Mammals
MammalsMammal, common name applied to any warm-blooded animal
belonging to the class that includes humans and all other animals that
nourish their young with milk, that are covered with varying amounts of
hair, and that possess a muscular diaphragm. Mammals have the most
highly developed nervous systems of all animals. Most members of the
group have four appendages, usually legs. These may be adapted for use
as swimming appendages, as in seals, or as wings, as in bats. Some
Types,, however, have two limbs that have been reduced to small
vestiges beneath the skin, as in whales, or have been lost altogether, as
in sea cows. All mammals, except the egg-laying monotremes, produce
live young that undergo the early stages of development within the body
cavity of the mother. Some mammals are helpless at birth, others are
able to walk and even run immediately, and may be born fully furred
and with their eyes and earsopen. The largest mammal, the blue whale,
Often exceeds 30 m (100 ft) in length, and the smallest shrews, mice,
and bats are often less than 5 cm (less than 2 in) in length.
In many mammals the color of the skin or fur blends with the animal�s
natural surroundings. In others there is great contrast with the natural
surroundings to favorvisual signals that provide information about
theidentity of a species, and about the gender, age, orsocial status of an
individual. The skin also functions as a sensory and excretory organ and
contains specialized glands. Mammary glands, which are present in
fully developed form in all adult female mammals.
Aquatic mammals, such as whales, dolphins, and sea cows, have no
sweatglands. Sweat glands are usually located at the base of hairs,
except those in regions of skin bordering mucous membranes, such as
the sweat glands surrounding the edges of the lips and covering the
genitalia. Many mammals, however, have few functional sweat glands;
in dogs and cats, for example, only the glands on the soles of the feet are
functional.
The eyes, ears, and nose of mammals also have their external endings in
the integument. All mammals have two eyes, but the eyes of several
burrow-dwelling mammals, such as moles, have lost their function
partially or completely or have become covered with skin.
All mammals reproduce sexually, and two types of reproductive acts are
used to bring about sexual conjugation. In the primitive egg-laying
mammals, excretory and genital organs open into a common orifice,
called the cloaca. Transfer of sex cells from the male to the female is
accomplished by bringing the cloacae into apposition. In all other
mammals, however, the male sex cells are transmitted by copulation.
After fertilization, development of offspring takes place entirely within
the body of the mother in all mammals except the monotremes, which
produce leathery-shelled eggs with large yolks, and in many marsupials,
in which the gestation period only lasts about 10 to 15 days, with most
of the development taking place in the mother's pouch afterbirth.
Mammalian young are not prepared to pursue an independent existence
immediately following birth but must be nursed during infancy.
Mammals probably appeared on the earth during the early Mesozoic era.
Most zoologists believe that mammals evolved from a group of extinct
mammal-like reptiles, which existed during the Triassic period. The
earliest animal fossils that have definitely been identified as mammals
were found in rocks from the Jurassic period. During the Jurassic
period, five distinct orders of mammals existed. One order was made up
of small, rodent tike mammals, having gnawing front teeth and grinding
teeth with several cusps. A second order consisted of small, carnivorous
mammals, having molar teeth equipped with three simple, cone like
cusps, that became extinct before the end of the Eocene epoch. A third
group of small insectivorous mammals are the probable ancestors of
present-day mammals. Of the mammalian subclasses that still exist,
themonotremes are unrepresented by fossil remains.