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FEATURES AND ADAPTATION OF MAMMALS TO THEIR ENVIRONMENT

Mammals possess many features that are shared with other animals. These include the possession of backbones (vertebral column) and being warm-blooded which they have with vertebrates and birds respectively. They however have features which are peculiar to them. These are presented here.

1. The integument: This is the external covering of the animal and from it hairs grow. The hairs cover the whole body of mammals, but it is sparsely distributed in the others. The hairs also serve as insulators warming the animal up against cold and thus help in maintaining the body temperature of the mammal. Possession of hairs is one of the characteristic of Mammals.  Hairs are projections out of the Mammalian integument.  Hairs are important in Mammals due to their usefulness in the maintenance of a constant internal body temperature.  The internal body temperature of mammals lies between 35 and 40^o C.  The maintenance of a constant internal body temperature termed homoithermy is achieved partly by the functioning of hairs.  The hair covering on the body surface of mammals is referred to as fur.  The mammalian fur is made up of two kinds of hairs:

 a.         Under hairs -    These sense and soft hairs they tray a layer of air which conserves

                                    heat on the surface of the skin and thus insulate the body

b.        Guard hairs –   These are coarse and horny relative to the underhairs.  They are for  protection against wear and for giving colour to the animal body.  The coloration itself is caused by pigmentation in the guard hairs.

MODIFICATION OF HAIRS

   1.      Sensory vibrissae – These are found under the eyes and on the snout of most mammals.

   2.      Bristles -     Found in hedgehogs

   3.      Spines    -  Found in spiny anteaters, porcupines and some other mammals.

 Further about hairs:

-      Hairs are exclusively found in Mammals

-      Hairs are not found in some parts of the body except the palms of the hand and soles of         

       the feat.  Also not found in the parts on the buttocks of some mammals e.g. baboons.

 Also from the integument are three different kinds of glands namely the sweat, sebaceous and mammary glands the mammary gland is usually well developed in females where it produces milk and function in suckling the young. This is where mammals derived their name.  Mammals possess mammary glands.  In the males, these glands are rudimentary or absent while they are fully developed and secret milk in the adult female.  Mammals derive their name from this feature.  Only Mammals possess mammary glands.  The number of mammary glands possessed by the mammals and their position on the body defers depending on the species concerned.  The number varies from two to twelve or more in terms of position, these glands may be

(i)                 Pectoral when they are found in the upper part of the body.

(ii)               Auxiliary

(iii)             Pectoral- abdominal when they are found on the ventral and along the long axis of the body

(iv)             Inguinal when they are in the ventral section and between the two hind legs.

Sexes and Reproduction: In all mammals the sexes are separate and reproduction is sexual and is effected by internal fertilization resulting in the formation of foetus. The foetus which so develops after a while is born alive. This is called viviparity. An exception from this is seen only in the egg-laying mammals found in Australia. In mammals, the sexes are separate.  There is marked sexual dimorphisms in most mammals due to physiological and anatomical difference.  For instance the female has well developed mammary glands manifested as breasts while the males have rudimentary mammary.  Also the female have a vaginal opening which leads into the uterus through the vulva while the males have a corpulatory organ, Penis and a pair of testes commonly in a sac called the scrotum which is usually located outside the abdomen.  Fertilization is internal in all mammals and an embryo results.  Most mammals are viviparous in which they give birth to live young ones after a period of pregnancy or gestation, the exception being the primitive mammals  -   monotremes, which lay eggs just like birds.  The monotremes are therefore oviparous.  Since gestation period is the time el takes the embryo to develop up to the birth of the young, monotremes have no gestation period, however the incubate their eggs.  In monotreme eggs, extraembryonic membranes namely amnion, chorion and allantois are present during development. 

In placentals, the gestation period varies with respect to species.  During gestation the development embryo called foetus is nourished by means of placenta.  The placenta is also called after-birth as it is expelled after the young one is born.  The birth of the young mammal is called parturition.  After parturition there is parental care of the young mammal by its parent during which it is taught some skills that are important for survival.  In mammals the chances of survival are quite high, they therefore give birth to only a few young at a time.  The number of young ones delivered at any one time by a mammal is described as litter size.  The litter size of some species is always one for example the marsupial phalanger, Dacrylopsits sp. In most mammals, there are definite mating seasons.  This happens when the animal is on heat and it coincides with the oestroes cycle.  Man is an exception to this.  In man, the oestrus cycle is the menstral cycle because it terminates in the shedding off of the uterine linning endometrium) with some blood which is termed menstruation.

3. Jaw Mechanism, Heterodont Dentition and Digestive System: Mammals possess a unique jaw mechanism in which the lower jaw is made up of a single bone called dentary.  In mammals only does the dentary articulate directly with the skull.  This arrangement gives the animal a powerful jaw mechanism that can grind and cut up food. Mammals posses well-developed dentitions and complex digestive system which begins with a pair of fleshy lips bordering the mouth, lead up to the stomach, small intestine, large intestine and terminates at the anus or cloaca (is primitive mammals i.e. the monotremes).  The mouth houses the teeth which varies in types, arrangement, and numbers with respect to various mammals.  The totality types of the types, numbers and arrangement of the teeth of an animal is referred to as its dentition.  Mammals have two sets of teeth throughout their lifetime that is an initial set of milk teeth which are grown at infancy and are later replaced lay a new set of permanent teeth.  The possession of two successive set of  teeth as found in mammals is referred to as diphyodont dentition.  Typically in mammals the teeth are differentiated into different types according to their functions and the food habits of the animal.  This is referred to as heterodont dentition. 

Mammals normally possess four types of teeth namely incisors, canines, premolars and molars.  The description of the dentition of mammals follows a standard pattern.

The maximum number of teeth in mammals is 44 as seen in sections of the dental formula of mammal are shown.  A tongue is attached to the floor of the mouth. The tongue has numerous sensory reception corpuscles referred to as taste buds.  The tongue is therefore responsible for taste.  Salivary glands discharge their secretions – saliva into the buccal cavity to aid and commence the process of digestion.  There three salivary glands namely (i)  Parotid   (ii) Submaxillary and (iii) Sublingual.  The food chopped and beaten by the teeth during mastication are rolled with the aid of the tongue into bolus and swallowed through the oesophagus into the stomach.  In most mammals, the stomach is simple but it is divided into chambers in some.  For example in rumninants (e.g. cattle, deer etc) and whales where it is divided to four chambers.  

4.      Pinnae: The auditory system is composed of three parts namely the Inner ear Middle and an Outer ear.  There are three bones in the middle ear namely Maleus (hanner), Incus (anvil) and stapes.  The Inner ear is a system of tubes called the semicircular canal which are three number and functions in maintaining balance for the animal.  The Inner and Middle ear are connected to the outer ear by the Eustacian canal which leads outside of the animal body and connects with an external fleshly pinna (plural pinnae).  The pinnae are found exclusively in mammals.

5.      Nervous system: this is composed of a well developed brain and 12 pairs of cranial nerves. This is one characteristic that marks mammals out among other vertebrates.  The most anterior portions of the brain, the cerebral hemispheres are pained, very large and expended in most other invertebrates, these hemispheres are small.  The cerebral hemispheres provides for a high level of coordination in all activities excluding leaving and retentive memory.  These in turn are made possible by the sheets of nervous tissue consisting of several mullions of nerve cells called the grey matter.  In mammals also, the senses of sight, smelling and hearing are well developed

6.      Circulatory/Transport System: this is composed of a heart of four chambers (two aurides and two ventricles), left aorta and non-nucleated biconcave red blood cells. The heart of mammals is found in a pericardial cavity and it is fully divided into four chambers with two upper atria and two lower ventricles (figure--)  There is complete separation of the veinous and arterial blood.  Mammals share this feature and arrangement of the birds.

7.      Respiration and the diaphragm: Respiration is effected through a pair of lungs and voice box. The Diaphragm: this is a sheet of membrane separating the thoracic from the abdominal region of the mammal. It is found only in mammals. It founds in gaseous exchange. This is a muscular sheath which separates the thoracic from the abdominal cavity of the animal.   The heart and lungs are in the thoracic cavity which run stomach, intestine in the urinary system.  Functionally, the diaphragm is very important in the breathing mechanism of the animal.  When the diaphragm contracts it becomes cured him a inverted letter (See Figure) and flattens out in the relaxed state.  The contracted diaphragm forces a reduction in the volume of the lungs, hence an increase in lung pressure which forces out air (exhalation).  When the reverse happens, that is the diaphragm relaxes, pressure is reduced in the lungs and so volume increase and draws in air (inhalation), thus complete the cycle of gaseous exchange or breathing.

8.      Skeletal System of mammals is characterized by two occipital condyles, seven cervical vertebrae and an elongated tail made of caudal vertebrae.

9.  The excretory system is composed of the kidneys (usually one pair) and ureters that leads into a bladder. Mammals presses a pair bear-shaped metanephric kidneys located in the dossal part of the abdominal cavity.  When dissected a mammalian kidney shows the following parts:

1. Cortex -  This is outer and contain the renal corpuscles, convoluted tubes and blood vessels.
2. Medulla – This is divided into triangular wedges called renal pyramids.  The broad tissues of these pyramids are divided towards the cortex and narrow apices referred to as renal papillae are oriented towards the centre of the kidney and opens into the calyx and pelvis.
3. Ureter -   Each kidney has a ureter, a duct leading into the pelvis empties fluid into the Urinary bladder.

Urinary bladder -  This is an organ where urine is stored before existing to the outside.

 Urethra -  This is another duct.  It drains the urinary bladder and conducts its content to the outside.

 The functional unit of each kidney is the nephron.  Each kidney has about 1,500.000 nephrons consisting of a closed bulb called Bowman’s capsule or glomerular capsule. 

Functions of the Kidney 

There are three main functions of the kidney

      1.                  Glomerular filtration

2.                  Tubular reabsorption

3.                  Tubular secretion

 10. Movable eyelids. All mammals have movable eyelids.Mammals have movable eyelids. This is a characteristic shared with birds. Mammals have two eyelids while birds have three. The main function of the eyelids in mammals is to protect the eye. This function of protection is against physical injury, and drying of the cornea. The cornea is protected from drying by the lachrymal fluid called tears produced by the lachrymal glands located beneath the eyelid. Blinking of the eye helps to spread the lachrymal fluid across the cornea and thus lubricating it.

11. All mammals are endothermic and homeothermic. Endothermy refers to the ability of animal to maintain a constant body temperature irrespective of the temperature of its environment. Endothermic animals are also known as warm blooded animals. Mammals and birds are the main endothermic animals, although some fish species such as Sharks and Tuna are also endothermic.

Homeothermic animals maintain a stable body temperature higher than that of their surroundings. These animals achieve stable body temperature by regulating their metabolism. Only birds and mammals are homeothermic animals.

 

ASSIGNMENT

Various mammal species have mammary glands of different numbers. On the bases of their number and arrangement identify the different groups of mammals.

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