mammals).
The Kidney
The kidney is the organ in charge to regulate the water and electrolyte state of the animal. When there is too much water, the kidney produces dilute urine, whereas in water scarcity conditions, concentrated urine is produced. The kidney has the function to excrete metabolic end products in the form of urea. Cetaceans get their water either from food or from seawater. In this last case, they are required to process large volumes of highly concentrated urine, and therefore they have a specialized lobulate or reniculate kidney able to do that (Ortiz, 2001).
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- The Ocean Portal team Reviewed by Nick Pyenson. (2020, March 26). Whales. Smithsonian Ocean. Retrieved December 3, 2021, from https://ocean.si.edu/ocean-life/marine-mammals/whales.
Credits:
Created with images by MrsBrown - "nature sea water" • wolfganglucht - "orcas killer whales whale watching" • Aktim - "whales killer whales orcas" • Derrick Neill - "A skull of Orcinus orca, the killer whale" • Jeroen - "Three orcas in a row, telegraph cove at Vancouver island, British Columbia, Canada."