Mammals

Characteristics of Mammals

Mammalian traits include, among others: specialized glands, modified jaw and inner ear bones, urinary bladder, and hair.

Learning Objectives

Summarize the distinguishing characteristics of mammals

Key Takeaways

Key Points

  • The various traits which are used to define mammals include: the presence of hair; the integument system which contains specialized secretory glands; the skeletal and muscular systems; the heart and brain structure.
  • Mammals contain specialized glands which have various functions: secretion of chemical compounds used for communication; glands that produce milk; glands that produce perspiration used for thermoregulation; and glands that produce sebum, which is used for lubrication.
  • Mammals have four-chambered hearts that are defined by the ability to regulate the heart beat with the presence of specialized pacemaker cells.
  • A mammal’s hair has many purposes, including insulation, sensory perception, protective coloration, and social signaling.
  • Mammals possess many unique skeletal structures including a single lower jaw bone that joins the skull at the squamosal bone and three bones in the inner ear.

Key Terms

  • vibrissa: any of the tactile whiskers on the nose of an animal
  • sebum: a thick oily substance, secreted by the sebaceous glands of the skin, that consists of fat, keratin and cellular debris
  • diphyodont: having two successive sets of teeth (deciduous and permanent), one succeeding the other
  • sinoatrial node: the impulse-generating (pacemaker) tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm
  • integument: an outer protective covering such as the feathers or skin of an animal, a rind or shell

Characteristics of Mammals

The presence of hair is one of the most obvious traits of a mammal. Although it is not very extensive on certain species, such as whales, hair has many important functions for mammals. Mammals are endothermic so hair provides insulation to retain heat generated by metabolic work by trapping a layer of air close to the body. Along with insulation, hair can serve as a sensory mechanism via specialized hairs called vibrissae, better known as whiskers. These attach to nerves that transmit information about sensation, which is particularly useful to nocturnal or burrowing mammals. Hair can also provide protective coloration or be part of social signaling, such as when an animal’s hair stands “on end. ”

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Mammalian fur as insulation: Polar bears use their fur for warmth. While their skin is black, their transparent fur appears white, providing camouflage while hunting and serving as protection by hiding cubs in the snow.

Mammalian integument, or skin, includes secretory glands with various functions. Sebaceous glands produce a lipid mixture called sebum that is secreted onto the hair and skin for water resistance and lubrication. Sebaceous glands are located over most of the body. Eccrine glands produce sweat, or perspiration, which is mainly composed of water. In most mammals, eccrine glands are limited to certain areas of the body; some mammals do not possess them at all. However, in primates, especially humans, sweat figures prominently in thermoregulation, regulating the body through evaporative cooling. Sweat glands are located over most of the body surface in primates. Apocrine glands, or scent glands, secrete substances that are used for chemical communication, such as in skunks. Mammary glands produce milk that is used to feed newborns. While male monotremes and eutherians possess mammary glands, male marsupials do not. Mammary glands are probably modified sebaceous or eccrine glands, but their evolutionary origin is not entirely clear.

The skeletal system of mammals possesses many unique features. The lower jaw of mammals consists of only one bone, the dentary. The jaws of other vertebrates are composed of more than one bone. In mammals, the dentary bone joins the skull at the squamosal bone, while in other vertebrates, the quadrate bone of the jaw joins with the articular bone of the skull. These bones are present in mammals, but they have been modified to function in hearing and form bones in the middle ear. Other vertebrates possess only one middle ear bone, the stapes. Mammals have three: the malleus, incus, and stapes. The malleus originated from the articular bone, whereas the incus originated from the quadrate bone. This arrangement of jaw and ear bones aids in distinguishing fossil mammals from fossils of other synapsids.

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Bones of the mammalian inner ear: Bones of the mammalian inner ear are modified from bones of the jaw and skull.

The adductor muscle that closes the jaw is composed of two muscles in mammals: the temporalis and the masseter. These allow side-to-side movement of the jaw, making chewing possible, which is unique to mammals. Most mammals have heterodont teeth, meaning that they have different types and shapes of teeth rather than just one type and shape of tooth. Most mammals are diphyodonts, meaning that they have two sets of teeth in their lifetime: deciduous, or “baby” teeth, and permanent teeth. Other vertebrates are polyphyodonts: their teeth are replaced throughout their entire life.

Mammals, like birds, possess a four-chambered heart. Mammals also have a specialized group of cardiac fibers located in the walls of their right atrium called the sinoatrial node, or pacemaker, which determines the rate at which the heart beats. As for blood, mammalian erythrocytes (red blood cells) do not have nuclei, whereas the erythrocytes of other vertebrates are nucleated.

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Mammalian heart: Mammals possess a four-chambered heart, with two atria and two ventricles, that circulates blood through the body.

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.

Evolution of Mammals

The modern mammals of today are synapsids: descendants of a group called cynodonts which appeared in the Late Permian period.

Learning Objectives

Outline the evolution of mammals

Key Takeaways

Key Points

  • Synapsids are defined by a single opening in the skull and the fact that they are endothermic.
  • Mammals are the only living synapsids, derived from a lineage in the Jurassic period.
  • Two groups of mammals include the eutherians, which are closely related to placentals and the metatherians, which are more closely related to the marsupials.
  • Mammalian lineages from the Jurassic include Dryolestes, related to placentals and marsupials, and Ambondro, related to monotremes.
  • Later synapsids had specialized structures for chewing, including teeth, cheeks that can hold food, and a secondary palate, which gave them the ability to chew and breathe at the same time.

Key Terms

  • eutherian: the mammals more closely related to animals like humans and rodents than to marsupials
  • metatherian: belonging or pertaining to the infraclass Metatheria of marsupials

Evolution of Mammals

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. The lineage leading to today’s mammals split in the Jurassic. 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).

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Cynodonts: Cynodonts, which first appeared in the Late Permian period 260 million years ago, are thought to be the ancestors of modern mammals.

Since Juramaia, the earliest-known eutherian, lived 160 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 320 and 315 million years ago. The sauropsids are today’s reptiles and birds, along with all the extinct animals more closely related to them than to mammals. 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 mid-1990s, 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. A secondary palate is not found in pelycosaurs, but is present in cynodonts and mammals. The jawbone also shows changes from early synapsids to later ones. The zygomatic arch, or cheekbone, is present in mammals and advanced therapsids such as cynodonts, but is not present in pelycosaurs. The presence of the zygomatic arch suggests the presence of the masseter muscle, which closes the jaw and functions in chewing.

Living Mammals

Living mammals can be classified in three subclasses: eutherian, metatherians and monotremes.

Learning Objectives

Name and describe the distinguishing features of the three main groups of mammals

Key Takeaways

Key Points

  • Monotremes include the platypus which are defined by their ability to lay eggs instead of giving birth to live young.
  • Metatherians are classified as the marsupials which possess a pouch where the premature young reside and nurse while continuing to develop.
  • Eutherians are the most common type of mammal and are defined by the presence of a complex placenta which connects the developing fetus to the mother during gestation.

Key Terms

  • marsupial: a mammal of which the female has a pouch in which it rears its young, which are born immature, through early infancy
  • placental: a mammal having a placenta; most members of Mammalia

Living Mammals

Living mammals can be classified into three major classes: eutherians, monotremes, and metatherians. The eutherians, or placental mammals, and the metatherians, or marsupials, together comprise the clade of therian mammals. Monotremes form their sister clade.

There are three living species of monotremes: the platypus and two species of echidnas, or spiny anteaters. The leathery-beaked platypus belongs to the family Ornithorhynchidae (“bird beak”), whereas echidnas belong to the family Tachyglossidae (“sticky tongue”). The platypus and one species of echidna are found in Australia; the other species of echidna is found in New Guinea. Monotremes are unique among mammals as they lay eggs rather than giving birth to live young. The shells of their eggs are not like the hard shells of birds, but are leathery, similar to the shells of reptile eggs. All monotremes possess a cloaca which serves as the opening for the intestinal, reproductive, and urinary tracts. Additionally, monotremes have no teeth.

Marsupials are found primarily in Australia,although the opossum is found in North America. Australian marsupials include the kangaroo, koala, bandicoot,Tasmanian devil, and several other species. Most species of marsupials possess a pouch in which the very premature young reside after birth, receiving milk and continuing to develop. Marsupials differ from eutherians in that there is a less complex placental connection. The young are born at an extremely early age and latch onto the nipple within the pouch.

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Tasmanian devil: The Tasmanian devil is one of several marsupials native to Australia. (credit: Wayne McLean)

Eutherians are the most widespread of the mammals, occurring throughout the world. There are 18 to 20 orders of placental mammals. Some examples are Insectivora, the insect eaters; Edentata, the toothless anteaters; Rodentia, the rodents; Cetacea, the aquatic mammals including whales; Carnivora, carnivorous mammals including dogs, cats, and bears; and Primates, which includes humans. Eutherian mammals are sometimes called placental mammals because all species possess a complex placenta that connects a fetus to the mother, allowing for gas, fluid, and nutrient exchange. While other mammals possess a less complex placenta or briefly have a placenta, all eutherians possess a complex placenta during gestation.

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A red fox: Red foxes are eutherian (placental) mammals because the mothers nourish their young via a placenta during fetal development. The placenta enables a mother to exchange gases, fluids, and nutrients with the growing embryos.