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 Soricidae and Chiroptera, and can weigh as little as 3 grams. The largest mammal, and indeed the largest animal to ever inhabit the planet, is the Balaenoptera musculus, 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)

 

 

The foot pads of many mammals provide cushioning using hydrostatic structures, essentially working as fluid-filled cushions.

     
  "Human heel pads and other mammalian foot pads make use of hydroskeletal support; our pads, which provide impact damping, some energy storage, and protection for bones, work as fluid-filled cushions--see, for instance, Ker (1999). They're complexly viscoelastic--if you want a stable reading of your height, you should stand for almost two minutes to allow your pads to creep into stability (Foreman and Linge 1989)." (Vogel 2003:417)
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The cylindrical veins and arteries of mammals play a crucial role in the smooth pumping of blood due to their elastic walls.

   
  "In a typical mammal body, the cylindrical arteries and veins which carry the blood have walls, containing the elastic fibre elastin, which expand to accommodate the spurts of blood pumped by the heart and then shrink again, pushing the blood onwards. The heart alone could never propel the blood all the way round the body if the blood vessels had rigid walls -- the blood would be stopping and starting all the time, instead of flowing. Strokes occur when the elasticity is lost." (Foy and Oxford Scientific Films 1982:23)

"The aortic wall of all vertebrates except agnathans contains a rubbery protein called elastin that allows the vessel to expand under the high pressures associated with cardiac contraction. In expanding, energy from the blood is temporarily stored in the elastin as elastic energy, but is promptly returned to the blood when the elastin recoils in diastole. This recoil acts as a second pump, forcing the blood on downstream and smoothing out pressure fluctuations. The total work of the heart is reduced as long as the transfer of energy into and out of the elastin is efficient. Elastin achieves both efficiency and long-range deformation with a high molecular mobility, although it is not clear how this mobility is achieved. Covalent crosslinks that unite individual molecules in an insoluble extracellular network ultimately limit this mobility and so allow the network to return to its original dimensions without permanent strain. Both high molecular mobility and insolubility are unusual for a protein, but they are understood to be necessary for elastomeric performance." (Chalmers et al. 1999:301)
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The sweat glands of many mammals aid thermoregulation through evaporative cooling.

     
  "Sweat glands play an extremely important part in temperature control. Shaped like a tube, knotted at the bottom and opening out of the epidermis at a 'pore', sweat glands secrete a colourless liquid which evaporates on the surface of the skin removing excess heat…There are two kinds of sweat glands: apocrine, associated with hairy skin, and eccrine, associated with smooth. Apocrine glands seem to be concerned mainly with producing scented secretions, and are progressively replaced in the more advanced mammals - gorillas, chimpanzees, and especially man - with eccrine glands, whose secretion dilutes and spreads that of the apocrine glands." (Foy and Oxford Scientific Films 1982:79)
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The bioadhesive glues used by mammals, plants, and mussels for adherance to mucosal surfaces (mucoadhesion) are made up of sticky proteins.

   
  "Bioadhesion may be defined as the state in which two materials, at least one of which is biological in nature, are held together for extended periods of time by interfacial forces. In the pharmaceutical sciences, when the adhesive attachment is to mucus or a mucous membrane, the phenomenon is referred to as mucoadhesion." (Smart 2005:1557)
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Cells produce heat by uncoupling of mitochondrial respiration resulting in shivering.

   
  "Waste of cell energy as heat can be achieved by uncoupling of mitochondrial respiration. Uncoupling proteins, which belong to the mitochondrial carrier family, are able to transport protons and thus may assume a thermogenic function." (Mozo et al. 2005)
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The uterus of female mammals can expand and contract to accommodate its contents thanks to spiral muscle fibers in its central myometrial layer.

     
  "Similarly, the uterus of female mammals must expand and contract with gestation and birth, often an order of magnitude (ten-fold). The hooped fibers of chitin in the locust are paralleled in the interior circular muscle fibers of the uterus. Of the three layers of the uterus, the central myometrial layer is responsible for the expansion and contraction of the uterus. It is composed of connective tissue, mainly smooth muscle fibers with an external layer laid longitudinally and an internal layer laid circularly at the base which then spirals in both directions around the uterine body (which might even be a logarithmic spiral…).

The lessons from these 'hooped' chitin fibers and spiral muscle fibers could be incorporated into a polymer packaging material, thereby allowing for expansion and contraction of the packaging depending on the size of its contents. The result of packing multiple items into a shipping case would be the absolute minimization of air space between objects created by the packaging alone. Additionally, the same packaging product could be specified for a large variety of object sizes, i.e. the bag holding the baby shoe would be the same SKU as the one holding the basketball shoe or the soccer ball." (Biomimicry Guild unpublished report)
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The eyelids of mammals provide lubrication for the eye using teardrops that are applied during blinking.

   
  "Being a particularly delicate instrument, the eye needs protection -- usually, an eyelid. Most mammals have two eyelids, one above and one below, but some - such as horses and deer - have a third, inner eyelid, the nictitating membrane, which may move upwards or sideways across the eyeball. Both types of eyelid can be closed to protect the eye from a blow, or from dirt; in closing - blinking - they wipe the eyeball clean and lubricate it with teardrops." (Foy and Oxford Scientific Films 1982:124)
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White blood cells of mammals roll along blood vessel walls, and anchor when they find an infection or cell damage via cell-adhesion molecules (CAMs) with variable affinity.

     
  "Dan Hammer of the Univ. of Pennsylvania in Philadelphia is studying how white blood cells roll their way through the bloodstream, yet are able to anchor themselves where they are needed. He hopes that if he can devise materials that mimic the cells' roll-and-stick ability, he'll be able to devise a new targeted drug-delivery system. White blood cells have surface proteins called selectins that stick out of the cell surface. Fluid pushes the cell along--bonds form in front and are broken in the back, resulting in the cartwheeling motion." (Courtesy of the Biomimicry Guild)

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The external ear-flaps of many mammals aid hearing by concentrating sound waves.

 
  "It is only among mammals that ears become noticeable, even striking, because of the visible external ear-flaps behind the narrow opening of the outer ear tube…The most obvious use of the ear-flap, though not necessarily the most important, is to gather and concentrate sound waves." (Foy and Oxford Scientific Films 1982:167)
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Muscles are contractile tissues that produce force and cause motion through a process involving electrical impulses and metabolization of glucose, producing ATP and lactic acid.

   
  "The muscle consumes oxygen and fuel that can be transported via a circulation system; the muscle itself supports the chemical reaction that leads to mechanical work; electrochemical circuits can act as nerves, controlling actuation; some energy is stored locally in the muscle itself; and, like natural muscle, the materials studied…contract linearly." (Madden 2006:1559)
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Mast cells of mammals reduce long-term inflammation by secreting a protein known as interleukin-10.

   
  "Allergic contact dermatitis, such as in response to poison ivy or poison oak, and chronic low-dose ultraviolet B irradiation can damage the skin. Mast cells produce proinflammatory mediators that are thought to exacerbate these prevalent acquired immune or innate responses. Here we found that, unexpectedly, mast cells substantially limited the pathology associated with these responses, including infiltrates of leukocytes, epidermal hyperplasia and epidermal necrosis. Production of interleukin 10 by mast cells contributed to the anti-inflammatory or immunosuppressive effects of mast cells in these conditions. Our findings identify a previously unrecognized function for mast cells and mast cell–derived interleukin 10 in limiting leukocyte infiltration, inflammation and tissue damage associated with immunological or innate responses that can injure the skin." (Grimbaldeston et al. 2007:1095)
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Antlers are strong and resist fracture via complex microstructure.

   
  "Antlers, elaborated annually, advertise age, size, and sex; as Currey points out, structures of cardboard would do the trick. They do get used, and sometimes broken, when males bang heads together. Good strength and work of fracture helps prevent such breakage. Low stiffness ought to be a virtue as well, minimizing transmission of impacts to the skull. High extensibility can be viewed either as part of the same coin or as the inevitable concomitant of high strength and low stiffness. Low density (from low mineral content) may give a slight advantage in carrying these huge sex symbols around, reducing both mass and turning moments." (Vogel 2003:349)
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The eyes of mammals are protected from dirt and impacts by eyelids.

   
  "Being a particularly delicate instrument, the eye needs protection -- usually, an eyelid. Most mammals have two eyelids, one above and one below, but some - such as horses and deer - have a third, inner eyelid, the nictitating membrane, which may move upwards or sideways across the eyeball. Both types of eyelid can be closed to protect the eye from a blow, or from dirt; in closing - blinking - they wipe the eyeball clean and lubricate it with teardrops." (Foy and Oxford Scientific Films 1982:124)
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Alveoli in mammalian lungs manage surface tension through use of a wetting agent whose concentration varies with alveolar expansion.

   
  "The individual alveoli have somewhat the same problem as the pair of lungs--why doesn't one alveolus expand to the point of explosion…before the others begin to inflate?…Lungs filled with air take more force to inflate than do lungs deliberately filled with a salt solution. With air inside, the outward pressure difference across the alveolar walls must work against tissue and the surface tension of the layer of water inside the alveoli. The latter opposes the formation of additional air-water interface as the alveoli expand. The surface tension, though, is drastically reduced by a wetting agent secreted by cells in the alveolar walls. But, and here's the trick, the effectiveness of the wetting agent depends on its concentration, which falls as the alveoli expand. Thus the force of surface tension rises sharply as an alveolus inflates, opposing further inflation. As a result of this wetting agent (or surfactant or detergent), the alveolar wall has a functionally curved stress-strain plot…and the requisite nonlinear elasticity." (Vogel 2003:53)
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White blood cells of mammals adhere tightly to target cells by increasing their surface area using arm-like projections and shape deformation.

   
  "Dr. Shasha Klibanov, Dr. Jonathan Lindner, and graduate student Jack Rychack of the University of Virginia are studying how leukocytes bind at high speeds to areas of infection. Physicians want to use microbubbles in combination with ultrasound to locate tumors or inflammation in the body. The microbubbles appear as a highlighted signal within the tissues or organ, enhancing the image. However, the microbubbles have low binding ability, so pass the target site and don't adhere efficiently. The researchers found that leukocytes have 'arms' that help bind them to the surface of an infection, and the blood cells deform to increase the surface contact area, increasing their adhesion to the infection. The researchers have modified the microbubbles to increase their surface area and adding micron projections to mimic leukocyte arms." (Courtesy of the Biomimicry Guild)
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The teeth of grazing mammals wear down but not smooth because of a side-by-side layered arrangement of enamel, dentine, and cementum.

           
  "Grazing has perhaps elicited the most dramatic dental specializations in mammals. About twenty million years ago, grasses and grasslands appeared on earth. Grass (and, incidentally, wood) provides poor fodder. It yields little energy relative to its mass, so a grazer has to process huge volumes. Much of that energy comes as chemically inert cellulose, which mammals hydrolyze only by enlisting symbiotic microorganisms in rumen or intestine. It's full of abrasive stuff like silicon dioxide and has lengthwise fibers that demand cross-wise chewing rather than rapid tearing. Long-lived grazers, concomitantly, have especially special teeth, with their components typically layered side by side, as in figure 16.5b. This odd-looking arrangement ensures that, while teeth may wear down…they won't wear smooth. The harder material (enamel, most particularly) will continue to protrude as the softer materials (cementum and dentine) wear down between them." (Vogel 2003:333)
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