Mammals Characteristics, Examples, and Classification

What are mammals?

Mammals are animals that belong to the class Mammalia and they are characterized by the presence of hair or furs and the possession of mammary glands. Animals that are mammals are one of the most evolved species in the animal kingdom, belonging to the Vertebrata order.

Definition of mammals

Mammals are those species of animals that fall under the order Vertebrata, with unique characteristics such as the presence of mammary glands for young ones to suckle on, being warm-blooded, and the presence of hair on their skin.

Characteristics of mammals

  • Diversity
  • Geographic range
  • Habitat
  • Physical description
  • Development
  • Reproduction
  • Lifespan/Longevity
  • Behavior
  • Communication and perception
  • Food habits
  • Predation
  • Ecosystem roles

In order to fully grasps the subject topic, mammalian animals will be discussed under the above-listed characteristics.


Mammalia is a class of about 5000 species divided into 26 orders and is seen in different shapes and sizes. The smallest mammals are shrews and bats, and they can weigh as little as 3 grams. The blue whale, which can weigh up to 160 metric tons, is the world’s largest mammal and, indeed, the largest animal to have ever lived on the planet (160,000 kg). Thus, the largest and smallest mammals differ in mass by a factor of 53 million.

In the aspect of mammals that fly, glide, swim, run, burrow, or jump, they have adapted morphologies that allow them to locomote efficiently; hence, these animals have evolved a wide range of forms that allows them to perform plenty of functions like the ones listed previously.

Geographic range

Animals of this class can be found everywhere in the world, in every ocean, and on numerous oceanic islands around the world.


Mammalian species have evolved to live in nearly every habitat on the planet. Some can be found in all terrestrial biomes, from deserts to tropical rainforests to polar icecaps. Many species are arboreal, meaning they spend the majority of their time in the forest canopy. One group of mammals (bats) has even evolved powered flight, which is only the third time in vertebrates that this ability has evolved.

Many of these animals are semi-aquatic, living near lakes, streams, or ocean coasts (e.g., seals, sea lions, walruses, otters, muskrats, and many others). Whales and dolphins (Cetacea) are fully aquatic and can be found in all oceans and rivers around the world. Whales are found in polar, temperate, and tropical waters, both nearshore and in the open ocean, and at depths ranging from the water’s surface to more than one kilometer.

Physical description

Animals that are mammals have hair at some stage of their development, and the majority of these animals have hair their entire lives. Adults of some species lose most or all of their hair, but hair is present at least during some stages of development in mammals such as whales and dolphins. Mammalian hair, which is composed of a protein called keratin, serves at least four purposes.

For starters, it slows the exchange of heat with the surrounding environment (insulation). Secondly, sensory hairs (whiskers or “vibrissae“) alert an animal when it comes into contact with an object in its environment. Vibrissae are frequently densely innervated and well-supplied with muscles that regulate their position.

Thirdly, the color and pattern of one’s hair can influence one’s appearance. It may be used to disguise predators or prey, to warn predators of a defensive mechanism (for example, a skunk’s conspicuous color pattern serves as a warning to predators). The hairs on mammals can also be used to communicate social information like threats and sex (male or female). Fourthly, the hairs of mammals offer some protection, either by acting as an extra layer of protection (against abrasion or sunburn,) or by taking the form of dangerous spines that deter predators as seen in porcupines and spiny rats).

Development of mammals

There are 3 main kinds of mammals, each of which is distinguished by a key feature of embryonic development. Monotremes (Prototheria) lay eggs, which is the most primordial form of reproduction in mammals. After a very short gestation period, marsupials (Metatheria) give birth to highly altricial young (8 to 43 days). The young are born at a young age in terms of morphological development, and as they develop, they attach to the mother’s nipple and spend a proportionately greater amount of time nursing. Whereas the gestation period lasts significantly longer in placental mammals (Eutheria).

All through gestation, eutherian children communicate with their mothers via the placenta, a complex organ that connects the embryo to the uterus. All mammals are completely reliant on their mothers for milk once they are born. Aside from these few generalizations, these animals have a wide range of developmental and life genealogy patterns that differ between species and broader taxonomic groups.

Reproduction in mammals

Most mammalian species are either promiscuous (both males and females have multiple mates in a given reproductive season) or polygynous (one male mate with plenty of females). Seeing as females incur such high costs during gestation and lactation, males can frequently produce much more progeny in a mating season than females. As a result, polygyny is the most common mating system in this class of animals, with very few males fertilizing numerous females and several males fertilizing nothing whatsoever. This circumstance creates the basis for intense male-male competition in many species, as well as the possibility for females to be picky about which males sire their offspring.

Approximately 3% of mammalian species are monogamous, with males mating with only one female per season. Males in these situations provide at least some care for their offspring. Mating systems within species frequently differ depending on local environmental conditions. When food is scarce, males may mate with a single female and care for the young.

Other mammalian mating systems, such as polyandry, can also be found in these classes of animals. For example, common marmosets and African lions exhibit cooperative breeding, in which groups of females, and occasionally males, share care of young from one or more females.

Most Mammalia breed seasonally and are influenced by environmental cues like day length, resource intake, and temperature. Females of some species often store sperm until the conditions are favorable for fertilization, at which point their eggs are fertilized. Other eggs of this class of animals may be fertilized shortly after intercourse, but embryo implantation into the uterine lining may be delayed a phenomenon known as delayed implantation. The third type of delayed gestation is “delayed development,” in which the embryo’s development is temporarily halted.

Another point on reproduction as a Mammalia characteristic is the fact that females feed their newborns with milk. Milk is a substance that is rich in fats and protein, that is given once the young are born (or hatch in the case of monotremes) through the process known as lactation. Lactation is far more energy-intensive than gestation because females must produce this high-energy substance and gestation does not require such a high-energy substance. Lactating females must produce enough milk for their offspring to keep their body temperatures stable while also growing and developing. Mammals must maintain their own body temperatures after birth, no longer relying on their mothers for thermoregulation as they did during pregnancy, in addition to protecting their young from predators. In some species, the young remain with their mothers for an extended period of behavioral development and learning even after lactation.


Mammals differ in lifespan, just as they do in size. Smaller creatures have shorter lives, while larger animals have longer lives. Bats are an exception to this pattern; they are small mammals that can survive for one or more decades in natural circumstances, which is significantly longer than the natural lifespans of much larger mammals. In the wild, mammalian lifespans range from a year or less to 70 years or more. But bowhead whales have been known to live for more than 200 years.


Mammalian behavior varies greatly between species. Their activity patterns reflect their high energy demands as endotherms require more energy intake than ectotherms of comparable size. Thermoregulation is crucial in dictating mammalian behavior, for example, animals in colder climates must stay warm, whereas animals in hot, dry climates must stay cool and conserve water. Mammals rely on behavior to help them maintain physiological balance, in order for them to move around their environment in a variety of ways, which include swimming, running, bounding, flying, gliding, burrowing, and climbing.

Social behavior varies a lot as well, for example, some mammals live in herds of tens, hundreds, thousands, or even millions of individuals while others are usually solitary unless they are mating or raising young. Mammalian activity patterns cover the entire spectrum of possibilities, such that their behavior can be nocturnal, diurnal, or crepuscular.

Communication and Perception

In a broad sense, mammals rely on olfaction, hearing, tactile perception, and vision as important sensory cues. Many facets of mammalian ecologies, such as foraging, mating, and social communication, rely on olfaction. Pheromones and other olfactory signals are used by many mammals to transmit information about their reproductive condition, territory, or individual or group identity. The use of these pheromones and other olfactory signals to communicate is commonly known as scent-marking in mammals. These pheromones are frequently transmitted through urine, feces, or glandular secretions. There are some mammalian species like skunks that use odors to protect themselves from predators which are particularly sensitive to foul-smelling chemical defenses.

Mammalian hearing is usually well advanced and it is the primary mechanism of perception in some species. Echolocation, or the ability to discern objects in the external environment by listening to echoes from an animal’s sounds, has evolved across several groups. For example, in microchiropteran bats (Chiroptera) and several toothed whales and dolphins (Odontoceti), echolocation is the primary perception channel used in foraging and navigation.

Countless mammals are vociferous (vocal) and use sound to communicate with one another. Vocalizations are being used in a broad range of social settings, including communication between mother and offspring, communication between potential mates, and many more. Alarms at the appearance of a predator, aggression in dominance interactions, territorial defense, and reproductive state can all be communicated through vocalizations. In some groups, most notably humans, communication via vocalizations is quite complex.

Mammals recognize their surroundings via tactile input from their hair and skin. Specialized hairs like whiskers or vibrissae serve a sensory function, of alerting an animal when it comes into contact with an object in its surroundings. Vibrissae are frequently densely innervated and well-supplied with muscles that regulate their position. The skin is also an essential sensory organ as well, especially those areas of the skin that are sensitive to tactile stimuli, which assist in specific functions such as foraging. Touch of the skin has several communication functions and is frequently linked to social behavior (e.g., social grooming).

A well-developed vision is more prominent among mammals even in many species that live underground or use echolocation.

Food Preferences

Mammals consume a wide range of organisms as a group, for instance, many mammals can be carnivores, herbivores, or omnivores. They feed on both animals and plants like fruit, nectar, foliage, wood, roots, seeds, and fungi. A small number of mammals can have a significant impact on the populations of their food items because mammals are warm-blooded animals and they require more food than cold-blooded animals.


Predation is a major cause of death for many mammals except for a few top predators. Mammals are preyed upon by a wide range of other organisms, including other mammals. For example, predatory birds or birds of prey, and reptiles are two other groups that eat mammals. Numerous species adapt to predation by using avoidance strategies such as cryptic coloration, limiting foraging to times when predators may very well be scarce, or by being social. Some mammals, such as skunks, have defensive chemicals or some form of protective armor or physical defense.

Mammary Glands

These are enlarged sweat glands that are modified to consist of ducts and glandular tissues that secrete milk through the nipples. Female Mammalian species nurse their young through the use of this gland. The milk provided by the mammary gland has essential proteins, sugars, fats, vitamins, and salts for the children. However, not all mammals have nipples, for instance, milk is secreted by ducts in the abdomens of monotremes such as the platypus, which diverged from other mammals early in evolutionary history.

Though present in both males and females, mammary glands fully develop only in females in most mammal species, explaining the existence of smaller nipples on males (including male human beings). The Dayak fruit bat and the Bismarck masked flying fox are the exceptions to this rule because males of these species can lactate and occasionally assist in the nursing of newborns.

Single-Boned Lower Jaws

Mammalian lower jawbones are made up of a single piece that connects directly to the skull. This bone is known as the dentary because it houses the lower jaw’s teeth. In other vertebrates, the dentary is just one of several bones in the lower jaw that does not connect directly to the skull. Mammals have a powerful bite thanks to their single-pieced lower jaw and the muscles that control it thereby stressing the significance of the dentary. It also enables them to use their teeth to tear and chow down their prey (as wolves and lions do) or to grind down tough vegetable matter (like elephants and gazelles).

One-time replacement of a Tooth Or teeth

Diphyodonty is a characteristic shared by most mammals in which teeth are replaced only once during the lifetime of an animal. Newborn and young mammal teeth are smaller and weaker than those of adults. Therefore, this first set of teeth, known as deciduous teeth, falls out before adulthood and is gradually replaced by the second set of larger, permanent teeth. Although not all mammals are diphyodont, there are some that are polyphyodonts (animals that replace their teeth continuously throughout their lives). Such mammals include elephants, manatees, and kangaroos.

presence of 3 (three) bones in the middle ear

The 3 unique inner ear bones of mammals are; the incus, malleus, and stapes, and they are also known as the hammer, anvil, and stirrup. These tiny bones transmit sound vibrations from the tympanic membrane (also known as the eardrum) to the inner ear, where they are converted into neural impulses and processed by the brain.


The diaphragm of mammalian species are arguably more developed than those of birds, and unquestionably more evolved than those of reptiles. This implies that mammals can breathe and use oxygen more efficiently than other vertebrate orders, which, when combined with their warm-blooded metabolisms, allows for a broader range of activity and better utilization of available ecosystems.

Four-Chambered Hearts

Mammals, like all vertebrates, have muscular hearts that contract multiple times to pump blood, which gives oxygen and nutrients to every cell while removing waste products like carbon dioxide. Only mammals and birds have four-chambered hearts, which are more efficient than fish’s two-chambered hearts or amphibians’ and reptiles’ three-chambered hearts.

Classification and examples of mammals

  • Eutheria
  • Metatheria
  • Prototheria

Mammalia is the animal kingdom’s largest class. They are divided into three subclasses based on reproduction as listed above.


This subclass of mammals gives birth to their young. The young are developed within the mother and receive nutrition from the mother’s placenta. Furthermore, it is made up of 19 orders, a few of which are as follows:

Examples of mammals


Mammals in this subclass give birth to underdeveloped young that remains in their mother’s pouch until they mature. Marsupials and kangaroos are two examples. They are grouped in seven different orders:

Mammalia examples
Marsupial modes
New world opossum
South American rat opossum


The subclass Prototheria, also known as Monotremes, is made up of egg-laying mammals. It has one order with six species. Monotremata is the order, and duckbilled platypus, Echidna are two examples of mammals in this order.

General classification of mammals

Scientists have also classified mammals on a broad scale, making it easier to learn about mammals and their distinguishing features.

Examples of Mammalia
Lion, Tiger, Dog
Kangaroo, Koala, Wombat
Chimpanzee, Gorilla, Monkey
Squirrel. Mouse, Porcupine
Dolphins, Whales
Other mammals
Seal, Walrus, Sea-lion

Aquatic mammals (water mammals)

  • Blue Whale
  • Harbor Porpoise
  • Humpback Whale
  • Orca
  • Sperm Whale
  • Spinner Dolphin
  • West Indian Manatee

The above-listed are some examples of mammals in the ocean. This simply means that they are mammals that live in water.

Mammals that can fly

The only mammals competent of actual flight are those in the order Chiroptera, also known as bats. (A few other mammal species, such as flying squirrels, are associated with flight, but they are not able of sustained flight; rather, their appendages allow them to glide through the air for short ranges.) Bats, the second-largest order of mammals, have a diverse range of species and can be found on every continent except Antarctica. Although bat species are extremely diverse and widespread, there are approximately two major suborders: megabats, which are much larger and live primarily on fruit, and microbats, which are much smaller and live primarily on insects.

Roles of mammals in the ecosystem

The nearly 5000 mammal species fill a wide range of ecological roles or niches. There are carnivores, omnivores, and herbivores that have the predator-prey relationship, as well as species that make or significantly alter their environment, and thus the habitat and structure of their communities. Mammals frequently play an ecological role that is disparately large in comparison to their numerical abundance, thanks in part to their high metabolic rates.

Several mammalian species play a critical role in seed dispersal or pollination and they also serve as keystone predators in their communities. The roles of mammals in the ecosystem are so diverse that generalizations across the group are difficult. Despite having a lower species diversity than other animal groups, mammals have a significant impact on global biodiversity.

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