Many of the characters common to mammals do not appear in other animals. Some of them, of course, can be observed also in birds—a very high (in respect to both maximum and mean values) metabolic rate and activity level or complexity of particular adaptations such as advanced parental care and social life, increased sensory capacities, and new pathways of processing sensory information or enormous ecological versatility. Fine differences between birds and mammals suggest that the respective adaptations are homoplasies—that is, they evolved in both groups independently.
Other mammalian characteristics are synapomorphies of Amniota, the characteristics shared because of common ancestry. The amniotes, a group including reptiles, birds, and mammals, are the terrestrial vertebrates in which embryonic development takes place under the protection of fetal membranes (amnion, chorion, allantois). As in other amniotes, mammals are further characterized by an increased role of parental investment, internal fertilization, keratinized skin derivatives, an advanced type of kidney (metanephros) with a specific ureter, an advanced type of lung respiration, and the decisive role of dermal bones in skull morphology. Of course, at the same time, mammals share a large number of characteristics with all other vertebrates, including the general body plan, solid inner skeleton, the design of homeostatic mechanisms (including pathways of neural and humoral regulation), and functional integration of particular developmental modules. Mammals also share with other vertebrates the patterns of segmentation of trunk skeleton and muscles and the specific arrangements of the homeobox genes organizing the body segmentation as well as a lack of their expression in the head region, etc. These characters are synapomorphies of vertebrates, which are at least partly retained not only in some amniotes but throughout all other vertebrate clades. With respect to mammals, these are symplesiomorphies, the primitive characters that do not reveal closer relations of the class but on its broadest phylogenetic context.
Mammals also exhibit a large number of qualities that are fully unique to them, the autapomorphies. The autapomor-phies are the characteristics by which a taxon can be clearly distinguished and diagnosed. Thus, though many characteristics of mammals are not specific just to them, answering the question "what is a mammal?" means first demonstrating the autapomorphies of that group. A simplified list of them includes:
(1) The young are nourished with milk produced by (2) mammary glands. These glands appear in all female mammals, and are the structure from which the class Mammalia got its name. (3) Obligatory vivipary (in Theria, i.e., marsupials and placen-tals) is the reproductive mode with a specialized organ interconnecting the embryo and maternal tissues, the chorioallantoic placenta (in Eutheria, i.e., placentals). (4) Hairs, covering the body, grow from deep invaginations of the germinal layer of epidermis called follicles. Similar to other amniotes, the hair is composed of keratin and pigments, but its structure is unique for mammals. (5) Skin is rich in various glands. Most mammals have sweat glands (contributing to water balance and cooling the body surface), scent glands, and sebaceous glands. (6) The specific integumental derivatives, characteristic of particular groups of mammals, are composed either exclusively of keratin (such as claws, nails, and hoofs, which protect the terminal phalanx of the digits and adapt them to a
specific way of locomotion or foraging) or of keratin in combination with dermal bone structures (horns of bovids and antlers of cervid artiodactyls, which play a considerable role in social signaling). A large variety of integumental derivatives are included in defensive adaptations: dermal armors of armadillos or keratinized scales of pangolins, spines modified from hairs in echidnas, hedgehogs, tenrecs, porcupines, or spiny mice, or the accumulations of hairlike fibers keratinized into a horn structure in rhinoceroses. (7) Limb position and function are modified to support specific locomotory modes of mammals such as jumping, galloping, or sustained running and can be specifically rearranged. The extreme rearrangements are seen in bats, which fly using a forelimb wing, and in specialized marine mammals, pinnipedian carnivores, cetaceans, and sirenia, whose forelimbs take the shape of a fin (the external hind limbs are absent in the latter two groups). (8) Pectoral girdle is simplified in comparison to the non-mammalian state: coracoid, precoracoid and interclavicle bones are lost (except for monotremes, which retain them) or partly included in the scapula. Also the clavicle, the last skeletal element that fixes the limb to the axial and thoracic skeleton, is lost in many groups. With these rearrangements the forelimbs get new locomotory qualities (such as extensive protraction), supporting abilities such as climbing and fine limb movements and providing a new spectrum of manipulative functions from cleaning hair to a variety of prey manipulations. (9) The bones of the pelvic girdle are fused into a single bone, with enlarged and horizontally prolonged ilium.
(10) A great degree of regional differentiation of the vertebral column. All mammals (except some edentates and manatees) have seven cervical vertebrae with the first two (atlas and axis) specifically rearranged to support powered head movements. (11) The vertebral column is strengthened against lateral movements but is greatly disposed to the vertical flexion. This is seen first of all in the lumbar section, whose vertebrae, in contrast to the non-mammalian ancestors, lack ribs. (12) The mammalian skull is bicondylous (the first vertebra, atlas, joints the skull via paired occipital condyles located on the lateral sides of the large occipital foramen), with (13) an enlarged braincase, (14) massive zygomatic arches (formed by the jugale and squamosum bones), and (15) a spacious nasal cavity with a labyrith of nasal turbinalia covered by vascularized tissue important both for olfaction (ethmoidal turbinalia) and/or heat and water exchange during breathing (maxillary turbinalia). (16) The nostrils open at a common structure called the nose, obviously the most prominent point of the head. The ancestral form of the nose, the rhinarium, is a hairless field of densely circular-patterned skin surrounding the nostril openings. The rhinarium is particularly large in macrosmatic (highly developed sense of smell) mammals (such as carnivores or artiodactyls), in lagomorphs, some rodents, and bats. In strepsirhine primates it is incised by a central groove, the phlitrum, while in some other groups such as in macroscelids or in elephants, the nose is prolonged and attains a number of supplementary functions. In contrast, all these structures are absent in cetaceans in which the nasal cavity is reduced and the nostrils (or a single nostril opening in Odontoceti) appear at the top of the head and their function is restricted to respiration. (17) Left and right maxillary and palatal bones are fused in early development and form the secondary bony palate, which is further extended by a fleshy soft palate. These structures provide a complete separation of the respiratory and alimentary tracts. The early appearance of such a separation is one of the essential prerequisites for suckling milk by a newborn and, hence, it seems probable that the secondary palate first appeared simply as an adaptation for this. (18) The heart is a large four-chambered organ (as in birds) with the left aorta persistent (not the right one, as in birds). (19) Erythrocytes, the red blood cells, are biconcave and lack nuclei. Thrombo-cytes are transformed to nonnucleated blood platelets.
(20) Lungs have an alveolar structure, ventilated by volume changes performed by the counteraction of two independent muscular systems, and a (21) muscular diaphragm, unique for mammals. (22) The voice organ in the larynx, with several pairs of membranous muscles, is unique for mammals. It is capable of very specialized functions such as the production of various communicative signals or high-frequency echolocation calls in bats and cetaceans. (23) There are three ossicles in the middle ear (malleus, incus, stapes). The former two are unique to mammals and are derived from the elements of the pri mary mandibular joint—articulare and quadratum—which still retain their original function in the immediate mammalian ancestors. The third bone of the primary mandibular joint, the angulare, changes in mammals into the tympanic bone, which fixes the tympanic membrane and finally enlarges into a bony cover of the middle ear—the bulae tympani. (24) The sound receptor (Corti's organ of the inner ear) is quite long and spirally coiled in mammals (except for monotremes) and surrounded bypetrosum, a very compact bone created by a fusion of several elements. (25) With an enlarged braincase, the middle ear and tympanic membrane are thus located deeper in the head and open to the external environment by a long auditory meatus terminating with (26) a large movable external auricle. Auricles (pinnae) are specifically shaped in particular clades and contribute to the lateral discrimination of the auditory stimuli and directionality of hearing. They may be absent in some aquatic mammals (cetaceans, sirenia, walruses), while they are extremely pronounced and diversified in other groups such as bats, for which the acoustic stimuli (echoes of the ultrasonic calls they emit) are by far the most important source of spatial information. (27) In contrast to other am-niotes, the lower jaw, or mandible, is composed of a single bone, dentary or dentale, which directly articulates with the temporal bone of the skull at the (28) dentary-squamosal joint. This arrangement not only fastens the jaw joint to resist the forces exerted during strong biting but also simplifies the functional rearrangements of jaw morphology responding to different demands of particular feeding specializations. (29) In all mammals, the posterior part of the mandible extends dorsally into the ramus mandibulae, which provides an area of attachment for the massive temporal muscles responsible for the powered adduction of the mandible.
(30) Essentially, all mammals have large teeth despite considerable variation in number, shape, and function in particular groups and/or the fact that some mammals secondarily lack any teeth at all (anteaters of different groups, and the platypus). Teeth are deep-rooted in bony sockets called alveoles. Only three bones host the teeth in mammals: the pre-maxilla and maxilla in the upper jaw and the dentary in the lower jaw. (31) Mammalian dentition is generally heterodont (of different size, shape, etc.). Besides the conical or unicuspidate teeth (incisors and a single pair of canines in each jaw) mammals also have large complex multicuspidate molars (three in placentals, four in marsupials, in each jaw quadrant) and pre-molars situated between canines and molars whose shape and number varies considerably among particular groups. The latter two teeth types are sometimes called "postcanines" or "cheek teeth." (32) The molars are unique to mammals. The basic molar type ancestral to all particular groups of mammals is called tribosphenic. It consists of three sharp cones connected with sharp blades. In combination with the deep compression chambers between blades, such an arrangement provides an excellent tool both for shearing soft tissues and crushing insect exoskeletons. This type of molar is retained in all groups feeding on insects, such as many marsupials, ten-recs, macroscelids, true insectivores such as moles, shrews or hedgehogs, bats, tree shrews, and prosimian primates, but the design of the molar teeth is often extensively rearranged in other groups. The multicuspidate structure of molars bears enormous potential for morphogenetic and functional re-
arrangements, one of the prerequisites of the large diversity of feeding adaptations in mammals. (33) Mammalian dentition is diphyodont. This means that there are two generations at each tooth position (except for molars): the milk or deciduous teeth of the young and the permanent teeth of an adult mammal. Diphyodonty solves a functional-morphological dilemma: the size of teeth, an essential factor in feeding efficiency, is limited by the size of the jaws. While the jaws can grow extensively, the posteruption size of the teeth cannot be changed due to the rigidity of their enamel cover, which is the essential quality of a tooth. With diphyodonty, the size of the late erupting permanent teeth can be maximized and adapted to adult jaw size while the deciduous dentition provides a corresponding solution for the postweaning period. Dental morphology and the patterns of tooth replacement are specifically modified in some clades. In marsupials, only one milk tooth—the last premolar—comes in eruption, while the others are resorbed prior to eruption. Dolphins, aardvarks, and armadillos have a homodont dentition without any tooth replacement. No tooth replacement occurs in small and short-living mammals with greatly specialized dentition, such as shrews or muroid rodents (deciduous teeth are resorbed instead of eruption), while in some large herbivores tooth replacement can become a continuous process by which the tooth row enlarges gradually by subsequent eruption of still larger molar teeth in the posterior part of the jaws. In elephants and manatees, this process includes a horizontal shift of the erupting tooth, which thus replaces the preceding cheek tooth. All these processes are well synchronized with the growth of jaws, the course of tooth wear, and subsequent prolonging of time available for tooth development. (34) A general enlargement of the brain related perhaps not only to an increase in the amount of sensory information and/or a need to integrate sensory information from different sources, but also to more locomotory activity, high versatility in locomo-
tory functions, a greatly diversified social life, and a considerably expanded role for social and individual learning. (38) The extended spectrum of behavioral reactions and their interconnections with an increased capacity of social and individual learning and interindividual discrimination should also be mentioned. In fact, this characteristic is very significant for mammals, as are the following two: (39) Growth is terminated both by hormonal control and structural factors. The most influential structural aspect of body growth is the appearance of cartilaginous epiphyseal discs separating diaphyses and epiphyses of long bones. With completed ossification, the discs disappear and growth is finished. Corresponding mechanisms determine the size of the skull (except in cetaceans, which have a telescoped skull in which the posterior bones of the cranium overlap each other). (40) Sex is determined by chromosomal constitution (XYsystem, heterogametic sex is a male).
Almost all of these (and other) characteristics undergo significant variations and their modifications are often largely specific for particular clades of mammals. What is common for all is perhaps that in mammals all the characters are more densely interrelated than in other groups (except for birds). The morphological adaptations related to locomotion or feeding are often also integrated for social signaling, physiological regulation, or reproductive strategy, and often are controlled by quite distant and non-apparent factors. Thus, the excessive structures of ruminant artiodactyls, such as the horns of bovids and antlers of deer, are undoubtedly significant in social signaling, in courtship and display behavior, and frequently are discussed as excessive products of sexual selection. However, the proximate factor of these structures, the hereditary disposition for excessive production of mineralized bone tissue, can actually be selected rather by its much less obvious effect in a female: her ability to produce a large, extremely precocial newborn with highly mineralized long bones that enable it to walk immediately after parturition. The female preference for the excessive state of the correlated characters in a male, his large body size and display qualities, possibly supported by social learning, supplement the mechanisms of the selection in quite a non-trivial way. Such a multi-layered arrangement of different factors included in a particular adaptation is indeed something very mammalian.
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