About
the Author
Sample
Entry from
How
Animals Are Alike
And
Yet Different
A.K.A.
The
Animal Kingdom
A
Computer Database & Handbook
(c)
1992 & 1994 D.D.
Mammals
[Class Mammalia]
APPROXIMATE
NUMBER OF KNOWN SPECIES WORLDWIDE
4,000
ORGANISMS
Egg-laying
mammals: Platypus and spiny
anteater.
"Marsupial"
[pouch-bearing] mammals: Opossum,
kangaroos, bandicoot, koala, wombat, Tasmanian devil, Tasmanian wolf
(extinct in recent times), etc.
Placental
mammals: Insectivorous
mammals, including shrews, moles, hedgehogs, tree shrews, and flying
lemur; bats (comprising about 1 of every 7 known species of mammals, many
of which, however, are endangered); primates, including lemurs, tarsiers,
monkeys, apes, and people; carnivorous mammals, including cats, mongooses,
hyenas, dogs, weasels, skunks, otters, raccoons, bears, seals, and
walruses; mammoths (extinct) and elephants; sea cows; odd-toed ungulates,
including horses, tapirs, and rhinos; even-toed ungulates, including hogs,
peccaries, and hippos as well as camels, deer, cattle, sheep, goats, and
antelope; armadillos, sloths, and South American anteaters; aardvark;
cetaceans, including whales and porpoises; rodents (the most diverse and
numerous mammals), including squirrels, rats, mice, beavers, and Guinea
pigs; rabbits and hares; etc.
ENVIRONMENTS
Marine,
freshwater, terrestrial, and aerial (Although some small mammals glide,
bats truly fly.).
DESCRIPTION
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 by blubber, a large size, and
surrounding seawater). There are 4 occasionally modified legs, typically mounted not
outward (as in lower "tetrapods") but downward, for efficient
locomotion. There is a
strong-jawed mouth with bony specialized teeth (the adults having a
"permanent" set), a tongue, and a "secondary palate"
[the roof of the mouth, allowing breathing while continuously eating].
The eyes have lids and tear glands.
There is a well-defined neck and an often short tail.
Mammals are
no smaller than the smallest shrew: If
these "warm-blooded" creatures (See below.) were any smaller,
they would lose precious body heat so quickly that they would die (hence,
a rapid metabolism -- constantly replenishing lost body heat -- for
shrews).
Egg-laying
mammals have some reptilian 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
secreted through the skin of the female, not through "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 advanced, "placental" competitors (below):
In fact, the evolution of marsupials on Australia is 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 placental wolf.).
Placental
mammals develop even more fully within the uterus of the mother -- there
is no pouch (let alone external eggs).
Primates have
5 fingers and toes, usually with "opposable" [grasping] thumbs
and finger- and toe-nails. The usually large eyes face forwards (for 3-D
vision, important in their native treetops), and the snout is usually
small.
"Catarrhines"
[Old World monkeys, apes, and people] have a large brain, an at least
partly hairless face, nostrils that are close together and pointing
downwards, 2 "premolar" teeth, and never a
"prehensile" [grasping] tail.
Old World
monkeys (such as macaques, baboons, and mandrills) walk on all fours but
often sit on the hard, often colored pads on their rump.
Their teeth are often used for grinding tough foods; but there are
large pointed "canine" teeth, used for defense.
Apes (the
gibbon, the gorilla, the orangutan, and the chimpanzee) have large brains
(even larger than in other animals of their large body size).
Although tailless, apes are well-adapted to a life in their native
forests, with grasping feet and long-fingered hands, long arms, and broad
chests for swinging through trees. The
powerful skulls and muscular jaws of apes are especially adapted for
chewing plants and other coarse foods, although there are large, pointed
"canine" teeth, for defense.
Human beings
have feet, legs, and a curved backbone adapted for an upright stance and a
"bipedal gait" [that is, we can walk]. Our skull is less massive than that of apes (or of extinct
"ape people") -- we often soften our foods by cooking them over
fire (whose power has been harnessed only by human beings).
We use our grasping hands to fashion all sorts of tools, with which
we have made even more complicated tools, including such weapons as
knives, guns, and nuclear weapons -- we have smaller canine teeth than
apes. And -- pound for pound --
we have the largest and perhaps most complex brain in the Animal
Kingdom (Only the brain of dolphins is perhaps more complexly folded.).
Although "racial" differences between human beings may look
significant and although cultural differences do exist, there is more
genetic variation within so-called races of human beings than between
so-called races of human beings: Genetically speaking, there are no such
things as races of human beings -- we are all of the same subspecies,
"Homo sapiens sapiens" (Other subspecies of human beings, such
as "Homo sapiens neanderthalensis", are long extinct.).
FEEDING
HABITS
Scavenging,
filter-feeding (as for "whalebone" whales), herbivorous,
carnivorous, and omnivorous (such as human beings).
MOTION
The muscle
system of mammals is very complex and coordinated.
Unlike lower
"tetrapods" [four-legged vertebrates], such as amphibians (12),
whose walking motion resembles the squirming swimming motion of fish (11),
mammals have legs mounted underneath the body, for more efficient running
(important for both would-be predators and would-not-be prey).
Although all
mammals have specialized legs (such as those bearing claws or hoofs), the
legs of some mammals have been greatly modified by evolution, producing
such specialized limbs as the wings of bats, the flippers of seals, and
the arms (with hands, bearing "opposable" [grasping] 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 very efficient gut, to get the most out of their food 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.
Unlike most
lower vertebrates, a mammal has a set of specialized teeth -- biting
"incisors" up front, then pointed "canines", followed
by tearing or grinding "premolars" and "molars" (In
addition, food is "predigested" by "enzymes" in the
"saliva" within the mouth of mammals.). Groups of mammals are
often identified by their "dentation" [arrangement of teeth].
For example, carnivores and herbivores have very different types of
teeth; elephants have incisors modified into long tusks (and a nose and
upper lip modified as a handy trunk); and although rodents and rabbits at
first 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 (making them the largest
animals ever to have evolved on Planet 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) and are exchanged across the extensive infoldings in the lungs.
A muscular "diaphram", under the lungs, 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"
(coming from the mouth and nasal passages) and the "trachea"
[windpipe] (leading to the 2 "bronchi", entering the lungs).
An "epiglottis" covers the larynx when the mammal
swallows, so that food goes down the esophagus, not the windpipe.
Most whales
and other "cetaceans" have the nostrils modified as a
"blowhole", atop the body.
CIRCULATION
Mammals have
a heart with 4 chambers -- 2 "atrias" (one for receiving blood
from the veins, the other for receiving blood from the lungs) and 2
"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 (12). This system,
and a relatively high blood pressure, helps mammals lead their typically
very active lifestyle.
Mammals are
"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), thus helping
to maintain our "high metabolic rates" [our relatively active
lifestyles]. In addition,
such behaviors as shivering, panting, or sweating help us 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 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, however, there is no "cloaca" chamber, receiving
outputs from both the urinary and digestive system:
Instead, the cloaca divides during development into the urethra and
the "rectum", of the digestive system.).
Terrestrial
vertebrates typically secrete less precious water from their kidneys than
aquatic species do.
COORDINATION
Genetic.
Hormonal:
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 are under hormonal control.
Nervous:
The complex lifestyles of mammals requires much coordination.
For example, various bats can navigate in dark caves and find
flying moths as prey at night by means of "echolocation" [a
natural sonar] -- the bat emits "ultrasonic sound waves"
[silent to the human ear]; and the echoes returned are deciphered by the
brain of the bat to judge the size, distance, and direction of travel of
foreign objects. Various
whales and other cetaceans also have such abilities (In addition, the
large sound-generating organ in their "forehead" can probably
produce shock waves in the water, to stun fish for eating.).
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 primates and cetaceans are the
most emotional, aware, coordinated, communicative, and intelligent of all
animals.
Social
Control: The instinctive
herding behavior of various mammals helps protect individuals, especially
by confusing predators (some of which, such as packs of dogs, are
well-coordinated hunting teams); the families and extended groups of
mammals, often with well-established "hierarchies" of dominant
individuals, are noted for generally taking excellent care of their young;
and human societies, coordinating the behavior of individuals over space
and time by such means as artificial languages (The beginning of a
civilization is generally dated from its invention of writing.), have made
us the most dominant single species on the planet.
Additionally, as evidenced by the fossils of prehistoric
"Neanderthal" people, human beings have always tended to care
for the sick or old amongst their numbers; and even after the death of
loved ones, most people have naturally shown a concern for the well-being
of others, as by providing burial grounds and ceremonies for an
"afterlife".
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 to the "marsupium" pouch of
the mother, in which it suckles on nipples while developing into a
"fetus". 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 (which develops 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 ways of mammalian life on Earth.
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