Mammalia  

Mammals can be found on all continents, in all oceans, and on many oceanic islands of the world.

Biogeographic Regions: nearctic (Introduced , Native ); palearctic (Introduced , Native ); oriental (Introduced , Native ); ethiopian (Introduced , Native ); neotropical (Introduced , Native ); australian (Introduced , Native ); antarctica (Native ); oceanic islands (Introduced , Native ); arctic ocean (Native ); indian ocean (Native ); atlantic ocean (Native ); pacific ocean (Native ); mediterranean sea (Native )

Other Geographic Terms: cosmopolitan

Eyelids cleanse eyes: mammals
 

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)
  Learn more about this functional adaptation.

Skin properties derive from arrangement of components: mammals
 

The skin of mammals may derive its unique mechanical properties and other characteristics from the arrangement of its stratum corneum keratin intermediate filaments into cubic rod-packing symmetry.

 
  "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.

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).

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.

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.

Other Physical Features: endothermic ; heterothermic ; homoiothermic; bilateral symmetry ; polymorphic ; venomous

Sexual Dimorphism: sexes alike; female larger; male larger; sexes colored or patterned differently; female more colorful; male more colorful; sexes shaped differently; ornamentation