CETACEANS: CHARACTERISTICS, NERVOUS SYSTEM, CIRCULATORY - BIOLOGY - 2025

The cetaceans are placental mammals that live in the water. They are made up of 80 species, mostly marine, except for some dolphins that live in fresh water.

This group of carnivorous animals includes dolphins, porpoises, and whales. Among these is the blue whale, the largest animal on earth, weighing 190 tons and measuring between 24 and 30 meters in length.

The ancestor of cetaceans lived in the Eocene, around 50 million years ago. It is the Pakicetus, a primitive artiodactyl whose common element with cetaceans is the structure of its inner ear.

The evolution of this species continued, being the Basilosaurus the first aquatic cetacean, which had numerous large sharp teeth, which allowed it to grind its food.

Cetaceans have suffered from the relentless attack of humans, who hunt them to market their meat, fat and oil. This has resulted in many species being in danger of extinction, such as the blue whale and the sperm whale.

In addition, these animals also die from other causes related to man: the impact of their body against fishing boats, the damage they suffer from the gear used in snow crab fishing and climatic variations due to environmental pollution.

General characteristics

Sperm whale Physeter microcephalus. Taken and edited from: Gabriel Barathieu.

Skin

Its body, which is streamlined in shape, lacks fur; however, they do have some hair follicles on the lower jaw and muzzle. Their skin can be of black and white tones, going through the greyish ones. Underneath it is a thick layer of fat and oil.

Fins

Cetaceans have a dorsal fin, with the exception of those that live in the polar region, since this would prevent them from swimming under the ice.

The caudal fin or tail is formed by two lobes of connective tissue, it has a horizontal position and moves it from top to bottom, for its propulsion. The pectoral fins are supported by bones, giving the animal stability, as well as allowing it to have lateral movements.

Face

Its jaw and teeth form an elongated structure, originating in some species a bony structure similar to a beak, while in others it is arched. They lack an external ear, presenting only one ear hole on both sides of the head.

Organs of the respiratory system

Their respiration is pulmonary, so they need to surface to make the gas exchange. The nostrils are at the top of the head, constituting the spiracles. The opening of these is by the voluntary action of the muscles, therefore, the cetaceans decide when they will breathe.

The trachea is made up of cartilaginous rings. The lungs are not lobed and their size is similar to that of mammals.

Physiology d

One of the most important aspects of cetaceans is their diving physiology. These organisms are air breathers, so they must hold their breath during their long dives.

In addition to morphological adaptations, such as the displacement of the nostrils towards the dorsal part of the head to form the blowhole, and the adoption of muscles to open and close this blowhole, there are functional adaptations for diving.

One of these adaptations is the myoglobin content in skeletal muscles. Myoglobin is a muscle protein that can associate with, and thus store, oxygen. Myoglobin acts as the primary source of oxygen for the muscles during periods of apnea.

This protein is approximately 25 times more abundant in the musculature of cetaceans than in the musculature of terrestrial vertebrates. It is also abundant in seabirds. Additionally, the hemoglobin concentrations in their blood are higher than those of terrestrial vertebrates.

An anatomical-physiological adaptation is the presence of rete mirabile (admirable networks), which are masses of tissue that contain high concentrations of blood vessels and that can function as a storage center to increase oxygen reserves during the dive.

Additionally, the lungs of cetaceans have the ability to almost completely collapse during the dive. After the collapse they are able to recover. The function of this collapsed lung is to help avoid problems with the solubility of nitrogen in the air. Nitrogen in lung air could cause decompression syndrome when rising to the surface.

Thermal regulation

Cetaceans store large amounts of fat in the form of layers under the skin, whose function is to serve as a thermal insulator. Additionally, the rete mirabile of the dorsal and tail fins help to exchange body heat with the environment during swimming.

Taxonomy

Mysticetes

Known as baleen whales for having baleen in their upper jaw, with which they filter the water and obtain small fish for food. Sexually they are dysmorphic animals, presenting notable differences in the external physiognomy between the male and the female.

Although they can be very bulky and heavy marine animals, some species are capable of swimming at high speeds. It includes the superfamilies:

-Balaenoidea

Family: Balaenidae (glacial right whale).

Family Cetotheriidae (pygmy right whale).

-Balaenopteroidea

Family: Balaenopteridae (humpback whale).

Family: Eschrichtiidae (gray whale).

Toothachets

Beluga whale. Delphinapterus leucas. Taken and edited from: Greg Hume (Greg5030).

They are animals that can live in the sea or in fresh waters. They are characterized by the presence of conical teeth in their jaw and by their ability to communicate and perceive the environment where they are. In some species there are external morphological differences between the male and the female.

Their body is aerodynamic, allowing them to swim up to 20 knots. It includes the superfamilies:

-Delphinoidea

Family: Delphinidae (killer whale and crossed dolphin).

Family: Monodontidae (beluga and narwhal).

Family: Phocoenidae (porpoise)

-Physeteroidea

Family: Physeteridae (sperm whale)

Family: Kogiidae (dwarf sperm whale)

-Platanistoidea

Family: Platanistidae (Indus dolphin)

-Inioidea

Family: Iniidae (Amazon dolphin)

Family: Pontoporiidae (silver dolphin)

-Ziphyoid

Family: Ziphiidae (Peruvian beaked whale)

Nervous system

This is divided into two: the central nervous system, formed by the brain and spinal cord, and the peripheral nervous system, where the nerves extend outside the central nervous system, enervating the different limbs and organs of the body.

The cerebral cortex has a high number of convolutions. The spinal cord is cylindrical, it has a thickening in the cervical region, which corresponds to the area of ​​the pectoral fins

The senses

View

The eyes are flattened and the pupils allow them to see objects both in water and in the air. In some species, binocular vision, except in dolphins that can move independently.

Smell

In general, cetaceans have very little developed this sense. In the mysticetes there are olfactory nerves, but they lack an olfactory bulb. In odontocetes there are neither nerves nor bulb.

Taste

Receptors for sensations are all over the skin of the animal, but in cetaceans they are mainly located on the head, genital organs and pectoral fins.

In addition to these mechanoreceptors, some mysticetes have structures in their jaws and jaws called vibrissae, which also capture tactile stimuli.

Ear

This is the most developed sense in cetaceans, as they are able to identify the direction of sound they hear. This is thanks to the structure of the inner ear, where the bones that make it up are separated from the skull, which interferes with the reception of acoustic stimuli.

To have a greater hydrodynamics they do not have an ear. The odontocetes capture the sound waves by an oily substance that they have in the jaw, to later be transferred to the middle ear.

Circulatory system

It is made up of veins, arteries, and the heart, which has four chambers, 2 atria, and 2 ventricles. In addition, it has structures called rete mirabile or wonderful net, located in greater numbers on the dorsal and caudal fins.

Its circulation is divided into two: major and minor. In the latter, oxygen-depleted blood is pumped from the heart to the lungs, where it is oxygenated and returns to the heart again.

From there it is sent to the rest of the body (greater circulation) to transport oxygen to the different organs, returning again to the heart, with non-oxygenated blood.

The main problem in cetaceans is thermoregulation. The body tries to counteract this with a layer of fat located under the epidermis, reducing the external appendages and developing a countercurrent circulation.

In this type of blood exchange the blood flows in opposite directions, where the rete mirabile acts to facilitate heat exchange. "Hot" blood circulates through the arteries, which comes from inside the body, and reaches the wonder network, where the "cold" blood flows in the opposite direction, cooled by the external water.

Digestive system

The esophagus is a long, thick-walled tube. The goblet cells that are inside secrete mucus, a lubricating substance that facilitates the passage of food through that organ.

The stomach is divided into three chambers: anterior, middle, and posterior. The anterior stomach is a strong muscle that contains bones and tiny stones that break down food. It also has anaerobic bacteria that ferment food, assisting in the digestive process.

Digestion continues in the middle and posterior chambers, where enzymes and specialized cells are found to facilitate this process.

Cetaceans do not have an appendix, their function being replaced by the anal tonsils, a group of lymphatic organs. The liver can have two or three lobes and do not have a gallbladder. Your pancreas is elongated and joins the intestine through the pancreatic duct.

Teeth and beards

Some cetaceans have teeth, like the sperm whale, while others have bales on their upper jaw, like whales.

The teeth are all the same size (homodont) and permanent (monofiodont), varying, depending on the species, their shape, quantity and size. Dolphins have conical teeth, whereas in porpoises they are flat.

The barbs are used as a filter to capture small animals. They are shaped like filaments and are made of keratin. They grow from the upper jaw, being eroded by the tongue and prey.

Reproductive system

The vagina is elongated and is, next to the anal opening, inside a genital pocket, located near the vagina. The mammary glands are also in that pocket, forming the so-called mammary grooves.

The ovaries are located in the abdominal cavity. In the female dolphin, the left ovary is more developed, while in the mysticetes both function.

The testicles and penis are inside the abdominal cavity, near the kidneys. The erection of the penis is due to the muscles that form it, very different from the rest of mammals, which occurs thanks to the vasodilation of the blood vessels of the corpus cavernosum.

Its reproduction is internal, as in placental mammals. Copulation occurs when the male and female contact their abdominal area, the penis is stretched and is inserted by the male into the female's vagina.

Once the ovum is fertilized, the placenta will form, responsible for feeding and providing oxygen to the fetus. The duration of gestation is more or less than a year, although in some whales it could culminate at 18 months. At delivery, the fetus leaves the tail, contrary to what happens in most mammals.

Habitat

Cetaceans are aquatic animals, most of them are marine that usually inhabit coasts or the open sea. Others inhabit rivers and lakes in Asia, South America, and North America.

While some marine species, such as the blue whale and the killer whale, can be found in almost every ocean, others are found locally, such as Hector's dolphin, whose habitat is the coastal waters of New Zealand.

Bryde's whale lives in specific latitudes, which are often tropical or subtropical waters. Several groups of cetaceans live only in one body of water, such is the case of the clock dolphin, which does so in the Southern Ocean.

There are species where the feeding and reproduction area are different, so they are forced to emigrate. This is the case of the humpback whale, which lives in the polar region during the summer, migrating to the tropics in the winter to reproduce.

Feeding

Cetaceans are carnivores and taking into consideration that they are divided into two groups, those with teeth and those with beards, their diet will be related to this characteristic.

Toothed species use their teeth to capture their food, which is usually large prey such as fish, squid, or other marine mammals.

Baleen whales take in a large amount of water, which they filter for small prey, plankton, krill, and a variety of invertebrate species. The food is trapped in the baleen, being removed by the whale with its tongue and then ingested.

Feeding methods

Aground

Used by some dolphins and killer whales, who carry their prey to land to capture it.

Bubble cloud

It consists in that the animal, when locating a school of fish, releases a curtain of bubbles, to push the prey towards the surface, in order to be able to capture it. This technique is used by humpback whales.

Quick hit

Used by humpback whales and refers to the blow they make with their tail against the surface of the water, which concentrates the prey in front of the animal. Then the whale swims through the area, capturing its food.

Fish stroke

The bottlenose dolphin, using its nose, strikes the prey to stun it and capture it.

Communication

Most species of cetaceans are gregarious, that is, they tend to live in groups. For example, orcas have been singled out among the groups of mammals that form the most cohesive groups. This gregarious behavior is more marked in odontocetes.

In mysticetos, the numerous and / or permanent groupings are more strange. In some species, associations are formed only during the mating, breeding season, or also temporary associations for hunting purposes.

Communication is essential to be able to maintain some level of group cohesion. In animals, communication can be of various types; through chemical (olfactory), visual, tactile or auditory messengers.

Chemistry

The ability to communicate through chemical messengers is common and important within land mammals. However, in aquatic environments this type of communication is rare. Cetaceans are microsmatic, or they can even become completely anosmatic, that is, unable to smell.

Smell and the anatomy of the olfactory organ are not suitable for communication in an aqueous medium. Cetaceans, like other marine mammals, must close their nasal openings while in the water, which makes it difficult or impossible to smell.

Due to this, this type of communication is not very developed in cetaceans, however, it has been suggested that belugas release pheromones in stressful situations. Some researchers also believe that dolphin feces and urine could contain these types of chemical messengers.

The perception of chemical stimuli would be more related to taste than smell. The presence of taste buds is documented for cetaceans. Some studies have shown that bottlenose dolphins are able to differentiate solutions with different types of flavors.

Visual

In cetaceans, visual communication is a short-range alternative for the exchange of information. Cetaceans show behavior patterns that can be associated with intraspecific communication mechanisms.

Visual communications can be simple, such as coloring patterns, body postures, or parts of the body that exhibit sexual dimorphism. They can also be more elaborate, through sequences of movements.

Among the simple signals, color patterns appear to be more important in smaller cetaceans. These coloration patterns are very evident in dolphins and can be used for species recognition, as well as individual and social recognition.

Sexually dimorphic body signs and features vary between species. These include, for example, the presence of protruding teeth in the upper jaw of the males of some species of toothed whales, or the forward-sloping dorsal fin of male spinner dolphins.

The most elaborate behaviors include threatening gestures by opening the mouth, jumping out of the water, as well as adopting different body postures. Cetaceans can use the latter method to communicate with individuals of the same species as well as with other species.

Body postures and behavior changes can also be used as cues for group actions.

Tactile

This type of communication is important in cetaceans; Among the signals used are touches and caresses, for this they can use different parts of the body, such as snouts or fins.

These signals are used frequently during sexual interactions. They can also be used in mother-child communications, as well as in other social interactions.

They can also be aggressive signals, such as biting and pushing. The intensity of the signal, its frequency, the emitter, the place it attacks, vary with the information to be emitted.

Captive odontocetes are very receptive to body contact. Coaches use gentle strokes and touches to help reinforce learning in training.

Acoustics

This is the most important type of communication among cetaceans, due to the ease of transmission of sound in the water. This communication can be vocal or non-vocal.

Non-vocal acoustic communication

This type of communication can be achieved by hitting the surface of the water with the fins or the tail, also making sounds with the teeth or breathing, emitting bubbles, even jumping out of the water.

The jumps out of the water produce a sound that can be reached for several kilometers and can have different functions, such as helping to maintain acoustic contact, they can also help to create sound barriers to disorient their prey.

Spinner dolphins generate noise that travels in many directions and at different distances. Its main function seems to be to maintain acoustic contact with its peers, since these noises increase during night hours, when visual contact is more difficult.

Tonina, Inia geoffrensis. Taken and edited from: Oceancetaceen. The signs of threat or danger are often achieved by hitting the water on many occasions with the tail (odontocetes), or with the pectoral fins (mysticetes). In the latter case, the signal does not always have a dangerous connotation and may sometimes serve as invitations to socialize.

Vocal communication

The vocal sounds of the mysticetes and odontocetes are very different from each other. These sounds, in the former, have several functions, including maintaining long-range contacts, sexual claims, threats, and greetings.

There are three forms of sounds among the mysticetes; low-frequency moans, thumps and squeaks, and whistles. Additionally, humpback whales are responsible for the well-known "whale songs".

Humpback whale songs are made by male whales. These songs are very long, and can reach up to half an hour. The songs contain elements that repeat periodically, vary by geographic area, and change annually.

Only the males sing and during the same time they all sing the same song; they generally only sing outside the breeding season. The song is probably a love claim pointing out the singer's health and general condition, as information for the possible partner.

The odontocetes, for their part, produce two types of signals, pulsed sounds and narrow-band sounds. Pulsatiles are known as clicks and are involved in echolocation. Narrowband sounds are known as whistles and their primary function appears to be communication.

Many species of odontocetes, however, do not whistle. Some species of odontocetes produce stereotyped calls. These calls are issued by particular members of the population and are called dialects by researchers.

The dialects are shared by "acoustic clans" within the population. Additionally, different clans can exist in the same population. For example, in the South Pacific whale population of the Physter macrocephalus species, there are at least six acoustic clans.

References

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