THE WORLD OF LIFE with Douglas Drenkow

The body of mammals typically bears hair (evolved from reptilian scales, still present on the tails of some rodents and primitive species); however, hair is almost entirely lost in whales (insulated instead by blubber, a large size, and the surrounding seawater). There are four legs, occasionally modified. Typically, the legs are not mounted outward (as in lower tetrapods) but rather downward, for much more efficient locomotion. There is a strong-jawed mouth with specialized, bony teeth (the adults having a "permanent" set), a tongue, and a "secondary palate" (the roof of the mouth, facilitating breathing while continuously eating). The eyes have lids and tear glands. There is a well-defined neck and an often short tail.

Mammals must be no smaller than the smallest shrew: If these "warm-blooded" creatures (See below) were any smaller, they would lose precious body heat more quickly than they could re-generate it (hence, shrews, like hummingbirds, have a very rapid metabolism -- constantly replenishing lost body heat -- and a very pugnacious nature -- each shrew protecting a territory with vital supplies of food).

Dimetredons (extinct, mammal-like reptiles) had a large sail on the back (presumably to help control body temperature, like the large, thin ears on an African elephant) and a somewhat mammal-like skull.

Lycaenops (extinct) also had a mammal-like skull; in addition, its legs moved underneath the body, as on modern mammals, instead of to the side, as on other reptiles and amphibians (descended from swimming, squirming fish).

Egg-laying mammals have various reptilian features, including the presence of some scales (as on the tail of the "duckbilled" platypus), a "cloaca" (a single chamber for the exit of the urinary, digestive, and reproductive tracts), and a habit of laying eggs. Milk is simply secreted through the skin of the female, not through distinct nipples (as in higher mammals).

Marsupials have a "marsupium" pouch on the underside of the female, in which the very young -- after climbing out of the "uterus" (womb) -- develop, attached to nipples. Marsupial mammals are found mostly on Australia, which drifted away from the other continents before the evolution of more advanced, "placental" competitors (below): Thereafter, the evolution of marsupials on Australia was remarkably similar to the evolution of placental mammals elsewhere (For example, the bandicoot fills the same sort of environmental "niche" as a rabbit; a kangaroo grazes and leaps like an antelope; and the extinct Tasmanian wolf was much like a typical, placental wolf).

Placental mammals develop even more fully within the uterus of the mother -- there is no pouch (let alone external eggs).

FEEDING HABITS

Scavenging, filter-feeding (ex. "whalebone" whales), herbivorous, and/or carnivorous (Various mammals are "omnivorous", eating almost anything; indeed, opossums survived the Age of the Dinosaurs in large part not only by being able to hide from the generally larger beasts but also by being able to feed on whatever was available...how they also survive quite well amongst human habitations).

MOTION

The muscle system of mammals is very complex and coordinated.

Unlike lower "tetrapods" (four-legged vertebrates), such as amphibians, whose walking motion resembles the squirming, swimming motion of fish, mammals have legs mounted underneath the body, for more efficient walking and running (important for both would-be predators and would-not-be prey). This important innovation perhaps first evolved in such mammal-like reptiles as the extinct Lycaenops.

Although the legs of all mammals are specialized to some degree (such as those bearing claws or hoofs), the legs of some mammals have been greatly modified by evolution, producing such highly specialized limbs as the wings of bats, the flippers of seals, and the arms (with hands, bearing opposable thumbs) of us primates.

The tail of mammals is often reduced in size, although it may be grasping in "arboreal" (tree-inhabiting) species or finlike (as in whales).

DIGESTION

Mammals have a gut that is very efficient in extracting nutrients from their foods, for their typically active lifestyles.

Cattle and other "ruminants", which consume low-calorie, hard-to-digest grasses, have a four-chambered stomach, in which bacteria help digest the food and from which food is occasionally regurgitated (as "cud"), for re-chewing.

The jaw of a mammal is typically very powerful, fundamentally different in design (and development and evolution) than that of lower vertebrates. Such differences, in jaws and teeth, first appeared in such mammal-like reptiles as the extinct Lycaenops.

Unlike most lower vertebrates, a mammal has a set of highly specialized teeth -- biting "incisors" up front, pointed "canines" behind them, and tearing or grinding "premolars" and "molars" towards the rear (In addition, food is "pre-digested" by "enzymes" in the "saliva" within the mouth of a mammal). Mammals are often grouped and identified by their "dentation" (arrangement and types of of teeth): For example, carnivores and herbivores have very different types of teeth, adapted to their foods; elephants have incisors modified into long tusks (and a nose and upper lip modified into a handy trunk); and although rodents and rabbits appear to be of the same group, rodents have just one pair of gnawing "incisors" in each jaw, whereas rabbits have two pair in the upper jaw (a small pair behind the front). The largest whales have no teeth but instead have horny "whalebone" (growing down from the upper jaws), which filters enough tiny "plankton" (microscopic plants and animals) from seawater to support their over 100 tons of weight: These, the largest animals ever, survive by eating some of the smallest life on Earth.

RESPIRATION

In mammals, gases are inhaled and exhaled through nostrils, with the "hard palate" (roof of the mouth) allowing breathing while the mouth is full (Typically, mammals feed continuously to support their relatively active lifestyle). Gases are exchanged across the extensive infoldings in the lungs of a mammal. A muscular "diaphram", under the lungs, efficiently controls breathing.

Speech and other mammalian sounds are made possible by vibrating "vocal cords", found in the "larynx". The larynx is the airway found in between the "pharynx" (extending from the mouth and nasal passages) and the "trachea" (the windpipe, leading to the two "bronchi", entering the lungs). An "epiglottis" covers the larynx when the mammal swallows, so that food from the pharynx goes down the "esophagus", into the stomach, not down the trachea, into the lungs.

Most whales and other "cetaceans" have the nostrils modified as one or two "blowholes", atop the body.

CIRCULATION

Mammals have a heart with four chambers -- two "atrias" (one for receiving blood from the veins, the other for receiving blood from the lungs) and two "ventricles" (one for pumping blood to the lungs, the other for pumping blood out into the arteries) -- there is no inefficient mixing of "oxygenated" and "de-oxygenated" blood, as in amphibians. This system, and a relatively high blood pressure, helps mammals lead their typically active lifestyles.

Mammals are (and their immediate, reptilian ancestors presumably were) "endothermic" (so-called "warm-blooded") -- biochemical reactions release heat, carried by the blood throughout the tissues, producing a relatively high and constant temperature within the body (insulated with hair or blubber); this helps maintain a mammal's "high metabolic rate" (reflected in a relatively active lifestyle). In addition, such behaviors as shivering, panting, or sweating help mammals maintain a relatively constant body temperature; and certain mammals undergo a complex "hibernation" overwinter, reducing their metabolic activity, although not becoming truly "cold-blooded" (unable to exert any biochemical control over their body temperature).

EXCRETION

Nitrogen-rich and other wastes (as in our urine) are cleansed from the blood of a mammal by the many tubules within a pair of kidneys and are excreted (via a urinary bladder) through the "urethra", of the urinary system. Unlike other vertebrates, mammals have no "cloaca" chamber, receiving outputs from both the urinary and digestive systems: Instead, the cloaca divides during the development of a mammal into the urethra, of the urinary system, and the "rectum", of the digestive system.

Excess nitrogen in mammals is typically excreted as "urea", which is relatively soluble in water.

Terrestrial vertebrates typically secrete less precious water from their kidneys than aquatic species do.

COORDINATION

The growth, development, and behavior of mammals is under genetic and hormonal control, influenced by the environment.

The females of most placental mammals are sexually active only during certain times of the year, whereas the females of primates periodically shed the lining of their uterus and are sexually active year-round -- both processes involve complex interactions amongst hormones.

The complex lifestyles of mammals requires much nervous coordination.

Various bats can navigate in dark caves and find flying moths as prey at night by means of "echolocation" (a natural form of sonar) -- the bat emits "ultrasonic sound waves" (silent to the human ear); and the echos returned to the often large ears of the bat are almost instantly deciphered by the brain of the bat to judge the size, distance, and/or direction of travel of foreign objects. Various whales and other cetaceans use the large, sound-generating organ in their "forehead" for similar echo-navigation and perhaps also for producing shock waves in the water that stun the fish they eat.

Before they became extinct, the most advanced mammal-like reptiles (related to Lycaenops) had an inner ear like that of mammals instead of like that of other terrestrial vertebrates -- that is, the eardrum was connected to the inner ear by three tiny bones instead of just one, thus amplifying the sound by increasing the leverage.

Higher thought processes evolved in higher vertebrates, as the upper, "cerebral cortex" of the brain enlarged and became more complicated (as evidenced by its complex infoldings). In particular, we mammals rely heavily upon behavior learned when young and remembered into adulthood; and by most measures, primates and cetaceans are the most emotional, aware, coordinated, communicative, and intelligent of all life on Earth.

The instinctive herding behavior of various mammals helps protect the individuals of the herd, especially by confusing predators (some of which, such as packs of dogs, are well-coordinated hunting teams).

The families and extended societies of various mammals -- often with instinctively established and recognized "hierarchies" of dominant individuals -- are noted for generally taking excellent care of their young (Please see the discussion of human societies with the article on primates).

REPRODUCTION

Mammals reproduce sexually, with the sexes separate; and as terrestrial vertebrates (or as species descended from them), mammals "copulate" (mate with physical contact).

The fertilized eggs of primitive, egg-laying mammals are typically laid in aquatic environments and sat-upon during "incubation" (the period before hatching).

The fertilized eggs of higher mammals are borne within the "uterus" (womb) of the female. In marsupial mammals, the developing "embryo" is nourished by a milk secreted within the uterus; and after being born at a very early age, the embryo crawls up into the "marsupium" pouch on the mother, in which it suckles on nipples and develops. In most mammals, however, external membranes of the embryo merge with the lining of the uterus to form a "placenta", through which food and oxygen are delivered to the embryo (developing into a "fetus") and wastes and carbon dioxide are taken away. The "mammary glands" (breasts) of female mammals produce milk, delivered through nipples, to nourish the young after birth. Typically, mammals have longer "childhoods" (even after sexual maturity) than any other animals -- undoubtedly to learn the extremely complex and successful ways of mammalian life on Earth.

Representatives

Vertebrates (Vertebrata)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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