Mammalia

Text © Prof. Angelo Messina

 

English translation by Mario Beltramini

 

The mammals (Mammalia) form a class of Vertebrates whose members are characterized mainly for the presence of mammary glands, hence the name, and for being equipped with an internal metameric skeletal structure, bony and/or cartilaginous (vertebral column).

The mammals gather warm-blooded species (homeothermic), being animals that maintain constant their body temperature, regardless of the variations of the ambient temperature.

Another characteristic typical of almost all mammals, is that of having the body covered with hairs, exclusive structures of these animals, of very varied constitution and that form a fur.

From the taxonomic and systematic point of view, the class includes very varied and well known forms, such as Mice, Cats, Dogs, Horses, Sheep, Bats, Deer, Lions, Elephants, Kangaroos, Whales, Seals, Monkeys, Man and many other forms, as well as a high number of extinct species and orders.

The dimensions of the members of the class of the mammals are extremely variable; some like Mice, Hamsters, Shrews, Bats, reach a comprehensive length, tail excluded, even less than 5 cm and a weight of only a few grams. Vice versa, among the Cetaceans, the Blue whale (Balaenoptera musculus, Linnaeus, 1758), with its 30 and more metres in length and a weight generally of around 120-130 tons and even more, is the biggest still existing known animal. Generally, however, most species reach small or modest dimensions. Typically tetrapod animals, that are four-limbed, in some cases modified or wholly disappeared in the course of the evolution, the mammals have differently shaped integument, usually thin, equipped with numerous sebaceous, sweat, odoriferous and mammary glands.

In most mammals, externally, the body is distinguishable in different main anatomic regions: head, neck, trunk, in turn subdivided in thorax, abdomen and pelvis, and tail if present. And still, as aforementioned, the body of the mammals is equipped with four limbs, one pair anterior and another posterior. We will systematically represent below the essential attributes that characterize the class of the mammals.

HEAD

The head of the mammals is provided with a well pronounced snout, usually large in size if compared to the remainder of the body. The size of the head is related to the volume of the encephalon, which in most species is well developed, especially the neocortex. In relation to defense or predation activities, the body usually has a good, at times very ample, mobility.

The three-toed sloth (Bradypus), genus of mammal of the order of the Pilosa living in the tropical forests of Central America, may have an almost 360° mobility of its head thanks to special joints in its neck.

In mammals, the skeletal structure of the head, skull is completely different from that of the Reptiles, with a significant reduction in the number of the bones, and is characterized by the tendency to extend over the bones of the face. The skull has two occipital condyles through which it articulates with the first cervical vertebra, the atlas.

Mouthparts

The mammals mouth is usually surrounded by fleshy lips, variously developed.

The mouth vault is composed of the secondary palate, which in its upper part is supported by the maxillary processes and of the palatines (hard palate), whilst in the back part does not have the bony support (soft palate): this allows breathing even while feeding.

The two branches of the mandible are formed by only one bone each (dental) and articulate directly with the squamous bone of the head, unlike the Reptiles where such articulation is realized through other bones. In the fore part of the mandible, the two dentals are merged through ligaments, cartilage or synostosis.

The Meckel’s cartilage disappears almost completely and usually is included in the middle ear originating one of the ossicles of the auditory system, the malleus.

Inside the mouth open the ducts of the salivary glands (parathyroid, sublingual, submandibular, infraorbital); very developed in the herbivores, these glands are reduced in the carnivores, whilst are totally absent in the forms living in an aquatic environment.

The mouth is usually equipped with a defined number of teeth (up to 56 in the Marsupialia) that are implanted in alveoli on the jaw and on the mandible (thecodont set of teeth).

The marked differentiation of the teeth (heterodonty), in form and size, in the various positions of the dental arches, reflects their functional specialization; in fact, in most mammals 4 types of teeth are recognizable: the incisors, placed in front, suitable for biting and cutting; the canines, long and pointed, suitable for grabbing and piercing; the premolars and the molars having the crown shaped for crushing the food.

The set of teeth of the various species of mammals is commonly is expressed with a dental formula, represented by a fraction on whose numerator are indicated the teeth of the upper hemiarch, on the denominator those of the lower hemiarch following the order: incisors, canines, premolars and molars.

A set of teeth presenting all four categories of aforementioned teeth is defined complete.

When the numerical reduction of teeth is accompanied by the disappearance of one or more of these categories, the set of teeth is called incomplete; in such cases the room (or the rooms) without teeth existing in the dental arches is called diastema.

The instances of teeth all the same (homodonty), as occurs in the Cetacea Odontocetes, as well as those of reduction or of total disappearance of the teeth, as occurs in the Armadillos, Anteaters and Sloths, for such reason called Toothless, Edentate or Badly toothed, are secondary evolutionary acquisitions, consequence of adaptations to particular diets.

In some mammals, like in the Platypus and in the Echidna (Monotremata) and in the Whales (Cetacea), the teeth are present only in the juveniles, whilst the adults present respectively a horny beak and baleens, laminar corneal structures suspended from the jaws.

The majority of mammals throughout their life cycle present two sets of teeth (diphyodont), a first juvenile set of teeth temporary or deciduous (“baby” or milk teeth), and a second permanent set of teeth, rightly, permanent.

In any case, however, the molars are exclusive to the second set of teeth.

In some species, the disappearance of one of the two sets of teeth (baby or definitive) is a consequence of the passage from a primitive two sets of teeth condition (diphyodont) to a second one in which occurs only one tooth eruption (monophyodont); this situation is observed in the Cetacea Odontoceti.

Usually, the teeth have defined growth, however some species of the class have a continuously growing dentition, where the tooth wears down in its free portion whilst it grows at the base.

Are continuously growing the incisors of the Rodentia (Mice, Rats, Squirrels, Hamsters, Guinea Pigs, Beavers, Marmots, Coypus, Porcupines, etc.) and of the Lagomorpha (Rabbits and Hares).

Also the canines of the Suidae (Pigs, Wild boars, Warthogs, Babirusas) and the upper incisors of the Elephantidae (Elephants) are transformed into variously developed fangs, usually more in the males.

The canines of the Hippopotamus (Hippopotamus amphibius Linnaeus, 1758) are sharp and cutting and protrude outwards, constituting a terrible weapon.

Endowed with continuous growth, in the males of the Hippopotamus the canines may reach even the length of 50 centimetres per a weight of 3 kg, whilst in the females they are definitely smaller and usually weigh 1 kg.

The canines of the Walrus (Odobenus rosmarus Linnaeus, 1758) are very long and in the males they may reach even one metre in length and exceed the 5 kg in weight.

The tongue, located on the buccal floor, is a muscular organ more frequently mobile. and appears various in shape and size in the various groups.

Several myrmecophagous species, like the Giant anteater (Myrmecophaga tridactyla Linnaeus, 1758), known also as Ant bear, are equipped with a long and sticky tongue in relation to the diet formed by Ants and Termites that they feed on in large quantities.

The surface of the mammals’ tongue is covered by taste buds of various shapes (corolliform, filiform, jeafy, etc.) that render it a very versatile organ for touch, taste and swallowing.

In several zoophagous species, the tongue is provided of rough papillae in particular utilized for the cleaning.

Several species, such as Dogs and Wolves (Canidae), when panting, utilize the tongue also as an organ of thermoregulation.

Horns

Some mammals have on the head bone protrusions, called horns, that may be one or two uneven like in the Rhinoceroses (Rhinocerotidae) or, more frequently, a couple (Cervidae, Bovidae) and others.

Only the male of the antelope (Tetracerus quadricornis Blainville, 1816) is armed with two pairs of horns.

Depending on the species, the horns have various conformations (simple, ramified, smooth, girdled) and often different in the two sexes or exclusive of the males and having defensive and offensive functions.

The Rhinoceroses are armed with one or two median horns resting on a bone relief of the nasal septum and that are considered homologous to intimately merged hairs.

In the Cervidae the horns are ramified and originate starting from a dense connective tissue on the bumps of the frontal bones of the skull.

In most species the horns represent a secondary sexual character exclusive to males and usually are deciduous, as by the end of each reproductive season they fall to regenerate in the following year.

Conversely, in the family of the Bovidae (Antelopes, Goats, Sheep, Oxen, etc.) the horns are not deciduous and are formed by a persistent horny case, of epidermal origin, that covers a central part with ossified tissue originated from the proliferation of the central bones. The horns made like this are therefore called hollow and normally are present in both sexes and are not ramified.

And more, the horns are continuously enriching and in the species living in regions characterized by a marked seasonal cycle they display growth rings related to the seasons of the year.

In the Pronghorn (Antilocapra americana Ord. 1815), only membre of the Antilocapridae, the horns, bifid in the male, are formed by a perennial bony process covered by a deciduous bony case renewing each year.

In the Giraffe (Giraffidae) the horns are very short and are always covered by skin on which numerous hairs grow.

NECK

In the mammals, the neck is the anatomical part connecting the head to the trunk and containing important vital organs like the trachea and esophagus. The neck plays a fundamental rôle in supporting the head, allowing its mobility and favouring essential functions such as the breathing, the communication and the swallowing.

Usually the neck is well distinct and more developed in the herbivorous species, as we can see in the Cervidae, in the Bovidae and in the Equidae, or even significantly elongated like in the Giraffidae.

Instead, the neck is reduced or little evident in the aquatic ones (Cetacea) and in the delving ones (Talpidae).

Characteristic of almost all mammals is the skeletal support of the neck formed by seven cervical vertebrae regardless of its length.

In few species, such as tridactylous Sloths (Bradypus), the neck contains 8-9 cervical vertebrae, whilst the didactylous Sloths (Choloepus) have 5 to 7 of them, allowing them a rotation of the neck up to 270°-360° to defend themselves. The Manatees (Trichechus) have 6 cervical vertebrae.

TRUNK

In the mammals, the trunk is the region of the body on whose sides are placed symmetrically a pair of forelimbs and a pair of hind limbs.

From a morphological point of view, the trunk of the mammals is very varied in appearance, from long and slender, like in the Mustelidae, to stocky and stout. as in the Hippopotamidae, in the Rhinocerotidae, and others more.

In turn the trunk of mammals is divided in thorax, abdomen, pelvis, including the main body cavities where the viscera are contained. The appearance of the diaphragmatic muscle, exclusive to mammals, subdivides the inner cavity in anterior thorax cavity, and posterior abdominal cavity.

During the evolutionary path of the mammals, the skeletal and muscular structure of the trunk has undergone even substantial modifications allowing on the one hand a proper support of the inner organs and on the other one the capacity of rapid movements also to animals with a considerable body mass.

This has been possible with the appearance of the ribs and of the robust abdominal muscles in the terrestrial Vertebrates and the specialization of the shoulder girdle and of the pelvic girdle that allow an adequate connection and mobility of the limbs with respect to the axial skeleton.

Moreover, the passage from a quadrupedal gait, pronograde, typical to many mammals, to a bipedal gait, orthograde, typical of some Hominidae, has been possible also due to the progressive verticalization of the trunk in respect to the limbs, with adjustments to the spine, the pelvic girdle and the shoulder girdle. This has also allowed them to even abdominal adduction, abduction and rotation of the limbs movements.

TAIL

The tail appears very various as per development, shape and functions; it can be rich in hairs (Squirrels, Horses, etc.) or naked (Mice). In the Ungulates, the tail is usually long and apically provided with bristly hairs, useful for chasing away the annoying insects.

In some cases the tail is flattened and used as a rudder, like in the Beavers and in the Cetacea, or compressed laterally like in the Muskrat (Ondatra zibethicus Linnaeus 1766). The opossums, American marsupials

Didelphidae, and some Monkeys are equipped with a long and prehensile tail they use for grasping branches.

In some species, like in the Kangaroos (Macropodidae), the tail is very strong and developed, so much to be able to support the whole weight of the body.

In other mammals the tail is little developed, like in the Short-tailed opossum (Monodelphis Burnett, 1830), Short-tailed mongoose (Herpestes brachyurus Gray, 1837), Rabbits (Oryctolagus cuniculus Linnaeus, 1758) and others more.

Finally, some mammals, like Anthropoid apes (Gorilla, Chimpanzee) and Man, have a vestigial residue or lack a tail altogether (anurous).

LIMBS

A characteristic that unites almost all mammals is the presence of two pairs of limbs very varied in shape and development, hence the name of the superclass they belong to, the Tetrapoda.

The limbs of mammals, commonly called legs, are specialized for deambulating, running, jumping or swimming or, to a lesser extent, flying. As well as for the basic bone structure, the legs of most Mammals have the characteristic of having the extremities with five fingers, Pentadattilia, primitive characteristic shared with many vertebrates of terrestrial habits.

An exception are the members of the families Suidae and Hippopotamidae that have 4 toes, whilst Bovidae,

Cervidae, Giraffidae and Camelidae have two of them. Long and agile, like in the Deers, Antelopes, Gazelles, teh legs are huge and very stocky, like in the Elephants and in the Hippopotami.

Kangaroos, Kangaroo rats, Hares and other mammals have the hind legs remarkably more robust and more developed than the fore ones, suitable for hopping. In relation to their underground habitat, the Moles have legs short and with wide soles suitable for digging.

A marked evolutionary differentiation of the limbs is noted in Whales and Dolphins (Cetacea), Dugong (Dugong dugon Müller, 1776) and Manatee (Trichechus Linnaeus 1758) of the order of the Sirenia, where the forelimbs have transformed in pectoral fins whilst the back ones have reduced to small internal vestigial bones.

In the Bats (Chiroptera) the forelimbs have specialized for the active flight with the extremely elongated toes, from the second to the fifth, that subtend a thin skin membrane, the so-called patagium, that extends along the body, the hind limbs and frequently also the tail. As well as a flight organ, the patagium also works as a structure of thermoregulation, thanks to a rich net of blood vessels.

The feet, normally provided with 5 toes, at times less, have a structure varying depending on the type of locomotion, as they are suitable for running, climbing, swimming, flying, digging.

In the running mammals we see a narrowing and an elongation of the foot, as well as a reduction in the number of the toes.

The sole of the feet usually is hairless and has a thick corneum layer, in particular at the level of the fleshy pads or the fingertips.

The toes have cornified dermal formations, the nails, in their different shapes of claws or falcula, hooves or ungulae, unguiculate or laminar nails. Normally, the nails are continuously growing.

The way to move on land of the mammals is quite various and, depending on the type of locomotion distinguish: the Plantigrades who, like the Bear, walk leaning the entire plantar and palmar surface, the Digitigrades, who move leaning only two toes (like, for instance, the Canids).

Mammals who walk on their fingertips, protected by strong nails and hooves are called Unguligrades.

Depending on whether the support on the ground is done on the tip of an even number of toes or on an odd number, the Unguligrades are distinguished in Artiodattili (Camels, Antelopes, Pigs, etc.) and Perissodattili (elephants, rhinoceroses, horses, etc.).

SENSE ORGANS

Vision

The eyes, one pair in number, are usually protected by two mobile eyelids, one upper and one lower, bordered by eyelashes. In some species, such as Cats, Polar bear, Beavers, Seals, Walruses, Aardvarks, is also present one “third eyelid”, the nictitating membrane, that, unlike the other two, moves horizontally. The nictitating membrane is transparent and serves to protect the eye and at the same time to allow the vision and the positioning when the animal is submerged. In most mammals, man included, the nictitating membrane is reduced to a small vestigial residue located at the inner corner of each eye (plica semilunaris).

The size of the vision organs varies greatly, from very big in the species leading a nocturnal life, like in several Prosimians (Prosimiae), to very reduced in the digging forms, to hypogeous life, such as Moles, Shrews (Insectivora) and Marsupial moles, or Notoryctes (Notoryctes Stirling, 1891).

Implanted laterally on the head, like in the species feeding on grasses, the eyes allow to spot from both sides possible foes and predators, whilst located in anterior position (Chiroptera, Carnivora and Primates) allow a binocular vision essential for depth perception.

The structure of the eyes is clearly similar to that of the other Vertebrates: however the sclerotic is not ossified, but is formed by dense fibrous tissue and has no comb, that is conversely present in the Reptiles and in the Birds. The pupil contracts in a vertical slit in the Felines and horizontal in the Ungulates.

The sense of vision is not the most important sense in the mammals, on the contrary, only a reduced number of them are able to see perfectly even immovable objects. The ability to perceive the colours in many members of this class is far inferior to that of many Fishes and Reptiles and of almost all Birds. The anthropoid apes and to a greater extent, the Man (Hominidae), are perfectly receptive fto colours.

Olfaction

Usually the nostrils open at the extremity of the head, but in the aquatic forms they may be moved further back, in a dorsal position.

Several species, like Whales, Dolphins, Orcas (Cetacea), Manatees and Dugongs (Sirenia) have nostrils that have evolved in a blowhole located at the end of the head; this adaptation to aquatic life allows these mammals to be able to breathe while swimming in surface, just like Camels and Dromedaries can close their nostrils to protect them from the sand. Moreover, the presence of some sacs under the blowhole allow the Cetacea to orient themselves and hunt, emitting sounds and interpreting the returning echoes like a real biological sonar. Typically, in the Mammals the nose is supported by the nasal bones and by cartilages; at times the fleshy mass may form a long trunk like in the elephants. The sense of smell is particularly developed in some Mammals, especially in relation to their diet habits.

The predatory species utilize the sense of smell for following the path of the prey, whilst those who feed on vegetables employ this sense as an effective means for promptly identifying the presence of a foe. In many members of the class of the mammals, the sense of smell plays a primary rôle in community life and in sexual relations. In these forms equipped with a very acute sense of smell (macrosmatic) the nasal cavities are rather developed and internally have a mucous membrane with many plicas and rich in fusiform cells.

In the species of mammals where the sense of smell has poorly developed (microsmatic) like, for instance, the man, the nasal cavities are reduced and the mucous membranes have no plicas; finally, animals that, like the Whales, have almost completely lost this sense are called anosmatic.

Hearing

The organ of hearing of the mammals, ear, is fundamentally responsible for perception of the sounds and the localization of the sourde producing them.

The ear also has the important function of collecting the information concerning the position and the movement of the body in space in order to maintain the posture and allow the coordination of the movement.

Thanks to a highly specialized ear, the mammals are endowed with exceptional hearing faculties, able to perceive sounds of very weak intensity, just a few decibels for many species among which also Dog and Cat.

Also the detectable frequency spectrum varies from the very low sounds, up to 20 Hz (infrasounds), to those very high, even beyond the 150 mil Hz (ultrasounds).

Various mammals, such as Bats, Dolphins, Orcas and others are able to perceive the environment and orientate, identify obstacles and prey, emit ultrasounds and evaluate their return echos (biosonar). The whales, particularly the gray whales and the blue whales, for communicating emit infrasounds that may travel for hundreds of kilometres.

Symmetrically located on the sides of the head, behind the jaw joint and anterior to the mastoid process, the hearing organ is subdivided in outer, middle and inner ear.

The outer ear is the visible part of the organ and is called pinna, mainly formed by a cartilaginous plate covered by skin and shaped like a shell.

Each pinna carries on the important function of collecting and conveying the sound waves into the ear canal towards the eardrum.

The position of the two pinnas allows to catch the sound stereoscopically and to therefore localize the origin. The pinna is particularly mobile in many species, such as Dogs and Cats, that may rotate it up to 180°. In Man, even if reduced and poorly mobile, the pinna is in any case capable of ensuring the hearing stereoscopic capacities.

The pinna reduces and becomes totally absent in the burrowing forms (Moles, Ground squirrels, and others) and in the aquatic ones, such as Whales, Seals, and Sirenians. The pinna continues with the middle ear from which is separated by the tympanic membrane (tympanum).

The middle ear characterizes for the presence of three ossicles: the malleus and the incus, exclusive to the mammals, and then the stapes. The ossicles have the function of transmitting the vibrations of the tympanic membrane caused by the sound waves through the oval window, in order to set in motion the fluid placed in the cochlea of the inner ear, thus transducing the energy of the pressure done by the sound signal.

The inner ear of the mammals is a complex structure carved into the temporal bone, formed by the bony labyrinth and by the membranous labyrinth, that contains within it a liquid (endolymph). The membranous labyrinth is characterized by the development of the cochlea, spiral-shaped, but the Monotremes. Ultimately, the inner ear performs the crucial functions in the perception of sounds through the cochlea) and of balance (through the vestibular duct with semicircular canals, utricle and saccule).

INTEGUMENTARY SYSTEM

The integumentary system of the mammals, or tegument, is a complex organ covering the entire body and protects it and characterizes for its exclusive annexes of the class like hairs, nails, horns and glands of various kinds (sebaceous, sudoriparous, mammary). The teguments perform a fundamental rôle for the defense from the outer agents and the social communication.

Hairs

The body of most mammals is usually covered by hairs forming a thick fur coat.

As happens for the feathers of the plumage of the Birds, in the coat stand out, long and robust bristly hairs or hairs of the contour that form the so-called giarra, that cover, protecting them from the use over time, the woolly hairs, shorter and thinner, down or soft substance or undercoat.

The giarra determines as a whole the contour of the body of the animal, whilst the down performs the function of thermally insulating the body. Called also mantle or hair, teh fur of the mammals greatly varies in length, colour, hair density and texture. Long and thick in many arctic species and of cold climate, the fur is short and with thinner hairs in the fur of the Mammals living in tropical areas.

In some species however the covering of hair may be limited to some parts of the body (like in the Elephants and in Man), or can be represented by a sparse down or be wholly missing, like in the Naked mole-rat (Heterocephalus glaber Ruppel, 1842), some Cetacea, armored rhinoceroses, mainly the Indian rhinoceros (Rhinoceros unicornis Linnaeus 1758) and others more.

Must be mentioned some hairless domestic races of mammals like the Canadian Sphynx or Naked cat, and the Guinea Pig or Domestic cavy (Cavia porcellus Linnaeus, 1758), races derived from a natural genetic mutation.

In some species of mammals the hairs are modified in relation to aquatic life, like Cetacea and Pinnipeds.

In several mammals the hairs are transformed in rigid defensive spines, like Echidna or Spiny anteaters, Monotremes Tachyglossidae native to Australia and New Guinea, Porcupines or Old World porcupines (Hystrix Linnaeus, 1758), Rodents of the family Hystricidae, and Hedgehogs (Erinaceus Linnaeus, 1758).

Among mammals, the Pangolins (Manis Linnaeus, 1758), known also as Scaly anteaters, characterize for having the back covered a shell of epidermal horny scales, arranged in way to allow the animal to curl up into a ball if scared.

Also the Common armadillo or Lucilla (Dasypus novemcinctus Linnaeus, 1757), diffused in central-southern America, presents the body protected by an articulated shell of epidermal plaques supported by bony plaques. The Armadillo cannot curl up like a ball.

Imbricated horny scales mixed with hairs can be found in the tail of many Marsupialia and of many Rodents.

Many mammals, around the mouth, the nose and the eyes, are provided with tactile vibrissae, long and rigid hairs with the base rich in numerous sensory nerve endings, structures that allow them to orientate, communicate and hunt even in the dark.

Moulting

Usually the hairs of the fur are periodically renewed with a process of moulting that usually is gradual so that the skin never remains bare.

In some species the loss of the hair begins from a more or less circumscribed point and extends like a wave all over the body, in others, instead, the moulting proceeds in a scattered and irregular manner.

Several mammals of the cold regions or of the high altitudes, in autumn change the summer coloured mantle with a winter one, white and longer. This phenomenon favours camouflaging among the ice and the snow and in the meantime reduces the dispersion of the body heat.

In the following spring, a new moulting restores the coloured summer fur.

Each hair originates from a piliferous papilla, placed at the base of a piliferous follicle, a deep cavity covered with epidermis allocated in the thickness of the dermis. Into the follicle opens a small sebaceous gland whose oily secretion, called sebum, provides to lubricate the hair. On each follicle inserts a small erector muscle through which the hair may be straightened under the stimulation of various factors, such as fear, excitement, cold, etc.

Typically the hair is formed by 3 layers of atrophied cells, cuticle, cortex and medulla.

The cuticle (epidermal), more external, is formed by a layer of dead lamellar cells, cornified and imbricated.

The cortex, the middle layer that usually represents most part of the hair, is formed by cellular residues where stand the various pigment granules that are essentially responsible for the colour of the hair: yellow, red, brown or black. The absence of pigment granules causes the albinism.

The medulla is the central part of the hair and is well developed and with sacs filled with air in the biggest hairs, reduced or completely absent in the smaller ones; the increase of the air-filled sacs causes the blanching of the hair.

Skin glands

In relation to its vital functions of protection, thermoregulation, secretion, excretion and sensitivity, the integument of mammals is particularly rich in multicellular glands that, from a functional point of view, are distinguished in sebaceous, sudoriparous, odoriferous, lacrimal, mammary. The sebaceous glands, acinous and holocrine, are usually associated with the piliferous follicles and produce a greasy and oily secretion (sebum) that maintains soft hairs and epidermis and represents a protection against external agents keeping them morbid.

The sudoriparous glands are essential for the thermoregulation and the disposal of waste. They may be of two main types: apocrine and holocrine.

The sudoriparous glands of apocrine type, present in all extant mammals and considered phylogenetically the most primitive, have the secreting part located in the deepest layers of the epidermis, or even in the subcutaneous layers, and are equipped with long excretory ducts.

In several species of mammals, man included, the apocrine sudoriparous glands develop only when the sexual maturity is reached and are limited preferably in the genital region, around the nipples and in the armpits.

These glands do not seem to be involved in the body thermoregulation processes, but their secretion formed by water, some mineral salts and organic compounds, favours the recognition of the individuals of the two sexes during the reproductive season.

Conversely, the holocrine sudiparous glands are present from birth and in most mammals having them, they are usually localized in the regions deprived or poor of hairs (nose, fleshy pads on the soles of the feet).

In some mammals, such as Horse, Monkeys, Man and others more, the holocrine sudoriparous glands are vastly distributed all over the body.

The holocrine sudoriparous glands play a fundamental rôle in the processes of regulation of the temperature of the body. Their watery secretion, particularly abundant when the environmental temperature is high, spreads on the body producing with its evaporation an effective heat dispersion from the surface. The Cetacea and the Pinnipedia do not have these glands.

The odoriferous glands, present in many mammals, vary in location; perianal in the Rabbits, in the Beavers, in the Mustelids and in others, suborbital and metatarsal in the Cervides, placed between the toes and at the base of the tail in the Canids, between the toes in the Ovines, etc. The function of the secretions of the odoriferous glands, that may be scarce and delicate, lile in the Squirrels (Sciuridae), or abundant and and intense, like in the skunks (Mustelidae), are utilized to mark individual territories, or to facilitate the matings or also as defense tool.

Even if rare, some mammals are provided with cutaneous glands that secrete a poisonous substance or in any case toxic utilized for blocking the preys.

Among these, the most dangerous, even if not fatal for the man, is the Platypus (Ornithorhynchus anatinus Shaw, 1799), whose males are armed with a horny hooked spur placed on the heel of the hind legs. Each spur, hollow and mobile, is connected with an alveolar gland located in the upper part of the thigh, whose poisonous secretion is mainly active during the reproductive season.

Moreover, among the Primates, the Slow loris (Nycticebus) is provided on the inner part of the elbows with glands producing an oily secretion that is distributed on its own body and on those of the babies through the so-called tooth comb while cleaning the mantle. Actually, the secretion of the glands itself is not poisonous, but becomes such with the bite when it is mixed with the saliva of the Loris.

Also the Solenodons (Solenodon), Caribbean night Mammals similar to Shrews, inject poisonous saliva through the teeth.

Finally, it should be remembered that several species of Shrews and Moles, like the Short-tailed shrew (Blarina brevicauda Say, 1823), produce toxic saliva to paralyze prey.

The lacrimal glands of the mammals are associated with the eyes and produce a secretion, the tears, that moistens, and protects the surface of the organs of sight.

The mammary glands, more commonly called udders, represent a characteristic unique to all mammals and originate from modified sudoriparous glands.

But the Monotremes, who do not have them, in the remaining mammals the udders are equipped with nipples, skin structures specialized in breastfeeding.

Associated with the reproductive system, the udders reach full development only in the females, at the time of puberty, and are ruled by the endocrine system under the stimulus of the hormonal changes associated with the youngs’ delivery.

In the males of the mammals, man included, the udders are less developed and, even though having nipples and reduced glandular tissue, are not functional for breastfeeding.

The presence of mammary glands also in the males is explained by the fact that the embryonic development is initially common in the two sexes, only at a later stage the mammary glands differ and get functional only in the females.

Depending on the species, the number of udders of mammals varies from a pair like in the Equidae and in most Primates, Man included, up to several pairs.

Cats have 4 pairs of udders, Dogs 5 pairs, Tailless tenrecs (Tenrec ecaudatus Schreber, 1778) have up to 32 of them.

Usually the udders are arranged along the so-called “mammary ridge” and can be located in the pectoral zone, like in the Primates, man included, abdominal or inguinal, as happens in Cats, Dogs, Horses, and Cows and others more.

CIRCULATORY SYSTEM

In the mammals the blood circulation is double and complete with the heart divided into two atria and in two distinct ventricles, separated by a septum. In this way, the arterial blood coming from the lungs does not mix with the venous as instead occurs in the Amphibians and in part of the Reptiles. Typically, the aorta originates from the left ventricle and bends in an arc to the left.

The red blood cells are usually oval and do not have nucleus; in the Camelidae (Camels, Dromedaries, Alpacas, Llamas and Vicunas) have an elliptic shape.

RESPIRATORY SYSTEM

The respiratory system of the mammals characterizes for the spongy and alveolar structure of the lungs which allows a greater exchange surface of oxygen (O₂) and carbon dioxide (CO₂).

Another exclusive peculiarity of the mammals is the presence of a muscular diaphragm that separates the thoracic cavity containing heart and lungs, from the abdominal one where the viscera are contained.

The larynx is equipped with the vocal cords and, through the glottis and the epiglottis, communicates above with the pharynx.

DIGESTIVE SYSTEM

The digestion of food takes place on the long alimentary canal that from the mouth extends up to the anal opening, differentiating itself in various portions (pharynx, oesophagus, stomach and intestine). To the digestive system are associated important glands, like salivary glands, pancreas, liver and glands of the wall.

The stomach is simple in most mammals.

Conversely, in the ruminants displaying the double chewing of the food (ruminazione), the stomach is subdivided into 4 compartments: rumen, reticulum, omasum and abomasum.

It should be remembered that in the Ruminants, the food is initially roughly chewed and sent to the rumen.

Subsequently, from the rumen the food is regurgitated into the mouth in small masses (food boluses); here it is completely chewed and then swallowed again and sent in the remaining cavities of the stomach.

In this regard, it is to be said that the Camelidae, family of teh suborder of the Tylopoda (Camels, Dromedaries, Llamas, Alpaca, Vicunas and Guanacos), considered pseudo-ruminants even if they present the double chewing of the food, have a stomach divided in rumen, reticulum and abomasum. Characteristic of these mammals is to have in the walls of the rumen special “aquifer cells” for the accumulation of water.

The intestine follows the stomach and is short in the forms feeding on other animals.

Otherwise, in the species feeding on vegetal substances the intestine is considerably long, even more than 10 times the length of the body. This anatomical feature is indispensable for the fermentation, and therefore the digestion, of the cellulose, structural polysaccharide of the vegetal cells, difficult to decompose.

NERVOUS SYSTEM

In the mammals the central nervous system, encephalon, is highly evolved and significantly more developed and more complex than in the other classes of Vertebrates.

Intended for the reception, processing and response to stimuli, the encephalon characterizes for the high development of the telencephalon, with large cerebral hemispheres, and complex cerebral cortex, frequently equipped with convolutions, grooves and folds, that remarkably increase its surface.

In the placental mammals, the two telencephalic hemispheres are connected through a typical commissure or corpus callosum.

Relatively small in some forms, the telencephalon is usually well developed and covers the remaining parts of the encephalon.

The development of the telencephalon reaches its maximum in the Primates. Particularly developed is the neocortex, or isocortex that represents the most recent and evolved part of the brain of the mammals, a distinctive feature compared to the remaining Vertebrates. Variously extended, the cerebral neocortex reaches its maximum development in the Primates and to a greater extent in the Man where it forms about the 90% of the cortex, and is responsible for higher cognitive functions like memory, learning, language, sensory analysis.

The functionality of the nervous system is complemented by the action of a particularly active endocrine glandular system.

SKELETON

The skeleton of the mammals is mostly composed of bone tissue.

Even if limited, the cartilage tissue remains in the adults in some parts to guarantee flexibility, to reduce the lattice friction and to provide structural support, as in the articular surfaces, in the ribs, nose, larynx, trachea, bronchi.

Certain tendons include small ossifications called sesamoid bones, the largest of which is the rotula, or patella, of the knee.

The skull of the mammals characterizes for a high specialization, with an ample neurocranium to accomodate a voluminous brain.

In the spinal column are distinguished the typical regions of the Tetrapods: cervical, thoracic, lumbar, sacral lumbar.

The cervical vertebrae in almost most mammals are 7 and their number is independent of the length of the neck. This anatomical feature concerns alseo species with very long necks, such as Giraffes, or very short, like Whales.

The thoracic region is formed by 13 vertebrae on which articulate the ribs.

The lumbar vertebrae are 7, the sacral ones are 3 and are fused (sacrum) for the attachment of the pelvic girdle.

The caudal ones, up to 20 in several species of mammals, in the anthropoid Apes and in the Man, reduce to a small bone (coccyx) formed by rudimentary vertebrae.

The vertebral bodies are usually aphipian or aceli, but at times more or less opistoceli (Ungulates) and separated by intervertebral discs allowing the movements of flexion of the whole column.

The sternum, thin medioventral bone typical of the class, and the 13 pairs of ribs, form the rib cage, a flexible scaffolding that, in addition to protecting the inner vital organs, performs the breathing movements.

The scapular or pectoral cingulum is characterized by the presence of a robust spine in the shoulder blade.

The pelvic cingulum, rigidly connected with the sacrum, includes on each side ilium, ischium and pubis.

Each forelimb is formed by distinct humerus, radius and ulna, 7 carpal bones, 5 metacarpal, of which the innermost is short, and the phalanges of the fingers.

The bones forming each of the hind limbs are distinct femur, tibia and fibula, 7 tarsal bones of the ankle, 4 long metatarsal bones (and a rudiment of the innermost), and the phalanges of the fingers.

REPRODUCTIVE SYSTEM and REPRODUCTION

The mammals are animals with separate sexes, with a sexual dimorphism that in several cases is noticeably accentuated.

Usually, compared to females, the males display very striking external features, such as manes and beards, bigger horns, more flashy mantle, voice with a broader and deeper tone, etc.

In many mammals, the two sexes have the same size, however in some species the males are bigger than the females, in others, instead, it’s the contrary.

Notable examples of differences in size between the two sexes are found in the Northern elephant seal (Mirounga angustirostris), where the male is more than three times heavier than the female, and in the Peninsular tube-nosed bat (Murina) where, conversely, it’s the female being 1,4 times bigger than the male.

The females of the Marsupialia differ from the males for the presence of the marsupial pouch or marsupium.

In the vast majority of species, the mammals are viviparous animals, as the females give birth to puppies whose embryonic development completes totally (Euteri) or partially (Marsupialia) inside the maternal body.

Unlike the remaining mammals, the Protoleri are oviparous and the females lay their fecundated eggs whose embryonic growth occurs instead outside the mother’s body.

The Prototherians, included within the mammals because having mammary glands with which they breastfeed their little ones, are represented by the unique order Monotremata to which only 5 extant species: the Duckbill or Platypus (Ornithorhynchus anatinus Shaw, 1799) and the Echidnas or Spiny anteater (Tachyglossus).

The male gonads, commonly called testicles, are paired and are responsible for the production of spermatozoa and of sex hormones, mainly testosterones.

In all mammals the original location of the testicles stands inside the abdominal cavity but in most species they become external after a migration to the interior of the scrotum (testicular descent), final location.

The scrotum is a skin sac placed at the base of the penis that guarantees its temperature lower than the body’s one, necessary for the development process of the spermatozoa (spermatogenesis).

In the mammals, the testicles move to the scrotum at different times depending on the species, usually between the advanced fetal stage and the first period of postnatal life.

Singular is the case of the Rodentia where the testicles grow and move into the scrotum only during the reproductive period; then they reduce and go back to the abdomen.

The copulatory organ, the penis, is erectile and, more or less, free in most species.

The males of several mammals have a retractable penis, that when resting is staying inside a sheath or preputial pouch.

Unlike the other mammals, the Monotremata have internal testicles and the penis is contained inside the cloaca and is everted only for the copulation.

This anatomical feature has established itself as presenting the advantage of protecting the penis from external agents.

The copulatory organ, longitudinally crossed by the ureter, in many instances is supported by a particular penile bone (baculum or os penis) whose shape varies depending on the species.

The prostate is always present among the glands attached to the male genital tract.

The female reproductive system, in addition to the ovaries, includes the genital tract, represented by a pair of oviducts, by the uterus and by the vagina, where the egg can be fecundated by the spermatozoon and develop in embryo.

In the most primitive form the female genital tract undergoes a gradual fusion into unpaired median structures; this concerns more primitively the vagina, then the uterus with different degrees of fusion (uterus bipartite, bicornuate, up to the simple uterus).

The vagina, double in the Monotremata and in the most primitive Marsupialia, presents an erectile organ (clitoris), often equipped with a small clitoral bone.

In relation to viviparity, the females of almost all members of the mammals produce very small and poor in vitellus eggs (oligolectic) This is to be related to the fact that the embryo develops inside the maternal body through the placental formations.

Otherwise, the Monotremata, having no placental formations, produce eggs with abundant yolk (telolecithal), similar to those of the Reptiles and of diameter from 1,3 to 2,4 cm. In this case this biological characteristic finds the logical explanation in the oviparity of these primitive mammals.

The female of the Platypus (Ornithorrhynchus anatinus Shaw, 1799) lays in a cavity of the soil 2 eggs at a time provided having a calcareous shell.

In turn,the female of the Echidna (Tachyglossus aculeatus Shaw, 1792) usually holds only one egg, rarely 2, inside a ventral pouch.

Extra-embryonic annexes

As happens in the Reptiles (Reptilia) and in the Birds (Aves), also in the females of the mammals form extra-embryonic annexes essential for nutrition, respiration and protection of the fetus during pregnancy.

Specifically, the allantois is an extra-embryonic annex that acts as a respiratory and excretory organ and participates in the formation of the umbilical cord, of the blood vessels and of the urinary bladder.

The amnion, also called amniotic sac, is a membrane wrapping the embryo and maintaining it in a liquid amniotic environment thus playing a fundamental rôle for the integrity of pregnancy.

The outer case or chorion, is an extraembryonic annex formed by an outermost membrane that covers the embryo playing a fundamental protective and nutritive rôle.

In the Metatherians (Marsupialia) happens a significant development of the yolk sac that sticks to the chorion on which develop however few primary villi that at times connect with the uterine mucosa and originate an omphalo-placenta or placenta chorio-vitellin.This structure avers not very effective and ensures an embryo nutrition very inadequate and only working for a short time.

For this reason in the Marsupials the delivery is premature and the newborns, small and very immature, are obliged to migrate very early inside the marsupium where they complete their own development.

Differently happens in the Eutherians or Higher mammals, indicated also with the name of Placentate, where the fetus develops entirely inside the mother’s body, thanks to the formation of an allantoic placenta.

The allantoic placenta forms after the fact that the chorion gets fixed to the wall of the uterus by means of very numerous villi and fuses intimately with the allantoid, in turn richly vascularized, instead of with the yolk sac as occurs in the Marsupialia.

Efficiency of the allantoic placenta is such to allow a long stay of the embryo in the uterus and consequently its advanced development.

In the mammals, the number of puppies born in every pregnancy is usually inversely proportional to the size of the animal. Usually, the large-sized species deliver one child a year, whilst the small-sized species are more prolific and characterize for a shorter gestation period and with several annual pregnancies.

In most classes, the reproductive activity is cyclic and is controlled by hormones. The females display phases of sexual activity (oestrus) that usually occur in spring or in winter, in relation to the maturation of the ovarian follicles, alternating with periods of inactivity (anestrus).

Several species of mammals reproduce only once per year, some more than once.

In some members of the class (Rodents, Carnivores, etc.) the new borns are weak, often have the eyelids closed and also lack fur (inept offspring).

In other mammals (Artiodactyls, Perissodactyls, etc.) instead, the puppies are born with the body already covered by hairs, have open eyes and are able to walk not long after birth (precocious or suitable offspring).

After their birth, the children of the mammals are the object of special care for a very varied period of time (parental care).

The care of the offspring is entrusted only to the mother or to both parents with whom may associate also members of the same species. Usually parental care is simple in the less evolved and more prolific forms, whilst they are very specialized in those with long gestation and with a reduced number of children.

The first and most important moment of parental care is by sure that of the suckling. In this period, and even after, the puppies, in addition to being fed, receive from the mother information of primary importance for their survival.

The duration of breastfeeding is quite various depending on the group and usually lasts until when the puppies develop an efficient set of teeth, thus becoming able to feed autonomously.

In many mammals have been observed protective cares concerning the defense against predators, choice of shelters, etc., and educational cares, suitable for transmitting the sons experiences useful for their survival.

DISTRIBUTION AND BEHAVIOUR

The present mammals are found practically in almost all available habitats at all latitudes of our planet. Most species of Mammals do live in the merged lands, but there are quite several who have adapted to the aquatic life (Cetacea, Pinnipedia, Sirenia, etc.). Some forms of mammals are able to perform a real active flight like the Bats (Chiroptera).

Several mammals are able to perform a gliding flight, like the Southern greater glider (Petauroides volans Kerr, 1792) among the Marsupialia, the Flying squirrels (Pteromyini Brandt, 1855) among the Rodentia and the Flying lemurs or Colugos among the Dermoptera.

Many mammals live on the ground, many are arboreal, several are hypogeous.

DIET

The diet of the mammals is extremely varied.

Many forms, like the Ungulates and most Rodentia feed exclusively or mainly on plant substances, such as herbs, leaves, little twigs, bark, seeds, pollen, nectar, fruits.

Several species of mammals are predators of other animals.

The Felidae and the Mustelidae are carnivorous, the Pinnipedia and the Cetacea Odontoceti eat essentially fish; the Talpidae, Soricidae and several small Bats are mainly or exclusively insectivorous.

Some mammals have a mixed diet and feed on animals as well as on vegetable substances.

The species with a specialized diet are very rare. Some mammals feed on plankton (Cetacea Misticeti), others on blood, like the Vampire bats (Desmodus Wied-Neuwied, 1826) among the Chiroptera. Very few species are monophagous like the Koala (Phascolarctos cinereus Goldfuss, 1817) marsupial nourishing only of eucalyptus leaves.

Usually the species of mammals with a broad food spectrum (eurytrophic) can live stably in the same territory being able to feed on various food during the different seasons.

Otherwise, the forms with a more strictly defined diet (stenophagous) are obliged to suffer from seasonal shortage of their usual food making migrations, or entering in summering or in hibernation, minimizing the vital functions.

Various species of seals (Pinnipedia), whales (Cetacea Mysticeti), Reindeers (Rangifer tarandus Linnaeus, 1758) and of bats (Chirotera), carry on latitudinal migrations, at times of great magnitude.

Differently it happens in some temperate regions where Cervidae, Bovidae and other mammals effect altitudinal migrations, settling in high mountains in the summer months and moving to the nearby valley during the winter.

Local relatively reduced migrations are observed in many Rodentia and Lagomorphs and in some Chiroptera.

Conversely, other mammals, during the seasons where food is scarce and the environmental conditions are not good, are able to pass to an inactive life, becoming temporarily heterothermic, and survive utilizing reserve substances, previously stored in the tissues. If this period of inactivity occurs during the summer months, the phenomenon is called aestivation.

The summer forms, most of whom feed on vegetables, usually retreat in underground dens where they sleep. In this way during the aestivation the metabolic processes slow down with consequent saving of the stored energy.

The phenomenon of aestivation is limited to a few species that live in environments with very warm and dry climate, such as Rodentia, Insectivores, and Marsupialia.

Inlike hibernation, the aestivation allows to overcome long periods of environmental hostility.

Otherwise, the hibernation, or torpor, occurs during the cold months.

The hibernating animals get even more markedly heterothermic, slowing down significantly the frequency of the rate of breathing, the rhythm of the heartbeat, etc.

Many Carnivores, like the Bears, keep sleeping for almost the entire winter period but their metabolic rate, and consequently their body temperature do not drop dramatically, so their sleep is to be considered as a winter sleep, rather than a real hibernation.

COMMUNICATION

The mammals communicate with each other via various signals, olfactory, vocal, visual, acoustic, tactile etc. The activity of communication is more developed in those species living in social groups.

Various activities, particularly those related to the reproduction and to the cohesion of the group, are often associated with chemical signals carried out by pheromones formed by the secretion of the aforementioned odoriferous glands.

It is through the pheromones that many species recognize their own offspring or of the members of their group.

Many mammals utilize facial and body language, like the position of the body, of the ears and of the tail, the exposition of coloured parts, as visual messages to express fear, excitement, irritation, etc.

Several species utilize their own voice for transmitting information with different meanings: maintain the cohesion within a pack, favour the meeting of males and females for mating, localize the parents or the offspring, and also to report a danger, to intimidate potential foes.

The Bats (Chiroptera) emit short ultrasound trains (50 kHz) that, reflected by nearby objects, guide them also when flying and in locating the prey; the same happens in some Shrews (Soricidae).

Some big Cetacea, Elephants, Rhinoceros, Hippopotami, Giraffes, Alligators and others produce infrasounds, with low frequency around 20 Hz, that can be detected even at kilometers far away.

The language, usually formed by stereotypical signals, is more varied in the Primates where it gets articulated in the Man.

Other rather used methods of communication are the noises produced in various ways, like the flapping of the tail against the surface of the water of the Beaver (Castor Linnaeus, 1758).

Several mammals, like the Prairie dogs (Cynomys Rafinesque, 1817), utilize tactile signals of the kissing model to recognize the members of their own pack.

Few species of mammals lead a nomadic life; usually each individual occupies a well defined space, family area, inside which it moves carrying out there all its own activities (eating, resting, reproductive, leisure, etc.).

The size of the family area space occupied by the mammals varies depending on the species and is influenced by various factors, such as the size of the animal, its mobility, its eating habits, the sex, the age, the season, the population density. The family area is a few square meters in some small Rodents and Insectivores. Otherwise, in the big Carnivores and in the Cetacea the family area extends for several square meters.

Many mammals have developed a sense of territoriality (territorialism) and actively defend from the intrusion of other individuals their own shelter and the surrounding territory.

In some species the defense behaviour manifests only during the reproductive period and of the caring of own offspring, in othess, instead, is permanent.

Many mammals utilize shelters for resting, caring osn offspring and repairing from the hostile weather conditions.

Some species exploit the natural cracks between the rocks (Carnivores, Bats, etc.), others the cavities of the trees (Opossum, Hamsters, Marmots, Ground squirrels, etc.).

Various Wood mice (Apodemus sylvaticus Linnaeus,1758), Squirrels (Sciuridae) and Hamsters (Cricetidaei) build nests among the foliage of the trees.

Some mammals, like Moles, Ground squirrels, Badgers, Skunks and more, dig tunnels in the ground.

The Beavers (Castor Linnaeus, 1758), considered in the imagination as the engineers of Nature, build up dams and shelters in the water.

The individuals of the same species do not live in isolation but often they gather to form more or less stable social groups. The simplest form of aggregation is the family group formed by a couple and its own children. In some mammals, the children leave their parents immediately after the weaning, in others, instead, they remain with the mother until reaching sexual maturity. In some like the Elks (Alces), Otaries (Otariidae) and others, the individuals merge in packs that migrate united during the Mating season.

During this time, usually ech male joins a group of females and of children, forming its own harem it defends against other males.

Some mammals join to form the so called gangs, more complex and numerous social organizations, for the search of food, the reproduction and the protection.

Within the gangs, among the single members forms a precise hierarchy dominated by a leader, who controls its activities.

Each gang has a well delimited territory that the members defend all together. The association in gangs is well developed within various species of Monkeys, in particular the Macaques.

THE MAN AND THE MAMMALS

Since prehistoric times, the relationships linking Man to the other mammals are multiple and linked to his subsistence: for the food, the clothes, the transportation, the work in the fields and other necessities.

With a process that began several centuries ago, several species of mammals have been tamed and raised in captivity by Man for satisfying his own necessities.

The Sheep, the Goats, the Oxen provide milk and meat. The Horses, the Oxen, the Camels, the Llamas and others more, are employed as saddle, packsaddle and draught animals.

The hair of the mantle of Sheep, Goats, Camels, and of other mammals is transformed into yarns for the preparation of fabrics. The skins of many mammals are tanned and transformed into leathers and furs.

Otherwise, several species cause damage to crops and to livestock.

Moreover, some species of mammals may transmit serious diseases to the man (zoonosis), such as plague, typhus, rabies, toxoplasmosis, salmonellosis, brucellosis, leishmaniasis, hydatid disease, and several others.

On the other hand, many species are very useful in the field of medical and pharmacological research, for the preparation of serums and vaccines and for the testing of new drugs.

Several mammals like Dogs, Cats, Horses, Donkeys, Rabbits and others, play an important rôle also in the supportive therapies to improve physical and psychological health of the persons (pet therapy).

On its part human activity has led to extinction of several species of mammals, especially those of medium or large size and with diurnal habits.

Despite the protectionist measures enacted in several countries, the numerical consistency of many species of mammals continues to decrease significantly.

Continuing the reckless destruction of forests in all continents, it seems very likely that in a not too distant future most of the big sized species will be able to survive only inside parks and reserves.

Presently, various species of mammals with a wide Euro-Mediterranean distribution, are in a worrying state of threat.

The Eurasian lynx (Lynx lynx Linnaeus, 1758), whose range until some time ago extended from the Pyrenees, and Alps up to the Siberia forests, has disappeared from the Alpine arc already at the beginning of the 20th century due to the human persecution.

The Common wolf (Canis lupus lupus Linnaeus, 1758) is nowadays relegated to the Eastern Alps.

The Eurasian wolf (Canis lupus italicus Altobello, 1921), subspecies diffused in central-western Europe in Italy on the Western Alps and on the Apennines is suffering.

The Sicilian wolf (Canis lupus cristaldii Angelici & Rossi, 2018), a subspecies endemic to the woody mountain areas of Sicily, has already become extinct in the XX century.

The Eurasian Brown bear (Ursus arctos arctos Linnaeus, 1758) whose range includes all northern Eurasia, in Italy is confined to the Alps of western Trentino and to the boundary between Friuli, Austria and Slovenia.

Another subspecies is the Marsican brown bear (Ursus arctos marsicanus Altobello, 1921) endemic to central-southern Italy where it survives in the Abruzzo National Park.

The Mediterranean monk seal (Monachus monachus Hermann, 1779), of which are occasionally sighted individuals, along the coasts of almost all Mediterranean countries, is a species at high risk of extinction of which survive in nature less than 700.

Other big mammals, presently drastically reduced in number with a consistent risk of extinction, are the Red deer (Cervus elaphus Linnaeus, 1758), the Alpine ibex (Capra ibex Linnaeus, 1758) and the Roe deer (Capreolus capreolus Linnaeus, 1758).

The Mouflon (Ovis gmelini musimon) is present in all Europe and is everywhere threatened as object of heavy hunting.

And also the Chamois (Rupicapra rupicapra, Linnaeus, 1758), now present in the mountain systems of central and southern Europe, the Pyrenian chamois (Rupicapra pyrenaica pyrenaica Bonaparte 1845), confined to the Pyrenees, and the Abruzzo chamois (Rupicapra pyrenaica ornata Neumann, 1899) endemic to central Apennines, all in need of continuous protection and assistance for guaranteeing their survival.

Only some micromammals, despite the hunting activity, have not undergone any nymerical decrease, rather they were favoured thanks to the rarefaction or disappearance of many of their predators.

 

CLASSIFICATION OF THE MAMMALS

Presently to the class of the Mammalia is assigned an unspecified number of species, between 5.500 and 6.500, amply diffused in most environments on the planet, from the warmest to the coldest, that have colonized also the depths of the oceans and the highest peaks of the mountains.

However, this number has a purely indicative value as it varies depending on the scientific source and of the frequent discovery or reclassification of new species.

The beginning of the evolutionary history of the Mammals is traced back to more than 300 million years ago starting from a branch of the Synapsida, a vast group of terrestrial tetrapod vertebrates looking a large lizard, very widespread in that time and where are included also the present Mammals.

It is necessary to clarify that, despite their external look, the prehistoric Synapsida were not real Reptiles (Reptilia) but a distinct and parallel evolutionary line.

Without going into speculative details of learned phylogenetic dissertations, this is not the place, and purely conscious that the origin of the Mammalia is a very complex paleontological question and in some ways not yet consolidated, we share the opinion of the scholars widely agreeing in believing that the modern mammals are a monophyletic group that has originated in the late Triassic, about 225 million years ago, rightly from a grouping of Synapsida, that of the Cynodontia.

So called because of their most evident morphological characteristic of having “dog tooth”, the Cynodontiait laid their eggs like the reptiles and it is thought probable they were warm-blooded animals and had the body covered with hair.

It was precisely in the Triassic that the Cynodontia, split in two evolutionary lines that, for ease of display we shall call non mammalia form Cynodontia, and mammalia form Cynodontia.

The non mammalia form ones, during their evolutionary path, did occupy several ecological niches, both as carnivores as well as herbivores.

Nevertheless, even though they survived the great mass extinction of the Permian-Triassic occurred about 252 million years ago, the non mammalia forms of Cynodontia have gradually extinguished during the beginning of the Lower Cretaceous, about 140 million years ago.

The decline of the non mammalia form of the Cynodontia and its subsequent disappearance are blamed on the competition with the dinosaurs and above all with that of the true Mammals, born in the meantime rightly through evolution of the mammaliform Cynodontia.

In fact, while the evolutionary history of the non mammaliaform branch of the Cynodontia ended with its extinction, that of the mammaliaform Cynodontia conversely continued and confirmed diversifying itself more and more in the true mammals.

But, at the dawn of their appearance in the living scene, the ancestors of the present mammals were represented by few small forms, just 10-20 cm long per a weight of a few grams.

For the most part similar in appearance to that of a shrew or of a little mouse, like the Morganucodon (Morganucodon Khne, 1949), the first mammals had night habits and lived hunting insects.

Moreover, they had the body covered by a thick fur which protected them from the cold.

The small size and the darkness helped them a lot in escaping the carnivorous dinosaurs.

Bigger dimensions, comparable to those of a large cat were reached by Kayentatherium (Kermack, 1982), Cynodontia extant in the Lower Jurassic included among the probable ancestors of the mammals. Several features of this animal already prefigured those of the present mammals, like the body partially covered by hair, the mouth with specialized teeth, differentiated into incisors, canines and molars, and the semi-erect posture.

In any case, for the primitive mammals it was not an easy life, especially for the numerous and aggressive presence of terrible predators.

Were particularly feared Microraptor, Sinornithosaurus and Deinonychus, carnivorous dinosaurs of small and medium size, ferocious predators even of small mammals.

Specifically Microraptor was very small in size, about the size of a crow, and typically had the body covered by feathers and was equipped with four wings and characteristic feathers on the legs.

It was widespread in the subtropical and temperate forests of north-eastern Asia where it led a semi-arboreal life. It extinguished about 120 million years ago during the Lower Cretaceous.

Larger dimensions, comparable to those of a large turkey were reached by Sinornithosaurus, carnivorous feathered dinosaur, also feathered, as evoked by the scientific name meaning “Chinese bird-lizard”. Typically provided with long canines with perhaps poisonous bite, was diffused in north-eastern China, where it populated the humid and temperate wooded environments. Also the extinction of this genus dates back to about 120 million  years ago.

Another particularly fearsome predator for the small and primitive Mammals was Deinonychus, carnivorous Dinosaur with feathered body reaching a size comparable to that of a wolf. Equipped with large sickle-sized claws, hence the scientific name meaning “terrible claw”, this dinosaur was widespread in North America and lived preferably in wooded areas where it went hunting in packs. It was extinguished only over 100 million years ago.

Closes this brief review of predators of mammals in prehistoric times,Velociraptor, carnivorous dinosaur that populated the desert dune environments of Central Asia.

As big as a large turkey, Velociraptor had the body covered by feathers and on the forelimbs carried feathers similar to those of the present birds.

The mouth was armed with about 30 sharp and serrated teeth and the second toe of each foot  had a large hooked and retractile claw with which hooked and immobilized the prey.

It got extinct about 71 million years ago.

Apart from the merciless hunt suffered until the disappearance of the aforementioned predators, occurred progressively starting from 120 million and ended 70 million years ago, the primordial mammals have been however obliged to continue to live in the shadow of the strong dinosaurs competition, who practically dominated the terrestrial, marine and aerial environments.

But, about 66 million years ago a catastrophic event occurred that would have radically changed the landscape of vegetal and animal biodiversity on our Planet and also the destiny of our small ancestors.

It is the Cretaceous-Paleogene extinction event, probably caused by the impact of a massive asteroid of 10-15 km as testified by the old Chicxulub impact crater in the Yucatán Peninsula in the Gulf of Mexico.

That was a terrible impact that unleashed tsunamis and devastating fires, and raised an immense cloud of dust that has obscured the sun for a long “nuclear winter”.

In turn, deprived of the sunlight, and consequently unable to perform the process of photosynthesis, the plants died with the consequent catastrophic collapse of the ecosystems. It is estimated that this happened over a period of time between a few months and some years.

This caused the disappearance of the large herbivores who lacked the plants they had been feeding on until then and with them also the carnivores, their predators.

This is how the great tragedy of the mass extinction did occur which caused the disappearance of about 75% of the species living on the Earth, including all the dinosaurs unable to fly.

It was rightly the mass extinction that determined the extraordinary evolutionary success of these small prehistoric animals triggering their adaptive radiation.

In fact, already previously liberated with the disappearance of their predators, therefore also from the competition of all Dinosaurs due to their total elimination caused by the mass extinction, the mammals, since then small and forced nocturnal, were able to enjoy the sunlight and had at their disposal extremely vast territories to colonize and abundant food resources.

So they were also able to increase their size and occupy the so many ecological niches left empty by the many extinguished forms of Dinosaurs.

In this way began the great adventure of the mammals that, no longer oppressed by the predation and by the competition, have rapidly developed thanks to the acquisition of important characteristics, such as the fully differentiated dentition, the braincase developed on the back of the head and the almost erect, orthograde, posture. characters that would have allowed and oriented the magnificent evolutionary path of the Mammalia with the differentiation of new taxa, towards the Hominidae up to the genus Homo.

Nowadays, talking of mammal classification is not easy at all also due to the strong difference of views among the scholars and in the light of the instruments and the methods now used for the taxonomic analysis.

For a long time, to the traditional classification based on the comparative morphological evaluation has been added the modern one that utilizes genetics and molecular biology approaches for understanding the evolution and the kinship relations (phylogenetics) and also of paleontology for reconstructing its evolutionary path.

We precise that in this context reference will be made to a simplified classification based on the traditional taxonomic categories.

Therefore, will not be taken into consideration intermediate categories of various levels like supercohort, cohorts Magnorder, Grandorder, Mirorder and so on.

Making our point clear about their origin and taxonomy, we shall agree with the opinion that divides the class of the Mammalia, in 2 subclasses: Prototheria or Ovipares, and Theria or Placentate, in turn subdivided in infraclasses.

Prototheria

They form a subclass that includes oviparous forms, that have conserved a more primitive organization, more similar to that of the Reptiles, but at the same time highly specialized.

In particular the Prototheria are characterized by the fact that the females lay eggs that are incubated in a nest, like the platypus, or are kept in a typical skin pouch, as happens in the echidnas.

For this feature, some mistakenly consider the Prototheria the connecting ring between mammals and reptiles.

Moreover, the Prototheria are equipped with mammary glands that, unlike all other mammals, are not organized in real breasts and lack nipples.

The pups, which hatch from the fertilized eggs, blind, hairless and immature, cling to their mother and leak its milk that oozes through several small cutaneous orifices.

On the other hand, it must be said that with the two cerebral hemispheres they are not connected with the corpus callosum.

Due to this characteristic, the Prototheria differ quite a bit from the reptiles and get closer to marsupials.

Other singular elements distinctive of the subclass are the presence of nipple-less mammary glands and not organized in real udders as conversely occurs in all other mammals.

Moreover, all members of the Prototheria are equipped with the cloaca, that is one single opening where get the urinary and genital tracts along with the intestinal canal.

In this regard, it should be remembered that the cloaca is a cavity organ present in many other Vertebrate animals, like Fishes, Amphibians, Reptiles and Birds, however the exclusive structural peculiarities of the Prototheria cloaca don’t take on the meaning of their kinship of phylogenetic value with any of these groups.

The adults of the forms belonging to the subclass have a beak-shaped snout and their mouth is toothless.

In any case, it is to be noted that the kids of the members of this subclass, like the fossil forms, are equipped with molars with three cusps.

And more, the males of the Prototheria are typically armed on the inner part for the hind legs of a special hook-shaped horny spur and channeled, connected with a femoral or crural gland that secretes a powerful poison.

This weapon is mainly utilized during the reproductive time, in the fights between males for territory possession and for the conquest of females.

To the subclass of the Prototheria, widespread in Oceania, are attributed only 6 species, all united in the unique order of the Monotremata, whose scientific name (one “single hole”) refers to the presence of the cloaca.

Following are the essential distinctive features of the species ascribed to this order.

The Platypus (Ornithorhynchus anatinus Shaw, 1799), only species of the genus, has a very unique appearance, as evoked by the common name, bird snout, and  by the scientific name meaning duck-billed bird.

The Platypus is endemic to eastern Australia and Tasman Island where it lives in small watercourses and rivers. Excellent swimmer it feeds on larvae of insects, worms, shrimps and occasionally also of small mammals it localizes detecting their body electricity (electrolocation).

The remaining species of this grouping, known as echidnas or spiny anteaters are ascribed to two gener and are diffused in Australia and in the islands of Tasmania and of New Guinea.

The Short-beaked echidna (Tachyglossus aculeatus Shaw, 1792) is characterized by the presence on the back and on the sides of long, more than 5 cm, spines, of yellow colour with black tips.

The body is a little more than 50 cm long with the tail measuring almost 10 cm. The snout is thinned, hairless and of black colour.

The legs are robust and equipped with 5 claws each for digging and dismantling the termite mounds.

The mouth, narrow and toothless, has a long and protractile tongue, covered by viscous saliva where get stuck ants and termites and other small invertebrates, its main food.

The Short-beaked echidna lives in Australia, and in the islands of Tasman and of New Guinea where it is found in various environments, such as mountain forests and deserts.

The Long-beaked echidna of Sir David (Zaglossus attenboroughi Flannery & Groves, 1998) is the smallest species in the genus.

It has thick fur, usually of brown colour, and the paws are equipped with five claws, suitable for digging.

This species is typical of the Cyclops Mountains located in the island of New Guinea.

And more, the Western long-beaked echidna (Zaglossus bruijni Peters & Doria, 1876) measures about 90 cm per about 15 kg of weight, and is characterized for having long and pointed snout, turned downward.

The back is covered by coarse spines and the legs have three claws or big nails utilized for digging.

The mouth has no teeth and has a big viscous tongue with which it seizes earthworms that form its main food.

The species is confined in the westernmost region of New Guinea.

Finally, the Eastern long-beaked echidna (Zaglossus bartoni Thomas, 1907) is the biggest mammal of the order.

It distinguishes from the other species of the genus as having the forelegs provided of five robust nails and the hind ones of four it utilizes for digging the soil looking for worms which form the main food. The body, equipped with spines, is covered by a thick black fur and has no tail. The species is endemic to the island of New Guinea.

Theria

Called also placentates, they are viviparous animals with placenta, a temporary structure forming in the mother’s uterus during pregnancy and connects the mother to the foetus through the umbilical cord.

The placenta is the fundamental organ for the development of the embryo’s vital processes, like nutrition, oxygenation, elimination of waste substances, production of the hormones necessary for fringing to a successful conclusion the pregnancy.

The subclass of the Theria is divided into the infraclass of the Metatheria (marsupials) and in that of the Eutheria (euplacentata) where almost all present mammals are included.