Class Mammalia
mammals

The Class Mammalia includes about 5000 species placed in 26 orders. Systematists do not yet agree on the exact number or on how some orders and families are related to others. The Animal Diversity Web generally follows the arrangement used by Wilson and Reeder (2005). Exciting new information, however, coming from phylogenies based on molecular evidence and from new fossils, is changing our understanding of many groups. For example, skunks have been placed in the new family Mephitidae, separate from their traditional place within the Mustelidae (Dragoo and Honeycutt 1997, Flynn et al., 2005). The Animal Diversity Web follows this revised classification. Whales almost certainly arose from within the Artiodactyla (Matthee et al. 2001; Gingerich et al. 2001). The traditional subdivision of the Chiroptera into megabats and microbats may not accurately reflect evolutionary history (Teeling et al. 2002). Even more fundamentally, molecular evidence suggests that monotremes (Prototheria, egg-laying mammals) and marsupials (Metatheria) may be more closely related to each other than to placental mammals (Eutheria) (Janke et al. 1997), and placental mammals may be organized into larger groups (Afrotheria, Laurasiatheria, Boreoeutheria, etc.) that are quite different from traditional ones (Murphy et al. 2001). (Dragoo and Honeycutt, 1997; Flynn et al., 2005; Gingerich et al., 2001; Janke, Xu, and Arnason, 1997; Matthee et al., 2001; Murphy et al., 2001; Nowak, 1991; Teeling et al., 2002; Vaughan, Ryan, and Czaplewski, 2000; Wilson and Reeder, 1993)

All mammals share at least three characteristics not found in other animals: 3 middle ear bones, hair, and the production of milk by modified sweat glands called mammary glands. The three middle ear bones, the malleus, incus, and stapes (more commonly referred to as the hammer, anvil, and stirrup) function in the transmission of vibrations from the tympanic membrane (eardrum) to the inner ear. The malleus and incus are derived from bones present in the lower jaw of mammalian ancestors. Mammalian hair is present in all mammals at some point in their development. Hair has several functions, including insulation, color patterning, and aiding in the sense of touch. All female mammals produce milk from their mammary glands in order to nourish newborn offspring. Thus, female mammals invest a great deal of energy caring for each of their offspring, a situation which has important ramifications in many aspects of mammalian evolution, ecology, and behavior. (Klima and Maier, 1990; Vaughan, Ryan, and Czaplewski, 2000)

Although mammals share several features in common (see Physical Description and Systematics and Taxonomic History), Mammalia contains a vast diversity of forms. The smallest mammals are found among the shrews and bats, and can weigh as little as 3 grams. The largest mammal, and indeed the largest animal to ever inhabit the planet, is the blue whale, which can weigh 160 metric tons (160,000 kg). Thus, there is a 53 million-fold difference in mass between the largest and smallest mammals! Mammals have evolved to exploit a large variety of ecological niches and life history strategies and, in concert, have evolved numerous adaptations to take advantage of different lifestyles. For example, mammals that fly, glide, swim, run, burrow, or jump have evolved morphologies that allow them to locomote efficiently; mammals have evolved a wide variety of forms to perform a wide variety of functions. (Vaughan, Ryan, and Czaplewski, 2000)

Mammals can be found on all continents, in all oceans, and on many oceanic islands of the world. (Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

Different species of mammals have evolved to live in nearly all terrestrial and aquatic habitats on the planet. Mammals inhabit every terrestrial biome, from deserts to tropical rainforests to polar icecaps. Many species are arboreal, spending most or all of their time in the forest canopy. One group (bats) have even evolved powered flight, which represents only the third time that this ability has evolved in vertebrates (the other two groups being birds and extinct Pterosaurs).

Many mammals are partially aquatic, living near lakes, streams, or the coastlines of oceans (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 of the world, and some rivers. Whales can be found in polar, temperate, and tropical waters, both near shore and in the open ocean, and from the water's surface to depths of over 1 kilometer. (Nowak, 1991; Reichholf, 1990a; Vaughan, Ryan, and Czaplewski, 2000)

Several synapomorphies unite all mammals as a monophyletic group. These include 3 middle ear bones, hair, and the production of milk by mammary glands. While the monophyly of Mammalia is not disputed, there has been a rich history of debate regarding the relationships of groups within this class. The accepted classification of major groups of mammals is currently in flux. Traditionally, placental mammals (Eutheria) and marsupials (Metatheria) were thought to form a monophyletic clade sister to monotremes (Prototheria). However, recent molecular evidence suggests that monotremes and marsupials may be sister taxa, together forming a clade that diverged from placental mammals approximately 130 million years ago. (Janke, Xu, and Arnason, 1997; Vaughan, Ryan, and Czaplewski, 2000)

Our understanding of inter-ordinal relationships is also changing rapidly. A recent hypothesis that is gaining support from new molecular evidence is that placental mammals arose in Gondwana (as opposed to the traditional view that placental mammals first evolved in the Northern Hemisphere). The split between Africa and South America is thought to have further divided placental mammals into two major groups (Afrotheria and Boreoeutheria + Xenarthra). Within this scheme, relationships within and among orders are significantly different from traditional taxonomies (e.g. Afrosoricida is considered highly paraphyletic under this hypothesis, indeed Afrosoricida has been split into three new orders: Afrosoricida, Erinaceomorpha, and Soricomorpha). (Murphy et al., 2001; Wilson and Reeder, 2005)

Hypotheses of lower-level relationships are also changing rapidly. For example, evidence continues to accumulate indicating that microchiropterans, one of the two major groups of bats (Chiroptera), do not form a monophyletic clade. This hypothesis has important implications for the evolution of echolocation, the main mode of perception in all microchiropterans. If microchiropteran bats do not share a single common ancestor, then either echolocation in bats has evolved twice, or evolved once in a group ancestral to all modern bats and was subsequently lost in the Megachiroptera. (Teeling et al., 2002; Vaughan, Ryan, and Czaplewski, 2000)

Another example of intra-ordinal revisions of traditional classification schemes deal with relationships within the Carnivora. While skunks have long been recognized as a subfamily within Mustelidae (weasels, otters, polecats, martens, badgers, etc.), new molecular evidence indicates that skunks lie outside the group and may warrant elevation to familial rank. Furthermore, stink badgers, also traditionally considered mustelids, are now considered a member of the skunk family. This hypothesis calls into question the geographic origin of skunks. Skunks are only found in the New World, whereas stink badgers are found in Southeast Asia. A common ancestor of these two groups probably occured in Asia. The more traditional view held that the New World distribution of skunks reflected a New World ancestor. (Dragoo and Honeycutt, 1997; Flynn et al., 2005)

These two examples indicate the importance of continuously re-evaluating phylogenetic hypotheses as new data emerges. Although it makes developing a stable classification scheme difficult, forming new hypotheses about relationships among organisms can lead to valuable insights into many other questions regarding the evolution of organisms and their traits. Mammalian systematics is currently an active and thriving area of research. The Animal Diversity Web generally follows the classification scheme of Wilson and Reeder (2005), but as new hypotheses emerge and become well-supported by available evidence, the classification we recognize also changes. (Dragoo and Honeycutt, 1997; Vaughan, Ryan, and Czaplewski, 2000; Wilson and Reeder, 1993; Wilson and Reeder, 2005)

All mammals have hair at some point during their development, and most mammals have hair their entire lives. Adults of some species lose most or all of their hair but, even in mammals like whales and dolphins, hair is present at least during some phase of ontogeny. Mammalian hair, made of a protein called keratin, serves at least four functions. First, it slows the exchange of heat with the environment (insulation). Second, specialized hairs (whiskers or "vibrissae") have a sensory function, letting an animal know when it is in contact with an object in its environment. Vibrissae are often richly innervated and well-supplied with muscles that control their position. Third, hair affects appearance through its color and pattern. It may serve to camouflage predators or prey, to warn predators of a defensive mechanism (for example, the conspicuous color pattern of a skunk is a warning to predators), or to communicate social information (for example, threats, such as the erect hair on the back of a wolf; sex, such as the different colors of male and female capuchin monkeys; or the presence of danger, such as the white underside of the tail of a white-tailed deer). Fourth, hair provides some protection, either simply by providing an additional protective layer (against abrasion or sunburn, for example) or by taking on the form of dangerous spines that deter predators (porcupines, spiny rats, others). (Klima and Maier, 1990; Vaughan, Ryan, and Czaplewski, 2000)

Mammals are typically characterized by their highly differentiated teeth. Teeth are replaced just once during an individual's life (a condition called diphyodonty). Other characteristics found in most mammals include: a lower jaw made up of a single bone, the dentary; four-chambered hearts; a secondary palate separating air and food passages in the mouth; a muscular diaphragm separating thoracic and abdominal cavities; a highly developed brain; endothermy and homeothermy; separate sexes with the sex of an embryo being determined by the presence of a Y or 2 X chromosomes; and internal fertilization. (Klima and Maier, 1990; Vaughan, Ryan, and Czaplewski, 2000)

Often, characteristics of skulls and dentition are used to define and differentiate mammalian groups. To make these easier to comprehend within the accounts of lower mammalian taxa, we provide links to dorsal, ventral, and lateral views of the skull of a dog on which the major bones, foramina, and processes have been labelled. Closeups of the basicranial region, orbital region, and lingual and labial views of a mandible are also available. A partially labeled full skeleton of a raccoon has also been prepared.

There are three major groups of mammals, each is united by a major feature of embryonic development. Monotremes (Prototheria) lay eggs, which is the most primitive reproductive condition in mammals. Marsupials (Metatheria) give birth to highly altricial young after a very short gestation period (8 to 43 days). The young are born at a relatively early stage of morphological development. They attach to the mother's nipple and spend a proportionally greater amount of time nursing as they develop. Gestation lasts much longer in placental mammals (Eutheria). During gestation, eutherian young interact with their mother through a placenta, a complex organ that connects the embryo with the uterus. Once born, all mammals are dependent upon their mothers for milk. Aside from these few generalities, mammals exhibit a diversity of developmental and life history patterns that vary among species and larger taxonomic groups. (Vaughan, Ryan, and Czaplewski, 2000)

Just as mammals vary greatly in size, they also vary greatly in lifespan. Generally, smaller mammals live short lives and larger mammals live longer lives. Bats (Chiroptera) are an exception to this pattern, they are relatively small mammals that can live for one or more decades in natural conditions, considerably longer than natural lifespans of significantly larger mammals. Mammalian lifespans range from one year or less to 70 or more years in the wild. Bowhead whales may live more than 200 years. (Grzimek, 1990)

Mammalian behavior varies substantially among species. As endotherms, mammals require more energy intake than ectotherms of a similar size, and mammalian activity patterns reflect their high energy demands. For example, thermoregulation plays an important role in dictating mammalian behavior. Mammals that live in colder climates must keep warm, while mammals that live in hot, dry climates must keep cool and conserve water. Behavior is an important way for mammals to help maintain physiological balance.

There are mammal species that exhibit nearly every type of lifestyle, including fossorial, aquatic, terrestrial, and arboreal lifestyles. Locomotion styles are also diverse: mammals may swim, run, bound, fly, glide, burrow, or climb as a means of moving throughout their environment.

Social behavior varies considerably as well. Some mammals live in groups of tens, hundreds, thousands or more individuals. Other mammals are generally solitary except when mating or raising young.

Activity patterns among mammals also cover the full range of possibilities. Mammals may be nocturnal, diurnal, or crepuscular. (Vaughan, Ryan, and Czaplewski, 2000)

Generally, olfaction, hearing, tactile perception, and vision are all important sensory modalities in mammals. Olfaction plays a key role in many aspects of mammalian ecology, including foraging, mating and social communication. Many mammals use pheromones and other olfactory cues to communicate information about their reproductive status, territory, or individual or group identity. Scent-marking is commonly used to communicate among mammals. They are often transmitted through urine, feces, or the secretions of specific glands. Some mammals even use odors as defense against mammalian predators (e.g. skunks), which are especially sensitive to foul-smelling chemical defenses. (Apfelbach and Ganslosser, 1990; Vaughan, Ryan, and Czaplewski, 2000)

Typically, mammalian hearing is well-developed. In some species, it is the primary form of perception. Echolocation, the ability to perceive objects in the external environment by listening to echoes from sounds generated by an animal, has evolved in several groups. Echolocation is the main perception channel used in foraging and navigation in microchiropteran bats (Chiroptera) and many toothed whales and dolphins (Odontoceti), and has also evolved to a lesser degree in other species (e.g., some shrews). (Apfelbach and Ganslosser, 1990; Vaughan, Ryan, and Czaplewski, 2000)

Many mammals are vocal, and communicate with one another or with heterospecifics using sound. Vocalizations are used in communication between mother and offspring, between potential mates, and in a variety of other social contexts. Vocalizations can communicate individual or group identity, alarm at the presence of a predator, aggression in dominance interactions, territorial defense, and reproductive state. Communication using vocalizations is quite complex in some groups, most notably in humans. (Apfelbach and Ganslosser, 1990; Vaughan, Ryan, and Czaplewski, 2000)

Mammals also perceive their environment through tactile input to the hair and skin. Specialized hairs (whiskers or "vibrissae") have a sensory function, letting an animal know when it is in contact with an object in its external environment. Vibrissae are often richly innervated and well-supplied with muscles that control their position. The skin is also an important sensory organ. Often, certain portions of the skin are especially sensitive to tactile stimuli, aiding in specific functions like foraging (e.g., the fingers of primates and the nasal tentacles of star-nosed moles). Touch also serves many communication functions, and is often associated with social behavior (e.g., social grooming). (Vaughan, Ryan, and Czaplewski, 2000)

Vision is well-developed in a large number of mammals, although it is less important in many species that live underground or use echolocation. Many nocturnal animals have relatively large, well-developed eyes. Vision can be important in foraging, navigation, entraining biological rhythms to day length or season, communication, and nearly all aspects of mammalian behavior and ecology. (Apfelbach and Ganslosser, 1990; Vaughan, Ryan, and Czaplewski, 2000)

As a group, mammals eat an enormous variety of organisms. Many mammals can be carnivores (e.g., most species within Carnivora), herbivores (e.g., Perissodactyla, Artiodactyla), or omnivores (e.g., many primates). Mammals eat both invertebrates and vertebrates (including other mammals), plants (including fruit, nectar, foliage, wood, roots, seeds, etc.) and fungi. Being endotherms, mammals require much more food than ectotherms of similar proportions. Thus, relatively few mammals can have a large impact on the populations of their food items.

Predation is a significant source of mortality for many mammals. Except for those few species that are top predators, mammals are preyed upon by many other organisms, including other mammals. Other groups that typically eat mammals are predatory birds and reptiles. Many species cope with predation through avoidance strategies such as cryptic coloration, by restricting foraging to times when predators may not be abundant, or through their sociality. Some mammals also have defensive chemicals (e.g., skunks) or bear some type of protective armor or physical defense (e.g., armadillos, pangolins, New World porcupines and Old World porcupines). (Vaughan, Ryan, and Czaplewski, 2000)

The ecological roles, or niches, filled by the nearly 5000 mammal species are quite diverse. There are predators and prey, carnivores, omnivores, and herbivores, species that create or greatly modify their habitat and thus the habitat and structure of their communities [e.g., beavers damming streams, large populations of ungulates (Artiodactyla and Perissodactyla) grazing in grasslands, moles digging in the earth]. In part because of their high metabolic rates, mammals often play an ecological role that is disproportionately large compared to their numerical abundance. Thus, many mammals may be keystone predators in their communities or play important roles in seed dispersal or pollination. The ecosystem roles that mammals play are so diverse that it is difficult to generalize across the group. Despite their low species diversity, compared to other animal groups, mammals have a substantial impact on global biodiversity. (Reichholf, 1990a; Vaughan, Ryan, and Czaplewski, 2000)

Some mammal species are considered to have a detrimental impact on human interests. Many mammals that eat fruit, seeds, and other types of vegetation are crop pests. Carnivores are often considered to be a threat to livestock or even to human lives. Mammals that are common in urban or suburban areas can become a problem if they cause damage to automobiles when they are struck on the road, or can become household pests. A few species coexist exceptionally well with people, including some feral domesticated mammals (e.g., rats, house mice, pigs, cats, and dogs). As a result of either intentional or unintentional introductions near human habitation, these animals have had considerable negative impacts on the local biota of many regions of the world, especially the endemic biota of oceanic islands.

Many mammals can transmit diseases to humans or livestock. The bubonic plague is perhaps the most well-known example. Plague is spread via fleas that are carried by rodents. Rabies, which can be transmitted among mammalian species, is also a significant threat to livestock and can kill humans as well.

Mammals are a vital economic resource for humans. Many mammals have been domesticated to provide products such as meat and milk (e.g., cows and goats) or fiber (sheep and alpacas). Many mammals are kept as service animals or pets (e.g., dogs, cats, ferrets). Mammals are important for the ecotourism industry as well. Consider the many people who travel to zoos or to all corners of the world to see animals like elephants, lions, or whales. Mammals (e.g., bats) often help control populations of crop pests. Some species like Norway rats and domestic mice are vitally important in medical and other scientific research; because humans are mammals, other mammals can serve as models in human medicine and research. (Vaughan, Ryan, and Czaplewski, 2000)

Overexploitation, habitat destruction and fragmentation, the introduction of exotic species, and other anthropogenic pressures threaten mammals worldwide. In the past five centuries at least 82 mammal species have gone extinct. Currently, the International Union for Conservation of Nature and Natural Resources (IUCN) has listed about 1000 species (roughly 25% of all known mammals), as being at some risk of extinction. Several factors contribute to a species' vulnerability to human-induced extinction. Species that are naturally rare or require large home ranges are often at risk due to habitat loss and fragmentation. Species that are seen to threaten humans, livestock, or crops may be directly targeted for extirpation. Those species that are exploited by humans as a resource (e.g., for their meat or fur) but are not domesticated are often depleted to critically low levels. Finally, global climate change is adversely affecting many mammals. The geographic ranges of many mammals are shifting, and these shifts often correlate with changes in local temperatures and climate. As temperatures rise, which is especially pronounced in polar regions, some mammals are unable to adjust and are consequently at risk of losing their environment. (Reichholf, 1990c; Vaughan, Ryan, and Czaplewski, 2000)