Mammals are warm-blooded, air-breathing, milk-producing vertebrates. The name "mammal" actually refers to the female's mammary glands which provide milk for her young. This characteristic sets off mammals amoung warm-blooded, back-boned animals. Mammals are hairy; young are born alive. Most have varied teeth, for cutting, tearing, or grinding. The mammal's skull is unique; the brain is more complex than in other animals.
Being warm-blooded means that a relatively constant blood temperature is maintained irrespective of the outside air or water temperature. This usually means warm-blooded, in fact, since the average blood termperature range for active mammals is 90 to 104 degrees Fahrenheit.
Mammals with different diets require different types of teeth. The primitive placental mammalian tooth count was 44, consisting, in each jaw, of: 3 incisors, 1 canine, 4 premolars, and 3 molars on each side. Incisors are for cutting, canines for tearing, premolars and molars are for grinding or shearing. Rodents, which gnaw but do not tear, have lost their canines.
Have differentiated teeth means that mammals can chew up their food and prepare it for quick degestions, thus increasing its availability as a source of energy. This makes mammals more active than reptiles, which, after swallowing their prey unchewed, may be sluggish for a week or two till the meal is slowly digested.
Increased activity means greater ease in catching prey and in escaping from predators, both desirable in the struggle for existence. This activity is also aided by the development of the diaphragm which promotes deep breathing, and hence more rapid aeration of the blood and a quicker metabolism.
Because of their prolonged internal development, newborn mammal young are often quite helpless. They require parental, or at least, maternal, care for a period after birth. This affords the parent an opportunity to teach the young, and the species is thus enabled to break away from the pattern of pure instinct and stereotyped responses characteristic of the reptiles. By increasing the emphasis on learned response, a way to greater variety in behavior has opened which means greater opportunity for the use and development of intelligence.
The largest mammal is the 100-foot, 100-ton blue whale. This is the largest animal that has ever lived, and in length and in weight it ouranks by perhaps 50% the largest dinosaur. The smallest mammal in America is the pigmy shrew, which, when adult, weighs barely 1/12 of an ounce, less than the weight of a dime.
There are about 3,500 mammal species in the world today, representing 118 families; of these there are in North Amercia about 385 species in 40 families.
Squirrels are classified as members of the rodent order, one of the fourteen orders of mammals and the largest group among all mammals. The squirrel family is one of 31 families of rodents and consists of about 250 different specifies, including chipmunks and marmots. Squirrels are classified by their scientific name: sciuridae, a Latin designation originally taken from the Greek words meaning "shadow tail."
| ORDER | DESCRIPTION | No. OF SPECIES | No. OF FAMILIES |
|---|---|---|---|
| MONOTREMES | platypus and spiny anteater | 3 | 2 |
| MARSUPIALIA | opossums, bandicoots, kangaroos, wombats, and other marsupials | 265 | 15 |
| INSECTIVORA | hedgehogs, moles, shrews | 406 | 9 |
| CHIROPTERA | bats | 879 | 18 |
| RODENTIA | rodents | 1625 | 31 |
| EDENTATA | anteaters, tree sloths, armadillos | 28 | 3 |
| LAGOMORPHA | rabbits, hares, pikas | 60 | 2 |
| CARNIVORA | dogs, foxes, wolves, seals, weasels, cats, and other carnivores | 266 | 9 |
| CETACEA | whales, dolphins, porpoises | 76 | 10 |
| PROBOSCIDEA | elephants | 2 | 1 |
| SIRENIA | dugongs, manatees | 4 | 2 |
| PERISSODACTYLA | horses, zebras, tapirs, rhinoceroses | 16 | 3 |
| ARTOPDACTYLA | antelope, deer, gazelles, giraffe, pigs, hippos, and other ruminants | 185 | 9 |
| PRIMATES | lemurs, monkeys, apes, humans | 199 | 15 |