MAMMALS: CHARACTERISTICS, CLASSIFICATION, SYSTEMS, REPRODUCTION - BIOLOGY - 2024

The mammals (Mammalia) are a class of animals includes more than 5000 species of warm - blooded vertebrates mammary glands and hair covering their bodies. Mammals have achieved the colonization of very diverse habitats, including the oceans and air environments.

Mammals are divided into 26 orders. One of them corresponds to the monotremes, seven to the marsupials, and the 18 orders to placental mammals. They are characterized by presenting a diverse series of shapes and sizes. This overwhelming morphology ranges from a tiny 1.5g bat to a massive 200,000kg blue whale.

Bats are the only mammals with the ability to fly. Source: Gilles San Martin from Namur, Belgium, via Wikimedia Commons

Mammals present a series of characteristics that distinguish them as a group. These characteristics are basically divided into soft anatomical and skeletal or skeletal characteristics.

Soft anatomical features

Glands

Tlacuach, Mexico

The skin of mammals and the specializations present in them are the characteristics that distinguish the group. In general, mammalian skin is thick, composed of an epidermis and a dermis.

The body of mammals is rich in glands. The most distinctive are the mammary glands, whose function is the production of a nutritive substance to feed the young.

Once the milk production appeared in the mammals, it was retained by all the members of the group. Several authors, including Charles Darwin, have speculated about the appearance of lactation. It could be that sweat changed its composition little by little, until a nourishing milky substance appeared.

In addition, there are sebaceous glands that produce fat and are associated with hair and sweat. The latter are classified as eccrine and apocrine.

The eccrine glands secrete a watery substance that helps to cool the individual, while the apocrine glands are associated with hair and secrete milky or yellowish substances. Its activity is related to the reproductive state.

There are also scent glands. These produce different substances that actively participate in the processes of chemical communication, social interactions and defense.

Hair

Mexican gray wolf

After the mammary glands, hair is the most notable feature of mammals.

This grows from a follicle of epidermal origin and continuously, thanks to the rapid proliferation of cells in the follicle. The protein that forms hair is keratin, the same that we find in nails and other structures.

Hair participates mainly in the body's thermoregulation. Together with the subcutaneous fat layer they confer thermal insulation. Coat coloring also participates in camouflage and social interactions.

All mammals have hair at least at some stage in their life. The density of the hair varies according to the species. For example, in humans hair has been reduced to very specific areas of the body, and in whales the reduction has been extreme, with hair found only as sensory bristles.

In some mammals the hair has been modified and has acquired secondary functions, such as the quills in the porcupine. In this case, the hair has hardened and participates in the protection of the body. There are even more extreme modifications of the hair, such as that found in the horns of rhinos.

Skull characteristics

Human skull

In mammals we find a series of very useful bony characters to distinguish them. The characters that we generally associate with mammals - call it hair or mammary glands - are not preserved in the fossil record, therefore it is necessary to have a series of characteristics that can be observed in the skeleton.

Mammals have three ossicles in the middle ear: the hammer, the anvil, and the stapes. In the rest of the amniotes we only find one: the columella (stapes).

There are only a couple of bones in the jaw, called dental. These articulate on each side of the head. They have a secondary palate that allows feeding the animal at the same time as breathing. Mammalian teeth are heterodonts and difiodonts.

The first term refers to the set of clearly differential teeth in incisors, canines, premolars and molars, while "diphiodonto" means that there are two sets of teeth: the so-called "milk teeth" and the permanent ones. Within the tetrapods, mammals are the only ones with the ability to chew.

The occipital condyle is double, the ancestral condition being a single condyle. There is a complex of vertebrae atlas and axis, which allows movements of the head.

Skeleton characteristics

The vertebral column of mammals is divided into discrete regions: cervical region, with 7 or 9 vertebrae; thoracic region, with 12 or 13 vertebrae and ribs strictly restricted to this area; lumbar region; sacral region and caudal region. The heel has a characteristic bone that connects it to the Achilles tendon, called the calcaneus.

Classification

Mammals are grouped into the class Mammalia. This is divided into two subclasses: Prototheria and Theria. The Prototheria subclass includes the Ornithodelphia infraclass. In the subclass Theria we find the subclass Metatheria and Eutheria. The Eutheria infraclass is in turn divided into several orders.

We will now describe the most important characteristics of the orders of mammals. The exposed classification was taken from Hickman (2001).

-Subclass Prototheria

Infraclass Ornithodelphia

It is made up of the monotremes. It only includes the order Monotremata, where we find oviparous mammals - which lay eggs. The most notable members of the order are the platypus and the echidna.

The front part of the mouth is modified like a beak, and the adult forms lack teeth. The limbs have also been modified for aquatic life or for excavation.

Compared with the rest of the mammals, they lack certain anatomical features, such as the tympanic ampulla, without rinary, without vibrissae and without nipples. The penis is internal and has a cloaca.

-Subclass Theria

Infraclass Metatheria

Kangaroo

The metatheria are the marsupials. These individuals have a very short development in the mother's uterus, and continue their development outside the mother clinging to the mammary glands inside the marsupium.

There are about 270 species, mostly located in Australia and a smaller proportion (about 70) in America.

Within the living orders we have Didelphimorphia, Paucituberculata, Microbiotheria, Dasyuromorphia, Peramelemorphia, Notoryctemorphia and Diprotodontia.

Infraclass Eutheria

It is formed by placental mammals, it includes the following orders:

Order Insectivara

Hedgehog

440 species of shrews, hedgehogs, tenrecs and moles form the order of insectivores. Most of these animals spend their lives in underground environments. They are distributed throughout the world, with the exception of New Zealand and Australia.

Order Macroscelidea

It is made up of 15 species of elephant shrews. These organisms have limbs and an elongated snout, adapted for the search and consumption of insects. They are distributed throughout Africa.

Order Dermoptera

Although they are commonly known as "flying lemurs", they are not related to lemurs or primates. They do not have active flight - like that of bats -, instead they are gliding animals, like squirrels.

Chiroptera Order

Myotis planiceps. Image via naturalista.mx

They are the bats. These organisms are the only mammals with the ability to actively fly. Four of its phalanges are elongated, inserting a membrane between them that allows flight.

Historically, they have been divided into microchiroptera and macrochiroptera. About 70% of its species are insectivorous, and the remaining members feed on fruit, pollen, nectar, leaves, amphibians, fish and even other mammals. They are found almost everywhere in the world, except for the poles.

Order Scandentia

There are 16 species of arboreal shrews that resemble a squirrel in morphology. Despite its name, not all of its members are adapted for a tree life. In fact, there are totally terrestrial species.

Order Primates

Source: pixabay.com

About 300 species of prosimians, monkeys, apes, and us humans. Its most distinctive feature is brain development. Most species have an arboreal lifestyle, with the exception of humans.

In the group, the presence of five fingers has become generalized, and each digit is covered by a flat nail with protective functions, both in the hind limbs and in the lower limbs.

With the exception of humans, all primates have their bodies covered with hair almost entirely.

There are two suborders of primates. The first is the Strepsirhini, which includes the true lemurs, the aye-aye (the first taxonomists confused this endemic specimen of Madagascar with a squirrel), the lorines, and the pottos or potos.

The second suborder is the Haplorhini, which comprises six families of primates. Its representatives include tarsiers, marmosets, old and new world monkeys, gibbons, gorillas, chimpanzees, orangutans, and humans.

Xenarthra order

Anteater

Xenarthra include nearly 30 species of anteaters, armadillos, and sloths. They are inhabitants of America, both north and south.

Order Pholidota

Pangolin. By Sandip kumar, from Wikimedia Commons

They include 7 species of pangolins distributed in Asia and Africa. They all belong to the genus Manis. They are a very particular group of mammals, characterized by the presence of framework-shaped scales.

Order Lagomorpha

Source: pixabay.com

Lagomorphs include rabbits, hares, pikas, or rock rabbits. They have significantly elongated incisors, similar to rodents. However, these have an additional pair. All members of the order have a totally herbivorous diet and are distributed throughout the world.

Order Rodentia

Source: pixabay.com

Rodents are the order of most numerous mammals (in number of individuals and number of species). They include squirrels, rats, mice, marmots, and the like. They have a pair of incisors that grow throughout the life of the animal and are adapted to gnaw. They have an impressive capacity for reproduction.

Order Carnivora

It is a very heterogeneous group, made up of dogs, wolves, cats, bears, weasels, seals, sea lions and walruses.

Order Tubulidentata

This order includes a single species endemic to Africa: the aardvark or oryteropus (Orycteropus afer).

Order Proboscidea

Herd of Elephants in the Serengeti

It includes elephants, both those from Asia and elephants from India. In addition to their long trunk, they have modified incisors in two fangs.

Order Hyracoidea

The order of hiracoid mammals includes seven species of daman. They are herbivorous animals found in African areas and also in Syria. They are reminiscent of a rabbit, with reduced ears. The digits have been reduced, with four fingers on the forelimbs and three on the hindlimbs.

Order Sirenia

They comprise the large marine animals known as sea cows and manatees. There are four species of these that are found on the tropical coasts of East Africa, Asia, Australia, Florida, the Amazon River, among other regions.

Order Perissodactyla

Perissodactyls are placental mammals with odd hooves (one or three), including horses, donkeys, zebras, tapirs and rhinos.

Order Artiodactyla

Artiodactyls are placental mammals with paired hooves, including pigs, camels, deer, giraffes, hippos, antelopes, cattle, sheep, goats, and the like.

The orders Perissodactyla and Artiodactyla are known as ungulates, a term that refers to their hooves.

Order Cetacea

Cetaceans are made up of placental mammals whose forelimbs have been modified into a fin shape to allow swimming. The hind limbs are absent and the hair reduction is considerable. The most renowned representatives are whales, dolphins, porpoises and the like.

There are two suborders: the toothed whales belonging to Odontoceti and the baleen whales of the order Mysticeti.

Digestive system

Mammals have a very wide feeding range, and each group has adaptations that allow them to maximize the extraction of nutrients from their diets.

The digestive system begins with the cavity for receiving food: the mouth. In mammals, heterodont teeth help to properly grind food. The digestion process begins in the mouth, with the enzymes present in the secretions of the salivary glands.

The already ground food reaches the stomach where the digestive process continues. In this step, the nutritive matter is called chyme. This partially digested mass mixes with gastric acids.

The chyme becomes chyle, and this step results in absorption. The nutrients are absorbed in the intestine.

Trophic habits and adaptations of the digestive system

Most insectivorous mammals are small, such as shrews and a high percentage of bats. The intestines of these animals are usually short. Similarly, the denture is designed for the crushing of exoskeletons, with pointed ends.

Herbivorous mammals feed on grass and other plant elements. These individuals are characterized by a simple stomach, longer intestines, and a pronounced cecum.

Ruminants have a stomach with four chambers, the intestines are long and behind the cecum they form a spiral loop.

The simplest digestive system is found in carnivores, where the intestine is short and the cecum is small.

Circulatory system

The mammalian circulatory system is closed and double, with systemic and pulmonary circulation. The heart has four chambers: two ventricles and two atria. This organ evolved in mammals independently of the four-chambered heart of reptiles and birds. Oxygenated blood is kept separate from non-oxygenated blood.

In the blood, mature red blood cells lose their nucleus, giving rise to cells without a nucleus, made up mostly of hemoglobin.

Nervous system

The mammalian nervous system is composed of a central nervous system and a peripheral nervous system. There are twelve pairs of cranial nerves. The mammalian brain is covered by three meninges, called the dura mater, arachnoid mater, and pia mater.

Both the brain and the cerebellum are very well developed and of considerable size, if we compare them with the rest of the amniotes. The optic lobes are separated.

Respiratory system

The main organ involved in mammalian respiration is the lung. This consists of a system of alveoli. The respiratory system is responsible for orchestrating the passage of air through the lungs and is composed mainly of the larynx, the secondary palate, and the muscles of the diaphragm.

In mammals we find turbinate bones in the nasal cavities. These are responsible for moistening and heating the air that enters the body.

Excretory system

The kidneys are metanephric with the urethra leading to a bladder. These paired organs are the major organs involved in excretion.

The mammalian kidney is divided into an outer cortex and an inner medulla. The region of the cortex contains the nephrons, which have the same function as in the rest of the amniotes. It is the functional unit of the kidney and is responsible for filtration, reabsorption and secretion.

In mammals, the kidney is capable of producing more concentrated urine than in birds, for example. Mammals are ureotelic, since the main nitrogen waste is urea.

Reproduction

Mammals have separate sexes and internal fertilization. Only echidnas and platypus lay eggs. The rest of the species are viviparous.

In males there are even testicles and in females ovaries. Copulation occurs through a special organ in the male: the penis. Monotreme mammals have a cloaca.

Mammals have a placenta, which consists of a protective and nourishing envelope that surrounds the embryo.

Origin and evolution

The origin of mammals is one of the best transitions reported in the fossil record, from a small, hairless and ectothermic ancestor, to a hairy specimen with the ability to regulate its internal temperature.

Paleontologists have found the bone features described above to identify mammals within the record.

Mammals and their ancestors are characterized by having a synapsid skull - with a pair of openings in the temporal region. This pair of openings is related to the insertion of the mandibular musculature. Synapsids were the first group of amniotes to radiate and acquire terrestrial habits.

Pelycosaurs

The first synapsids were pelycosaurs, organisms that resemble lizards (although this name could lead to confusion, we are not referring to any type of dinosaur). These animals had herbivorous and carnivorous habits.

Therapsids

One of the earliest synapsid carnivores was therapsids - erroneously called "mammalian reptiles," along with pelycosaurs. This group was the only one to survive after the Paleozoic.

Cynodonts

Cynodonts are a very particular group of therapsids that survived the Mesozoic era.

In this group, characteristics related to the high metabolic rates typical of mammals evolved; the jaw began to specialize, increasing the power of the bite; heterodonts appear, which allow the animal a better food processing; turbinate bones and secondary palate appear.

The secondary palate is a very important innovation in the evolutionary trajectory of mammals, as it enables young pups to breathe while sucking their mothers' milk.

In cynodonts, the loss of ribs occurs in the lumbar area, a fact that is related to the evolution of the diaphragm.

At the end of the Triassic period, a series of small mammals similar to a mouse or a shrew appear. In these specimens, an enlarged skull, an improved design of the jaws and difiodontos stand out.

From the jaw to hearing: appearance of the three small bones of the middle ear

One of the most important transformations in mammals was the appearance of the three small bones in the middle ear, specialized for the transmission of vibrations. The stapes is homologous to the hiomandibular of other vertebrates, it already had functions related to the perception of sounds in the first synapsids.

The hammer and the anvil originated respectively from the articular and the square, two bones that ancestrally participated in the union of the jaw.

Mammal radiation

For millions of years, the diversity of mammals was repressed with the gigantic reptiles that ruled the earth: dinosaurs. After the extinction of this group, the mammals - which were probably small and nocturnal, similar to today's shrews - managed to diversify quickly.

The ecological niches that were left empty after the mass extinction of the dinosaurs, were occupied by mammals, leading to immense adaptive radiation.

Different characteristics of mammals, such as endothermy, their intelligence, their adaptability, the fact of giving birth to live young, and being able to feed them with milk, contributed to the remarkable success of the group.

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