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)