At first sight, this is not a difficult question. Every child is able to identify an animal as a mammal. Since its earliest age it can identify what is a cat, dog, rabbit, bear, fox, wolf, monkey, deer, mouse, or pig and soon experiences that with anyone who lacks such a knowledge there would be little chance to communicate about other things as well. To identify an animal as a mammal is indeed easy. But by which characteristics? The child would perhaps explain: Mammals are hairy four-legged animals with faces.
Hair, or fur, probably the most obvious mammalian feature, is a structure unique to that group, and unlike the feathers of birds is not related to the dermal scales of reptiles. A mammal has several types of hairs that comprise the pelage. Specialized hairs, called vibrissae, mostly concentrated in the facial region of the head, perform a tactile function. Pelage is seasonally replaced in most mammals, usually once or twice a year by the process called molting. In some mammals, such as ermines, the brown summer camouflage can be changed to a white coat in winter. In others, such as humans, elephants, rhinoceroses, naked mole rats, and aardvarks, and in particular the aquatic mammals such as walruses, hippopotami, sirenia, or cetaceans, the hair coat is secondarily reduced (though only in the latter group is it absent completely, including vibrissae). In the aquatic mammals, the role of the pelage is performed by a thick layer of subcutaneous adipose tissue by which the surface of body is almost completely isolated from its warm core and the effect of a cold ambient environment is substantially reduced. Thanks to this tissue, some mammals can forage even in cold arctic waters and, as a seal does, rest on ice without risk of freezing to it. In short, the essential role of the subcutaneous adipose layer and pelage is in thermal isolation, in preventing loss of body heat. Mammals, like birds, are endotherms (heat is generated from inside of the body by continuous metabolic processes) and homeotherms (the body temperature is maintained within a narrow constant range).
The body temperature of mammals, about 98.6°F (37°C), is optimal for most enzymatic reactions. A broad variety of functions are, therefore, kept ready for an immediate triggering or ad hoc mutual coupling. All this also increases the versatility of various complex functions such as locomotion, defensive reactions, and sensory performances or neural processing of sensory information and its association analysis. The constant body temperature permits, among other things, a high level of activity at night and year-round colonization of the low temperature regions and habitats that are not accessible to the ectothermic vertebrates. In short, endothermy has a number of both advantages and problems. Endothermy is very expensive and the high metabolic rate of mammals requires quite a large energetic intake. In response, mammals developed a large number of very effective feeding adaptations and foraging strategies, enabling them to exploit an extreme variety of food resources from insects and small vertebrates (a basic diet for many groups) to green plants (a widely accessible but indigestible substance for most nonmammals). At the same time, mammals have also developed diverse ways to efficiently control energy expenditure.
Besides structural adaptations such as hair, mammals have also developed diverse physiological and behavioral means to prevent heat and water loss, such as burrowing into underground dens; seasonal migrations or heterothermy; and the controlled drop of body temperature and metabolic expenditure during part of the day, or even the year (hibernation in temperate bats, bears, and rodents as well as summer estivation in some desert mammals). So, considerable adaptive effort in both directions increases foraging efficiency and energy expenditure control. When integrated with morphological, physiological, behavioral, and social aspects, it is an essential feature of mammalian evolution and has contributed to the appearance of the mammalian character in many respects.
Four legs, each with five toes, are common not only to many mammals, but to all terrestrial vertebrates (amphibians, reptiles, birds, and mammals), a clade called Tetrapoda. Nevertheless, in the arrangement of limbs and the modes of locomotion that it promotes, mammals differ extensively from the remaining groups. The difference is so clear that it allows us to identify a moving animal in a distance as a mammal even in one blink of an eye. In contrast to the "splayed" reptilian stance (i.e. horizontal from the body and parallel to the ground), the limbs of mammals are held directly beneath the body and move in a plane parallel to the long axis of the body. In contrast to reptiles, whose locomotion is mostly restricted to the lateral undulation of the trunk, mammals flex their vertebrate column vertically during locomotion. This arrangement enables a powered directional movement, such as sustained running or galloping, very effective for escaping from a predator, chasing mobile prey, or exploring spatially dispersed food resources. The respective rearrangements also bring another effect.
By strengthening the vertebral column against lateral movement, the thoracic cavity can be considerably enlarged and the thoracic muscles released from a locomotory engagement, promoting changes to the effective volume of the thoracic cavity. With a synergetic support from another strictly mammalian structure, a muscular diaphragm separating the thoracic and visceral cavity, the volume of the thoracic cavity can change during a breathing cycle much more than with any other vertebrates. With the alveolar lungs, typical for mammals, that are designed to respond to volume changes, breathing performance enormously increases. This enables a mammal to not only keep its basal metabolic rate at a very high level (a prerequisite for endothermy) but, in particular, to increase it considerably during locomotion. In this connection, it should be stressed that the biomechanics of mammalian locomotion not only allow a perfect synchronization of limb movements and breathing cycles but, with the vertical flex of the vertebral column, are synergetic to the breathing movements and support it directly. As a result, the instantly high locomotory activity that characterizes a mammal increases metabolic requirements but at the same time helps to respond to them.
The face is the essential source of intra-group social information not only for humans but for many other mammal groups. The presence of sophisticated mechanisms of social integration and an enlarged role in interindividual discrimination and social signaling are broadly characteristic of mammals. Nevertheless, each isolated component contributing to the complex image of the mammalian face says something important regarding the nature of the mammalian constitution, and, moreover, they are actually unique characters of the group. This is particularly valid for fleshy cheeks and lips, the muscular belt surrounding the opening of a mouth. The lips and the spacious pocket behind them between the cheeks and teeth (the vestibulum oris) are closely related to feeding, and not only in that they enlarge the versatility of food processing in an adult mammal. The lips, cheeks and vestibulum oris are completely developed at the time of birth and since that time have engaged in the first behavioral skill performed by a mammal. Synergetic contraction of lip and cheek muscles producing a low pressure in the vestibulum oris is the key component of the suckling reflex, the elementary feeding adaptation of a newborn mammal. All mammals, without exception, nourish their young with milk and all female mammals have large paired apocrine glands specialized for this role-the mammary glands, or mammae. Nevertheless, not all mammalian newborns actually suck the milk. In the egglying monotremes (the Australian duck-billed platypus and spiny anteaters), mammary glands lack the common milk ducts and nipples, so young do not suck but instead lick the milk using their tongue. All other mammals, both marsupials and eutherians, together denoted as Theria, bear a distinctive structure supporting suckling-the paired mammary nipples.
The nipples originate independently from mammary glands, they are present both in males and females, and their number and position is an important character of individual clades. The therian mammals are all viviparous. For the most vulnerable period of their lives they are protected first by the intrauterine development with placental attachment of the embryo and then by prolonged postnatal parental care. A milk diet during the latter stage postpones the strict functional control on jaws and dentition and enables postnatal growth, the essential factor for the feeding efficiency of an adult mammal. At the same time this provides extra time for development of other advanced and often greatly specialized mammalian characteristics: an evolving brain and the refinement of motor capacities and behavioral skills. Thanks to the extended parental investment that mammalian offspring have at the beginning of their independent life, they enjoy a much higher chance for post-weaning survival than the offspring of most other vertebrates. The enormous cost of the parental investment places, of course, a significant limit upon the number of offspring that can be produced. Despite the great variation in reproductive strategies among individual mammalian clades, in comparison to other vertebrates (excepting elasmobranchians and birds), the mammals are clearly the K-strategists (producing few; but well-cared for, offspring) in general.
The other components of the mammalian face provide correspondingly significant information on the nature of these animals. The vivid eyes with movable eyelids, external auricles, nose, and last but not least long whiskers (vibrissae, the hairs specialized for tactile functions), show that a mammal is a sensory animal. Most extant mammals are noctural or crepuscular and this was almost certainly also the case with their ancestors. In contrast to other tetrapods, which are mostly diurnal and perceive almost all spatial information from vision, mammals were forced to build up a sensory image of the world from a combination of different sources, in particular olfaction and hearing. Nevertheless, vision is well developed in most mammals and is capable of very fine structural and color discrimination, and some mammals are secondarily just optical animals. For example, primates exhibit a greatly enlarged capability for stereoscopic vision. In any case, all mammals have structurally complete eyes, though the eyes may be cov-ered by skin in some fossorial mammals (such as blind mole rats, or marsupial moles) or their performance may be reduced in some respect. In comparison with other vertebrates, the performance of vision is particularly high under low light intensities, and the eyes are quite mobile. The latter character may compensate for a reduced ability of head rotation in mammals due to the bicondylous occipital joint contrasting to a monocondylous joint in birds or reptiles. The eyes are covered by movable eyelids (not appearing in reptiles), significant both in protecting the eyes and in social signaling. The remaining two structures-nose and auricles-are particularly unique for mammals and are related to the senses that are especially important for mammals: olfaction and hearing. Not only the nose and auricles themselves, but also the other structures associated with the senses of smell and hearing feature many traits unique to mammals.
Mammals construct much of their spatial information with the sole aid of olfactory, acoustic, or tactile stimuli combined with information from low-intensity vision. This task necessitated not only a considerable increase in the capacity and sensory versatility of the respective organs, but also the refinement of the semantic analysis of the information they provide. As a result, the brain structures responsible for these tasks are greatly enlarged in mammals. The tectum mesencephali, a center for semantic analysis of optical information, bi-lobed in other vertebrates, is supplemented by a distinct center of acoustic analysis by which the tectum of mammals becomes a four-lobed structure, the corpora quadrigemina. The forebrain or telencephalon, a structure related to olfactory analysis, is by far the largest part of the mammalian brain. Its enlargement is particularly due to the enlarging of the neocortex, a multi-layered surface structure of the brain, which further channels inputs from other brain structures and plays the role of a superposed integrative center for all sensory, sensory-motor, and social information.