Characteristics and Classification of Mammals Mammals are distinguished from other back-boned animals by having hair, which in a few kinds may be scanty, and by the fact that their females give milk and suckle the young. Also, in all but the egg-laying platypus and echidnas, the young are born alive. The term "mammal" is an attempt in one word to put into common language the idea that all of the thousands of different forms sharing the attributes just mentioned make up a single great class, distinct from the reptiles, birds, amphibians, and fishes. The Class Mammalia thus includes the beasts of field and forest, the aquatic whales, seals, and sirenians, the bats, and mankind. This idea is a relatively new one, dating from the Swedish biologist Linnagus in the second half of the eighteenth century. It has been accepted by most educated people but is still foreign to a large proportion of humanity. DISTINCTIVE CHARACTERS OF MAMMALS Mammals are grouped into Orders on the evidence of similarity of structure, reinforced by what has been discovered about the fossil history of the different groups. The members of orders are in most cases obviously more closely allied to each other than to other mammals. Thus, the flesh-eaters belong for the most part to the Order Carnivora (literally, flesh-eaters) ; the monkeys, the gnawing animals, the hoofed animals, the flying animals (bats) likewise are members of distinct orders. But exceptions occur; the pouched animals, including pouchless 1
Z MAMMALS OF THE PACIFIC WORLD ones whose ancestors had pouches, comprise meat-eaters, gnawers, monkey-like creatures, and forms with hoof-like toes like the kangaroos.
For prompt and easy recognition it is best to dwell chiefly upon those differences of shape or behavior which can be observed easily. In the field most mammals are unfortunately killed almost as soon as discovered, so it is best to emphasize differences of shape, size, and color. The structure of certain of the parts of mammals is frequently more distinctive and easy to see than that of other parts ; the teeth and limbs notably provide definite indications of species, and usually agree with the evidence of other parts and with fossil evidence. Teeth. Although the toothed whales may have as many as fifty teeth on each side of each jaw, other mammals never have more than that number all together. The Australian meateating pouched animals or marsupials may have a total of forty-six teeth (our opossum has four more) ; most other mammals have far fewer — at the most forty-four. The spiny and scaly anteaters have none, but lap up their insect prey with a long, sticky tongue; and the whalebone whales have replaced teeth with a horny set of sieves that retain engulfed small fish and shrimp-like animals for swallowing.
The teeth of mammals, except the simple, peg-like teeth of the toothed whales, consist of several sorts. They comprise front cutting-teeth, or incisors ; pointed, often fang-like eyeteeth, or canines; narrowed cheek-teeth in the side of the jaw near the front, or premolars ; and broader crushing-teeth near the back, or molars. In some kinds of animals the teeth of one class may assume the shape and function of another class. In many mammals all but the molars develop in two successive series, a milk or juvenal series, and a permanent or adult series which replaces the former. In pouched animals one tooth only in each jaw is so replaced. The toothed whales have only permanent teeth. Squirrels and porcupines among the gnawing mammals replace one or two cheek-teeth at about the time they
CHARACTERISTICS OF MAMMALS 3 become adult, but the rat family and most remaining gnawers have no milk teeth.
The incisors, or cutting-teeth, may number three, two, or one on each side; some marsupials even have four upper incisors. They may be chisel-like or ever-growing, as in rodents Incisors Fig. I— Mammalian teeth, exemplified hij the teeth of the Marsupial Cat and rabbits; pincer-like, as in some insect-eaters; broad and spade-like, as in cattle and some monkeys ; or small and almost useless, as in some bats. They are often about equal in size and shape, as in meat-eaters and monkeys; or the middle mcisor may be much larger than the other, as in the rabbit, or lost completely, as in the upper jaw of some bats. Upper in-
4 MAMMALS OF THE PACIFIC WORLD cisors are absent in cattle and deer, while in the elephant the lower ones are missing and the two upper incisors are modified to form the tusks.
The canines, or eye-teeth, are usually longer and more rounded in cross section than the other teeth. While the upper incisors are set into the front upper jaw bone, the upper canine is the first tooth in the maxillary or side bone of the upper jaw. The lower canine fits in front of the upper one. Bats, meateaters, the seal group, and most monkeys, especially males, have large canine teeth. These teeth are tusk-like in pigs and dagger-like in the mouse-deer. They are the killing teeth of beasts of prey, the fighting teeth of these and animals such as pigs and monkeys which tear and hurt rivals. Rabbits, rodents, and most kinds of kangaroos have no canines ; cattle and most true deer have only lower ones, similar in shape to the incisors. In each toothrow the cheek-teeth, from few to as many as seven in number, stand behind the canine, or the gap (diastema) which is present when this tooth is undeveloped. In elephants only one enormous grinder with parts of others is used at a time. The tooth, as it wears away, travels forward in the jaw and the next behind it moves upward and forward gradually to take its place. These teeth are very large. Their grinding surfaces may have fifteen or twenty cross-ridges of hard enamel which make the teeth efficient mills to grind leaves and twigs. The grinders of other plant-eaters such as rhinoceros and tapir, cattle and deer, have different patterns ; but the ridges, whether they are shaped like the Greek letter tt or are crescent-like, serve similar purposes.
The cheek-teeth of many meat-eaters are reduced in number, and a pair of very large ones on each side works together like shears to cut up meat. The cheek-teeth of bears, otters, and many of the civet family have mound-shaped cusps, better suited for crushing than shearing. Insect-eaters and the insect-eating bats have sharp-cusped teeth for crushing and cutting up hardbodied insects.
CHARACTERISTICS OF MAMMALS 5 In mammals that eat almost everything — plant food, small mammals, insects, and eggs — the cheek-teeth also have rounded, mound-like cusps. Such are present in the pigs, some rats, bears, and most primates. The leaf monkeys have sharp-cusped teeth which chop up leaves. In many rats the cusps form crossridges or zig-zag patterns.
Limbs. The limbs, next to the teeth, provide the best external guides for recognizing and classifying mammals. The several different kinds of limbs are all modifications of single basic patterns of the fore and hind limbs. By reduction or enlargement of various parts, involving changes in proportions, hands, flippers, wings, paws, and hooves have evolved. Only whales and sirenians, which have lost all but internal traces of the hind pair, lack four limbs. The front limbs of these animals have become steering paddles, while the tail has become flattened and enlarged to provide propulsion. The seals and their allies likewise have converted their front limbs into paddles. Their broadened hind feet are used somewhat like the whale's tail, moving up and down in the water.
The hoofs of hoofed mammals, both those with odd and those with even numbers of toes, are nothing but enlarged fingernails and toenails. In most of them the central toe, or pair of toes, is enlarged in order to bear most of the weight. Moreover, the bones of the hand and foot are lengthened, a condition which is an advantage in running.
The slow-moving, massive elephant, which has hoof-like nails, walks partly on the tips of the toes and partly on a pad of fatty tissue. The foot-bones are short, the limb-bones long and massive.
Meat-eaters, rabbits, rodents, insect-eaters, flying lemurs, scaly and spiny anteaters, and pouched mammals have claws. Some meat-eaters, rabbits, and rodents walk on their toes ; in these, pads are developed under the joint between the free finger or toe and the hand or foot, and other pads are formed at the tip of each toe. The other clawed mammals, including the bear
Fig. 2 — Examples of mammalian feet A. Marsupial cat: Clawed, five-toed foot, the entire sole is used in walking; B. Kangaroo: Lengthened foot; the first toe is absent, the second and third united, while the fourth toe is the greatest; C. Monkey: Opposable first toe; friction ridges on the sole which is used in walking; D. Dingo: Lengthened foot in which walking is done on the clawed four toes. Foot pads well developed; E. Deer: Even-toed, hoofed and lengthened foot. The weight is borne by the third and fourth toes, while the second and fifth toes, dew-claws, are much smaller and set higher in the foot; F. Tapir: Odd-toed hoofed mammal. The chief toe is the middle one, while the third and fourth are less developed or even absent.
CHARACTERISTICS OF MAMMALS 7 and weasel families among the meat-eaters, walk on their flat feet and hands.
Monkeys and their allies usually walk on their palms and soles, and most have nails rather than claws. The lemurs and tarsiers have claws on some toes and nails on others. The gibbons and orang-utan swing from branch to branch suspended by their long arms ; they seldom come to the ground. Bats have hook-like claws on their hind feet and thumbs. The rest of the fore limb is greatly modified. The bones of the arm and hand are lengthened; the third finger alone may be longer than the bat's body, and the forearm (the part between wrist and elbow) is almost as long. This bony framework is covered with nearly naked, elastic skin extending between the hand and ankle, and frequently also between the legs and tail. The skin between the hind legs and tail is called the interfemoral membrane. It is used as a brake and rudder. The hind limbs are so constructed that the knee bends backward. CLASSIFICATION AND SCIENTIFIC NAMES It is a part of human nature to group things in orderly arrangements and to abstract general ideas. The animal world is so complex that unless it is arranged and thought about systematically, it cannot be well understood. Men have been classifying mammals in various ways for thousands of years. They divided them into those that were good or bad to eat, or those that were harmless or dangerous. In the Bible a different classification was made: animals with divided hoofs which chewed the cud were considered fit to eat ; the rest were "unclean." Modern classification is based instead on fundamental similarity of structure and what we know about the evolution and descent of animals from common ancestors.
Since man is a mammal, he may be chosen to illustrate different stages in a simplified classification. The Class Mammalia is divided into two Subclasses ; man is placed in the Subclass
8 MAMMALS OF THE PACIFIC WORLD Theria (often called Eutheria) with all modern mammals; the egg-laying duckbill and spiny anteaters are placed in the other Subclass, Prototheria. Man is next classified as a member of the Order Primates. Dogs and cats, on the other hand, go in the Order Carnivora ; and pigs, deer, and cattle are considered members of the Order Artiodactyla. Nineteen such Orders of living mammals are recognized ; sixteen of them are dealt with in this handbook. The Orders Hyracoidea (Hyraxes or conies of Africa and Palestine), Tubulidentata (African aardvarks), and Edentata (armadillos, sloths, and anteaters — South American for the most part) are not represented in the Pacific region.
The basic system of classification of living mammals is indicated by the accompanying diagram or phylogenetic tree (p. 9).
Many of the orders of mammals have no inclusive common name. Especially is this true when they contain mammals that appear quite different from one another ; even dogs and cats are different enough to make the idea of the Order Carnivora a difficult one. The less obvious relationships among mammals have been recognized only during the last two centuries, which is too short a time for the names given these groups to mean much to the average person. However, it is useful to have common names for groups, even if they are not familiar and if they are merely anglicized forms of scientific names. Whenever such names are introduced in this handbook an attempt at explanation will be made.
Each order of mammals is subdivided into families. Man belongs to a family group of his own, the Family Hominidse. Other families of the primates comprise the apes (Pongidae), gibbons (Hylobatidse), Old World monkeys (Cercopithecidas and Colobidae), the tarsier (Tarsiidag), the slow loris and some other lemurs (Lorisidas). There are still other families of South American and Mascarene primates, as well as extinct families known only from fossils.