Evolutionary Pressures in Mammals

EVOLUTIONARY PRESSURES IN MAMMALS

Mammals are naturally distributed on all continents except Antarctica. They live in all oceans.

EXTERNAL STRUCTURE AND LOCOMOTION

SKIN

The skin of a mammal consists of epidermal and dermal layers. It protects the animals from mechanical injury, microorganisms, and ultraviolet radiation of sun. Skin also play role in temperature regulation, excretion and water regulation. The skin also acts as sensory organ.

Hair

Hair is a keratinized structure. It is derived from epidermis of the skin. It is present in a hair follicle. Hair follicle is an invagination of the epidermis. A coat of hair is called pelage. The pelage consists of two kinds of hair. Long guard hairs protect the shorter under hairs. These shorter hair forms a dense insulating coat.

Pelage is reduced in large mammals living in hot climates (e.g.. elephants and hippopotamuses) and in some aquatic mammals (e.g., whales). They have fatty insulation. A few mammals like naked mole rats have no pelage.

1. Molting of hair: Hair is composed of largely dead cells. It is periodically molted.  In some mammals molting occurs gradually. Thus it may not be noticed. In other mammals hair loss occurs rapidly. Thus it changes the pelage characteristics. In the fall, many mammals acquire a thick coat of insulating under hair. It changes the color of pelage. For example, the Arctic fox takes a white or cream color with its autumn molt. It helps the fox to conceal in a snowy environment, The Arctic lox acquires a gray and yellow pelage afier spring molt.

2. Hairs as sensory structure: Hair is also important for the sense of touch. Hair roots have nerve cells Mechanical displacement of a hair stimulates these nerve cells. Sometimes, guard hairs are modified into thick shafted hairs called vibrissae. Vibrissae occur around the legs, nose, mouth, and eyes of many mammals. Their roots are richly supplied with nerve cells. Vibrissae are very sensitive to displacement.

3. Hair as insulatory structures: Air spaces are present in the hair shaft. Air is trapped between hair and the skin. It forms an effective insulating layer. Arrector pili muscle runs between the hair follicle and the lower epidermis. Arrector pili are a band of smooth muscle. It contracts and the hairs stand upright. It increases the amount of air trapped in the pelage and improves its insulating properties. Arrector pili are under the control of the autonomic nervous system. Autonomic nervous system also controls the “fight-or-flight^­ ” response of mammal. The hair (especially on the neck and tail) stands in threatening situations. It gives the perception of increased the size and strength.

4. Hair coloration: Hair color depends on the amount of pigment melanin and quantity of air in the hair shaft. The pelage of most mammals is dark above and lighterunderneath. This pattern makes them less conspicuous under most conditions. Some mammals use aposematic (warning) coloration.

Claws

Claws are present in all amniote classes. They are used for locomotion and offensive and defensive behavior. Claws are formed by the accumulation of keratin. Claw covers the terminal phalanx (bone) of the digits. In some mammals, claws are specialized to form nail or hooves.

Skin glands

Glandsds develop from the epidermis of the skin. There are following glands:

(a) Sebaceous glands (oil gland): They are associated with hair follicles. They produce oily secretion. These secretions lubricate the skin. It makes the skin and hair waterproof.

(b) Sudoriferous glands (sweat gland): Most mammals also possess these glands. There are two types of sudoriferous eland:

• Eccrine glands: These are small sudoriferous glands. They release watery secretions. It is used for evaporative cooling.
• Apocrine glands: These are larger sudoriferous glands. They secrete a mixture of salt, urea, and water. The microorganisms on the skin produce odorous smell from these compounds.

(c) Scent or musk glands: They are present around the face. feet. or anus of many mammals. These glands secrete pheromones. These pheromones are involved in defense, species and sex recognition, and territorial behavior.

(d) Mammary glands: Mammary glands are functional in female Mammals. But they are nonfunctional in males. Mammary gland secretes milk. The milk contains water, carbohydrates (lactose), fat, protein, minerals, and antibodies. Mammary glands are derived from apocrine glands. They contain large deposits of fats.

The mammary glands of Monotremes lack nipples. The glands discharge milk into depression in the belly . The young lap it up. In other mammals, mammary land open through nipples or teats. and theyoung suckle them for their nourishment.

THE SKULL AND TEETH

SKULL

The skulls or mammals skims reptilian pattern. It has some important modifications.

1. Modifications in the jaw

The method of jaw articulation is used to differentiate between reptiles and the mammals. The jaw articulates  with two small bones at the rear side of the jaw in reptiles. In mammals, these bones have moved into the middle ear. These bones with the stapes form the middle-ear ossicles. A single bone of the lower jaw articulates in the mammalian jaw.

2. Evolution of palate

A secondary palate evolved twice in land vertebrates. First it evolved in the archosaur lineage and then in the synapsid lineage. Palate has two parts:

(a) Hard Palate: In some therapsids. hard palate formed as a small, shelf like extensions lineage and then in the synapsid lineage. It partially separated the nasal and oral passageways.

(b) Soft palate: In mammals, the secondary palate extends posteriorly by a fold of skin. It is called softpalate. Soft palate completely separates the nasal passages from the mouth cavity. Some mammals chew their food.

The secondary palate extends more in mammals. It allows mammals to breathe during chewing. Breathing is stopped only during swallowing.

TEETH (DENTITION)

1. Arrangement and structure of teeth

There are important modifications in the structure and arrangement of teeth in mammals. Reptiles are homodont. In this case, the teeth are uniformly conical. Mammals are heterodont. Their teeth specialized for different functions. Reptilian teeth attach on the top or inside of the jaw. The teeth of mammals are present in sockets of the jaw. Most mammals have two sets of teeth during their lives:

(a) Deciduous or milk teeth: They are formed before or shortly after birth.

(b) Permanent teeth: The milk teeth are replaced by permanent teeth. Adult mammals have four kinds of teeth.

(i)   Incisors: They are the most anterior teeth in the jaw. They are chisel-like. They are used for gnawing or nipping.

(ii) Canines: They are long, stout, and conical, They are used for catching. killing, and tearing prey. Canine and incisors have single roots.

(iii) Premolars: They are present next to canines. They have one or two roots. They truncated surfaces for chewing.

(iv) Molars: They have broad chewing surfaces. They two (upper molars) or three (lower molars) roots.

2. Dental formula

Mammalian species have characteristic numbers teeth in adults. Zoologists use a denial formula to characterize taxa. Dental formula is an expression of the number of teeth of each kind in one-half of the upper and lower jaws. The teeth of the upper jaw are listec above the lower jaw. There is following order of teeth in dental formula: incisors. caniae. premolars. and molars. For example:

                         Human                            Beaver

2.1.2.3                                  1.0.1.3

2.1.2.3                                   1.0.1.3

3. Dentition in different mammals

Mammalian teeth (dentition) are specialized for particular diets.

(i)  Dentition in Edentata: In some mammals, the dentition is reduced. Some mammals have no teeth. For example, armadillos and the giant anteater (order Edentata) feed on termites and ants. Thus their teeth are reduced.

(ii)  Dentition in omnivores: Some mammals (e.g.. humans. order Primates, and pigs, order Artiodactyla) are omnivorous. They feed on different plant and animal materials. Their anterior teeth have sharp ripping and piercing surfaces. The posterior teeth have flattened grinding surfaces. It is used for rupturing of plant cell walls.

(iii) Dentition in herbivores: The herbivore mammals have flat, grinding posterior teeth. Their incisors and canines are modified for nipping plant matter. These are horses (order Perissodactyla), deer (order Artiodactyla). Some others use them for gnawing like rabbits, order Lagomorpha; beavers, order Rodentia. The incisors of rodents  grow throughout life. The rodents have enamel only on the from surfaces of their incisors. They keep the teeth sharp in front than in back. A gap called the diastema is present in these mammals.

Diastema separates the anterior food-procuring teeth from the posterior grinding teeth. The diastema is formed by the elongation of the snout. Thus the anterior teeth reach close to the ground or into narrow openings during feeding. The posterior teeth have a high exposed surface (crown). They have continuous growth. Thus they can grind the food for many years.

(iv) Dentition in carnivores: Predatory mammals use canines and incisors for catching, killing, and tearing prey. Predatory mammals include order Camivora (e.g., coyotes, dogs. and cats). Their fourth upper premolars and first lower molars form a scissor like shearing surface. It is called the carnassial apparatus. It is used for cutting flesh from prey.

VERTEBRAL COLUMN AND APPENDICULAR SKELETON

VERTEBRAL COLUMN

The vertebral column of mammals is divided into live regions.

1. Cervical vertebrae: Mammals have seven vertebrae. The first two cervical vertebrae we the atlas and axis. Five other cervical vertebrae follow these two vertebrae. The

giraffe and the whale have seven neck vertebrae. But their cervical vertebrae are greatly elongated or compressed. On other hand, tree sloths have six or nine cervical vertebrae. The manatee has six cervical vertebrae.

2. Thoracic vertebrae: The trunk is divided into thoracic and lumbar regions. The thoracic region contains the ribs. Most ribs connect to the thoracic vertebrae in the back. The ribs are attached to the sternum through costal cartilages. Other ribs attach only to the thoracic vertebrae. Or they may be attached to other ribs through cartilages. All ribs protect the heart and lungs. The articulation between the thoracic vertebrae provides the flexibility. This flexibility is needed for turning, climbing, and lying.

3. Lumber vertebra: Lumbar vertebrae have interlocking processes. These processes give support to the vertebrae. But they provide little freedom of movement.

4. Caudal vertebrae: These are tail vertebrae.

APPENDICULAR SKELETON

The appendicular skeleton of mammals rotates under the body. Thus their appendages are present directly beneath the body. Joints limit the movement of appendages. Therefore, bones can move in a single anteroposterior plane. Thus the tips of the appendages move in long arcs. The bones of the pelvic girdle are fused in the adult. This fusion has advantage for locomotion. But it causes a problem during the birth of offspring. The ventral joint between the two halves of the pelvis is called pubic symphysis. It is loosened before birth in a pregnant female. Thus it allows the pelvis to spread during birth.

MUSCLES

The appendages of the mammals are directly beneath the body. Therefore, the skeleton hears the weight of the body. Muscle mass is concentrated in the upper appendages and girdles. Many running mammals like deer have little muscle in their lower leg. Tendons run from muscles high in the leg to lower joint. Thus these muscles cause movement at the lower joints.

NUTRITION AND DIGESTIVE SYSTEM

The digestive tract of mammals is similar to other vertebrates. But they have many specializations for different feeding habits. The mammals have different Feeding habits. These habits show the ecological specializations in the mammals. These feeding habits are:

1. Carnivores: Most members of the order Carnivora feed on animal flesh. Thus they are carnivores. Example of carnivores are lion, tiger etc.

2. Omnivores: Some other mammals feed on both plants and animal products like bears and man. They are called omnivores.

3. Insectivores: Some carnivorous mammals are specialized for feeding on arthropods or soft-bodied invertebrates. Thus they are called insectivores. These animals are included in the orders Insectivora (e.g shrews), Chiroptera (bats), and Edentata (anteaters).

4.Herbivores: Herbivores teed mostly on vegetation. But they sometimes also eat vertebrates during feeding. Examples of herbivores as deer (order Artiodactyla) and Zebras (order Perissodactyla). The herbivores face difficulty in digestion of cellulose. Thus there are specializations in their digestive tract.

(a) Horses, rabbits and rodents have an enlarged cecum. Caecum is present at the junction of the large and small intestines. Cecum acts as a fermentation pouch. The microorganisms help in cellulose digestion in cecum.

(b) Sheep, cattle, and deer are called ruminants (ruminaie, to chew the cud). Their stomachs are modified into four chambers. The first three chambers are storage and fermentation chambers. They contain microorganisms which synthesize a cellulose-digesting enzyme cellulase. Fermentation produces gases. These gases are belched periodically. Some plant matter (cud) is regurgitated (back flow) and rechewed. Other microorganisms convert nitrogenous compounds or food into new proteins.

CIRCULATION,

GAS EXCHANGE AND TEMPERATURE REGULATION

CIRCULATION

Heart

Evolution of heart: The hearts of birds and mammals are almost similar. Both have four‑ chambered pumps. They have separate systematic and pulmonary circuits separate. The heart of both evolved from the hearts of ancient reptiles. However, their similarities are due to adaptations to active lifestyles. The evolution of similar structures in different lineages is called convergent evolution. The mammalian heart evolved in the synapsid reptilian lineage. On the other hand, the avian heart evolved in the archosaur lineage.

Blood circulation in fetus

There are important adaptations in the circulatory system of eutherian mammals for exchange of respiratory gases and nutrients in the fetus. It exchanges between maternal and fetal blood occur through the placenta. Blood is not actually mixed between maternal and foetal tissues. Nutrients, gases and wastes diffuse between fetal and maternal blood. There are two adaptations in the fetus.

1. A blood vessel brings blood from placenta and opens into right atrium of the fetus. It s highly oxygenated. The fetal lungs are not inflated. Thus there is high resistance to blood flow through the pulmonary arteries. Therefore, most of the blood entering the right atrium bypasses the right ventricle. It passes into the left atrium through a !valved opening between the atria. This valve is called foramen ovale.

2. However, some blood from the right atrium enters into the right ventricle and the pulmonary’ artery. But uninflated lungs produce resistance. Therefore, most of this blood is passed to the aorta through the ductus arteriosus. Ducats arteriosus is a vessel connecting the aorta and the left pulmonary artery.

3. The placenta is lost at birth. Thus the lungs are inflated. Resistance to blood Clow through the lungs is reduced. Therefore. blood flow to them increases. Flow through the ductus arteriosus decreases. It is gradually reduced to a ligament.

4. Blood flow back to the left atrium from the lungs also increases. The valve of the foramen ovale closes. It is gradually fuses with the tissue separating the right and left atria.

GAS EXCHANGE

Mammals have high metabolic rates. Therefore they have adaptations for efficient gas exchange. Most mammals have separate nasal and oral cavities and longer snouts.

Snout has greater surface area for warming and moistening the inspired air. Their respiratory passageways are highly branched. It has a large surface area for exchange of gases. Mammalian lungs resemble to a highly vascular sponge.

Mammalian lungs inflate by a negative-pressure mechanism. However, mammals possess muscular diaphragm. Diaphragm separates the thoracic and abdominal cavities. There are following mechanisms of inspiration and expiration.

1. Inspiration takes place by the contraction of the diaphragm and expansion of the rib cage. It decreases the intrathoracic pressure. Thus it allows air to enter the lungs.

2. Expiration takes place by elastic recoil of the lungs and relaxation of inspiratory muscles. It decreases the volume of the thoracic cavity. The contraction of other thoracic and abdominal muscles can produce forceful exhalation.

TEMPERATURE REGULATION (THERMOREGULATION)

Mammals are widely distributed over the earth. Some mammals face harsh environmental temperatures. Therefore, they dissipate excess heat at some times and conserve and generate heat at other times. There are following adaptations in mammal for thermoregulation,

1. Most mammals produce internal heat. Heat producing mechanisms of mammals are divided into two categories.

(a) Shivering thermogenesis: It is a muscular activity. It generates large amounts of heat but little movement.

(b)  Nonshivering thermogenesis: It involves heat production by general cellular metabolism. Heat is also produce by the metabolism of brown fat.