ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 18. CLASS MAMMALIA, OR MAMMALS (Mammalia)

1. General Characteristics of the Class Mammalia, or Mammals (Mammalia)

The most highly organized Class of vertebrates. The global fauna includes about 5,000 species of mammals (over 100 species in Ukraine). They are divided into terrestrial, underground, aquatic, and flying forms. The largest number of mammal species are terrestrial animals, which are distributed almost everywhere. The first representatives of mammals appeared on Earth during the Triassic period.

Theriology (from Greek therion — beast, logos — study), mammaliology, mammalogy — a branch of zoology that studies mammals.

Giants among mammals:

✵ terrestrial — the giraffe (up to 4.5 m tall); the African elephant (3.5 m tall, weighing 4-5 t);

✵ aquatic — the blue whale (up to 33 m long, weighing over 150 t).

Dwarfs among mammals: the Etruscan shrew (length — 3.8-4.5 cm, weight 1.2-1.7 g).

Progressive features of mammals:

1. Warm-blooded (homeothermic) animals with a well-developed capacity for thermoregulation.

2. Development of a body covering: fur or Hair, which helps retain heat. Presence of sebaceous and Sweat Glands in the Skin.

3. Emergence of Organs that ensure embryonic development within the mother's body and live birth.

4. Nursing offspring with milk (some Skin glands are modified into Mammary Glands).

5. High level of Nervous System development, particularly the Cerebral Cortex covered with numerous sulci, which provides ADAPTATION TO ENVIRONMENTAL conditions through behavioral changes.

6. Differentiation of the Vertebral Column into distinct regions and the shifting of limbs from the sides to underneath the body, raising the body off the ground (unlike reptiles).

7. Complete Separation of SYSTEMIC AND PULMONARY Circulation (four-chambered Heart, two circulatory loops, the left aortic arch is retained); red Blood Cells (erythrocytes) are non-nucleated.

8. Development of alveolar Lungs, which increased the intensity of gas exchange and, consequently, the overall metabolic rate.

9. The body cavity is divided by a Diaphragm into thoracic and abdominal compartments.

10. Teeth are differentiated and have roots. Well-developed Stomach (multichambered in some species) and elongation of the cecum.

11. Appearance of the outer ear (pinna) in the auditory system, and three auditory ossicles (malleus, incus, stapes) in the Middle ear.

Structural Features of mammals (Table 13):

The body shape of mammals varies greatly in size and appearance.

Table 13

Biological features of MAMMALS AS HIGHLY ORGANIZED VERTEBRATES

The body consists of a HEAD, neck, trunk, paired limbs, and a tail. The head features external ears (pinnae), whiskers (sensory hair), an elongated Nose, a Mouth with Lips, and eyes with two

eyelids and eyelashes. The shape and proportions of body parts vary among different species depending on their adaptation to the habitat and The Nature of movements associated with foraging and defense against predators. In mammals that spend all or most of their lives in Water (whales, seals, walruses), the body shape is streamlined (the head merges smoothly into the trunk, with an indistinct neck), and the limbs are modified into flippers. Terrestrial mammals adapted for fast running (horses, zebras) have a slender trunk, elongated limbs, and a long, mobile neck. Burrowing mammals (moles, shrews, ground squirrels) that live in tunnels have a cylindrical body, shortened limbs, and a barely noticeable neck. Species that climb tree branches (monkeys) possess prehensile limbs and tails. Some species (Order Chiroptera) are capable of flight: their forelimbs have evolved into unique wings.

The integument of mammals is a relatively thick skin consisting of several layers:

a) epidermis (Fig. 77) — stratified; the outer cells gradually keratinize (lose their nuclei, fill with keratohyalin granules) and desquamate (shed as dander); it reaches its greatest thickness in areas subjected to constant friction during walking and climbing. ^{1 2 3 4}

Fig. 77. Structure of mammalian skin (after Naumov, Kartashev, 1979):

1 — outer stratum corneum of the epidermis; 2 — stratum germinativum, or Malpighian layer, of the epidermis;

3 — dermis (corium); 4 — hair; 5 — sweat gland; 6 — duct opening of the sweat gland;

7 — sebaceous gland; 8 — arrector pili Muscle; 9 — Connective Tissue fibers;

10 — blood vessels; 11 — hair bulb.

Calluses often form in these areas (paw pads, ischial callosities of monkeys, knee calluses of camels). At the Base of the epidermis lies the stratum germinativum, or Malpighian layer, composed of epithelial cells that divide intensively and give rise to the overlying layers; its lower surface has invaginations into which the papillae of the corium (dermis) fit: this ensures a strong connection between both skin layers and increases their contact surface, which is crucial for the epidermis to receive nutrients and oxygen solely via diffusion from the Blood vessels of the corium (as the epidermis lacks blood vessels);

b) dermis (corium) — a layer thicker than the epidermis, permeated with blood vessels; it contains hair bulbs, sweat glands, Sebaceous Glands, scent glands (which are modifications of sweat or sebaceous glands), mammary glands (which are modified sweat glands), receptors (thermal, tactile, pain), and pigment cells;

в) the lowest, deepest layer of the corium — subcutaneous adipose tissue (hypodermis) — is formed by adipose connective tissue, which performs thermoregulatory and storage (as an additional energy source) Functions; the thickest layer of subcutaneous fat is found in cetaceans (where it can reach 30-40 cm) and pinnipeds; subcutaneous fat is generally developed more or less uniformly across the body (thinner on the head and limbs), but in camels, it is concentrated at the base of the humps on the back, and in fat-tailed sheep, on the tail.

Skin coloration is determined by pigments distributed as melanin granules within the Cells of the stratum germinativum, intercellular spaces, and specialized pigment cells (melanoblasts, melanophores).

In addition to the thickening of the epidermal stratum corneum (calluses), mammals develop specific keratinous structures: hair, claws, Nails, hooves, and horns.

The hair coat is developed on almost all PARTS OF THE body (absent on the lips, and in some species, on the soles) and performs a thermoregulatory function, acts as a tactile receptor, protects the skin from mechanical damage and certain parasites, improves the aerodynamic and hydrodynamic Properties of the body, and provides species-specific coloration. A continuous hair coat forms fur, which consists of long, thick, and resilient guard hairs and short, soft underfur, or guard hairs alone. Hair on the mammalian body is angled in a specific pattern, usually from head to tail. In sloths, which constantly hang upside down from trees, the hair is directed from the abdomen to the back. On certain areas of the body, mammals grow large, individual hairs on the head called vibrissae, or whiskers; they are part of the tactile organs. In some animals, guard hairs can modify into spines or quills (echidnas, hedgehogs, porcupines). The hair coat is not permanent and is periodically replaced by a new one through molting (shedding). In some species, a significant portion of the hair is replaced all at once, while in others, this occurs twice a year: in spring and autumn (changing not only the fur structure but often its color as well). In squirrels, for example, there are about 4,200 hairs per 1 cm² of skin in summer (guard hair 17.4 mm long, underfur 9.4 mm long), and 8,100 in winter (guard hair 25.9 mm long, underfur 16.8 mm long). Therefore, the thermal insulation quality of the winter coat is dramatically increased compared to the summer coat. Only a few mammals (cetaceans, elephants, rhinoceroses, hippopotamuses) have virtually no hair coat on their bodies, although during embryonic development, the fetuses of these animals are temporarily covered with hair primordia (indicating the secondary nature of its loss in adulthood).

The terminal Phalanges of the digits in most mammals are protected by keratinous claws (Fig. 78), which are derivatives of the epidermis.

Fig. 78. Terminal phalanges of mammalian digits (after Naumov, Kartashev, 1979):

I — nail; II — claw; III — hoof:

1 — digital pad; 2 — sole plate; 3 — claw plate; 4 — nail fold; 5 — terminal phalanx.

In arboreal mammals, claws are sharp and strongly curved, while in burrowing species, they are elongated and flat. In all felids (except the cheetah), the claw, along with the terminal phalanx, is retracted toward the dorsal surface of the middle phalanx by specialized tendons, preventing it from dulling during walking. In most primates, claws have evolved into nails (Fig. 78) that cover the tips of the digits only from above; a soft pad is developed underneath, enhancing the tactile sensitivity of the fingers.

The specialization of claws led to The formation of hooves (Fig. 78) — thick keratinous structures that almost completely encase the terminal phalanx. Hooves are particularly well-developed in cursorial (fast-running) species (antelopes, goats, horses).

Due to the extensive proliferation of the keratinized epithelium, massive horns are formed in rhinoceroses, as well as the horns of bovids — hollow keratinous sheaths fitted over bony cores that are fused with the frontal bones. Deer antlers, on the other hand, are bony structures derived from the corium: they are shed annually.

Like that of other terrestrial vertebrates, the mammalian Skeleton is divided into the Axial Skeleton and the associated rib cage, the Skull, and the Skeleton of the limbs (forelimbs and hindlimbs) and their girdles.

The axial skeleton is formed by vertebrae; the Characteristic Features of the vertebrae that make up the axial skeleton are:

✵ their platycoelous (flat-surfaced) shape;

✵ cartilaginous intervertebral discs are located between the vertebrae;

well-developed neural arches.

The axial skeleton consists of five regions:

1) cervical — 7 vertebrae; only manatees and Hoffmann's two-toed sloths (Choloepus hoffmani) have 6, while three-toed sloths (Bradypus) have 8-10; the first vertebra, the atlas, and the second, the axis, have the same Structure and function as in reptiles; the longest cervical vertebrae are found in giraffes, and the shortest in cetaceans;

2) thoracic — 12-15 vertebrae; Ribs attach to the thoracic vertebrae, some of which are connected to the flat Sternum, forming the rib cage; the last 2-5 thoracic vertebrae bear "false ribs" that do not reach the sternum; in bats and burrowing species with strong forelimbs (such as moles), the sternum features a small projection — a keel, to which the pectoralis Major and minor Muscles attach, driving the wings of bats or the forelimbs of moles;

3) lumbar — 2-9 massive vertebrae;

4) sacral — 4-10 fused vertebrae, of which only the first 2 are true sacral vertebrae, while the others are caudal;

5) caudal — 3-49 free vertebrae.

The mammalian skull consists of the facial and well-developed cranial regions. The facial region contains the orbits, upper and lower jaws, which bear teeth of various shapes and functions (incisors, canines, and cheek teeth) in special sockets called alveoli. Mammals develop a hard bony palate that separates the nasal passage from the Oral Cavity. This allows the animal to breathe while the mouth is full of food.

The Pectoral Girdle: two scapulae with fused coracoid? bones and two clavicles, which are present only in those mammalian species whose forelimbs are capable of diverse and complex movements (moles, bats, primates, cats, bears). Clavicles are absent in canids and ungulates.

The skeleton of the free forelimb consists of the arm (humerus), forearm (ulna and radius), and hand (carpals, metacarpals, and phalanges).

The Pelvic Girdle consists of two bones formed by the fusion of the ilium, ischium, and pubis. The pelvis is closed: the pubic and ischial BONES OF THE left and right sides fuse with each other along the midline, forming a symphysis.

The skeleton of the free hindlimb consists of the thigh (Femur), shin (Tibia and Fibula), and FOOT (tarsals, metatarsals, and phalanges).

Due to mammals living in diverse environments, their limbs can be highly modified. For instance, in aquatic mammals (pinnipeds, cetaceans), the forelimbs have evolved into flippers, and in bats, into wings. In animals adapted for fast running, the bones of the hand are positioned more or less vertically because they support themselves on their digits during movement (canids, felids, ungulates).

The musculature of mammals is highly differentiated. The most developed are the masticatory muscles, and the Muscles of the back and limbs. A distinctive feature of mammals is

the presence of a dome-shaped muscular partition — the diaphragm, which separates the thoracic cavity from the Abdominal cavity. The Evolution of the diaphragm allowed for a dramatic intensification of lung ventilation. Complex subcutaneous musculature is involved not only in thermoregulation (altering the state of the coat, curling the body into a ball) but also in animal communication: it controls the movement of vibrissae and provides facial expressions (mimic muscles), which play an important role in transmitting information, especially in carnivores and primates.

Many mammalian muscles contain Myoglobin, which provides an oxygen reserve. Its concentration is highest in cardiac and skeletal muscles. The highest myoglobin content is found in aquatic mammals, allowing them to remain underwater for long periods. The muscles of the sperm whale contain 8-9 times more myoglobin than those of terrestrial mammals. How does a sperm whale regulate its diving depth? In the sperm whale's head, which spans up to a third of its total body length, There is a cavity filled with a waxy substance called spermaceti. Thanks to spermaceti, the sperm whale can regulate its buoyancy. When preparing to dive, it increases Blood Circulation around this cavity; the spermaceti melts, and its density increases. The sperm whale becomes heavier than water and easily dives to depths of 1-2 km. When the sperm whale wants to resurface, it draws water into its nasal passages, cooling the spermaceti and decreasing its density, making it lighter than water so it floats to the surface.

The positioning of the limbs beneath the body allowed for an increased stride length and more efficient Muscle Function in mammals. The movement speeds of various mammalian species are:

✵ shrews — 4-7 km/h; elephants, rabbits — up to 40 km/h;

✵ fur seals — up to 27 km/h; antelopes, lions — up to 80 km/h;

✵ bats — 25-30 km/h; cheetahs — up to 105-115 km/h.

The Digestive System of mammals begins with the mouth apparatus (lips, which are unique to mammals, a muscular Tongue with taste buds, jaws, and teeth), which is used for capturing, gathering, and mechanically Processing food. The cavity between the lips and the jaws is called the Vestibule of the mouth. In ground squirrels, chipmunks, and monkeys, the vestibule expands to form cheek pouches where food can be temporarily stored. Within the jaw sockets, there are alveolar teeth of various shapes and purposes:

✵ incisors — have an elongated, sharpened edge; they help bite off a small piece of food from a larger chunk;

✵ canines — are conical and pointed, and larger than other teeth; they help hold and tear prey, and also serve for defense;

✵ premolars and molars — have a wide, bumpy or flat surface for grinding food.

Heterodonty (tooth differentiation) in mammals is an important adaptation that improves feeding and Digestion.

In terms of structure, all teeth are similar: they consist of a ROOT (incisors and canines have one root, premolars and lower molars have 2 roots, upper molars have 3 roots), a neck, and a crown covered with enamel. In most mammals, teeth are replaced at least once

during their lifetime. The first teeth are called deciduous (milk) teeth and have underdeveloped roots. Deciduous teeth are replaced by permanent ones.

The incisors of rodents and lagomorphs grow throughout their lives and are constantly worn down and sharpened, remaining constant in size and always sharp. Hares have only 28 teeth. Elephants retain only a pair of incisors in the upper jaw (tusks) and a pair of molars in each jaw; their total number of teeth is 6. The marsupial opossum has 50 teeth, the wild boar has 44, the wolf has 42, the cat has 30, and the mouse has 16. Baleen whales are completely toothless. They feed on plankton, catching their prey with the help of their tongue and special baleen plates (from 160 to 500), the bases of which are embedded in the gum tissue of the upper jaw, and the fringed edges are in the oral cavity.

In the oral cavity, food undergoes mechanical processing and is exposed to salivary Enzymes: amylase begins breaking down starch during chewing. From the oral cavity, food passes through the Pharynx and Esophagus into The Stomach.

The esophagus of most mammals is a thin-walled tube made of Cytology/cytology/32.html">Smooth Muscle tissue. Only in ruminant artiodactyls (cows, deer, goats, sheep, giraffes) does it have striated muscle, which allows food (cud) to be regurgitated from the stomach back into the oral cavity for additional chewing.

The stomach of most mammals is single-chambered, but in ruminant artiodactyls, which feed on coarse plant food, it is four-chambered (consisting of the rumen, reticulum, omasum, and abomasum). The first three compartments (rumen, reticulum, omasum) are lined with Stratified Epithelium and lack digestive glands. In these compartments, food Fermentation occurs under the action of enzymes from symbiotic Bacteria, which can only exist in a neutral or slightly alkaline environment. Gastric juice secreted by the stomach glands contains Hydrochloric acid (up to 0.4-0.5%) and enzymes — Pepsin (breaks down Proteins) and lipase (breaks down fats). Processing of the food bolus with gastric juice occurs only in the abomasum, in its acidic environment. The complex stomach of cetaceans, whose teeth cannot grind food, provides mechanical processing during peristaltic movements of its walls. The gastric juice of cetaceans is extremely active and decomposes (hydrolyzes) even highly resistant substances such as Chitin, which is indigestible for other animals.

From the stomach, food enters the intestine, which consists of three sections: the Small Intestine, the Large Intestine, and the rectum. The ducts of the digestive glands, namely the Pancreas and Liver, empty into the upper part of the small intestine, called the duodenum. Digestive glands participate not only in digestion by producing active enzymes, but also in general metabolic and excretory processes, as well as in their hormonal regulation. Glands in the walls of the small intestine secrete various digestive Enzymes for the final digestion of food. The absorption of major nutrients occurs in the small intestine. At the junction of the small and large intestines, a large cecum branches off, harboring symbiotic bacteria, Fungi, and Protozoa. The cecum is reduced in size or completely absent in species that feed exclusively on animal food. Undigested remains enter the large intestine, where water is primarily absorbed, and then proceed to the rectum. Fecal matter is formed there and expelled through the anus. The length of the intestine depends on the diet: in bats, the intestine is 1.5-4 times longer than the body; in insectivores (hedgehog, white-toothed shrew, shrew, mole), 2.5-4.5 times; in the wolf, 6.5 times; in rodents (squirrel, beaver), 5-12 times; in the horse, 12 times; and in the sheep, 29 times.

The smaller the animals, the more food they need and the more frequently they must feed. For example, the Etruscan shrew (weighing 1.2-1.7 g) consumes 2-4 times its body weight in food daily. It cannot survive without food for more than 5-8 hours.

The excretory system of mammals includes bean-shaped metanephric (pelvic) Kidneys, Ureters, a Urinary Bladder, and a Urethra. The smallest Structural and functional unit of the Kidney is the nephron. It consists of a glomerulus and Bowman's capsule. A renal tubule extends from the capsule and is divided into four sections: the proximal convoluted tubule, the Loop of Henle, the Distal convoluted tubule, and the collecting duct. The openings of the collecting ducts empty into the renal pelvises, from which the ureters originate. Urine is formed (first primary urine, then secondary urine) through two processes, namely diffusion and reabsorption, which occur in the nephron. In mammals, urea is excreted as the main product of Nitrogen METABOLISM; in this respect, mammals are closer to amphibians. Excess water and salts are also excreted with urine. The skin with its sweat glands and the intestinal tract also participate in Water-Salt Metabolism in mammals.

Desert mammals are capable of eating almost completely dry food and practically never drinking throughout their lives, satisfying their water needs solely through metabolic water produced by biochemical reactions in the body.

During favorable periods, camels store fat in their humps, which is consumed during dry periods, producing a certain amount of metabolic water. In addition, they lower their body temperature during rest and Sleep, which also significantly reduces water loss.

The Respiratory system of mammals consists of:

✵ the airways: paired nostrils; the Nasal cavity (function: trapping dust particles during inhalation — the mucosal epithelium secretes mucus that disinfects the air; the air is warmed or cooled (in the desert) by blood capillaries); the Larynx (its anterior and lateral walls form the thyroid Cartilage; a thin epiglottis adjoins the anterior margin of the thyroid cartilage, closing the entrance to the larynx during the passage of food through the pharynx; vocal cords are present); the Trachea (the walls consist of cartilaginous rings or half-rings that maintain a constant lumen for air passage); two Bronchi (the walls consist of cartilaginous rings; they branch into smaller tubes, forming the bronchial tree; the smallest tubes — bronchioles — end in tiny sacs — alveoli);

✵ alveolar lungs (alveoli measuring 25-400 μm, with numerous capillaries branching in their walls; this structure increases the respiratory surface area by 50-100 times compared to the body surface area; the relative size of the respiratory surface is larger in mammals with a more active lifestyle: in carnivores — 28 cm² per 1 g of body weight, in the sloth — 6.2 cm² per 1 g of body weight. Carnivorous mammals have 100-500 million alveoli in their lungs, while the sloth has 2 million).

Lungs play the primary role in mammalian gas exchange, but the mucosal surface of the airways and, to a small extent, the skin (through which about 1% of oxygen enters) also participate. Respiratory movements are carried out by the rib cage and the diaphragm.

Respiration also plays a role in thermoregulation. Rapid, shallow breathing increases evaporation from The surface of the Upper Respiratory Tract, which promotes heat dissipation.

The Circulatory system OF mammals is similar to that of birds: The Heart is four-chambered; There are two Circuits of blood circulation (Fig. 79).

Fig. 79. Diagram of mammalian blood circulation (after Naumov, Kartashev, 1979):

1 — right ventricle; 2 — left ventricle; 3 — right atrium; 4 — left atrium;

5 — pulmonary Arteries; 6 — capillary network in the lungs; 7 — Pulmonary Veins; 8 — aorta;

9, 10, 11, 13 — capillary network in body organs; 12 — artery; 14, 15 — veins.

Arrows indicate the direction of blood flow.

Only the left aortic arch arises from the left ventricle, giving off branching arteries. Arterial and venous blood do not mix. Mammals have a high metabolic rate and are warm-blooded (homeothermic).

The relative size of the heart is larger in active and small animals. In large species, the heart mass is 0.2-0.7% of body mass, in small ones — up to 1-1.5%; in bats — 1.3%. The heart rate also depends on the size of the animal. For example, the pulse rate per minute is 600 in a mouse, 140 in a dog, and 24 in a bull and an elephant. In aquatic mammals, the heart rate decreases upon diving (in seals, from 180 at the surface to 60-30 underwater), which allows for a more economical use of oxygen stores in the lungs and airways.

Hematopoietic organs of mammals: Red Bone Marrow produces erythrocytes, leukocytes, and platelets; the Spleen and Lymph Nodes produce lymphocytes.

The small erythrocytes of mammals lack nuclei, which increases their oxygen-carrying efficiency, as they consume 9-13 times less oxygen for their own respiration than avian erythrocytes and 17-19 times less than amphibian erythrocytes.

The Nervous System of mammals is similar in structure to that of all vertebrates. The Central nervous system consists of the BRAIN AND SPINAL cord, while the Peripheral Nervous System consists of the nerves branching from them. The mammalian brain is significantly larger in volume, which is due to the increased size of the Forebrain and Cerebellum. In the forebrain, most of the brain matter is concentrated in the cerebral cortex; it is the center of Higher Nervous Activity and the coordinator of other brain regions. In most mammals, the cerebral cortex forms gyri and sulci that increase its surface area. In primates and toothed whales, the number of sulci is particularly large. These animals are characterized by complex behavior (complex Conditioned and Unconditioned Reflexes). The relative size of the brain increases as body size decreases and animal activity increases. For example, in relatively large insectivores (hedgehog), the brain mass is about 0.6% of body mass, while in small ones (white-toothed shrew, shrew) it is up to 1.2%; in large cetaceans (baleen whales: blue whale, bowhead whale) it is about 0.3%, and in small ones (toothed whales: dolphins, killer whales) it is up to 1.7%. The brain mass of primates is 0.6-1.9% of body mass, and in humans, it is about 3%. In all mammals, the mass of the forebrain exceeds the mass of other brain regions: in various groups, it accounts for 52-72% of the total brain mass; in primates, this figure rises to 76-80%, and in humans, up to 86%. The ratio of brain to Spinal Cord mass is highest in humans (45 : 1), high in primates and cetaceans (10-15 : 1), and lower in carnivores, insectivores (3-5 : 1), and ungulates (2.5 : 1).

Mammals, like all other vertebrates, possess organs of sight, Hearing, balance, smell, taste, and Touch.

Vision is very well developed in mammals that inhabit open areas. Externally, the mammalian eye is covered by an outer fibrous tunic (sclera), which transitions into the transparent cornea at the front of the eye. Beneath the sclera lies the choroid, containing blood vessels that nourish the eye. Between the sclera and the choroid in some mammals, there is a layer of crystalline cells forming a tapetum lucidum that reflects light rays, causing the eyes to "glow" in reflected light (carnivores, ungulates). Anteriorly, the choroid transitions into the iris with the pupil (which acts as a diaphragm, regulating the illumination of the retina by changing the pupil size) and the ciliary body (muscles that accommodate the eye by changing the shape of the lens). The lens, which is lenticular in shape, is relatively small in diurnal mammals and significantly larger in nocturnal ones. Behind the choroid lies the retina, which consists of outer pigmented and inner photosensitive layers. The retinal receptors are rods (black-and-white vision) and cones (Color Vision). Many mammals are capable of distinguishing colors.

Smell and hearing are well developed in nocturnal and crepuscular forest-dwelling animals.

The olfactory organs are located in the upper-posterior part of the nasal cavity and consist of receptor cells with cilia. The SENSE OF SMELL in mammals is more effective than in other terrestrial vertebrates: chemoreceptors allow them to distinguish specific substances (scents) characteristic of a species, a group of individuals, or even individuals. A highly developed sense of smell is found in marsupials (kangaroos), insectivores (moles, hedgehogs, white-toothed shrews, shrews), rodents (squirrels, beavers, mice, rats), carnivores (canids), and ungulates; they are called macrosmatic. Most primates (monkeys and apes) and A number of other mammals have a less acute sense of smell, so they are called microsmatic.

The Organ of Hearing (Fig. 81) in mammals consists of three sections: the outer ear (the pinna and the external auditory canal, which ends at the tympanic membrane), the middle ear (with three auditory ossicles: malleus, incus, and stapes; it connects to the back of the oral cavity via the Eustachian tube, which ensures equal air pressure on both sides of the tympanic membrane), and the Inner ear (the cochlea, which contains The Organ of Corti; the function of the organ of Corti is the primary analysis and encoding of sound signals transmitted to the auditory center of the brain).

The hearing range of mammals is wider compared to birds; they use both ultrasonic (over 20 kHz) and low frequencies. For example, dogs and bats perceive ultrasound. This ability is used in dog training, and in bats, it enables spatial orientation. For echolocation, bats use low-frequency sounds down to 12 Hz (infrasound) in addition to ultrasound (4080 kHz). Toothed whales (dolphins, sperm whales, killer whales) have a wide sound range: from a few hertz to 20θ kHz. Baleen whales (blue whale, bowhead whale, humpback whale) emit sounds of 1-2 kHz with high intensity and duration.

Fig. 81. Diagram of mammalian ear structure (after Naumov, Kartashev, 1979):

1 — pinna; 2 — external auditory canal; 3 — tympanic membrane; 4 — auditory ossicles in the middle ear cavity;

5 — Eustachian tube; 6 — vestibular apparatus; 7 — cochlea; 8 — auditory nerve.

The Organ of Balance (vestibular apparatus) includes three semicircular canals and the utricle, connected to the inner ear (Fig. 81, 6).

The Organs of taste in mammals are located in the oral cavity on the tongue. They are highly developed in herbivores. Thanks to their taste organs, they can identify the edible plants they feed on.

Tactile sensitivity in mammals is provided by receptors for heat and cold (thermoreception), pressure, and touch. On parts of the body that most frequently come into contact with environmental objects, mammals have especially long and stiff hairs called vibrissae. Their roots are associated with nerve endings. Vibrissae reach their greatest development on the muzzle.

The reproductive organs of mammals are significantly more complex than those of other amniotes. Females have paired Ovaries, oviducts, a Uterus, a Vagina, and a vaginal vestibule (where the urethra opens). Males have paired Testes (located in the posterior part of the abdominal cavity in egg-laying mammals, some insectivores, proboscideans, cetaceans, sirenians, and rhinoceroses, or relocated to the Scrotum — a leathery pouch connected to the body cavity by the Inguinal Canal in marsupials, carnivores, ungulates, and primates), epididymides, vasa deferentia, and the urogenital canal of the Penis. Fertilization is internal, occurring within the female reproductive tract where the male's semen is deposited.

Based on the characteristics of reproduction and development, all mammals can be divided into three groups:

1) egg-laying mammals (monotremes) — they lay eggs, but for a certain period, the eggs remain in the female's reproductive tract; the eggs are covered with a leathery shell; after laying, the animals incubate them (platypus) or carry them in a skin pouch (echidna); after hatching, the young feed on milk secreted by numerous ducts of mammary glands without nipples;

2) marsupials (kangaroos, Tasmanian wolf, marsupial jerboa, koala) — there is only a rudimentary Placenta (the embryonic membranes only adjoin the uterine wall, but no villi are formed to penetrate deep into the uterine wall), so the embryo feeds on the egg yolk and secretory secretions of the uterine wall, which are insufficient for complete development.

Pregnancy is short-lived: 38-40 days in the giant kangaroo; 12 days in the opossum; the newborn is very small, weak, and helpless, so it continues its development in the mother's pouch; the newborn in the pouch finds a teat of the mammary gland and remains attached to it for a relatively long time.

3) placentals — a true placenta develops, which is an organ that provides a connection between the mother's body and the embryo. The embryo receives nutrients and oxygen from the mother and is cleared of Metabolic waste products) (Fig. 80);

Fig. 80. Rabbit embryo with membranes (on the twelfth day)

(after Naumov, Kartashev, 1979):

1 — embryo; 2, 3, 4 — membranes surrounding the embryo; 5 — placenta.

In animals, gestation is longer the larger they are: in insectivores — 13-19 days, in the house mouse — 18 days; in the vole — 1623 days, the muskrat — 25-26 days, in marmots — 30-40 days, in squirrels — 35-40 days, in bats — 54-73 days, in the leopard — 120 days, in the coypu — 130

days, in the pig -150 days, in the brown bear and walruses — 200 days, in the cow — 290 days, in cetaceans — 270-365 days, in the double-horned rhinoceros — 530-550 days, in the elephant — 600 days.

After birth, the offspring feeds on the mother's milk; mammalian milk contains all the substances necessary for the young's development: proteins, fats, CARBOHYDRATES, Vitamins, and salts (The rate of development is higher the more proteins and fats the milk contains).

Sexual maturity occurs earlier in small mammals: voles — at the age of 1,5-2 months, mice — 2-3 months, muskrats — 5 months, hares — around a year. Larger mammals begin to breed later: wolves, martens, foxes, sables — in their second year of life, tigers, bears, seals, cetaceans — in their 3-4-му year, deer — in their 2-4-му year, rhinoceroses, elephants — at 10-15 years, monkeys — at 10-12 years.

The overall fecundity of mammals is low, thanks to a high level and Various Forms of parental care (warming, protection, feeding, grooming, and "training"). In small rodents, which are characterized by high mortality, the number of young can reach 1012 individuals, and breeding continues year-round (up to 6 litters). Hares and squirrels give birth 2-3 times a year to 3-8 young. Wolves, foxes, cats, sables, and martens breed once a year and give birth to 3-6 young. Deer, seals, and dolphins give birth to 1-2 young once a year. Elephants, baleen whales, tigers, and lions breed once every 2-3 years and give birth to 1-2 young.

The annual cycle of mammals consists of several phases:

✵ preparation for breeding and the breeding process — maturation of Gametes and pair formation (permanent or temporary) for the breeding season; Selection of a breeding site (which may involve long-distance Migrations) or construction of specific "shelters";

✵ care of newborns — warming, protection, feeding, grooming, "training";

✵ preparation for wintering — intensive feeding to accumulate a fat layer; they may migrate in search of food; molting;

✵ wintering — some animals (certain carnivores, insectivores, bats, rodents) hibernate, while others remain active throughout the winter; in some species (brown bear, raccoon dog, ground squirrels), hibernation during the winter period may be interrupted during prolonged thaws, when they emerge from their shelters and lead an active lifestyle; in other animals (hedgehogs, bats), hibernation is continuous.