12.26: Mammal Characteristics - Biology

12.26: Mammal Characteristics - Biology

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One of these is not a mammal. Which one?

Mammals are a class of endothermic vertebrates. They have four limbs and produce amniotic eggs. Examples of mammals include bats, whales, mice, and humans. Clearly, mammals are a very diverse group. Nonetheless, they share many traits that set them apart from other vertebrates.

Characteristics of Mammals

Two characteristics are used to define the mammal class. They are mammary glands and body hair (or fur).

  1. Female mammals have mammary glands. The glands produce milk after the birth of offspring. Milk is a nutritious fluid. It contains disease-fighting molecules as well as all the nutrients a baby mammal needs. Producing milk for offspring is called lactation.
  2. Mammals have hair or fur. It insulates the body to help conserve body heat. It can also be used for sensing and communicating. For example, cats use their whiskers to sense their surroundings. They also raise their fur to look larger and more threatening (see Figure below).

Cat Communicating a Warning. By raising its fur, this cat is “saying” that it’s big and dangerous. This might discourage a predator from attacking.

Most mammals share several other traits. The traits in the following list are typical of, but not necessarily unique to, mammals.

  • The skin of many mammals is covered with sweat glands. The glands produce sweat, the salty fluid that helps cool the body.
  • Mammalian lungs have millions of tiny air sacs called alveoli. They provide a very large surface area for gas exchange.
  • The heart of a mammal consists of four chambers. This makes it more efficient and powerful for delivering oxygenated blood to tissues.
  • The brain of a mammal is relatively large and has a covering called the neocortex. This structure plays an important role in many complex brain functions.
  • The mammalian middle ear has three tiny bones that carry sound vibrations from the outer to inner ear. The bones give mammals exceptionally good hearing. In other vertebrates, the three bones are part of the jaw and not involved in hearing.
  • Mammals have four different types of teeth. The teeth of other vertebrates, in contrast, are all alike.

Dolphins are mammals that have adapted to swimming and reproducing in water.


  • Mammals are a class of endothermic vertebrates.
  • Mammals have four limbs and produce amniotic eggs.
  • The mammal class is defined by the presence of mammary glands and hair (or fur).
  • Other traits of mammals include sweat glands in their skin, alveoli in their lungs, a four-chambered heart, and a brain covering called the neocortex.


  1. List five traits that are shared by all mammals, including the two traits that are used to define the mammal class.

Characteristics of Mammals

The presence of hair is one of the most obvious signs 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, and hair provides insulation to retain heat generated by metabolic work. Hair traps a layer of air close to the body, retaining heat. 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.”

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, and 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 likely are 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 ([Figure 1]). 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.

Figure 1: Bones of the mammalian inner ear are modified from bones of the jaw and skull. (credit: NCI)

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, that is, 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. Mammalian erythrocytes (red blood cells) do not have nuclei, whereas the erythrocytes of other vertebrates are nucleated.

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, which is 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.

Mammary Glands

By Duke.of.arcH - Getty Images

Unlike other vertebrates, mammals nurse their young with milk produced by mammary glands, which are modified and enlarged sweat glands consisting of ducts and glandular tissues that secrete milk through nipples. This milk provides young with much-needed proteins, sugars, fats, vitamins, and salts. Not all mammals have nipples, however. Monotremes such as the platypus, which diverged from other mammals early in evolutionary history, secrete milk through ducts located in their abdomens.

Though present in both males and females, in most mammal species, mammary glands fully develop only in females, hence the presence of smaller nipples on males (including human males). The exceptions to this rule are the Dayak fruit bat and the Bismarck masked flying fox. Males of these species have the ability to lactate, and they sometimes help to nurse infants.

Evolution of Mammals

Figure 2. Cynodonts, which first appeared in the Late Permian period 260 million years ago, are thought to be the ancestors of modern mammals. (credit: Nobu Tamura)

Mammals are synapsids, meaning 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 (Figure 2).

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, which are specialized for chewing, 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. A secondary palate 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.

In the appendicular skeleton, the shoulder girdle of therian mammals is modified from that of other vertebrates in that it does not possess a procoracoid bone or an interclavicle, and the scapula is the dominant bone.

Mammals evolved from therapsids in the late Triassic period, as the earliest known mammal fossils are from the early Jurassic period, some 205 million years ago. Early mammals were small, about the size of a small rodent. Mammals first began to diversify in the Mesozoic Era, from the Jurassic to the Cretaceous periods, although most of these mammals were extinct by the end of the Mesozoic. During the Cretaceous period, another radiation of mammals began and continued through the Cenozoic Era, about 65 million years ago.

Mammals Classification

Mammals are vertebrate animals constituting the class of Mammalia and characterized by the presence of mammary glands, Which produce milk for feeding their young. The presence of hair or fur. Sweat glands. Glands specialized to produce milk, known as mammary glands. Three middle ear bones. A four-chambered heart.

Largest and Smallest Mammals

  • The largest mammal is the Blue Whale which lives in the ocean and can grow to over 80 feet long.
  • The largest land mammal is the Elephant followed by the rhino and the hippo (They can’t spend a lot of time in the water).
  • The smallest mammal is Kitty’s hog-nosed bat. This bat is 1.2 inches long and weighs less than 1/2 a pound. It is also called the bumblebee bat.

Dolphins are mammals, and warm-blooded. They breathe air using their lungs. Dolphins must make frequent trips to the surface of the water to catch a breath. The blowhole on top of a dolphin’s head acts as a “nose,” making it easy for the dolphin to surface for air.

Other characteristics of dolphins that make give birth to live young rather than laying eggs and they feed their young with milk. Even they have a tiny amount of hair, right around the blowhole. Whales and porpoises are also mammals.

Classification Of Mammals

Mammalia has the largest class in the animal kingdom. Based on their reproduction, they are classified into three subclasses:

  • Eutheria: under this subclass give birth to young ones. The young ones are developed inside the mother and derive nutrition through the placenta from the mother. Ex: Elephants, Cows.
  • Metatheria: Under this sub-class give birth to immature young ones, hence they stay in their mother’s pouch until they mature. Ex: Marsupials, and Kangaroos.
  • Egg-laying: Few mammals lay eggs, they are called monotremes. Monotremes include the platypus and the long-nosed spiny anteater.

They are the most adaptable animals on the planet. They are found on every continent and in every ocean. Mammals as a group use every possible form of locomotion. Terrestrial species walk, run, jump, climb, hop, swing, dig, and burrow. Aquatic ones swim, shuffle, and dive. Some are even fly.
Most mammals are intelligent, with some possessing large brains, self-awareness, and tool use. Mammals can communicate and vocalize in several ways, including the production of ultrasound, scent-marking, alarm signals, singing, and echolocation. Mammals can organize themselves into fission-fusion societies, harems, and hierarchies, but can also be solitary and territorial. Most mammals are polygynous, but some can be monogamous or polyandrous.

Biology - What Are the Characteristics of a Mammal?

There are many different animals that qualify as mammals. Some live in the sea, some live in trees, and some live in penthouse apartments with a view of the city skyline. Here is a quick rundown of the major characteristics shared by most mammals.

Mammals give birth to live babies, not eggs. If it lays an egg, it's probably not a mammal, unless it's a duck-billed platypus, which is a whole other article. Almost every other type of animal, even bugs and fish, lay eggs to make new babies. Aside from a few types of snakes and some unusual amphibians and birds, this is a trait that is uniquely of the mammalian family.

Mammals feed their young with milk. A mammal nourishes its young with milk produced in its own body. When you contrast this with other types of animals you will see that this is a striking difference. Birds, for example, must go forage for food and regurgitate it into the chick's mouth. Even mammals that live in the water, like whales and dolphins, feed their young milk from their mammary glands.

Mammals have hair. They originally started growing the hair as an insulation method. Mammals have to regulate their body temperature so that they don't get too cold or too hot. Some mammals, like monkeys, are still really hairy. Other mammals have evolved to having fairly little hair. And still other mammals insist on shaving their hair, or using depilatory cream.

Mammals are warm-blooded. Unlike a cold-blooded animal, a mammal must keep its core body temperature pretty stable to maintain life. Most mammals live in climates that are pretty stable, and that they can handle.

Mammals breathe air. Mammals use their lungs to take in fresh air. This is in contrast to some animals, for example fish, which breathe water. Just because an animal has lungs and breathes air does not make it a mammal. Many reptiles and amphibians also have this characteristic. Mammals that live in the water need to resurface from time to time to get a re-up of oxygen.

So there you go, if an animal meets all of these criteria, you're probably dealing with a mammal. A list of the most common mammals would include bats, rats, cows, monkeys, rhinos, elephants, sheep, cats, dogs, rabbits, whales, dolphins, giraffes, lions, and bears. If you've considered this list and taken a look at yourself, you have no doubt noticed that you meet all of these standards as well. Congratulations, you're a mammal! Of course there are peculiar animals that don't have one or more of these characteristics but are still considered mammals.

General features

The evolution of the class Mammalia has produced tremendous diversity in form and habit. Living kinds range in size from a bat weighing less than a gram and tiny shrews weighing but a few grams to the largest animal that has ever lived, the blue whale, which reaches a length of more than 30 metres (100 feet) and a weight of 180 metric tons (nearly 200 short [U.S.] tons). Every major habitat has been exploited by mammals that swim, fly, run, burrow, glide, or climb.

There are more than 5,500 species of living mammals, arranged in about 125 families and as many as 27–29 orders (familial and ordinal groupings sometimes vary among authorities). The rodents (order Rodentia) are the most numerous of existing mammals, in both number of species and number of individuals, and are one of the most diverse of living lineages. In contrast, the order Tubulidentata is represented by a single living species, the aardvark. The Uranotheria (elephants and their kin) and Perissodactyla (horses, rhinoceroses, and their kin) are examples of orders in which far greater diversity occurred in the late Paleogene and Neogene periods (about 30 million to about 3 million years ago) than today.

The greatest present-day diversity is seen in continental tropical regions, although members of the class Mammalia live on (or in seas adjacent to) all major landmasses. Mammals can also be found on many oceanic islands, which are principally, but by no means exclusively, inhabited by bats. Major regional faunas can be identified these resulted in large part from evolution in comparative isolation of stocks of early mammals that reached these areas. South America (the Neotropics), for example, was separated from North America (the Nearctic) from about 65 million to 2.5 million years ago. Mammalian groups that had reached South America before the break between the continents, or some that “island-hopped” after the break, evolved independently from relatives that remained in North America. Some of the latter became extinct as the result of competition with more advanced groups, whereas those in South America flourished, some radiating to the extent that they have successfully competed with invaders since the rejoining of the two continents. Australia provides a parallel case of early isolation and adaptive radiation of mammals (specifically the monotremes and marsupials), although it differs in that Australia was not later connected to any other landmass. The placental mammals that reached Australia (rodents and bats) evidently did so by island-hopping long after the adaptive radiation of the mammals isolated early on.

In contrast, North America and Eurasia (the Palearctic) are separate landmasses but have closely related faunas as the result of having been connected several times during the Pleistocene Epoch (2.6 million to 11,700 years ago) and earlier across the Bering Strait. Their faunas frequently are thought of as representing not two distinct units but one, related to such a degree that a single name, Holarctic, is applied to it.

Comparison between Artiodactyla and Perissodactyla | Mammals

Usually present. May be hollow or solid, located on frontal bone.

Third and fourth digits are larger and bear hoofs. The axis of the limb pass­ hoofed. The axis of the limb passes between the Nos. 3 and 4 digits (Paraxonic condition).

Third trochanter is absent.

Usually articulates with the calca­neum.

8. Dorsolumbar vertebrae:

General tendency of suppression of upper incisors.

Complicated by forming different chambers.

Sus, Camelus, Bos Capra , ovis , Axis axis.

Comparison # Perissodactyla:

Only the third digit is stout and down the third digit (Mesaxonic condition)

Third trochanter is present.

Does not articulate with the calca­neum.

8. Dorsolumbar vertebrae:

Present and used for cropping.

Equus zebra, Tapirus Rhinocerous Equae hemionus.

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It is commonly assumed that aerial insectivorous bats in the tropics respond to moonlight intensity by decreasing their foraging activity during bright nights due either to an increase in predation risk, or to a reduction in insect availability.

The effect of moonlight on bat activity can be measured both between nights and within a single night. However, few studies have simultaneously used both approaches, and most authors generally compare bat activity with lunar phases. Our main aim was to evaluate how moonlight influences aerial insectivorous bat activity at different time scales: between nights and within the same night. Activity of five bat species was measured using autonomous ultrasound recording stations and moonlight intensity percentages retrieved from the Moontool program nightly throughout a 53-day sampling period. Only one species (Myotis riparius) responded negatively to moonlight, while two species (Pteronotus parnellii and Saccopteryx leptura) increased their foraging activity in moonlight. For Cormura brevirostris and S. bilineata, moonlight intensity did not affect activity level. Bat activity was greater for all species at the beginning of the night, independent of the presence of the moon, indicating that foraging just after the sunset is adaptive. Thus, bat response to the effect of moonlight intensity is more apparent between nights than within a single night and may depend on species-specific traits, such as flight speed, flexibility in habitat use and body size.

The Evolution of Fur and Mammals

There are many different kinds of views regarding why fur evolved in proto-mammals and all of them have their strengths and disadvantages. In this blog post I will be writing about the one that seems more probable and realistic to me.

The main problem with trying to trace back the evolution of fur is the overwhelming lack of evidence. Due to its keratin structure, fur remains in an excellent condition in fossils for the first 100-150 million years, sometimes being preserved even better than bones. However as the time goes by, fur tends to merge into one giant mess, and it is no longer easy to tell it apart for scales, feathers, or any other currently unknown body coverings. As such, it is hard to pinpoint the exact point in time when fur appeared in proto-mammals.

About 150 to 200 million years ago the very first mammals were confined to the underground. Although direct evidence of fur has not been preserved for this long, creatures such as the Morganucodonsare still assumed to have had furry coats due to some of their other traits. For one, the Morganucodonspent its days sleeping in burrows and only came out at night to feed on insects. Such a lifestyle would be impossible for cold-blooded organisms, who are forced into a near-death state whenever there is a lack of sun and heat therefore, by method of elimination, the Morganucodon was, in fact, an homeothermic creature. However, as mentioned in the previous blog post, scales are extremely poor thermal insulators (after all, they were developed to let in as much heat and warmth as possible). In addition to this, the fossils show that the Morganucodonhad oil secretion ducts along its skin that are similar to those modern mammals and are presumed to have been used for grooming.

The proposed look of the Morganucodon.

But if you go slightly farther in time—to the realm of the early therapsids—you would be unable to find definite proof of a single furred creature (or at least one with some version of proto-fur). So what we have is not only a lack of direct evidence of fur, but also a lack of indirect evidence of the intermediary stages of its formation. It appears that over a short period of a million years (well… short it terms of evolution), fur spontaneously appeared—first in patches on the face of the early mammals and eventually covering their whole bodies.

If fur truly did evolve from scales as was once believed, we would find many different varieties of it in fossils. First we would see elongated scales with a different distribution of proteins. We would see these long scales becoming thinner and we would see them beginning to grow out of follicles. Furthermore, the first fur would not be growing in conjunction with scales, with single strands sticking out in between them, it would be a gradual transition. As such, I believe that fur was, just like feathers, an evolutionary novelty.

Its main purpose, however, was not to provide insulation or to attract mates. The first proto-mammals lived in very warm climates and thus could afford to live, sleep, and hide in dark burrows for short periods of times. Nevertheless, they faced a different problem: after living for hundreds of millions of years under the sun, their eyes were not well equipped for the limited light available in the burrows. They kept running into rocks and, like lemmings, falling off of cliffs (okay, the last one was an exaggeration… I love lemmings).

The solution was to develop some sort of tactile apparatus—in this case it was vibrissae, or their more common name, whiskers. The ancient mammals crawled about their burrows using their long and rigid hairs to feel their surroundings. This theory is supported by the indentations in the therapsids’ skulls that are akin to those of modern whiskered mammals. The Thrinaxodon, for example, lived 250mya and had pitted foramina—which are tiny openings that nerves pass through—on its skull. This species was also covered in scales for the most part. (Keep in mind that although this is strong evidence, it is still not proof: there are a couple species of modern lizards that have similar indentations, but do not have whiskers.)

Whiskers on the face of a Therapsid.

Originally these whiskers were located only on the head of the animals, protruding in between the scales, but eventually they spread to the rest of the body, giving the organism the ability to completely sense its environment. Over time, each hair became shorter, and the fur covered more and more area each generation. Finally evolution gave it another purpose—to protect the mammals from the cold weather and to help them retain heat.

Now that I’m done rambling about the evolution of fur, I will write about the one other defining features of mammals: the mammary gland. Since glands, in general, do not fossilize at all, theories about their evolution have to derive from their similarities to other existing organs. One popular theory states that they evolved from Apocrine sweat glands and were at first used to keep eggs moist (it also states that this organ evolved long before fur and other mammalian characteristics did). When the development of hard calcium shells replaced the previous rubbery ones, these sweat glands were repurposed—their new job was to provide nourishment to the newly hatched young, which, in turn, allowed the eggs they came from to become very small. The newborn hatchlings would then simply suck on the mother’s skin and the Apocrine sweat glands (now mammary gland) would provide them with lactose and other nutrients.

That’s it for today! Sorry for the shortage of pictures, I didn’t have much time to draw them, so I just found a couple on the internet. The next post will be about scales, nails, and horns and will be relatively short. Once again, I look forward to reading your comments.


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