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Mammals (Mammalia) are a class of warm-blooded, mostly viviparous vertebrates that suckle their young: the mothers produce milk and use it to feed their young. They are part of the Amniota and as such are most closely related to reptiles and birds. Mammals are distinguished from other animals by their protective fur, their well-developed brains and the specific construction of their skulls.

Modern mammals include more than 6400 species. Mammals include both flying (bats) and swimming (whales) and both carnivorous (predators) and herbivorous (ruminants, among others) species. The largest animal of all time, the blue whale, and the largest living land animal, the savanna elephant, are both mammals. Mammals are found on virtually the entire earth, except in the most extreme habitats. Humans, too, are mammals.

Many mammals exhibit intelligent behavior. They are able to communicate with each other in a variety of ways through sounds, smells and signals. Most mammals live in groups and form social structures, such as a harem relationship or a hierarchy. A few species are solitary and have a territorial lifestyle.

From the Neolithic revolution onward, many species of mammals were domesticated by humans. Domesticated mammals remain an important component of the human food supply (in the form of meat and dairy products) to the present day. Mammals also play a role in sports, such as hunting or racing, and are used as model organisms in science. More and more wild mammals are threatened with extinction, mainly due to poaching and habitat destruction.


Mammals are warm-blooded vertebrates with – at least potentially – a body covering composed of hair. They breathe with their lungs and they suckle their young with mother’s milk, the secretion of their mammary glands. Besides mammary glands, the skin also has sweat glands, which allow mammals to lose heat. With the exception of cloacadians, all species are viviparous. A characteristic feature is the construction of the lower jaw, which consists of a single bone, and the three ossicles.

Not all the features mentioned are unique; for example, some sharks also develop a placenta and birds are also warm-blooded. Both groups are examples of convergent evolution; both the warm-bloodedness of birds (descended from cold-blooded reptiles) and the placental development of young in sharks (an exception within the egg-laying cartilaginous fishes) arose independently of the parallel features in mammals. Unique to mammals are the fur, the ossicles, the mandible and the milk and sweat glands.

Mammals are diploid organisms (chromosomes occur in pairs). In mammals, the sex in almost all species is determined by the sex chromosomes: females have two X chromosomes and males have one X and one Y chromosome. Exceptions to this include the platypus, the echidna[2] and Tokudaia osimensis. The total number of chromosomes (2n) varies greatly: from 2n=6 in the Indian muntjac (Muntiacus muntjac), an Asian deer, to 2n=118 in Dactylomys boliviensis, a South American rodent in the porcupine rat family.


In all mammalian species, fertilization occurs internally, with a penis delivering sperm into the vagina. The sperm move through the cervix and uterus to the fallopian tube. Fertilization generally takes place in the fallopian tube. Although the more primitive members of the class (the cloacadians, for example, the platypus and the echidna) are oviparous, most mammals give birth to their young alive. In the marsupials (Metatheria), the young are very little developed after birth and often remain in a pouch for a long time after birth, where they continue to develop. In placental animals (Placentalia), a more developed young is born after staying in the uterus, where it is supplied with food and oxygen through the umbilical cord.

There is a great difference in the degree of development at birth. For example, many rodents, rabbits and several predators, such as bears, are born naked and blind in a burrow. For the first few weeks of their lives, they need care from the mother. Cats and other predators are also born with their eyes closed, but these animals already have fur. These animals also need maternal care, especially to learn to hunt. Herd animals such as ungulates give birth to young that are fully developed and can walk within an hour. Gestation periods vary from the marsupial mouse Sminthopsis macroura, whose young are born after 11 days, to African elephants, whose gestation period can be 22 months. Litter size ranges from several species of great apes, elephants and whales, which usually give birth to only one young at a time, to the Virginian opossum, which can give birth to up to eighteen to nineteen young at a time.

Body temperature

Mammals, like birds, are warm-blooded. This means that body temperature is kept constant (homeothermic) and is independent of outside temperature. In cold-blooded animals, body temperature is more dependent on the outside temperature.

The hypothalamus, a small area in the brain, regulates body temperature in mammals. It can do this by:

  • increasing/decreasing the rate of metabolism
  • dilating blood vessels (which transport heat to the skin)
  • raising/flattening of the hairs (which hold/release an insulating layer of air)
  • shivering to get warm
  • sweating and panting (through evaporation the animal loses heat).

A mammal can also regulate its temperature without the hypothalamus playing a role. For example, some species huddle together, crouch or take sunbaths to stay warm. In warm regions, where outside temperatures can get high in the afternoon, mammals seek shade, shelter in a burrow or protect themselves from the sun by covering themselves with a layer of mud, for example.

Warm-bloodedness enables mammals to survive in places where cold-blooded animals such as reptiles and amphibians cannot, such as the cold polar regions. However, warm-bloodedness also costs a lot of energy, requiring small mammals, such as shrews, to eat much more than reptiles of the same size. Some mammals sometimes temporarily sacrifice their homeothermia. For example, in many species, body temperature drops during hibernation.

Skin and hair

Mammals are the only animals that have hair-covered bodies. Most species have fur consisting of an insulating undercoat and a protective outer coat consisting of cover hairs. In a few mammals, including humans, the skin is more sparsely hairy. Whales are virtually hairless and even the naked mole rat is bald except for a few tactile hairs.

The main functions of the hairs are to retain and lose body heat. They may also have another function, as camouflage or for impressing opponents (for example, the mane of a lion). Some hairs have a different function, such as defense (the spines of hedgehogs and porcupines, for example) or for the sense of touch (whiskers, for example). Hairs are made up of the protein keratin. The horn of a rhinoceros also consists of keratin.

The skin consists of two layers, a dermis (dermis), containing blood vessels, glands and nerve endings, and an epidermis (epidermis), consisting of dead cells. Mammals have some glands unique to the group: in addition to mammary glands, they have sweat glands, which play a role in regulating body temperature, and scent glands, which many species of mammals use to secrete substances to communicate, such as through territory demarcation.

Skull and teeth

The middle ear contains three ossicles: the hammer, anvil and stirrup. The cheek arch is shaped to accommodate the jaw muscles. The upper jaw is firmly anchored to the base of the skull. The lower jaw is connected to the skull by a jaw joint. Each half of the lower jaw consists of one bone. Other vertebrates have one bone in the middle ear, at least three bones on each side of the lower jaw and no direct connection between lower jaw and skull. There are some groups of reptiles in which the bones in ear and mandible are mammalian, however. The best known of these groups are the extinct cynodons, which are considered the ancestors of mammals.

Mammals have heterodont teeth: teeth from the same set of teeth are different, and specialized for different functions. Animals with homodont teeth, such as reptiles, have similarly shaped teeth. Basically, every mammal has 44 teeth. In each half of the jaw are:

  • 3 incisors for biting
  • 1 canine tooth for grasping and tearing food
  • 4 false molars for chewing and grinding
  • 3 true molars for chewing and grinding

The teeth of a mammal are adapted to the diet of the species. In carnivores (meat eaters), the canines are very sharp. The molars of these animals are snap molars: they are used to cut meat. Herbivores (herbivores) have molars with folds. This allows them to grind grasses and other plants. Rodents generally do not possess canines. Omnivores (omnivores, including humans) have tuberous molars to properly grind both plant and animal foods.

For the identification of species, genera and other taxa, characteristics of the skull and teeth are often used, such as the size of certain skull dimensions, the shape of (parts of) the skull, the presence of foramina (holes in bones through which nerves run) and fossae (grooves) in certain places, and the presence of various lumps on the molars.



Based on their diet, mammals fall into three groups. First, there are the carnivores, which eat animal material. These may be other mammals, but several groups have specialized, for example, in fish (they are piscivores) and other aquatic animals or in insects and other invertebrates (called insectivores). A distinct group within the carnivores are the vampire bats, the only parasitic species of mammals, which live only on blood. A second group are the herbivores, which eat mainly plant material. These include grazers, as well as leaf eaters, fruit eaters, seed eaters and animals that live mainly on nectar. The third and final group, omnivores, includes animals such as bears and humans, which feed on both plant and animal foods. Incidentally, the boundary between these groups is not strict: the wolf, for example, is a carnivore, but also eats plant foods such as fruits at times, and the red squirrel supplements its herbivorous diet with eggs and young birds.

The size of a species partially determines its diet. The smallest mammals, such as shrews, marsupials and small bats, have relatively large skin areas and therefore retain less heat than larger mammals. To maintain their body temperature, they need a lot of energy and have a high metabolism. Because digestion of plant material is slower than that of animal material, the smallest mammals live mainly on insects and other small animals. Small herbivorous mammals live mainly on fruits, nuts, seeds, buds, shoots and roots, which store the reserve food of plants. Larger mammals have a relatively smaller skin surface area and can retain more heat. As a result, they can afford a lower metabolism and a more herbivorous diet is adequate.

The tougher parts of plants, such as leaves, twigs, grasses and stems, are more likely to be eaten by larger rather than smaller mammals because their digestion is relatively high in relation to the energy they contain. Larger carnivorous mammals can afford longer breaks between meals, while small insectivores such as shrews are almost constantly searching for food. Larger insectivorous mammals like anteaters and aardvarks could never find enough insects to live on that way. They live mainly on ants and termites, social insects that live together in large colonies.


Mammals know a wide range of communication tools, including sound, color, smell and behavior. Smell is a common means of communication. The odorants from skin glands serve to demarcate territory and provide information such as sex, age, status, mating readiness and diet. Individuals can also be distinguished from one another based on scent and can be smelled to which group an individual belongs. Several species of smaller predators, including the skunks, also use scent glands as repellents. The scent glands can be found all over the body, but are most often found in the anogenital region. In no other group of vertebrates does scent play such an important role in communication as it does in mammals. Mammals also communicate through sound, body postures and facial expressions, among other things (this is mainly true of primates).

Social structures

Mammals have several social structures. Solitary species, such as several felines, bears and moles, avoid other conspecifics and usually have their own territory which they defend against conspecifics. Specimens of different sexes seek each other out only during mating season, but adults of the same sex are not tolerated. Female tigers, for example, do not tolerate other tigresses on their territory. Male tigers are tolerated only when the female has no young. The territory of a male tiger usually overlaps with the territories of several females, but not with the territory of another male.

A large number of mammals are more or less social. The simplest form of cohabitation is a very loose group association, in which any individual can join or leave. Members of different groups can be exchanged without difficulty. In mammals, this form of cohabitation is quite rare. It is found in snow hares, among others.

A more common form of cohabitation is the large closed group, in which both males and females live. This form of cohabitation is found in baboons and rats, among others. In these groups, there is a social hierarchy among males – and often females as well – that determines who is dominant and who is subordinate. The most dominant animal is usually the leader of the group, and this one has the right to mate. This hierarchy is not constant and is constantly being fought over.

Certain species of mammals, such as gibbons, beavers, wolves and jackals, live in pairs throughout the year. Often these pairs are supplemented by offspring of the pair. Usually these are immature young or subordinate animals that do not reproduce. Only the parent pair has the right to reproduce within the group. These pairs are generally monogamous, but “cheating” occurs regularly.

In some mammals, including elephants and several marine predators, the sexes live separately from each other outside the mating season. Females usually live in (family) groups outside the mating season, males alone or in bachelor groups. Another common structure is the harem relationship, in which a male keeps several females. This behavior occurs in several ungulates, among others. A mixture of the two aforementioned forms of cohabitation occurs in many species. Male elephant seals and red deer live alone or in bachelor groups outside the mating season, forming harems during the mating season that they defend against other males.

In animals that must compete for the right to mate, (sham) fights take place between males during the mating season. In these species, males are much larger than females. Also, males often have imposing weapons such as tusks, horns and antlers, as well as manes to impress the other males. In animals that live in pairs, the distinction between males and females is less clear.

A very special form of society is the eusocial society, in which a colony consists of the offspring of one female and a few males. All other group members are infertile. In insects, this form of society is found in bees, ants and termites, among others. In mammals, this behavior is known in only two species, mole rats, Fukomys damarensis and the naked mole rat (Heterocephalus glaber).

Living in groups has many advantages. Enemies are spotted earlier, young can be raised together and the territory can be defended by several animals. Social predators hunt together, which increases the chances of finding prey. On the other hand, a group of animals is also more conspicuous and food must be shared with more animals.

Habitat and territory

Every mammal moves in a certain part of the biotope. The area in which they move, the habitat, is often bounded. The size of this habitat is determined by food supply and terrain. Many species inhabit the home range permanently and year-round, but some animals migrate between home ranges. East African wildebeest, for example, migrate with the rains. Whales migrate to warmer waters during the mating season; outside the mating season, they are found in nutrient-rich colder waters. Several species of seals live year-round on the high seas, but migrate to permanent breeding grounds during the mating season, where the animals form colonies.

Home ranges may overlap, but often a mammal protects (part of) its home range from rivals. This may happen year-round or only for a certain part of the year, mainly the mating season. The defended piece of land is called the territory. The territory is usually marked with scents, as well as optical cues, such as damaged trees and plants and leaving feces in conspicuous places such as a rock or clearing. Some species, such as gibbons, lions and red deer, also indicate the boundaries of their territory through loud noises.

Care of the young

In many species, young are born during the period when food is at an all-time high. This is because pregnancy and lactation cost the mother animal a lot of energy, and the young, once weaned, provide additional food competition.

The mother animal’s milk ensures that a small and helpless young is not left to fend for itself, but can be guided until it is big enough to fend for itself. This increases the chances of survival for young mammals. All energy gained by the young can thereby be utilized for growth. All mammals have parental care because of this. The degree of parental care varies by species. In some mammals, including the toepaja, care is limited to suckling, and the young are left alone for most of the day. In most other mammals, parental care is more intense. The young are fed, their fur is cared for, they are protected from enemies and learn such things as hunting and social interactions. Usually only the mother cares for the young, but in a large number of cases the father and/or other group members also care for them. In jumping monkeys (Callicebus), it is the father who takes the most care of the young. The mother’s role in these monkeys is limited to suckling.

Play is an almost typical phenomenon for young mammals, although it is also known from some young bird species. It is a particular form of behavior, involving seemingly aimless movements that are closely related to purposeful behavior. For example, the play of lion cubs consists of stalking and pouncing on each other, just as adult animals hunt. However, the young lion cubs rarely bite through and do not aim to kill the other cub. Play behavior is primarily observed in social species such as primates, elephants, whales, ungulates and social predators. Play can be disadvantageous: it takes a lot of energy, can lead to injury and playing animals are less attentive, making them easy prey. The benefits of play are not entirely clear, but may include improvement through practice of later behavior, improved physical fitness through exercise (young animals need less exercise than adults, and through play the animals stay fit) and the establishment of social roles. For example, hierarchy among adult elephants is established as early as infancy.


All mammals have periods of rest. Resting generally occurs where an animal is safe, such as hidden in a nest or burrow or in a herd, with other animals watching for danger. The time when an animal is active is determined by predator activity, prey activity, competition with other species and by temperature. Mammals are active mainly at dusk and at night. For example, many small rodents are active at night because they are then less visible to predators. Felines, in turn, are active at night because that is precisely when rodents become active. Bats are probably active mainly at night, to avoid competition with birds active during the day.

Animals that depend largely on their eyesight to find food and their way around, such as arboreal mammals like monkeys and squirrels, are predominantly diurnal. Ungulates living in herds are active throughout the day and may rest at any time during the day. As they do so, the other group members watch for any enemies. Other mammals can also be active throughout the day. Seals do not depend on light or dark for hunting, but on the tides. At low tide they rest, at high tide they go in search of fish and other animals. Underground animals like moles notice little of the difference between day and night and are active throughout the day, with set periods of rest and of activity. Shrews must be active throughout the day to find enough food to maintain their body temperature. They alternate short periods of rest with periods of activity throughout the day. During the day, shrews usually remain hidden among vegetation.


Mammals have great adaptability and can survive almost anywhere. Because they do not depend on the outside temperature for their body temperature, as is the case for reptiles and amphibians, mammals can survive even in colder areas such as around the poles and in high mountains. As a result, the order is found on all continents, as well as all oceans and most islands. Only in the deep sea and the interior of Antarctica are mammals not found permanently.

The order achieves its greatest richness of form on land. One order, that of the bats (Chiroptera), spends a significant part of their lives in the air. Other mammal groups live largely (seals, hippos, otters, beavers) or even permanently (whales, manatees) in the water. On land, however, mammals are the dominant life form. Most large and medium-sized land animals belong to this order; mammals are at the top of most food chains. They have filled most niches, both on and below ground and in trees.

Islands are more difficult for land mammals to reach; consequently, many oceanic islands do not naturally host mammals, often with the exception of bats. Even New Zealand has no naturally occurring mammals, with the exception of two bat species (a third species is extinct) and marine predators, which live on the coast. Man, with the help of means of transportation such as boats and later airplanes, managed to spread throughout the world and now has a cosmopolitan range. In the wake of humans, several mammals also traveled with them, including stowaways such as rats and mice and domesticated animals such as dogs, cats, goats and pigs.