Cetacea

The cetaceans (Cetacea) are an infraorder of placental mammals fully adapted to aquatic life. The name "cetacean" derives from the Greek κῆτος, kētos, which means "whale" or "sea monster" and was coined by Aristotle to refer to aquatic animals endowed with lung respiration. They have a fusiform body, similar to that of fish, which makes them more hydrodynamic. The forelegs have turned into flippers, while the hindlegs have disappeared as such, although some vestigial bones remain, not attached to the pelvis and hidden within the body. The caudal fin is horizontal and divided into two lobes. They are generally hairless and have a thick layer of fat that serves as thermal insulation. The cetacean clade contains about eighty species, almost all marine, except for 5 species of freshwater dolphins. Living cetaceans are subdivided into two orders, the mysticetes and the toothed cetaceans. A third parvorden, the Archaeocetes, only contains extinct species. Until recently cetaceans were classified as a separate order, but new current cladistic classifications have included Cetacea as an infraorder of the order Artiodactyla and in the suborder Whippomorpha or Cetacondonta together with the hippos, their closest relatives, and some extinct clades, since they both shared ancestors who lived during the Eocene and Paleocene. The order Artiodactyla including cetaceans is sometimes referred to as Cetartiodactyla. Among the mysticetes are the animals commonly called whales, the largest in the world; Specifically, the blue whale is the largest animal that has ever existed on Earth, even larger than the dinosaurs. On the other hand, among the toothed whales are dolphins and killer whales, some of whose species can be raised and trained in dolphinariums. The branch of biology that deals with the study of these animals is cetology.

Evolution and taxonomy

The traditional theory of cetacean evolution postulated that they derived from the mesonychids, a group of hoofed, wolf-like carnivorous ungulates close to artiodactyls. These animals had triangular teeth similar to those of fossil cetaceans, which is why scientists long believed that whales and dolphins were derived from this group.

Starting in the early 1990s, molecular analyzes of a large number of proteins and DNA sequences indicated that cetaceans should be included in the order Artiodactyls, with Hippopotamids being very close phylogenetically. Thus, the creation of the clade of cetartiodactyls, which includes both artiodactyls and cetaceans, was proposed. However, shortly thereafter a consensus began to emerge that redefining Artiodactyla to include cetaceans was preferable to create a new clade., and the use of Artiodactyla was recovered, this time with the same meaning as Cetartiodactyla ("traditional artiodactyls" + cetaceans). Thus, according to the most widespread point of view, cetaceans are part of the group of artiodactyls, currently considered monophyletic.

A 2001 study confirmed, based on molecular data, that cetaceans are not derived from mesonychids, but belong to the artiodactyl group. Some anatomical similarities that archaeocetes (ancient cetaceans) share with artiodactyls is morphology of the posterior molars and the bony ring in the temporal bone (bulla) and involucre, a feature of the skull previously associated only with artiodactyls, as well as the special construction of the talus (ankle bone) with a double-sided articular surface. roller, which was thought to be exclusive to artiodactyls, but was found in archaeocetes and transitional cetaceans. Mesonychids, another type of ungulate, did not show these anatomical similarities and therefore it was concluded that they were not direct ancestors of cetaceans.

The first ancestor of cetaceans is considered to be Pakicetus, a primitive artiodactyl that lived in the Lower Eocene, approximately 50 million years ago. This animal retained some aspects of early artiodactyls, such as the presence of triangular teeth that modern artiodactyls have lost. The link between Pakicetus and cetaceans is the structure of the ear bones. Furthermore, the teeth of Pakicetus are reminiscent of those of fossil whales.

Ambulocetus was the first ancestor of cetaceans to lead an amphibious life and had legs more adapted to swimming than moving on the ground. Instead, one of the first cetaceans to be fully aquatic was the basilosaurus, which lived approximately 38 million years ago. The first mysticetes, the ketoterids, appeared in the Oligocene, as did the first odontocetes, the kentriodontids.

Phylogeny with artiodactyls

The following cladogram shows the phylogenetic relationships of cetaceans to terrestrial artiodactyls based on genetic and molecular analyses.

Taxonomy

The clade of living cetaceans is subdivided into two orders:

• Mysticetos endowed with beards. Beards are structures present in the upper jaw that serve as a chryba and are composed of keratin. This structure allows to filter the plankton of the water.

They include the following families:

• • Balaenidae
  • Balaenopteridae
  • Eschrichtiidae
  • Neobalaenidae

• odontocetes endowed with teeth. They feed mainly on fish and/or squids. These cetaceans have an exceptional capacity in the perception of the environment around them through ecolocalization. They include the following families:

• • Delphinidae
  • Monodontidae
  • Phocoenidae
  • Physeteridae
  • Pontoporiidae
  • Kogiidae
  • Platanistidae
  • Iniidae
  • Ziphiidae

Phylogeny

The following cladogram shows the phylogenetic relationships of cetaceans:

Classification

The eighty species of cetaceans are divided into two parvordens and 12 families, to which must be added the extinct Parvorden Archaeoceti, adding five families to the total number of species.

Suborden	Family	Description
Archaeoceti † (primitive cetaceans)
	Pakicetidae †	Earth quadruple ancestors
	Ambulocetidae †	Semiaquatic quadruple ancestors
	Remingtonocetidae †	Semiaquatic quadruple ancestors
	Protocetidae †	Aquatic ancestors with seal appearance
	Basilosauridae †	Fully aquatic ancestors
Mysticeti (bearded crackers)
	Balaenidae	Greenland Whale and Frank Whale
	Balaenopteridae	Rorcuales yubarta
	Eschrichtiidae	Grey whale
	Neobalaenidae	Pygmy Whale
Odontoceti (teething)
	Delphinidae	Ocean dolphins and orcas
	Iniidae, Platanistidae and Pontoporiidae	Dolphins of the Amazon, Silver and Yangtse rivers
	Phocoenidae	Marsopas
	Physeteridae	Common Cachalote
	Kogiidae	Dwarf and Pygmy
	Ziphiidae	Zifios
	Monodontidae	Beluga and narval

Morphology

Having evolved from artiodactyl terrestrial ancestors, cetaceans have had to develop anatomical and physiological adaptations in order to have a fully aquatic life:

Description of the morphological characters of a myticetos (above) and a dentist (below).

• the body is fusiform and has taken a hydrodynamic form similar to that of a fish;
• has developed a dorsal fin on the back, formed by connective tissue;
• the previous legs have become pectoral fins and have taken shape of rowing;
• the end of the tail is flat and consists of two lobes;
• the rear legs are absent and there are only small bones hidden inside the body;
• they present a breath at the top of the head;
• hairs disappear completely after the first months of life;
• Atrial pavilions are absent;
• external genitals are hidden inside pockets.

Body shape and dimensions

The shape of the body of cetaceans is very reminiscent of that of fish. In fact, by convergent evolution they have developed a hydrodynamic spindle shape that allows them to move with agility in the aquatic environment, reducing friction with the water. Mysticetes have a more compact body than toothed whales, which are capable of swimming at a higher speed.

The cetacean infraorder includes some of the largest animals that have ever lived on Earth. Especially among the mysticetes, the body dimensions are considerable; the blue whale can reach 30 meters in length and is considered the largest animal of all time. Among the toothed whales, it is the sperm whale that reaches the largest size, reaching a length of about 20 meters in males. On the other hand, the smallest cetacean of all is the cochito, a porpoise that can reach a length of approximately 140 centimeters. The smallest mysticetes is the pygmy right whale, which reaches about 6 meters.

Table 1 summarizes the maximum length and weight attained by some cetaceans.

Skin

As in all mammals, cetacean skin is made up of epidermis, dermis, and hypodermis.

The epidermis is made up of a multilayered squamous epithelium, it is between ten and twenty times thicker than that of terrestrial mammals and its outermost layer is renewed about twelve times a day.

The dermis is made up of fibrous connective tissue and lacks hair follicles and sebaceous glands.

The hypodermis forms the adipose layer and is made up of loose connective tissue rich in adipocytes and collagen fibers. Its function is to prevent heat dispersion and serve as a reserve substance.

The surface of the skin of some toothed whales bears a few 'skin ridges', often visible even to the naked eye and distributed over the entire body, except for the head and, in some species, the ventral region. Its function is still unclear; it is believed that they could be involved in the reception of tactile stimuli, that they could have a hydrodynamic function, or both.

The skin of the head and pectoral fins of many whales is colonized by a series of skin parasites, mainly cyamids, known as whale lice and thoracic lice of the genus Coronula, Cryptolepas, Conchoderma, Xenobalanus and Tubicinella. While "lice" can interfere with whale sensory receptors and feed on their skin, thoracic lice do not appear to cause harm to cetaceans.

Fins

Dorsal fin

Dorsal fin of an orca (Orcinus orca).

Flow fin of a cachalote.

Almost all cetaceans have a dorsal fin on their backs made of connective tissue. Its function is to give stability to swimming, preventing the animal from tipping over during rapid lateral movements. This fin is absent in animals that live in the polar regions, such as narwhals, belugas, and bowhead whales, since it would not allow them to swim agilely under the ice.

The dorsal fin has different shapes and sizes in different cetacean species: it can be sickle-shaped, triangular or rounded. This feature is useful for identifying species. In addition, since the fin is often covered in scratches, cuts, and scars, it is used by researchers for photo-identification, a technique that makes it possible to recognize individual specimens of a species through photographs of anatomical features.

Caudal fin

The tail of cetaceans is made up of two lobes of connective tissue that form the caudal fin. Unlike that of fish, in cetaceans this fin is horizontal and moves up and down. This feature makes it possible to distinguish a cetacean from a fish at first glance. The tail serves as a means of propulsion through its vertical movement.

In this regard, sizes also vary between different species, so this feature can be used to identify species, especially large ones.

Pectoral fins

Pectoral fin of a yubarta.

In cetaceans, the forelegs have evolved into pectoral fins. Unlike the dorsal and caudal fins, the pectoral fins are supported by bones homologous to those of the forelegs of land mammals. The function of these fins is to provide stability when swimming and allow lateral movements.

The pectoral fins also vary in shape and size, and the swimming ability of different species depends on this feature. Species with small fins relative to body surface area, such as blue whales, specialize in swimming slowly in the open sea, while species with larger fins are capable of faster swimming and more agile maneuvering. The largest fins belong to the humpback, in which they reach a length equal to about a third of the animal's length. The large size of these fins help the animal perform acrobatic maneuvers to capture prey.

Hind legs

All cetacean species lack hind legs, with only small vestigial bones remaining inside the body that are not attached to the vertebral column. During embryonic development, however, all cetaceans present buds of these legs, the subsequent regression of which is due to as yet unknown causes.

In 2006, Japanese fishermen caught a bottlenose dolphin with a pair of fins located in the caudal region. The researchers think they represent further evidence that cetaceans evolved from terrestrial ancestors and that evolution had wiped out the hind legs. In this bottlenose dolphin, a mutation brought to the surface a character lost millions of years ago.

Face

The mandible and maxilla are elongated and form a beak-like structure, called a snout, which is quite visible in dolphins, while in mysticetes it is practically invisible.

In toothed whales, especially beaked whales, the snout is made up of rather compact bones. It is believed that this characteristic serves to increase the strength of the snout and prevent fractures during fights between males over females. Instead, according to other researchers, without the high mineralization of the bones, the snout is a rather fragile structure than it has a role in the reception of sound waves and is therefore important in echolocation.

In mysticetes, the snout is arched to allow the elongation of the wattles, which are attached directly to the jaw bone. The greatest arching occurs in balaeids, in which the baleen is exceptionally long.

Physiology

Nervous system

Cetaceans have a highly developed brain, with a size comparable to that of anthropomorphic primates, including humans.

The cerebral cortex of the cetacean brain has a high number of convolutions, especially in the case of toothed cetes, which have a greater number of convolutions than the human brain, although the thickness of the cortex is less.

There is a lively debate about how intelligent cetaceans in general and dolphins in particular are. According to some, these animals would be potentially capable of communicating with a language, while according to others, the size of the brain could be due to the presence of a very well-developed primary acoustic area. However, the great cognitive abilities of these animals are evident. These animals. For example, bottlenose dolphins are the only animals, along with humans and anthropomorphic apes, that can recognize themselves in a mirror, and demonstrate arithmetic abilities.

The spinal cord is cylindrical in shape and its length depends mainly on body size. The ratio between the body length and that of the medulla is approximately equal to that of humans. In the cervical region, in correspondence with the pectoral fins, there is a thickening of the medulla, while in the lumbar region the thickening is less. evident due to lack of hind limbs.

There are between 40 and 44 spinal nerves, in which the posterior roots are less developed than the anterior ones. This characteristic is due to the greater development of the ventral musculature of cetaceans compared to the dorsal musculature and the scarce presence of peripheral sensory receptors.

Sensory Organs

Cetacean eyes are flattened and the lens is spherical. The pupil of these animals allows them to see both underwater and in the air, despite the significant difference in density between the two environments. The eyes are located laterally on the head and while, in some cetaceans, vision it is binocular, in dolphins the eyes move independently of each other, although in bottlenose dolphins the presence of some overlapping areas has been demonstrated. Behind the retina there is a highly vascularized area, the tapetum lucidum, which, thanks to a layer of reflective cells, increases the amount of light that reaches the retina. Since the retina has as many cones as there are rods, it has been suggested that cetaceans can distinguish colours. However, it is still a highly controversial issue. Cones only represent 1% of the photoreceptors present in the eye and lack sensitivity to low wavelengths, so it is believed that these animals can only distinguish colors at conditions of good lighting. Some authors argue that the presence of both types of photoreceptors helps animals to better identify objects. Although lacrimal glands are absent, many cetaceans have glands in the conjunctiva that secrete a fluid that serves to protect the cornea from the salt present in seawater.

The sense of smell is greatly reduced in mysticetes and completely absent in toothed whales. Olfactory nerves are present in mysticetes, but the olfactory bulb is missing from the frontal lobe of the telencephalon, which is only present in the fetal stage. In toothed whales, on the other hand, there are neither nerves nor a bulb.

Cetaceans have taste buds on their tongues, although in reduced numbers compared to land mammals. Cetaceans are able to recognize the taste of various substances. Bottlenose dolphins have a sensitivity to sour taste about seven times that of humans, while sensitivity to sweet and salty tastes is about ten times greater. High sensitivity to salty tastes could help animals to orient themselves, thanks to the variations in salinity of marine waters.

The sense of touch is mediated by mechanoreceptors, which cover the entire body surface, but are located primarily on the head and near the pectoral fins and genitalia. In addition to the mechanoreceptors, many mysticetes have very subtle vibrissae on their jaws and mandibles, which also serve to receive tactile stimuli. In toothed whales, only vestigial hair follicles remain of these structures. The only thing that has well-developed vibrissae is the tucuxi (Sotalia fluviatilis), but in this species they are receptors that perceive the direction of the water current.

Hearing is the most highly developed sense of cetaceans, which are able to determine underwater from which direction sound is coming, an ability land mammals do not have. This is possible because the inner ear bones of these animals are well separated from the rest of the skull, which could interfere with the reception of acoustic stimuli. This separation is even more evident in toothed cetes than mysticetes. For greater hydrodynamics, cetaceans lack pinnae, while the middle ear and inner ear are similar in structure to those of other mammals. In toothed whales, sound waves are received by an oily substance in the jaw, from where they are transferred to the middle ear, where they reach the eardrum. In mysticetes, the transmission of sound through the mandible has not been demonstrated and probably the reception is done directly through the auditory canal.

Respiratory system

Breathing of a blue rorcual (Balaenoptera musculus).

Cetaceans, like all mammals, breathe air through lungs. For this reason, they need to regularly rise to the sea surface to expel CO2 and inspire O2.

The nostrils have moved to the top of the head and form the spiracles. This solution allows cetaceans to remain almost completely immersed during respiration. While in mysticetes the spiracle consists of two holes, in toothed whales there is only one. The opening of the blowhole is produced by the action of voluntary muscles, which is why, unlike other mammals, cetaceans have to decide when to breathe.

The expired air, heated by the lungs, when it comes into contact with the outside, condenses and forms lightning, visible from a great distance. As the shape, direction and height of the ray vary between species, cetaceans can be identified from a distance using this feature.

The trachea is made up of a series of cartilaginous rings attached to each other. In mysticetes, the rings are open and collapsible, unlike toothed whales, in which the rings are closed.

The lungs are sac-shaped, not lobed, and contrary to what one might think, are no larger than those of other mammals. The right lung is usually larger and longer than the left, in order to accommodate the heart inside the rib cage. The lung volume is lower than that of terrestrial mammals, in order to avoid the formation of gas emboli during ascent after deep dives. In fact, at great depths, the pressure presses the internal organs against the diaphragm, which causes the lungs, since they have a low volume, to empty almost completely. The alveoli are highly vascularized and allow them to absorb almost all the oxygen present. in the breathed air. The bottlenose dolphin is capable of absorbing approximately 90% of the oxygen present in the lungs, while humans only absorb 20%. Cetacean lungs have the ability to almost completely collapse with increasing depth and depth. Most of them complete collapse occurs at a depth of about a hundred meters. This characteristic allows cetaceans to avoid dangerous accumulations of nitrogen inside the blood, which could cause decompression syndrome or nitrogen narcosis, diseases well known by the divers. With increasing pressure, the solubility of nitrogen present in the inspired air also increases; during the ascent, with the decrease in pressure, the nitrogen returns to the gaseous form and could form dangerous bubbles in the blood. The collapse of the lungs avoids this problem, sending the air into the upper airways (bronchi and trachea), where it does not come into contact with the blood.

Cetaceans are able to stay underwater without breathing for much longer periods of time than other mammals. Some species, such as the sperm whale, can remain underwater for up to a little over two hours on a single breath of air. The following table compares the immersion times and the maximum depths reached by some cetaceans.

Circulatory system

The circulatory system of cetaceans does not differ much from that of land mammals. Deoxygenated blood is pumped from the heart into the pulmonary circulation via the pulmonary artery, which reaches the lungs. There, the blood is oxygenated and, through the pulmonary vein, returns to the heart, where it is sent to the systemic circulation, to then return to the heart through the vena cava.

In land mammals, blood reaches the brain through the carotids, while in cetaceans it is the anterior spinal artery that reaches the head and supplies the brain.

As in all mammals, the cetacean heart has four chambers, two atria and two ventricles. During the dive, the heart goes into bradycardia, that is, the heart rate is reduced. Bradycardia often begins shortly after the animal submerges and shortly before it resurfaces, the rate increases (tachycardia). In the bottlenose dolphin, the heart rate at the surface is about 110 bpm and drops to about 37 bpm during a dive of about four minutes; in killer whales, the rate at the surface is about 60 bpm, decreasing to 30 bpm after 15 seconds of immersion. Bradycardia is accompanied by reduced blood flow to the gut, muscles, and skin, which allows blood pressure remains almost constant and vital organs, such as the brain, kidneys, liver and heart, are well irrigated. To ensure a good supply of oxygen during dives, the muscles have myoglobin concentrations between three and ten times higher than those of terrestrial mammals. The distribution of myoglobin is not uniform in all the muscles of the organism; it is most abundant in the dorsal muscles near the tail and in the part of the muscles that is in closest contact with the vertebrae. In addition, cetaceans capable of deep diving have higher myoglobin concentrations than cetaceans that live near the coast and dive to less depth and for shorter durations.

Cetacean blood has a high concentration of hemoglobin, which ensures efficient oxygen transport during dives. Land mammals have blood hemoglobin values of between 11 and 14 g/hl, while cetaceans capable of making deep dives have values between 21 and 25 g/hl.

Skeleton

Blue whale skeleton.

Because they are not intended to support the weight of the body, the bones of cetaceans are relatively light and spongy. Inside them there is a high concentration of fats that contribute to buoyancy.

The vertebral column is made up of four regions: cervical, thoracic, lumbar, and caudal. Since the pelvic girdle is absent, there is no sacral region. The neck vertebrae, which are always set in number, are fused in most cetaceans, providing stability during swimming in exchange for less flexibility. In the bottlenose dolphin only the first two vertebrae are fused, whereas in a common beaked whale the first four are fused. The thoracic vertebrae vary in number between different species and even between individuals of the same species; the vertebrae of the lumbar region are much more numerous than in terrestrial mammals. The Dall's porpoise has 29-30 lumbar vertebrae, making it the cetacean that has the most, while the dwarf sperm whale, with only two vertebrae, is the species that has the most. fewer have; the number of vertebrae in the caudal region also varies between species. The pygmy right whale has 13, while the beaked whale has 49. The total number of vertebrae varies between 41 and 98. The thoracolumbar region is rather rigid due to the presence of subdermal connective tissue, while the vertebral column is more flexible than in the tail and head, allowing the dorsoventral movements responsible for swimming.

The ribcage is made up of a variable number of ribs and is very flexible, to allow the lungs to collapse during deep dives and prevent nitrogen buildup in the blood.

Orca circus.

The skull is telescopic and asymmetrical; the maxillary bones and jawbones are previously elongated to form the snout and the odontocetes later extend to welcome the melon, a mass of adipose tissue that is believed to have a role in echolocation; also in the odontocetes, the bones of the right side of the skull are always longer than those of the left band. It is a consequence that the right band has specialized in the production of sound and the left in the breathing.

Having derived from that of land mammals, the cetacean hind leg is made up of the same bones: the humerus, radius, and ulna. However, these bones are shorter and flatter than in land mammals, and the ulna and radius are longer than the humerus. All cetaceans exhibit some degree of hyperphalangia, especially in the middle toes. The greatest number of phalanges occurs in the pilot whales, which have between 3 and 4 on the first finger, between 9 and 14 on the second and between 9 and 11 on the third.

Digestive system

The extraordinary length of the cetacean digestive tract may be due to the large size of these animals or to the fact that it helps them maintain water balance. There is no correlation between the length of the digestive system and the type of feed to digest.

The esophagus is a long, thick-walled tubular structure, where there are goblet cells that secrete a lubricating liquid, mucus, to facilitate the passage of food.

The stomach is subdivided into several chambers, as in ruminants. While ruminants have four, in cetaceans there are three:

• previous stomach or pre stomach, homologist to rumen, reticle and libretto of ruminants;
• main stomach, homologist to the draw;
• posterior or pilric stomach, equivalent to the pilric region of the garlic.

The anterior stomach, which has no glands, is endowed with robust musculature and contains bones and small stones for grinding food. Anaerobic bacteria are also present that help the animal digest food through fermentation, just like in the rumen. The mid-stomach contains gastric glands that secrete pepsin, lipase, and hydrochloric acid. Gastric digestion continues in the posterior stomach, the walls of which are rich in goblet cells. The food bolus passes through the pylorus to the duodenum, the first part of the intestine, where the absorption of nutritive substances takes place.

Cetaceans lack an appendage, and their filtering function may be performed by a complex of lymphatic organs known as anal tonsils. It is not yet clear whether these tonsils exist in all cetaceans, although they are highly developed in bottlenose dolphins.

The liver can be bilobed or trilobed, and there is no gallbladder. The pancreas has an elongated shape, is attached to the intestine via the pancreatic duct, and is generally larger in females.

Teeth and beards

Illustration of a whale's mouth. Remove the beards that extend from the jaw in the jaw.

The main characteristic that distinguishes Odontocetes from Mysticetes is the presence of teeth in the former and beards in the latter.

The shape, number and size of the teeth of odontocetes vary from one species to another, but all are characterized by being homodonts and monophyodonts. The number of teeth can also vary within the same species: in sperm whales In common dolphins and pilot whales, the number of teeth varies between 6 and 18. Some toothed whales, such as sperm whales, only have teeth in their jaws, while some beaked whales have a single tooth in each hemimandible. The shape of the teeth also varies between families. Dolphins, for example, have conical and arched teeth, while porpoises have flat teeth. Male narwhals are well known for having a left-hand spirally rotated tusk, possibly giving rise to the myth of the unicorn. Its function is not well known, but it is thought that males use it for intraspecific combat for females.

Beards are filamentous structures of keratin that extend from maxillary of mysticets. They grow from their basal part and are constantly eroded by the action of the tongue and by the abrasion caused by the prey. They are used for balenoptérides and ballenides as a filter to catch small fish, plankonic organisms and kril. The length of beards varies between species. The longest are those of the frank whales, in which they can reach a length of three meters, while the shortest are those of the gray whale, in which they do not exceed 50 centimeters.

Genital system and reproduction

Disposition of genital and breast clefts in the mular dolphin.

The genital apparatus of cetaceans does not differ much from the typical structure of terrestrial mammals, but they have adaptations that mainly concern the external genitalia and mammary glands, hidden inside "genital pockets" to favor the hydrodynamicity.

Mother and son of orca in the Strait.

In females, the ovaries are inside the abdominal cavity. In mysticetes they are oval, elongated and convoluted, while in toothed whales they are spherical and smooth. In mysticetes it is possible to determine the number of ovulations produced in the past by observing and counting the corpora albicantes of the scars that remain in the ovary after the degeneration of the corpus luteum if the ovum is not fertilized. In terrestrial mammals these scars are not persistent, but in mysticetes they are permanent. In toothed whales, the left ovary is more developed and functional than the right, while in mysticetes both ovaries are fully functional. The vagina is long and is hidden inside a genital pocket, which also hides the anal opening. The long, flattened mammary glands are also hidden inside pockets called "mammary slits" and located on the sides of the vagina.

Unlike terrestrial mammals, the testicles of males are not on the outside, but inside the abdominal cavity, near the kidneys. They are cylindrical in shape and, when viewed in cross section, are oval or circular. The weight of these organs in comparison with the total body weight is the highest in the entire animal kingdom. The sum of the weight of the two testicles of right whales can reach 900 kilograms, that is, 10% of the animal's body mass. The penis, when not erect, is completely hidden inside the genital pocket. The erection is due to the musculature and not to the vasodilation of the blood vessels of the cavernous bodies as in all other mammals. The penis of the largest mysticetes can reach a length of approximately three meters and a diameter of 30 cm. Copulation generally occurs belly to belly and is very rapid.

In toothed whales, gestation lasts between 7 and 17 months, and there is a correlation between the size of the animal and the length of gestation, the rate of growth of the fetus, and the weight of the calf at birth. Large toothed whales, such as killer whales and pilot whales, have a longer gestation period. Many mysticetes, despite their large size, have shorter gestation periods than many toothed whales. Thus, the pregnancy lasts between 10 and 13 months. This is explained by the long annual migrations that the mysticetes make from the areas where they feed to the areas where they reproduce, hardly compatible with a gestation period of more than one year..

Thermoregulation

Like all mammals, cetaceans are homeothermic animals and therefore need to maintain a constant body temperature.

Water has a high thermal conductivity, which translates into a heat transfer rate some 24 times higher than air, which together with the lack of hair means that these animals have had to develop effective mechanisms to ensure thermoregulation. An important role is played by the thick layer of fat and connective tissue present under the skin, at the level of the hypodermis. This fatty layer acts as a thermal insulator that prevents heat dispersion very efficiently. In some toothed whales, such as dolphins and porpoises, this layer of blubber shows seasonal variations in thickness. During the warm season, when the water temperature rises, the layer loses volume, while in cold seasons it gains. The amount and type of lipids present determine the insulating capacity of the layer. The shelter of the harbor porpoise is made up of 80% fat, while in the Atlantic spotted dolphin it is only 55%. This difference makes the adipose layer of the porpoise four times more insulating than that of the spotted dolphin.

In the dorsal fin and caudal fin lobes there is a dense surface network of capillaries called rete mirabile, which contributes significantly to thermoregulation by acting as a countercurrent heat exchanger. The heat passes from the arteries, which carry warmer blood from inside the body, to the venous capillaries of the surrounding network, where blood that has been cooled by external water flows in the opposite direction.

Osmoregulation

Living in a hyperosmotic environment, that is, with a concentration of ions higher than that of body fluids, cetaceans (except platanistoïdeus) must avoid dehydration due to the phenomenon of osmosis.

The main organs in charge of water balance are the kidneys. In cetaceans, these organs are made up of a large number of small lobes, called "renicles" and are similar to the kidneys of bears and otariids. Each renicle is composed of a medullary and a cortical region. Although the anatomy of a cetacean kidney may allow it to produce highly concentrated urine, it has been shown that this does not occur. The urine of these animals is only slightly more concentrated than that produced by most terrestrial mammals. It has been hypothesized that dehydration stimulates increased metabolic water production through fat oxidation and that excessive water is Expelled by the kidneys, diluting the urine.

Some dolphins, such as common and bottlenose dolphins, are capable of drinking seawater, but these are exceptions. In fact, cetaceans generally do not drink, but instead absorb the water present in the food or obtain it through the metabolic pathways of carbohydrate, protein and lipid degradation.

Longevity

Most cetaceans can live for 20-30 years, but some far exceed that age. A fin whale reached the age of 116 years, while a 1999 study describes how several specimens of bowhead whales reach and exceed 100 years, and in at least one case it reached 216.

Distribution and habitat

Cetaceans live in all the world's seas and oceans, and some species live in lakes and rivers in North America, South America, and Asia. Some species, such as the orca, are cosmopolitan, others have a wide geographical distribution, but are not present in all the world's waters, and others live in even more limited areas. This is the case, for example, of the California porpoise, endemic to the northern part of the Gulf of California.

Some cetaceans live near the coast, in what is called the neritic zone, others live in the open sea, in the oceanic zone, and some species, such as the bottlenose dolphin, have different populations that live in one zone or the other. Also, some cetaceans live near river estuaries and others swim in fresh water.

Behavior

Migrations

Flow fin of the Mediterranean Amanita cachalote. Flow and dorsal fins allow long-term follow to cetaceans. Amanita has been visiting the Strait for 20 years.

Many mysticetes migrate from feeding areas to breeding areas. This is the case of humpback whales, which in summer swim and feed in the seas of the polar regions, which are abundant in krill, to later migrate to the equator in winter, where they mate and give birth to pregnant females. Corkeron and Connor, the mysticetes migrate, in addition to looking for food and giving birth in calm waters, also to protect the young from the attack of orcas. These predators are more abundant at high latitudes and do not follow whales on their migrations because they would stray too far from their main prey, pinnipeds.

Satellite brand detail in cachalote.

Among toothed whales, smaller species, such as striped dolphins, make small migrations, moving from shoreline to sea in search of food, while larger species, such as sperm whales, are capable of longer migrations. important.

The mechanisms by which cetaceans follow migratory routes are still poorly understood. It is believed that they can be based on the geomagnetic field, the position of the Sun, ocean currents or the location of the origin of very low frequency sounds.

Food

All cetaceans are predators and are at the top of the food chain. They have few natural enemies and the most dangerous is undoubtedly the human being. Mysticetes generally feed on small planktonic organisms and small fish, while toothed whales eat larger organisms such as cephalopods (especially squid) and fish. Orcas, a unique case among cetaceans, also feed on marine mammals, such as otariids or porpoises. From there derives one of the common names given to it, "killer whale".

Feeding strategies for mysticetes

A group of humps feed after having confused the prey by forming a bubble network.

Mysticetes have three different types of feeding strategies. Balenaids and the pygmy right whale feed by swimming slowly in shallow waters with their mouths open. In this way they filter a large amount of water and huge amounts of small copepods are trapped in the long beards. Balenopterids have a large mouth opening and a large number of folds, called "golar grooves", in the ventral region of the mouth and throat, which serve to increase the volume of water that cetaceans can hold in their mouths. The blue whale's mouth can hold up to seventy tons of water, equivalent to approximately 70% of the animal's body weight. The movements of the tongue create a negative pressure that sucks the water and the prey it contains into the mouth. mouth. Then, the mouth closes and the tongue expels the water and the prey, mainly krill, towards the baleen. The water is expelled to the outside, while the shots remain trapped and are later ingested. Humpback whales usually form groups to hunt and use a particular hunting system called bubble-feeding. When hunting small gregarious fish such as herring, one member of the group forms a series of bubbles produced by the air expired from the blowhole. The other whales swim under the school of fish and push them into the bubbles, which form a kind of net. This web appears to confuse prey, which compacts into a spherical structure that makes it easier to capture. At this time, the whales attack the fish from below, ingesting large quantities. To facilitate the execution of these maneuvers and coordinate with each other, the humpbacks communicate with each other by issuing a series of calls. Gray whales feed on small crustaceans that live within the sediments of the seabed. These whales swim with their backs turned downwards and use their tongues to "dredge" the bottom, ingesting water and sediment, which are then pushed by their tongues towards their baleen and expelled, while the shots are trapped and then eaten.

Odontocete feeding strategies and echolocation

Odontocetes feed on prey larger than mysticetes and use echolocation to detect it. These animals produce a series of high-frequency sounds, called "clicks," that are directed in the direction the head is pointing. When sound waves from clicks collide with prey, they bounce back. The returned echo is received by the jaw, which transmits the vibrations to the ear via an oily substance. The clicks are generated by three air sacs located on the animal's head and are amplified by the melon.

Sounds also serve to stun prey, especially fish from the clupeidae family such as herrings, which have developed the ability to perceive ultrasound and are therefore able to avoid capture by fleeing echolocation sounds. To prevent this, some toothed whales make sounds called bangs, which can reach 256 dB and disorient and stun fish. Bangs are produced by the same mechanism as echolocation, but some species also emit bangs through rapid jaw closure. While mysticetes feed primarily in shallow waters, many toothed whales descend to greater depths. Sperm whales and beaked whales dive to very great depths to hunt and eat squid, and the former are known to hunt giant squid (Architeuthis sp.). Orcas are capable of hunting animals much larger than them, using tactics that consist of surrounding the shot and dividing the work among the different members of the group during the attack. The orcas that live in Patagonia use a hunting method, called “voluntary stranding”, which consists of hunting the otariids that are directly on the beach, and they have developed an effective technique to return to the sea. This technique is not innate, but is taught by mothers to their young. Some bottlenose dolphins living in Shark Bay, Australia use a similar hunting technique to capture the fish they feed on: they circle the fish and push them inshore until they are stranded, making them easier to catch.

Social behaviors

Group formation

A glacial frank whale with her baby.

Many cetaceans associate and form groups made up of a variable number of individuals. Associations can be formed for defensive purposes or to carry out more efficient hunting techniques, but above all for reproductive reasons.

Mysticetes generally do not form large groups and often the highest level of social organization is constituted by the temporary association of a female with her young. An exception is the humpback, which, as described above, can form hunting associations using the bubble feeding technique.

The social behaviors of toothed whales are more complex and articulate. Many species form long-term, more or less complex associations. Groups can gather thousands of individuals and in some cases can be made up of associations between different species. Striped dolphins, for example, can form interspecific associations with other toothed whales, such as common dolphins and pilot whales.

Group of peaceful mule dolphins in the Red Sea.

The social structures of toothed whales are generally dominated by associations between females, who unite with males during reproduction. Fighting between males to conquer females is not uncommon, as evidenced by the presence of scars left on the skin of losers of conspecifics. Often, as with Australia's Shark Bay bottlenose dolphins, males can form small coalitions, called "alliances", which fight with other alliances and can "abduct" females from another group and force them into forced intercourse. Long-snouted dolphins associate in groups of about a hundred individuals. Within the group, subassociations of about twelve individuals are formed, which swim synchronously in a V-shaped formation, similar to that seen in flying geese. Orca groups are matriarchal associations led by the oldest mature female and consisting of at least one male, calves, and other females. Males born in a group continue to be part of it when they get older, but they only mate with females belonging to other groups. The members of the group communicate among themselves through a dialect that varies from one association to another and that is taught to the new generations. Sperm whales form similar associations, called "units", in which the males do not stay with the females and young, but at the age of about five years they undertake a long journey to higher latitudes, where there is more food, to complete its development. Thereafter, they move from unit to unit to mate with more females.

Communication

The main means by which cetaceans communicate is the production of sounds. Body language and tactile sensations also play an important role in communication.

Vocalizations

Spectrogram of the chanting of a yubarta.

Odontocetes, in addition to the clicks produced by echolocation, also emit low-frequency sounds, whistling and barking, similar to the barking of a dog. These sounds play an important role in communication. Some dolphins, such as the bottlenose dolphin, emit characteristic whistles, called "signature whistles", which identify each individual. Unlike the other dolphins, killer whales do not have a signature whistle, instead emitting whistles unique to each group. These cetaceans communicate within their own group by issuing a series of repetitive calls that constitute a true "dialect", which is taught to the new generations and makes vocal communications within the group more effective. Often these calls are used by killer whales to coordinate during the hunt. Sperm whales also use the clicks used for echolocation to communicate with each other, producing a series of 3-30 clicks of complex duration of about 2 seconds, called a "tail". Each individual has their own unique queue, so click queues can be used to recognize a particular individual.

Mysticetes are capable of emitting low-frequency sounds that can be heard over considerable distances. Humpbacks emit sounds of variable frequency, which form authentic "songs". Each chant lasts between seven and thirty minutes and is then repeated. There is no pause between one song and the next, so humpbacks can continue singing for hours. Each song is made up of a series of themes, phrases, and sub-phrases (see phrasing) and there are differences between the songs of the humpback whales that live in the North Atlantic, those of the North Pacific and those of the southern hemisphere. The songs of humpback whales have an important role in reproduction; it has been shown that only adult males sing (similar to what occurs in birds), which communicate through songs their availability to mate with females and their position. Apart from humpbacks other mysticetes may also sing, but with much simpler songs. The Greenland whale emits songs composed of few sounds that are repeated several times.

Tactile communication

Cetaceans have a large number of tactile endings throughout their bodies. The most sensitive parts of these animals are believed to be the fins, genital areas, and head, which have sensitivity comparable to that of human lips. Many cetaceans rub or caress each other, using their pectoral fins. This behavior could serve to reinforce social ties between members of the same group, which is why it would play the same role as grooming in primates.

Sex could also play the same role in reinforcing bonds between individuals. Cetaceans are one of the few animals that mate for reasons other than reproduction. Sexual intercourse between immature individuals has been observed, with calves attempting to mate with their mother within weeks of birth.

Body language

Despite not being as developed as the ear, the view is also a very important sense for cetaceans. Many of them communicate through a series of movements of the head, beating the maxillae with strength and opening the mouth, behaviors that usually indicate hostility to other individuals. Some of the dolphins produce bubbles under the water, expiring air from the spiral when they emit the "signature notes", probably to help their peers identify who is "talking". The coloring of the body can also be useful for identification. The humps have a coloring that varies from one individual to another. When they swim next to each other, these cetaceans can easily be recognized at a glance; in the dolphins of the genus Stenella colouring changes with age, allowing them to determine the age of their peers.

Surface Behaviors

An orca makes spyhopping in a hole in the ice.

All cetaceans make on the surface of the water a series of acrobatics and leaps, whose meaning is not yet clear. Some of these behaviors could help animals get rid of parasites, but it is not excluded that they are games or have a role in socialization.

Among the main behaviors are:

• Breaching: consists of jumping completely or partly out of the water. It's one of the most spectacular behaviors.
• Spyhopping: consists of keeping your head out of the water to your eyes and turning on yourself.
• Lobtailing (or Tailslapping) and Flipperslapping: they consist of repeatedly whipping the tail and pectoral fins against the water surface, causing a loud noise.
• Fluke-up: consists of keeping the tail perpendicular to the surface of the sea. It's typical of the cachalotes.
• Logging: consists of remaining still on the surface of the water. It is used to replenish or breathe before a deep dive.
• Bowriding: it is typical of the dolphins and consists of "cabalgar" the waves that leave the bows of the boats or the great whales when they swim on the surface.
• Porpoising: also typical of dolphins, it consists of making long and low jumps out of the water while swimming quickly.
• Tailspinning: consists of "andar" with the tail on the water behind. It is very used in the dolphins.

Relationship with humans

Tarentine coin of 500-480 B.C. that presents on one of the faces Taras, founder of the city, riding a dolphin.

For the Greeks, dolphins were related to the cult of Apollo and the Oracle of Delphi owes its name to this animal; After having done penance with Admetos for having killed Python, the guardian of the oracle, Apollo returned to Delphi in the form of a dolphin. Also in Greece, there were many cities that minted coins on which dolphins appeared. Among them was Taranto, founded according to mythology by Taras, who came to the city on the horse of a dolphin. Pliny explains how the dolphins watched the bathers from the shore to prevent them from drowning and that near Nimes, in Provence, the dolphins came to the fishermen's call for help in order to help them fish. These two myths could have a background of truth. Stories of dolphins saving humans from drowning are also well known today, and some bottlenose dolphins near Laguna in Brazil cooperate with fishermen to catch fish and both species benefit from this interaction.

Group of tourists watching the cetaceans aboard a boat. This activity, called cetacean sighting, is becoming increasingly popular.

Cetaceans have often been the protagonists of literary works, films and television series. Herman Melville's novel Moby Dick, which narrates the voyage of the whaler Pequod in search of a "white whale" that is actually a sperm whale, is very famous. He is also famous for Flipper, a bottlenose dolphin starring in movies and television series, as well as Free Willy, a 1993 film that tells the story of a boy's friendship with an orca, taken from its parents and trained by dolphinariums.

In recent years, the activity of watching cetaceans in nature has been spreading more and more. This activity is important for tourism, for scientific research and for the conservation of these animals. It is estimated that, since 1991, the number of people who participate has grown by approximately 12% per year and it is believed that this percentage could continue to grow in the future. Despite the fact that whale watching can be organized on an individual basis, it is mainly a commercial activity that involves some 87 countries and generates a turnover of approximately 1 billion dollars a year. In the Mediterranean, common dolphins, striped dolphins, bottlenose dolphins, pilot whales, pilot whales, Cuvier's beaked whales, fin whales, the Iberian orca, sperm whales and occasionally porpoises, Humpback whales can be observed. Also in the Mediterranean, whale watching has contributed to the signing of an agreement between France, Italy and Monaco for the creation of the Pelagos Sanctuary, a protected marine area, in the marine zone between Liguria, Tuscany, Corsica and France.

Conservation status

According to the IUCN Red List, fourteen cetacean species are seriously threatened with extinction. Of these, two are classified as critically endangered: the Chinese river dolphin and the California porpoise. Although the IUCN still Considering the Chinese river dolphin to be critically endangered, a research expedition on the Yangtze River in China in December 2006 led researchers to declare this species functionally extinct. According to these researchers, it would be the first total extinction of a cetacean and the first extinction of an animal weighing more than 100 kg in the last 50 years. However, on August 29, 2007, a tourist filmed a large dolphin swimming in the waters of the Yangtze. The experts determined that it was a Chinese river dolphin, which gave new hope for the survival of the species, which, however, would continue in a critical situation. The Amazon river dolphins (Sotalia fluviatilis and Inia geoffrensis) although they are not endangered they have seen a decline in their populations since the year 2000.

Under CITES, Appendix I includes all species protected by the 1986 IWC whaling moratorium. Therefore, trade in and take of these species are prohibited. All other cetaceans are included in Appendix II, so their trade and capture are only allowed if they are compatible with the survival of the animals.

The main threats to cetaceans are hunting, accidental capture, water pollution, competition with fishermen, collisions with large boats, disease, capture by dolphinariums and aquatic zoos, and the destruction of their habitat. habitat.

The Mediterranean and Black Sea, cetaceans are protected by the Agreement on the conservation of cetaceans in the Black Sea, the Mediterranean Sea and the contiguous Atlantic area (ACCOBAMS), promulgated in Spain as a Supplement to the BOE on July 2 of 2001 as "Instrument of ratification of the Agreement on the conservation of cetaceans of the Black Sea, the Mediterranean Sea and the contiguous Atlantic area, done in Monaco on November 24, 1996".

Hunting

White Atlantic flanks dolphins captured and killed in a hunting gown on the Faroe Islands.

The hunting of cetaceans, especially large ones, has very ancient origins. Already in the Neolithic, about 6,000 years ago, some populations in northern Europe hunted and fed on these animals. In the 16th and 19th centuries there was a large increase in the number of hunted specimens. Among the products recovered from the whales, the most important from a commercial point of view were blubber, turned into oil for lamps; beards, used to make corsets; or the spermaceti of the sperm whale, used to make perfumes. Currently, the main use of cetaceans is their meat, highly appreciated in Iceland, Norway and Japan.

Hunting has caused over the years a drastic reduction in the number of populations. The first species to be threatened were the easiest to catch, such as sperm whales, humpbacks, gray whales, and right whales; later, with the development of increasingly efficient harpoons, blue whales, common and northern or boreal fin whales were also threatened.

Smaller cetaceans, such as dolphins, have also been hunted. In Japan, massacres are carried out which, in addition to raising the outrage of Western public opinion, have led to a rapid decline in striped dolphin populations, drawing the attention of local fishermen to killer whales, bottlenose dolphins, and pilot whales.

As of 2015, the hunting of large cetaceans is regulated by the International Whaling Commission (IWC), which in 1986 approved a moratorium on hunting that is still in force. However, every year the member countries of the commission meet to decide if it is convenient to withdraw any species from the moratorium.

Bycatch

A marsopa trapped in a fishing net.

Large numbers of cetaceans, especially dolphins, drown after accidentally getting caught in fishing nets. This problem has only been recognized as such in the last three to four decades. Primary netting is believed to be one of the main threats to the survival of the California porpoise. In Italy and Spain, bycatch is mainly due to swordfish nets. Bycatch is also a problem for fishermen, who waste time removing mammal carcasses from nets, which are often damaged. and they are useless and the fishermen do not obtain any economic benefit from the captured cetaceans.

Apart from the nets, dolphins often die during tuna slaughter; since many times they swim together with these fish, sometimes being surrounded by the boats of the fishermen together with the tuna. Many die because of their unpredictable behavior or due to human error.

Competition with the fishermen

Some fishermen believe that cetaceans compete with them for the capture of fish and that is why they kill them on purpose. In recent years, this problem has become particularly noticeable in the Mediterranean, where attempts are made to keep dolphins away from the fishing area using instruments that emit sounds that repel them. However, this system could damage the hearing of animals.

The problem of competition with fishermen is used by countries favorable to the resumption of whaling, such as Japan or Norway, to affirm that hunting is necessary to avoid damage to the supply of fish by humans, taking into account the fact that, according to the FAO, approximately one million people in the world eat mainly fish. Others argue that the biggest competitors of fishermen are surely predatory fish and there is insufficient scientific data to quantify the contribution of cetaceans.

Boat collisions

Several large cetaceans are killed by collisions with large vessels, especially when they rest on the surface of the water and do not have time to flee. This problem has increased considerably with the increase in maritime traffic. In the Mediterranean, the cetaceans that collide most often with ships are sperm whales and white-tipped whales. The presence of scars on the skin of some animals shows that in some cases they manage to survive the accident.

Chemical and noise pollution

High concentrations of PCBs and heavy metals have been shown to be present in the tissues of many toothed whales. Concentrations of these substances greater than 100 mg/kg interfere with the endocrine and immune systems of animals, making them more vulnerable to disease and causing reproductive abnormalities. It seems that mysticetes are less sensitive than toothed whales to the effects of these substances. Another danger derives from oil spills in the sea, which can cause damage if ingested and can render the beards of the mysticetes useless.

Human-produced underwater noises can interfere with the activities of cetaceans, which base much of their reproductive and feeding behaviors on acoustic signals. The main marine noises are those caused by seismic tests, dredging of the seabed, underwater drilling and maritime traffic. These noises often travel miles underwater and can cause temporary or permanent hearing loss in cetaceans. A particularly important problem concerns military operations carried out in the oceans by the navy. The use of sonar experiments or the testing of new explosives cause enormous damage to cetaceans. A clear relationship has been drawn between the mass stranding of groups of beaked whales, which did not present any apparent symptoms apart from damage to the auditory system, with military operations that were taking place in the area where the animals lived.

Diseases

Cetaceans are very sensitive to diseases caused by Morbilliviruses and to the neurotoxins produced by some dinoflagellates responsible for red tides. In the 1990s, a Morbillivirus epidemic decimated Mediterranean populations of striped dolphins. As the PCB values analyzed inside the tissues of these animals were very high, it is believed that the infection was favored by the weakening of their immune system.

Catched by dolphinariums

Dolphins are captured to be taken to zoos or to be trained and exhibited in dolphinariums. The effects of these captures, removing a certain number of animals from their natural environment (including those that accidentally die during the operation) are added to those of deliberate hunting. The most commonly caught dolphins are the bottlenose dolphin, the Irawadi river dolphin and Sousa chinensis. In most countries, the conservation of dolphins in dolphinaria is regulated by law.

Stranded

Some volunteers are trying to prevent them from dehydrating some long-finaled black calderones in New Zealand.

Some toothed whales, especially long-finned black pilot whales, sperm whales, and beaked whales, become stranded—that is, they get stuck on dry land and can't make it back to sea. Very often, stranding causes the animal to die from dehydration or suffocation due to the collapse of the lungs under the weight of its body. The launching can be individual, when there is only one specimen, or massive, when an entire group is greedy. The strandings have been the center of a long debate among researchers for decades, with the aim of determining the causes, which are still not very clear today. They are not thought to be caused by a single cause, but by a combination of natural, biological, and behavioral events. The former include changes in ocean currents and tides, as well as the occurrence of storms, while biological factors include predation, diseases and disturbances of echolocation. Regarding behavioral factors, in highly social species it is possible that an animal in difficulties due to individual causes is followed to the mainland by other members of the group, who had been with him.

In 2004, German researchers linked the frequent stranding of sperm whales in the North Sea to solar activity. By analyzing the strandings of these cetaceans that took place between 1712 and 2003, the researchers realized that 97% of the strandings occurred during periods of minimal solar activity.

Another possible explanation for the strandings are the exercises of the military navy carried out through the use of medium frequency sonar, for the detection of submarines. In 1996, twelve beaked whales washed up on the coast of Greece and autopsid analysis revealed the presence of a pathology characterized by the presence of gaseous emboli inside the animal's organs. Once the cetaceans have beached, these emboli cause their death due to the serious damage caused to the circulatory system. The strandings have been related to military exercises that took place two days before they occurred. In 2002, another fourteen beaked whales that presented the same symptoms stranded on the Canary Islands and the strandings began just four hours after the start of military operations, confirming the close relationship between the use of sonar and the strandings and deaths of these animals.

History of ketology

Cetology is the branch of marine biology that deals with the study of cetaceans. Much of the knowledge that is derived from the study of stranded animal carcasses.

Cetaceans were already studied by Aristotle, who, referring to them with the term κῆτος (which previously had the meaning of "sea monster"), gave rise to the current term. Aristotle clearly distinguished cetaceans from fish, as viviparous animals that breathe with lungs and suckle their young. Among the many ideas conveyed by Aristotle is the distinction between baleen and toothed cetaceans, as well as the description of copulation, which is done belly to belly. Pliny the Elder, who studied these animals in his Naturalis historia , also knew that they breathed air through lungs, but attributed body sizes exceeding long the real ones and devoted more space to anecdotes and popular beliefs than to physiology.

In the Middle Ages, only a few Scandinavian and Icelandic scholars concerned themselves with cetaceans. In the Icelandic work Speculum regalae from 1240 they are presented again as man-killing, ship-destroying monsters.

The Renaissance, the study was based on the dissection of stranded animals, but the knowledge of Aristotle had not yet been recovered; it was not clear, specifically, whether cetaceans should be classified with fish or with mammals. Pierre Belon, in his Histoire naturelle des estranges poissons marins, considered them fish, while Guillaume Rondelet defined them as "aquatic quadrupeds".

Carl Linnaeus, in the tenth edition of his Systema Naturae of 1758, classified them as mammals.

In the book Recherches sur les fossils ("Research on Fossils") from 1823, the physician and paleontologist Georges Cuvier analyzed and described the skeleton of cetaceans, defining them as "legless mammals later”.

Between the 19th century and the XX, much of the information about migratory routes and morphology came from whalers, who knew very well the animals they had to hunt. In 1924, a scientific expedition called Discovery, which lasted almost 25 years, was commissioned to study the ecology of the Arctic regions and the reproductive behavior of cetaceans.

The difficulty of scientific investigation is increased because animals spend only a fraction of their time at the surface. In addition, when they swim or dive, they do not leave any traces, so it is also difficult to follow their movements. This problem is solved by dialing with satellite radio transmitters. Researchers studying these animals are usually equipped with hydrophones to listen for vocalizations, binoculars to observe the horizon, and photographic apparatus for photo-identification.