Species
Mammalia
IUCN
NCBI
EOL Text
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).
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.
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.
- Nowak, R. 1991. Walker's Mammals of the World. Baltimore: Johns Hopkins University Press.
- Vaughan, T., J. Ryan, N. Czaplewski. 2000. Mammalogy, 4th Edition. Toronto: Brooks Cole.
- Gingerich, P., M. ul Haq, I. Zalmout, I. Khan, M. Malkani. 2001. Origin of whales from early artiodactyls: Hands and feet of Eocene Protocetidae from Pakistan. Science, 293: 2239-2242.
- Dragoo, J., R. Honeycutt. 1997. Systematics of mustelid-like carnivores. Journal of Mammalogy, 78: 426-443.
- Janke, A., X. Xu, U. Arnason. 1997. The complete mitochondrial genome of the wallaroo (Macropus robustus) and the phylogenetic relationship among Monotremata, marsupialia, and Eutheria. Proc. National Academy of Sciences, 94: 1276-1281.
- Matthee, C., J. Burzlaff, J. Taylor, S. Davis. 2001. Mining the mammalian genome for artiodactyl systematics. Systematic Biology, 50: 367-390.
- Murphy, W., E. Eizirik, S. O'Brien, O. Madsen, M. Scally, C. Douady, E. Teeling, O. Ryder, M. Stanhope, W. de Jong, M. Springer. 2001. Resolution of the early placental mammal radiation using Bayesian phylogenetics. Science, 294: 2348-2351.
- Teeling, E., O. Madsen, R. Van Den Bussche, W. de Jong, M. Stanhope, M. Springer. 2002. Microbat paraphyly and the convergent evolution of a key innovation in Old World rhinolophoid microbats. Proc. National Academy of Sciences, 99: 1431-1436.
- Wilson, D., D. Reeder. 1993. Mammal Species of the World. Washington D.C.: Smithsonian Institution Press.
- Flynn, J., J. Finarelli, S. Zehr, J. Hsu, M. Nedbal. 2005. Molecular phylogeny of the Carnivora (Mammalia): assessing the impact of increased sampling on resolving enigmatic relationships. Systematic Biology, 54/2: 317-337.
- Klima, M., W. Maier. 1990. Body Structure. Pp. 58-84 in B Grzimek, ed. Grzimek's Encyclopedia of Mammals, Vol. 1, 1 Edition. New York: Mcgraw-Hill.
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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.
Foraging Behavior: stores or caches food ; filter-feeding
Primary Diet: carnivore (Eats terrestrial vertebrates, Piscivore , Eats eggs, Sanguivore , Eats body fluids, Insectivore , Eats non-insect arthropods, Molluscivore , Scavenger ); herbivore (Folivore , Frugivore , Granivore , Lignivore, Nectarivore ); omnivore ; planktivore ; mycophage ; coprophage
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Optimal branching of vascular vessels minimizes work: mammals
Vascular and respiratory vessels in mammals minimize the amount of biological work required to operate by being arranged hierarchically.
"The vessels found in mammalian cardiovascular and respiratory systems are usually arranged in hierarchical structures and a distinctive feature of this arrangement is their multi-stage division or bifurcation. At each generation, the characteristic dimension of the vascular segments will generally become smaller, both in length and diameter." (Barber and Emerson 2008: 179)
"The branching structures found in mammalian cardiovascular and respiratory systems have evolved, through natural selection, to an optimum arrangement that minimizes the amount of biological work required to operate and maintain the system. The relationship between the diameter of the parent vessel and the optimum diameters of the daughter vessels was first derived by Murray (1926) using the principle of minimum work. This relationship is now known as Murray’s law and states that the cube of the diameter of a parent vessel equals the sum of the cubes of the diameters of the daughter vessels." (Barber and Emerson 2008: 180)
[This mathematical structure is also found in trees and other organisms that exhibit branching]
Learn more about this functional adaptation.
- Barber RW; Emerson DR. 2008. Optimal design of microfluidic networks using biologically inspired principles. Microfluidics and Nanofluidics. 4: 179-191.
- Murray CD. 1926. The physiological principle of minimum work. I. the vascular system and the cost of blood volume. PNAS. 12: 207-214.
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Wetting agent reduces surface tension: mammals
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)
Learn more about this functional adaptation.
- Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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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.
Ecosystem Impact: disperses seeds; pollinates; creates habitat; biodegradation ; soil aeration ; keystone species
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Sweating aids thermoregulation: mammals
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)
"From the evidence of comparative mammalian physiology, we suggest that the very common apocrine sweat gland is not primitive but is both specialized and efficient as a cooling organ in an animal with a heavy fur coat and relatively slow movement. The remarkable thermal eccrine sweating system of humans probably evolved in concert with bipedalism, a smooth hairless skin, and adaptation to open country by the ancestors of H. sapiens." (Folk and Semken 1991:185)
Learn more about this functional adaptation.
- Foy, Sally; Oxford Scientific Films. 1982. The Grand Design: Form and Colour in Animals. Lingfield, Surrey, U.K.: BLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, London. 238 p.
- Folk GE; Semken A. 1991. The evolution of sweat glands. International Journal of Biometeorology. 35(3): 180-186.
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White blood cells adhere closely: mammals
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)
Learn more about this functional adaptation.
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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).
Known Predators:
- Aves
- Reptilia
- Mammalia
Anti-predator Adaptations: aposematic ; cryptic
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Filaments adopt geometric symmetry: mammals
The formation and dynamics of the keratin intermediate filaments in mammalian stratum corneum may be the result of membrane templating.
"Keratin is tough, adaptable, flexible, resistant to water, and provides a good protective covering for the rest of the body. These qualities also make it an ideal material for the moulding of claws, nails and hooves…" (Foy and Oxford Scientific Films 1982)
"A new model for stratum corneum keratin structure, function, and formation is presented. The structural and functional part of the model, which hereafter is referred to as 'the cubic rod-packing model', postulates that stratum corneum keratin intermediate filaments are arranged according to a cubic-like rod-packing symmetry with or without the presence of an intracellular lipid membrane with cubic-like symmetry enveloping each individual filament. The new model could account for (i) the cryo-electron density pattern of the native corneocyte keratin matrix, (ii) the X-ray diffraction patterns, (iii) the swelling behavior, and (iv) the mechanical properties of mammalian stratum corneum. The morphogenetic part of the model, which hereafter is referred to as 'the membrane templating model', postulates the presence in cellular space of a highly dynamic small lattice parameter (<30 nm) membrane structure with cubic-like symmetry, to which keratin is associated. It further proposes that membrane templating, rather than spontaneous self-assembly, is responsible for keratin intermediate filament formation and dynamics. The new model could account for (i) the cryo-electron density patterns of the native keratinocyte cytoplasmic space, (ii) the characteristic features of the keratin network formation process, (iii) the dynamic properties of keratin intermediate filaments, (iv) the close lipid association of keratin, (v) the insolubility in non-denaturating buffers and pronounced polymorphism of keratin assembled in vitro, and (vi) the measured reduction in cell volume and hydration level between the stratum granulosum and stratum corneum. Further, using cryo-transmission electron microscopy on native, fully hydrated, vitreous epidermis we show that the subfilametous [sic] keratin electron density pattern consists, both in corneocytes and in viable keratinocytes, of one axial subfilament surrounded by an undetermined number of peripheral subfilaments forming filaments with a diameter of ~8 nm." (Norlén and Al-Amoudi 2004:715)
Learn more about this functional adaptation.
- Norlen, L.; Al-Amoudi, A. 2004. Stratum Corneum Keratin Structure, Function, and Formation: The Cubic Rod-Packing and Membrane Templating Model. Journal of Investigative Dermatology. 123(4): 715-732.
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Specialized teeth wear down but remain effective: grazing animals
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)
Learn more about this functional adaptation.
- Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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