HomeVertebratesCetacean

Cetacean

Cetacea

Cetaceans are large aquatic mammals with streamlined bodies culminating in paddle-like tails called flukes. They are members of the infraorder Cetacea (under order Artiodactyla) and are further subdivided into two parvorders: The baleen whales or Mysticeti (including the blue whale, the humpback whale, the bowhead whale, and others) and toothed whales or Odontoceti (porpoises, dolphins, beluga and the sperm whale).

The term Cetacea is derived from the Latin word Cetus, which means’ a huge fish or a sea monster’ and indicates the unusually bulky bodies of these mammals. Cetaceans are fully carnivorous and feed on plankton, shrimp, squid, octopus, prawns, fish, and small marine mammals.

Description

Size

The largest cetacean is the blue whale (the largest known animal to have ever existed on land), which measures 98 ft (29.9 m) in length and weighs about 191 tons (173,272 kg). In contrast, the porpoise vaquita is the smallest cetacean, measuring about 5 ft long on average.

Body Plan

Like fish, cetaceans have a streamlined body and are well-adapted for a smooth aquatic lifestyle. Their forelimbs are modified into flippers, and almost all have a prominent dorsal fin (though some, like the Beluga whale, lack it). The flippers and the dorsal fin help these mammals maintain their stability and steer through the water. 

Their bodies are covered in a thick, insulating layer called blubber, which can be up to 1.6 feet thick in larger species. Beneath this layer, the male genitals and mammary glands are deeply embedded.

Head

Baleen whales usually have a massive elongated head (about 40% of their body mass) due to their long, overhanging jaws. They have two big holes on the top of their heads called the blowholes, which contain the nostrils. The eyes are found on either side of the head, below the nostrils. Such positioning of the blowhole helps the body remain submerged even while the whale surfaces for air.

The back of their skull is shortened and deformed, and the nasal passages extend perpendicularly through it. A set of cranial bones connect the skull to the nasal passage. The teeth or baleen of these whales are fitted into the upper jaw on the maxillary bone.

In toothed whales, the connective tissue layer of the brain extends into a mass of adipose tissue called the melon located in the forehead. This melon is particularly prominent in sperm whales and is modified into the spermaceti organ (hence their name, sperm whale), which produces a waxy liquid responsible for producing sound. Unlike baleen whales, they have a single blowhole on the top of their heads.

Brain

Cetaceans generally have large brain masses, the largest of which is found in sperm whales (8,000 cm3 and 7.8 kg in mature males). Due to their evolution underwater, cetaceans have modified brain shapes, characterized by laterally extending brain folds around the insula. 

The neocortex of many cetaceans consists of elongated spindle neurons homologous to humans. These neurons are thus believed to be involved in social conduct, emotions, judgment, and general intelligence in cetaceans.

Skeleton

Unlike compact bones in terrestrial mammals, the cetacean skeleton is made of lighter and more elastic cortical bone. To ensure buoyancy underwater, some heavy bony elements are replaced by cartilage and fat. For example, the sternum is completely cartilaginous, and the last two to three pairs of ribs are disconnected and thus hang freely.

Depending on the species, cetaceans have forty to ninety-three vertebrae. The cervical spine comprises seven highly reduced or fused vertebrae in these mammals.

Fluke

At the rear end of the body is a tail that culminates in a horizontally attached cartilaginous fluke. This structure aids in propelling the animal underwater.

Teeth

Instead of teeth, baleen whales possess characteristic bristles made of keratin that help filter small invertebrates from the water. In contrast, toothed whales, as their name suggests, have bony teeth shaped like cones, spades, pegs, tusks, and beaks.

In female beaked whales, the teeth are hidden in the gums, whereas in males, there are two visible short tusk-like teeth. Male narwhals, on the other hand, have additional vestigial teeth apart from their prominent tusks.

Organ System

Circulatory

Cetaceans have a closed circulatory system characterized by four-chambered hearts (two auricles and two ventricles) that circulate blood throughout the body.

Respiratory

These mammals respire through the lungs. They are deliberate breathers, meaning they must be awake to inhale and exhale. 

Cetaceans undergo a unique mechanism of exhalation. The stale air breathed out of the lungs is generally warmer than the surrounding air, and thus, when it is exhaled, it condenses into a cloud of steam upon contact with the external air. This cloud is often called a spout and may be released in different shapes, angles, and heights.

Digestive and Excretory

The stomach has three parts: the most anterior region, formed by a muscular gland and forestomach, followed by the main stomach and the pylorus. To ease digestion, the stomach is associated with a few glands that secrete digestive juices. Accessory glands, like the liver and gall bladder, also facilitate the digestive process.

Cetaceans release nitrogenous waste from their bodies using their long, flattened kidneys. Since these animals consume saline water, and the salt concentration in their blood is maintained lower than that in seawater, they excrete excess salt through the urine, often stored in urinary bladders before release.

Sensory

Vision

Cetaceans have eyes on either side of their heads, which provides good binocular vision. Their nearly spherical lenses enhance focus in low-light conditions underwater. Tear glands that produce greasy tears lubricate and protect their eyes from the salt in the water.

Auditory

Cetaceans have exceptional auditory senses. Although these mammals have no external ear or pinna, they possess a deep, narrow ear canal leading to the characteristic three compact ear ossicles or bones. These semicircular bones are small and slightly different from those in most land mammals.

A compact bony structure called the auditory bulla is found in a cavity within the ear. In toothed whales, this cavity comprises dense foam that cushions the bulla, which, in turn, is connected to the skull by ligaments. This arrangement prevents the sounds transmitted into the ear from seeping into the bones of the skull.

Olfactory

Although cetaceans have a poor sense of olfaction (and taste), some baleen whales can smell using their reduced but functional olfactory system.

Taxonomy

Scientific evidence suggests that cetaceans are phylogenetically closely related to artiodactyls (Order Artiodactyla). Hippos are their closest relatives, with similarities in posterior molars, the bony ring on the temporal bone (bulla), and the involucre (a skull component).

Both molecular and morphological evidence suggest that other artiodactyls are paraphyletic to cetaceans, although they are nested within Artiodactyla. They together form a clade, which is often called Cetartiodactyla.

Around 90 species of cetaceans have been classified under two parvorders: Odontoceti, or toothed whales, and Mysticeti, or baleen whales.

Evolution

Distribution and Habitat

While most cetaceans live in oceans, a few, like river dolphins, are found exclusively in brackish or freshwater. Marine species, such as the blue whale, humpbacked whale, and killer whale, have a worldwide distribution, while others are found in patches.

Diet

Cetaceans feed on a wide range of food, including plankton, shrimps, squid, octopus, prawns, fish, and small marine mammals. A few toothed whales, like orcas, feed on seals and other whales.

Behavior

Diving

Some cetacean groups, like rorquals, can stay underwater for up to 40 minutes, while others, like the bottlenose whales, can do so for two hours. Similarly, sperm whales dive between 60 to 90 minutes, up to depths of 3,000 meters (9,800 ft). The average diving range in cetaceans is 1,200 meters (3,900 ft).

Migration

Some cetaceans, like the gray whale, migrate to warm lagoons along Baja California during winter. Again, in summer, they travel 5,000–7,000 miles (8,000–11,300 km) along the coastline of Alaska to reach the feeding grounds of the Bering, Chukchi, and Beaufort Seas. The roundtrip covers a distance of almost 10,000 miles (16,000 km).

Most baleen whales migrate to the equatorial regions to give birth and, during this phase, completely depend on their stored fat for energy.

Sociality

Most cetaceans are extremely social and form groups or pods comprising ten to fifty individuals. During the mating season or surplus of resources, these groups often expand to a thousand individuals.

These pods are hierarchical, with the most powerful maintaining authority by forcefully pushing or biting the inferiors during stressful situations like food shortages. Some smaller cetaceans, like dolphins and porpoises, also play (sometimes with members of other species) by jumping in the air, somersaulting, surfing, or hitting each other’s fins.

Hunting

Killer whales and dolphins often set out in pods for hunting. Some cetaceans, like humpback whales and Bryde’s whales, engage in a unique hunting strategy called bubble netting.

Communication

Cetaceans communicate with their conspecifics using a variety of sounds, like groans, clicks, whistles, moans, and songs.

Echolocation

Toothed whales emit short bursts of high-frequency waves that are used to detect the presence of prey, conspecifics, or obstacles. Even in complete darkness, these cetaceans can chase fast-swimming prey or even sense their shape and size.

Whale Song

Some baleen whales emit characteristic high-pitched sounds to communicate with their pod members. While individuals may produce as many as 600 distinct sounds, the 52-hertz whale (an unidentified species) calls at an unusual frequency of 52 hertz.

Males of humpbacks, fin whales, and blue whales are found to produce a male-specific song to attract potential female mates and display their sexual fitness to them.

Decision-making

Some bottlenose dolphins and killer whales are often chosen as pod leaders based on the consensus of their group members. Similarly, certain groups of sperm whales are also involved in group decision-making.

Sleeping

Although cetaceans sleep, they cannot risk remaining unconscious for long. Thus, they exhibit unihemispheric slow-wave sleep (USWS), a mechanism in which one side of the brain is kept active to remain conscious even in sleep.

A 2008 study revealed that sperm whales sleep vertically under the water surface during the day and usually do not respond to passing objects unless in physical contact.

Lifespan

Compared to higher mammals, whales have remarkably high lifespans. The bowhead whale (Balaena mysticetus) lives as long as 200 years, with the oldest specimen being a 211-year-old male.

Reproduction and Life Cycle

Although most cetaceans mature between seven and ten years, those with relatively longer lifespans, like the sperm whale, mature at around twenty. In contrast, the La Plata dolphin takes only two years to mature sexually.

Reproduction in cetaceans is seasonal, usually during winter. The males become fertile at a time when females ovulate. While toothed whales are polygamous (bond with multiple partners), baleen whales are typically monogamous (bond with only one partner). Several toothed whales are sexually dimorphic, and the males differ considerably from the females. Such differences help in sexual selection by the females. As mammals, cetaceans undergo internal fertilization, in which the male inserts his penis into the female’s reproductive tract to release the sperm. 

Most cetaceans have a gestation period of 9 to 16 months (blue whales and porpoises gestate for 11 months). The embryo develops within the mother’s body, deriving nutrition from the temporary embryonic organ called the placenta.

Cetaceans usually give birth to one calf in a tail-first fashion (opposite of terrestrial mammals). When giving birth to twins, one typically does not survive because the mother can only produce enough milk for a single calf. Although relatively independent from birth, she carries the newborn to the water’s surface for its first breath. Since the calves have no lips, the mother splashes milk into the calf’s mouth. This milk is rich in fat (16 to 46%) and aids in the rapid growth of the newborns. 

In small cetaceans, the calves may continue to suckle for about four months, while in larger species, suckling may last for over a year.

Number of Chromosomes

All cetaceans have 22 pairs of chromosomes (2n = 44), except sperm and pygmy sperm whales, which have 21 pairs (2n = 42). They have four pairs of telocentric chromosomes (centromeres towards the tail), up to four pairs of subtelocentric, and four large submetacentric chromosomes. All the other chromosomes are small and metacentric (centromere in the middle).

Predators

Although cetaceans have few natural predators, sharks often kill them. Some species, like killer whales and bottlenose dolphins, also attack and kill juvenile cetaceans.

Adaptations

Ecological Importance

Whales play a vital role in maintaining aquatic biodiversity, even after they die and become carcasses.

Over the first few years, aquatic animals, like sharks and hagfish, scavenge on the flesh of the dead whale. Then, opportunistic feeders, like crustaceans and polychaetes, colonize the bones for a few years. Finally, sulfophilic bacteria act on these bones, releasing hydrogen sulfide and facilitating the growth of chemoautotrophic organisms, like mussels, clams, limpets, and sea snails. Thus, whales support a diverse community of marine organisms of approximately 400 species.

Interesting Facts

References Article last updated on 9th July 2024
Comments

Leave a Reply

Your email address will not be published. Required fields are marked *