January 12, 2011, 6:35 pm
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Cheetah in grassland, north central Namibia. Source: C.Michael Hogan.

Mammalia is a group of warm-blooded, air breathing vertebrates. With the common name mammal, each species is endowed with the characteristic of fur and three-boned middle ear; but the most remarkable element of group identity is an advanced brain element known as the neocortex, that functions as a center of complex cognition; no species except mammals have this well defined brain structure. Having pronounced inherent sexual dimorphism, the females have mammary glands capable of producing milk. There are approximately 5400 described mammalian species comprising around 1200 genera.

Mammals span a size range from the three centimeter Bumblebee Bat to the 33 meter long Blue Whale. Feeding habits vary widely among species, including carnivores and insectivores who prey on animals, to frugivores and granivores who eat fruit or seeds. Earliest mammals arose approximately 200 to 130 million years ago.

Physical characteristics

caption Australian fur seal, Kangaroo Island. Source: C.Michael Hogan

Shared prominent features of all mammals are:

  • Vertebrate bone structure
  • Warm-blooded
  • Fur on parts of their exterior
  • Three-boned middle ear architecture, used in advanced hearing
  • Air breathing
  • Neocortex brain structure, leading to advanced cognition

Most mammals give live birth, except for five species of the class Protheria. All female mammals possess mammary glands used for producing milk for the young. All female animals have nipples except for the five Prototheria (egg laying) species, who exude milk through the skin. Prototheria retain a palette of primitive reptilian characteristics including retention of shoulder girdle; certain skull bones and a cloaca (fused opening for urinary, intestinal and reproductive functions).

Metatheria (marsupials) have a unique form of dentition, where only the third upper and lower pre-molars are replaced with secondary teeth. Bone structures also have some unique features relative to placental mammals; for example, marsupial nose bones are quite large and expand at the posterior; marsupial palates usually exhibit two pairs of openings; and marsupial cranial cavities are smaller than placental mammals of similar size. The large nasal bone structure may have purpose in a greatly enhanced olfactory and pheremonal sensory system. Marsupials also have known specialized skin glands, which may be used in pheremonal communication.

Huxley and others observed as early as 1881 very detailed differences in the morphology among Prototheria, Metatheria and Eutheria. In particular, at that early time, the clear distinctions of Prototheria in having cloacal structures, differentiation in inner ear bone geometries, primitive development of the vertebral epiphyses, and a large iliac/sacral axis angle. 

All mammals possess a double occipital condyle; two knobs at the skull base, which fit into the topmost neck vertebra. Consverse other vertebrates manifest a single occipital condyle.


Some mammals are specialist feeders, restricting their diets variously to  animals or plants, or in some cases have an omnivorous bent. Many species of mammals are specialists to subgroups within their preferred prey kingdom; for example, certain mammals (known as insectivores) consume chiefly insects; others specialize to higher animals (and are called carnivores). Specialists in eating plant fruits are termed frugivores, while seed eating mammals are known as granivors. . Other herbivores specialize to eating grasses. Carnivorous mammals have a simple digestive tract, since the proteins, lipids, and minerals in meat require little specialized digestion. Conversely, plants contain complex carbohydrates, including cellulose. The digestive tract of an herbivore is host to bacteria that ferment these complex molecules. Such bacteria are either within the multi-chambered stomach or in a large cecum.

The size of a mammal is an additional factor in determining diet type. Since small mammals have a high ratio of heat-loss surface area relative to heat-generating mass, they have high-energy consumption requirements and a high metabolic rate. Mammals that weigh less than approximately 500 grams are chiefy insectivores, since they cannot tolerate the slow, complex digestive process of a herbivore. Larger animals on the other hand generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (those that prey on larger vertebrates) or a slower digestive process (herbivores). Furthermore, mammals that weigh more than 500 grams generally cannot collect enough [insects during their waking hours to sustain themselves. The only large insectivorous mammals (such as anteaters) are those that feed on massive insect colonies. (ants or termites)


All mammals reproduce by copulation with members of the opposite sex. Mothers give live birth, except for Prototherians, who lay hard shelled eggs. Sensory capabilities are important in the ancillary functions of mate finding, courtship and (where appropriate) den, burrow or nest construction. The majority of mammal species are either polygynous or promiscuous. This outcome is based upon females' incurring a high costs during gestation and lactation. The result is intense male-male competition in many species, and also for females to be highly  selective in mate decsions. Many mammals exhibit strong sexual dimorphism as a result of this high selectivity of females, sometimes giving rise to striking male morphologies.


This most primitive of the mammals are generally solitary except for male/female associations during the breeding season. Breeding seasons endure for two to three months, depending on species and climate, but this season always is limited to the time window of July to October. Males have no investment in parenthood beyond finding a mate and copulation; one species, Ornithorhynchus anatinus, performs moderately elaborate courtship behavior prior to mating.

Monotreme eggs typically measure 12 to 15 millimeters in diameter; there are generally one to three eggs produced at a time, and these are covered with a leathery shell. Incubation occurs external to the mother's body, although some Protherians have pouches and one species, Ornithorhynchus anatinus, uses burrows in riparian areas or pond banks for hatching locations. Incubation takes approximately 12 days. For most of the species that have a brood pouch, the young occupy the pouch for about two months; for all species there is a total weaning period of up to three months.


metatheria technically do not give birth via placental means; however, the mother produces a yolk sac placenta and gives birth to an underdeveloped offspring. These young lack fully developed posterior limbs, and subsequent to birth, they migrate to the marsupium where they suckle a nipple. Mouths of newborns metatherians are laterally fused, but open medially;  this architecture creates an O-shaped mouth which fits around the mothers nipple, that subsequently swells to secure the offspring for strong attachment.


Many of the aspects of mate finding and courtship are not dissimilar to the fundamentals of Metatheria; however, there is a vast differentiation of territoriality, home range size and courtship rituals among the various species of Eutheria. In terms of placental architecture Eutherian births exhibit a complex placenta, in contrast to Metatheria, whose placentae are simple.  Eutherian young are altricial or precocial, where Metatherian young are always highly altricial. The young of Eutherians are not incessantly attached to the mother via teats, in contrast to Metatherian young.

Eutherians have a wide variety of practices of territory formation, seasonal migration, and denning or colony formation depending on the species. For example, sea elehants and seals have terrestrial breeding practiices that follow considerable length seasonal marine migration. Polar bears have considerable length land migrations coupled with very lengthy marine migration prior to constructing birthing dens. 


Sensory faculties that are fundamental to mammalian behavior include hearing,vison, olfaction, and tactile perception. The auditory function is basic to livelihoods of many mammals, and for some species is the most critical sense. Many mammals have frequency discrimination far beyond the range perceived by humans. Bats and some marine mammals have highly specialized capabilities of echolocation, by which they emit sound signals and interpret the reflected sound waves to determin highly specific positions of objects.

Vision capability is extensively developed in many mammal species, although it subordinate to  echolocation, for those species who are endowed with that specialist function. The spectral capability of different mammals varies, such that full color viewing is not available to all species. Some nocturnal mammals have somewhat large, well-developed eyes. Vision can be vital in foraging, predation, navigation, locomotion, mate finding and communication.

Olfaction assists in foraging, mating and social communication. Many mammalian species employ pheromones to communicate regarding reproductive interests, territory or group identity. Scent-marking is a specific technique, which may be conducted by urination, defecation or rubbing of body parts to leave chemicals from skin glands.In other cases, certain mammals, such as skunks, deploy odors as a defense against  predators.

Mammals also receive external stimuli through tactile sensors in their hair and skin. Specialized whiskers or vibrissae behave much like the cilia of the inner ear, to send neural signals to the brain when the whisker is in contact with an external object. Vibrissae are typically richly innervated and even supplied with muscles that can vector their attitude. Skin is also a key sensory organ, useful in foraging and social communication.


All Prototheria are carnivorous, with their diets consisting of various small invertebrates. Ornithorhynchidae forage in the benthos of lakes and streams, employing their sensitive bills to find prey. They are generalist predators, whereas Tachyglossidae are specialists, preying upon either ants or termites (in the case of Tachyglossus) or worms (in the case of Zaglossus). Both short and long-beaked species of echidna are powerful diggers and robustly employ their claws and snouts to root through the earth to find prey.

Auditory function, olfaction, touch, and vision are senses used by Prototheria. Hearing and sight are well developed in Ornithorhynchidae and moderately well-developed in Tachyglossidea. The sense of touch is  important to a platypus that is searching at the bottom of a stream for prey or an echida that is rooting through soil for termites or worms. Platypus bills and echidna snouts are highly sensitive organs that are essential to effective foraging. Platypuses may also use electrical stimuli to locate prey. Olfaction is well-developed in echidnas and may be used in individual recognition. Prototherians occasionally produce certain simple vocalizations.


Metatherian social behavior and communication skills have conventionally been viewed as less complex than for placental mammals; some have postulated that this attribute is associated with a smaller cranial cavity and hence brain mass. However, more recent research on olfactory capabilities and skin glands suggest that marsupials possess complex scent communication. These studies are further supported by observations I have made of kangaroos and wallabies in the wild, where complex group guard behavior is demonstrated. Earlier indictments of lack of marsupial soical behavior may have been biased by observations of captive populations, whose learned behavior may have been impeded by lack of threat exposure.   


This most familiar set of mammals includes the bears, felids, canids, bovids, pinnipeds, rodents, bats and  numerous  other animals. The terrestrial fauna of this group are generally characterized by an upright gait, and adaptations for running, leaping, burrowing and many further athletic forms of locomotion. The aquatic mammals are capable of extraordinary swimming, diving and marathon migration in the world's seas and rivers. Chiroptida (bats) are known for their deft gliding and their arboreal inverted hanging from trees. Lemurs and other primates are often arboreal and capable of exceptional athleticism in almost horizontal aerial propulsion for leaps of up to twenty meters.

Biodiversity and conservation

caption Painted Hunting Dog, Botswana. Source: C.Michael Hogan

Of the approximately 5400 mammalian species, a considerable fraction are considered vulnerable, particularly many of the cetaceans, felids, bears, lemurs, seals and other large fauna. The fundamental driver of Holocene extinctions is the human population explosion, which has produced a continuing assault on mammals for prehistoric and historic exploitation of hides, meat, oil and other products. In current times the most significant cause of population reductions has been habitat destruction and habitat fragmentation, but activities such as trophy hunting, misguided taking for medicinal purposes and use of animal parts such as tusks for ivory carving, have added to the mammalian population declines.


caption Short-beaked Echidna. Queensland. Source: C.Michael Hogan Mammals have been classified in several different ways, the earliest based upon morphological features. While some of these older schemes persist in some textbooks, we will follow analysis from molecular biology that uses DNA markers to evaluate the closeness of true genetic features.

Mammalia divide into to major classes:

  • Prototheria (egg laying mammals)
  • Theria (live birth mammals)

In turn Theria are divided into two large groups:

  • Metatheria (marsupials)
  • Eutheria (placental mammals)

Protheria, or monotremes, have two orders of extant species: Platypoda (platypuses, with the sole family Ornithorhynchidae) and Tachyglossa (echidnas, species that are spiny anteaters, represented by the sole family Tachyglossidae).

Eutheria are classified into 21 orders as follows:

*Clade Atlantogenata
** Group I: Afrotheria
***Clade Afroinsectiphilia
****Order Macroscelidea: elephant shrews (Africa)
****Order Afrosoricida: tenrecs and golden moles (Africa, Madegascar)
****Order Tubulidentata: aardvarks (sub-Saharan Africa)
***Clade Paenungulata
****Order Hyracoidea: hyraxes (Africa, Arabia)
****Order Proboscidea: elephants (Africa, Southeast Asia)
****Order Sirenia: dugong and manatees (tropical)
**'''Group II: Xenarthra
***Order Pilosa: sloths and anteaters (Neotropical)
***Order Cingulata: armadillos (Americas)
*'''Clade Boreoeutheria
**'''Group III: Euarchontoglires: supraprimates
***Superorder Euarchonta
****Order Scandentia: treeshrews (Southeast Asia).
****Order Dermoptera: colugos (Southeast Asia)
****Order Primates: lemurs, bushbabies, monkeys, apes, humans (cosmopolitan)
***Superorder Glires
****Order Lagomorpha: pikas, rabbits, hares (Eurasia, Africa, Americas)
****Order Rodentia: rodents (cosmopolitan)
**'''Group IV: Laurasiatheria
***Order Erinaceomorpha: hedgehogs
***Order Soricomorpha: moles, shrews, solenodons
***Clade Ferungulata
****Clade Cetartiodactyla
*****Order Cetacea: whales, dolphins, porpoises
*****Order Artiodactyla: even-toed ungulates,e.g. pigs, hippopotami, camels, giraffes, antelopes, bovids
****Clade Pegasoferae
*****Order Chiroptera: bats (cosmopolitan)
*****Clade Zooamata
******Order Perissodactyla: odd-toed ungulates,e.g. horses, zebra, tapir,  rhinoceros
******Clade Ferae
*******Order Pholidota: scaly anteaters (Africa, South Asia)
*******Order Carnivora: carnivores (cosmopolitan)



The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. Molecular DNA marker studies suggest that these clades diverged from early common ancestors in the Cretaceous, but fossil evidence to date does not corroborate this hypothesis. The molecular research is consistent with mammal zoogeography.:

Following molecular DNA sequence analyses, the first divergence is deemed the Afrotheria 110 to 100 million years ago (Mya). The Afrotheria continued to diversify in the isolation of the African-Arabian continent. The Xenarthra, isolated in South America, diverged from the Boreoeutheria approximately 100 to 95 Mya. According to an alternative view, the Xenarthra has the Afrotheria as closest allies, forming the Atlantogenata as sister group to Boreoeutheria. The Boreoeutheria split into the Laurasiatheria and Euarchontoglires between 95 and 85 Mya; both of these groups evolved on the northern continent of Laurasia. After tens of millions of years of relative isolation, Africa-Arabia collided with Eurasia, exchanging Afrotheria and Boreoeutheria.

Formation of the Isthmus of Panama linked South America and North America, which facilitated the migration of mammalian and other species in the Great American Interchange. The traditional view that no placental mammals reached Australasia until about five Mya when bats and murine rodents arrived has been challenged by recent evidence and needs re-evaluation. These molecular results are debated because they are not supported by morphological data, and thus not endorsed by systematists.

Metatheria first appeared in the Cretaceous Period; both Metatheria and Prototheria made their way to southern Pangea and became isolated in present day Australia by the subsequent continental drift. These specialized species survived on this remote isolated continent in the absence of certain predation that evolved on the other continents. Extant metatherians diversified at the conclusion of the Cretaceous-Paleogene approximately 65 Mya.


The neocortex is the most recent element of the mammalian cerebral cortex to evolve. The balance of the brain of mammals is known as the allocortex, whose cellular architecture differs from the six-layer structure unique to neocortex. This six layer structure, not found in species other than mammals, is the center of higher cognition.

Mammals utilize two bones for hearing originally used for chewing by their fossil ancestors. Early mammal ancestors, the synapsids, exhibited a jaw joint composed of the articular (a minute bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). Most reptiles including crocodilians and dinosaurs use this system. Extant mammals have a different jaw, consisting of the dentary (the lower jaw bone which holds teeth) and the squamosal (a tiny skull bone). In mammals the quadrate and articular bones have evolved into the incus and malleus bones in the middle ear, enabling an enhanced auditory function.


Physical description


  • U.Heckner. 1990. Egg-laying mammals (Monotremes). pages 192-207



  • Brian K. Hall. 2010. Evolution: Principles and Processes. Jones & Bartlett Learning. 400 pages
  • Inés Horovitz, Thomas Martin, Jonathan Bloch, Sandrine Ladevèze and Marcelo R.Sánchez-Villagra. 2009. Cranial Anatomy of the Earliest Marsupials and the Origin of Opossums.  PLoS ONE. vol 4, issue 12
  • Mark Ridley. 2004. Evolution. Wiley-Blackwell. 751 pages


Hogan, C. (2011). Mammal. Retrieved from


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