Hair

Joining of the Jaw

Missing nucleus in Red Blood Cells.

How do they live?

All mammals have hair. Be it a whale or an elephant. They all have hair, irrespective of the size of the hairs. In mammals, the lower jaw is hinged to the skull as in all other vertebrates, but in this it is directly attached to the skull - not through another bone. In mammals, there are three bones that help in transmitting sounds to the brain. There is a diaphragm that is found below the lungs, separating the abdomen from the heart. The Red Blood Cells of mammals are unlike the red blood cells of any other vertebrate, in the fact that the mature ones do not have a nucleus in them.

All mammals are Viviparous ( viviparous animals are those that do not lay eggs but give birth to young ones ) with the slight exception of monotremes ( animals like echidna's and duck-billed platypus ).

In mammals like man ( placental mammals ), the young remains in the womb for quite some time, and thus are quite developed when they are born. But in Marsupials ( animals that have pouches and carry the young in them - animals like koala ), the young is born when quite undeveloped and is carried about in its pouch.

In mammals, the young learn from their parents as they learn from their elders. Since that are taken care of the mothers, their behavior matches that of their parents. They learn from the elders and are also trained by them.

The appearance of Mammals:

The appearance of mammals first occurred in the Triassic Period ( 220 Million years to 180 Million years ago). Their ancestors were reptiles. The immidiate ancestors were of a class Therapsida. These were small meat eating animals. The first animals were very small - not much larger than a rat.

Mammals were the dominant of vertebrates - form the time the dinosaurs had died out. They ruled since the past 70 million years. They have been the most dominant on terrestian plains. In the aquatic region, the fish are the most extensive and in the arieal, the birds. Mammals are warm blooded animals, and hence their body temprature does not vary according to the external temprature. Hence they have an advantage over the reptiles. Warm blooded animals can live in conditions where cold blooded animals cannot as they can maintain their metabolic rate. Hence they can go and sustain themselves in environments where cold blooded animals cannot.

There are two surviving classes of mammals - the Protortheria ( montremes - like duck- billed platypus ) and Theira ( Marsupials and Placenta ). There is yet another subclass that has been found, but only from fossil remains. Although they were generally primitive mammals, they have evolved into the herbivores with gnawing teeth and grinding teeth.

The last surviving monotremes are the spiny ant-eater and the duck billed platypus. They are a sub class of the ancient mammals. The monotremes seem out of order from the other mammals. They lack many of the characteristics of mammals. They lay eggs and incubate them. The milk is secreted through the fur, where an opening duct from the abdomen is present, and the milk is lapped up. They have no teeth, but a hard bill to make up for their short coming in this region. beak. They might be here today - as they are - because of lack of competition from the other, higher devoloped mammals that were found in the other parts of the world. They lived in the isolated island continent of Australia. They also lived in New Guinea.

The largest and most dominant of the mammals are the therians. They have conqured land, sea, air, and have inhabited almost every inhabitable place on the earth. There are four thosand species belonging to this group. They are sub grouped as Marsupials - those that carry the young in a pouch like koala's, and placentals - those that give birth to quite grown live ones.

Marsupials Placental

In marsupials, the young ( unweaned ) are nurtured in a pounc. They are born when they are still in their enbryonic stage and complete their devolopment attached to their mothers. They are found generally in the Americas and Australiaa. The best known of the marsupials are the koala's and kangaroo's. They were believed to have once been widespread, but ended up as the dinner of the wolves, cats, and other placental creatures.

In Mammals, reproduction involves the preperatio of the female's utrine wall ( where the young is kept ) for the reception of the egg which is already fertilized. The time period of the young's stay in the mother's body varies from 2 weeks in small animals to 1 year and 10 months in the elephant.

The resulting complex of fetal and maternal tissues is a true placenta, and by means of it the young stay within the mother's uterus for periods ranging from two weeks for the domestic hamster to 22 months for the African elephant.

From where did they evolve and when?

See our page on mammal evolution.