What do only mammals have

Some mammals, such as the anteater and the Baleen whales, do not have any teeth. Shrews have so little body fat they cannot go more than a couple of hours without food.

Missing a meal is a sure way to a quick death. A good night's sleep could be fatal! A polar bear looks white, but he isn't really. His long, shaggy hairs are colourless and hollow. Beneath his hair, the skin is black. Mammals are grouped by some basic characteristics that all of them share.

Mammals are endotherms , which means that their bodies can generate heat in order to maintain a suitable body temperature. They have special glands called mammary glands which produce milk for feeding their young after they are born.

Hair is another important feature in mammals and it is present on their body at least at some point in their life. They also have three bones in their middle ear. Their circulatory system how blood is sent around the body is highly efficient and a four-chambered heart is one of the exceptional features of mammals. All mammals breathe air, even the ones which live in the ocean. Their lungs play an important role in the exchange of oxygen and carbon dioxide.

Based on how their offspring develop, mammals are divided into two subclasses: Prototheria — These mammals lay eggs, incubate them for a certain period, after which the young ones are born. The platypus, and the echidna are the only prototheria on Earth. Theria — These mammals carry their offspring in the womb for a certain period before it is born.

This class is further divided into: Metatheria — Includes mammals which have abdominal pouches. These are also known as marsupials. They give birth to underdeveloped babies which are then carried in the pouches till they are fully developed.

Kangaroos and koalas are some well-known examples of marsupials. Eutheria — These mammals do not have pouches and the baby is kept nourished within the body until it is ready to be born. They are also known as placentals. Human beings, elephants, tigers, cows and bears are some examples of placentals. Words to know: Carnivore — an animal that eats other animals.

Characteristics — features or qualities that make somebody or something recognisable Circulatory system — the system consisting of the heart, blood vessels, and lymph vessels that pumps blood and lymph round the body Climates — a place with a particular type of weather, or the different kinds of weather in different places Endotherm — an animal that is able to maintain a constant body temperature despite changes in the temperature of its environment Environment — the surroundings or conditions in which a person, animal, or plant lives or operates Eutheria — mammals which do not have pouches.

The baby is kept nourished within the mother's body until it is ready to be born. Bats and mole rats can't regulate their body temperatures; whereas bees, birds and the hawk moth can. Other characteristics not unique to mammals include live birth, determinate growth and a four-chambered heart. Some sharks give live birth to their young; whereas, two mammals, platypus and the echidna, lay eggs.

Mammals are known to have determinate growth, but so do birds, and at least the American alligator. Four-chambered hearts aren't exclusive to mammals either, as both birds and crocodilians have them too. By using the site, you agree to the uses of cookies and other technology as outlined in our Policy, and to our Terms of Use.

Mammary Glands Mammals got their name from this evolutionary adaptation. Hair Hair on mammals is an adaptation that provides insulation to keep the animal warm, provide protection to the animal's skin, provides camouflage through color patterns and helps the animal by providing greater sensory input through touch.

Middle Ear Bones All mammals have three middle ear bones, commonly called the hammer, anvil and stirrup. Sweat Glands Sweat glands, while being unique to mammals, aren't present in all mammals. The kidneys of mammals have a portion of the nephron called the loop of Henle or nephritic loop, which allows mammals to produce urine with a high concentration of solutes; higher than that of the blood.

Mammals lack a renal portal system: a system of veins that moves blood from the hind or lower limbs and region of the tail to the kidneys. Renal portal systems are present in all other vertebrates except jawless fishes.

A urinary bladder is present in all mammals. Mammalian brains have certain characteristics that differ from other vertebrates. In some, but not all mammals, the cerebral cortex, the outermost part of the cerebrum, is highly folded, allowing for a greater surface area than is possible with a smooth cortex.

The optic lobes, located in the midbrain, are divided into two parts in mammals, whereas other vertebrates possess a single, undivided lobe. Eutherian mammals also possess a specialized structure that links the two cerebral hemispheres, called the corpus callosum.

The modern mammals of today are synapsids: descendants of a group called cynodonts which appeared in the Late Permian period. The evolution of mammals passed through many stages since the first appearance of their synapsid ancestors in the late Carboniferous period. Mammals are synapsids: they have a single opening in the skull. They are the only living synapsids as earlier forms became extinct by the Jurassic period. The early, non-mammalian synapsids can be divided into two groups: the pelycosaurs and the therapsids.

Within the therapsids, a group called the cynodonts are thought to be the ancestors of mammals. By the mid-Triassic, there were many synapsid species that looked like mammals. Synapsids from this period include Dryolestes more closely related to extant placentals and marsupials than to monotremes as well as Ambondro more closely related to monotremes.

Later, the eutherian and metatherian lineages separated. Metatherians are the animals more closely related to the marsupials, while eutherians are those more closely related to the placentals. Eutherians are distinguished from noneutherians by various features of the feet, ankles, jaws, and teeth. One of the major differences between placental and nonplacental eutherians is that placentals lack epipubic bones, which are present in all other fossil and living mammals marsupials and monotremes.

Cynodonts : Cynodonts, which first appeared in the Late Permian period million years ago, are thought to be the ancestors of modern mammals. Since Juramaia, the earliest-known eutherian, lived million years ago in the Jurassic, this divergence must have occurred in the same period. After the Cretaceous—Paleogene extinction event wiped out the non-avian dinosaurs birds are generally regarded as the surviving dinosaurs and several other mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Paleogene and Neogene, by the end of which all modern orders had appeared.

The synapsid lineage became distinct from the sauropsid lineage in the late Carboniferous period, between and million years ago. This does not include the mammal-like reptiles, a group more closely related to the mammals. Throughout the Permian period, the synapsids included the dominant carnivores and several important herbivores. In the subsequent Triassic period, however, a previously-obscure group of sauropsids, the archosaurs, became the dominant vertebrates. The mammaliaforms appeared during this period; their superior sense of smell, backed up by a large brain, facilitated entry into nocturnal niches with less exposure to archosaur predation.

The nocturnal lifestyle may have contributed greatly to the development of mammalian traits such as endothermy and hair. Later in the Mesozoic, after theropod dinosaurs replaced rauisuchians as the dominant carnivores, mammals spread into other ecological niches. For example, some became aquatic, some were gliders, and some even fed on juvenile dinosaurs.

Most of the evidence consists of fossils. For many years, fossils of Mesozoic mammals and their immediate ancestors were very rare and fragmentary; however, since the mids, there have been many important new finds, especially in China.

The relatively new techniques of molecular phylogenetics have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record.

Although mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation. This is because these soft tissues are not often preserved in the fossil record. Most study of the evolution of mammals centers, rather, around the shapes of the teeth, the hardest parts of the tetrapod body.

Other much-studied aspects include the evolution of the middle ear bones, erect limb posture, a bony secondary palate, fur and hair, and warm-bloodedness. A key characteristic of synapsids is endothermy, rather than the ectothermy seen in most other vertebrates.

The increased metabolic rate required to internally-modify body temperature went hand-in-hand with changes to certain skeletal structures.

The later synapsids, which had more-evolved characteristics unique to mammals, possess cheeks for holding food and heterodont teeth specialized for chewing by mechanically breaking down food to speed digestion and releasing the energy needed to produce heat. Chewing also requires the ability to chew and breathe at the same time, which is facilitated by the presence of a secondary palate.

It separates the area of the mouth where chewing occurs from the area above where respiration occurs, allowing breathing to proceed uninterrupted during chewing.