Elementary Zoology Part 24

OTHER MAMMALS

The mammals const.i.tute the highest group of animals, including man, the monkeys and apes, the quadrupeds, the bird-like bats and fish-like seals and whales; in all about 2500 species. They are found everywhere except on a few small South Sea islands. Only a few species, however, have a world-wide distribution. The name Mammalia is derived from the mammary or milk glands with which the females are provided and by the secretion of which the young of this cla.s.s, born free in all but a few of the lowest forms, are nourished for some time after birth. In size mammals range from the tiny pigmy-shrew and harvest mouse, which can climb a stem of wheat, to the great sulphur-bottom whale of the Pacific Ocean, which attains a length of a hundred feet and a weight of many tons. Mammals differ from fishes and batrachians and agree with reptiles and birds in never having external gills; they differ from reptiles and agree with birds in being warm-blooded and in having a heart with two distinct ventricles and a complete double circulation; finally, they differ from both reptiles and birds in having the skin more or less clothed with hair, the lungs freely suspended in a thoracic cavity separated from the abdominal by a muscular part.i.tion, the diaphragm, and in the possession by the females of mammary glands. In economic uses to man mammals are the most important of all animals. They furnish the greater portion of the animal food of many human races, likewise a large amount of their clothing. Horses, a.s.ses, oxen, camels, reindeer, elephants, and llamas are beasts of burden and draught; swine, sheep, cattle, and goats furnish flesh, and the two latter milk for food; the wool of sheep, the furs of the carnivores, and the leather of cattle, horses, and others are used for clothing, while the bones and horns of various mammals serve various purposes.

[Ill.u.s.tration: FIG. 147.--Dissection of the Mouse, _Mus musculus_.]

=Body form and structure.=--The mammalian body varies greatly. Its variety of form and general organization is explained by the facts that, although most of the species live on the surface of the earth, some are burrowers in the ground, some flyers in the air, and some swimmers in the water. Mammals never have more than two pairs of limbs; in most cases both pairs are well developed and adapted for terrestrial progression. In the aerial bats the fore limbs are modified into organs of flight; among the aquatic seals, sea-lions, walruses, and whales both sets are modified to be swimming flippers or paddles. In many of these aquatic forms the hind limbs are greatly reduced or even completely wanting.

Most mammals are externally clothed with hair, which is a peculiarly modified epidermal process. Each hair, usually cylindrical, is composed of two parts, a central pith containing air, and an outer more solid cortex; each hair rises from a short papilla sunk at the bottom of a follicle lying in the true skin. In some mammals the hairs a.s.sume the form of spines or "quills," as in the porcupine. The hairy coat is virtually wanting in whales and is very spa.r.s.e in certain other forms, the elephant, for example, which has its skin greatly thickened. The claws of beasts of prey, the hooves of the hoofed mammals, and the outer h.o.r.n.y sheaths of the hollow-horned ruminants are all epidermal structures.

The bones of mammals are firmer than those of other vertebrates, containing a larger proportion of salts of lime. Among the different forms the spinal column varies largely in the number of vertebrae, this variation being chiefly due to differences in length of tail. Apart from the caudal vertebrae their usual number is about thirty. The mammalian skull is very firm and rigid, all the bones composing it, excepting the lower jaw, the tiny auditory ossicles, and the slender bones of the hyoid arch, being immovably articulated together. The correspondence between the bones of the two sets of limbs is very apparent. The number of digits varies in different mammals, and also in the fore and hind limbs of a single species. Among the Ungulates the reduction in the number of digits is especially noticeable; the forefoot of a pig has four digits, that of the cow two, and that of the horse one. The two short "splint" bones in the horse are remnants of lost digits. The teeth are important structures in mammals, being used not only for tearing and masticating food, but as weapons of offence and defence. A tooth consists of an inner soft pulp (in old teeth the pulp may become converted into bone-like material) surrounded by hard white dentine or ivory, which is covered by a thin layer of enamel, the hardest tissue known in the animal body. A hard cement sometimes covers as a thin layer the outer surface of the root, and may also cover the enamel of the crown. The teeth in most forms are of three groups: (_a_) the incisors, with sharp cutting edges and simple roots, situated in the centre of the jaw; (_b_) the canines, often conical and sharp-pointed, next to the incisors; (_c_) next the molars, broad and flat-topped for grinding, and divided into premolars and true molars. There is great variety in the character and arrangement of these structures in mammals, their variations being much used in cla.s.sification. The number and arrangement of the teeth is expressed by a dental formula, as, for example, in the case of man

2--2 1--1 2--2 3--3 _i_----, _c_----, _p_----, _m_---- = 32.

2--2 1--1 2--2 3--3

[Ill.u.s.tration: FIG. 151.--A group of Rocky Mountain sheep, or "big horns," _Ovis canadensis_, including males, females and young.

(Photograph by E. Willis from specimens mounted by Prof. L. L. Dyche, University of Kansas.)]

The mouth is bounded by fleshy lips. On the floor of the mouth is the tongue, which bears the taste-buds or papillae, the organs of taste.

The sophagus is always a simple straight tube, but the stomach varies greatly, being usually simple, but sometimes, as in the ruminants and whales, divided into several distinct chambers. The intestine in vegetarian mammals is very long, being in a cow twenty times the length of the body. In the carnivores it is comparatively short--in a tiger, for example, but two or three times the length of the body.

The blood of mammals is warm, having a temperature of from 35 C. to 40 C. (95 F. to 104 F.). It is red in color, owing to the reddish-yellow, circular, non-nucleated blood-corpuscles. The circulation is double, the heart being composed of two distinct auricles and two distinct ventricles. Air is taken in through the nostrils or mouth and carried through the windpipe (trachea) and a pair of bronchi to the lungs, where it gives up its oxygen to the blood, from which it takes up carbonic-acid gas in turn. At the upper end of the trachea is the larynx or voice-box, consisting of several cartilages attaching by one end to the vocal cords and by the other to muscles. By the alteration of the relative position of these cartilages the cords can be tightened or relaxed, brought together or moved apart, as required to modulate the tone and volume of the voice.

The kidneys of mammals are more compact and definite in form than those of other vertebrates. In all mammals except the Monotremes they discharge their product through the paired ureters into a bladder, whence the urine pa.s.ses from the body by a single median urethra.

Mammary glands, secreting the milk by which the young are nourished during the first period of their existence after birth, are present in both s.e.xes in all mammals, though usually functional in the female only.

[Ill.u.s.tration: FIG. 152.--A group of moose, _Alce americana_, showing male, female, and young. (Photograph by E. Willis from specimens mounted by Prof. L. L. Dyche, University of Kansas.)]

The nervous system and the organs of special sense reach their highest development in the mammals. In them the brain is distinguished by its large size, and by the special preponderance of the forebrain or cerebral hemispheres over the mid- and hind-brain. Man's brain is many times larger than that of all other known mammals of equal bulk of body, and three times as large as that of the largest-brained ape.

In man and the higher mammals the surface of the forebrain is thrown into many convolutions; among the lowest the surface is smooth. Of the organs of special sense, those of touch consist of free nerve-endings or minute tactile corpuscles in the skin. The tactile sense is especially acute in certain regions, as the lips and end of the snout in animals like hogs, the fingers in man, and the under surface of the tail in certain monkeys. All the other sense-organs are situated on the head. The organs of taste are certain so-called taste-buds located in the mucous membrane covering certain papillae on the surface of the tongue. The organ of smell, absent only in certain whales, consists of a ramification of the olfactory nerves over a moist mucous membrane in the nose. The ears of mammals are more highly developed than those of other vertebrates both in respect to the greater complexity of the inner part and the size of the outer part. A large outer ear for collecting the sound-waves is present in all but a few mammals. A tympanic membrane separates it from the middle ear in which is a chain of three tiny bones leading from the tympanum to the inner ear, composed of the three semicircular ca.n.a.ls and the spiral cochlea. The eyes (fig. 150) have the structure characteristic of the vertebrate eye, consisting of a movable eyeball composed of parts through which the rays of light are admitted, regulated, and concentrated upon the sensitive expansion, retina, of the optic nerve lining the posterior part of the ball. The eye is protected by two movable lids. In almost all mammals below the Primates there is a third lid, the nict.i.tating membrane. In some burrowing rodents and others the eye is quite vestigial and even concealed beneath the skin.

=Development and life-history.=--All mammals except the Monotremes give birth to free young. The two genera of Monotremes produce their young from eggs hatched outside the body; _Tachyglossus_ lays one egg which it carries in an external pouch, while _Ornithorhynchus_ deposits two eggs in its burrow. The embryo of other mammals develops in the lower portion of the egg-tube, to the walls of which it is intimately connected by a membrane called the placenta. (In the kangaroos and opossums, Marsupialia, there is no placenta.) Through this placenta blood-vessels extend from the body of the mother to the embryo, the young developing mammal thus deriving its nourishment directly from the parent.

The duration of gestation (embryonic or prenatal development in the mother's body) varies from three weeks with the mouse, eight weeks with the cat, nine months with the stag, to twenty months with the elephant.

Like the birds, the young of some mammals, the carnivores for example, are helpless at birth, while those of others, as the hoofed mammals, are very soon able to run about. But all are nourished for a longer or shorter time by the milk secreted by the mammary gland of the mother.

=Habits, instinct, and reason.=--Despite the wonderful examples of instinct and intelligence shown by many insects and by the other vertebrates, especially the birds, it is among mammals that we find the highest development of these qualities and of reason. In the wary and patient hunting for prey by the carnivora, in the gregarious and altruistic habits of the herding hoofed mammals, in the highly developed and affectionate care of the young shown by most mammals, and in the loyal friendship and self-sacrifice of dogs and horses in their relations to man, we see the culmination among animals of the development of the functions of the nervous system. In the characteristics of intelligence and reason man of course stands immensely superior to all other animals, but both intelligence and reason are too often shown by many of the other mammals not to make us aware that man's mental powers differ only in degree, not in kind, from those of other animals.

Pure instinct is hereditary, and purely instinctive actions are common to all the individuals of a species. Those actions which the individual could not learn by teaching, imitation, or experience are instinctive.

The accurate pecking at food by chicks just hatched from an incubator is purely instinctive. Purely instinctive also is the laying of eggs by a b.u.t.terfly on a certain species of plant which may have to be sought for over wide acres, so that the caterpillars when hatched shall find themselves on their own special food-plant. Yet the b.u.t.terfly never ate of this plant and will never see its young. Such elaborate instincts as these have been developed from the simplest manifestations of sensation and nervous function, just as the complex structures of the body have been developed from simple structures (see Chapter XXIX).

The feeding and domestic habits and the whole general behavior of animals are extremely interesting subjects of observation and study.

And such observation intelligently pursued will be of much value. The point to be kept ever in mind is that all animal habits are connected with certain conditions of life; that in every case there is an answer to the question "why." This answer may not be found; in many cases it is extremely difficult to get at, but often it is simple and obvious and can be found by the veriest beginner.

=Cla.s.sification.=--The mammals of North America represent eight orders.

Three additional mammalian orders, namely, the Monotremata, including the extraordinary duck-bills (_Ornithorhynchus_) and a species of _Tachyglossus_ in Australia and Tasmania; the Edentata, including the sloths, armadillos, and ant-eaters found in tropical regions; and the Sirenia, including the marine manatees and dugongs, are not represented (except by a single manatee) in North America. In the following paragraphs some of the more familiar mammals representing each of the eight orders represented in North America are referred to.

=The opossums (Marsupialia).=--The opossum (_Didelphys virginiana_) is the only North American representative of the order Marsupialia, the other members of which are limited exclusively to Australia and certain neighboring islands. The kangaroos are the best known of the foreign marsupials. After birth the young are transferred to an external pouch, the marsupium, on the ventral surface of the mother, in which they are carried about and fed. The opossum lives in trees, is about the size of a common cat, and has a dirty-yellowish woolly fur. Its tail is long and scaly, like a rat's. Its food consists chiefly of insects, although small reptiles, birds, and bird's eggs are eaten. When ready to bear young the opossum makes a nest of dried gra.s.s in the hollow of a tree, and produces about thirteen very small (half an inch long) helpless creatures. These are then placed by the mother in her pouch. Here they remain until two months or more after birth. Probably all the North American opossums found from New York to California and especially common in the Southern States belong to a single species, but there is much variety among the individuals.

=The rodents or gnawers (Glires).=--The rabbits, porcupines, gophers, chipmunks, beavers, squirrels, and rats and mice compose the largest order among the mammals. They are called the rodents or gnawers (Glires) because of their well-known gnawing powers and proclivities.

The special arrangement and character of the teeth are characteristic of this order. There are no canines, a toothless s.p.a.ce being left between the incisors and molars on each side. There are only two incisor teeth in each jaw (rarely four in the upper jaw), and these teeth grow continuously and are kept sharp and of uniform length by the gnawing on hard substances and the constant rubbing on each other.

The food of rodents is chiefly vegetable.

Of the hares and rabbits the cottontail (_Lepus nuttalii_) and the common jack-rabbit (_L. campestris_) are the best known. The cottontail is found all over the United States, but shows some variation in the different regions. There are several species of jack-rabbits, all limited to the plains and mountain regions west of the Mississippi River. The food of rabbits is strictly vegetable, consisting of succulent roots, branches, or leaves. Rabbits are very prolific and yearly rear from three to six broods of from three to six young each. There are two North American species of porcupines, an Eastern one, _Erethizon dorsatus_, and a Western one, _E. epixanthus_.

The quills in both these species are short, being only an inch or two in length, and are barbed. In some foreign porcupines they are a foot long. They are loosely attached in the skin and may be readily pulled out, but they cannot be shot out by the porcupine, as is popularly told. The little guinea-pigs (_Cavia_), kept as pets, are South American animals related to the porcupines.

The pocket gophers, of which there are several species mostly inhabiting the central plains, are rodents found only in North America. They all live underground, making extensive galleries and feeding chiefly on bulbous roots. The mice and rats const.i.tute a large family of which the house-mice and rats, the various field-mice, the wood-rat (_Neotoma pennsylvanica_) and the muskrat (_Fiber zibethicus_) are familiar representatives. The common brown rat (_Mus dec.u.ma.n.u.s_) was introduced into this country from Europe about 1775, and has now nearly wholly supplanted the black rat (_M. rattus_), also a European species, introduced about 1544. The beaver (_Castor canadensis_) is the largest rodent. It seems to be doomed to extermination through the relentless hunting of it for its fur. The woodchuck or ground-hog (_Arctomys monax_) is another familiar rodent larger than most members of the order. The chipmunks and ground-squirrels are commonly known rodents found all over the country. They are the terrestrial members of the squirrel family, the best known arboreal members of which are the red squirrel (_Sciurus hudsonicus_), the fox-squirrel (_S. ludovicia.n.u.s_), and the gray or black squirrel (_S. carolinensis_). The little flying squirrel (_Sciuropterus volans_) is abundant in the Eastern States.

=The shrews and moles (Insectivora).=--The shrews and moles are all small carnivorous animals, which, because of their size, confine their attacks chiefly to insects. The shrews are small and mouse-like; certain kinds of them lead a semi-aquatic life. There are nearly a score of species in North America. Of the moles, of which there are but few species, the common mole (_Scalops aquaticus_) is well known, while the star-nosed mole (_Condylura cristata_) is recognizable by the peculiar rosette of about twenty cartilaginous rays at the tip of its snout.

Moles live underground and have the fore feet wide and shovel-like for digging. The European hedgehogs are members of this order.

=The bats (Chiroptera).=--The bats (fig. 153), order Chiroptera, differ from all other mammals in having the fore limbs modified for flight by the elongation of the forearms and especially of four of the fingers, all of which are connected by a thin leathery membrane which includes also the hind feet and usually the tail. Bats are chiefly nocturnal, hanging head downward by their hind claws in caves, hollow trees, or dark rooms through the day. They feed chiefly on insects, although some foreign kinds live on fruits. There are a dozen or more species of bats in North America, the most abundant kinds in the Eastern States being the little brown bat (_Myotis subulatus_), about three inches long with small fox-like face, high slender ears, and a uniform dull olive-brown color, and the red bat (_Lasiurus borealis_), nearly four inches long, covered with long, silky, reddish-brown fur, mostly white at tips of the hairs.

[Ill.u.s.tration: FIG. 153.--The h.o.a.ry bat, _Lasiurus cinereus_.

(Photograph from life by J. O. Snyder.)]

=The dolphins, porpoises, and whales (Cete).=--The dolphins, porpoises, and whales (Cete) compose an order of more or less fish-like aquatic mammals, among which are the largest of living animals. In all the posterior limbs are wanting, and the fore limbs are developed as broad flattened paddles without distinct fingers or nails. The tail ends in a broad horizontal fin or paddle. The Cete are all predaceous, fish, pelagic crustaceans, and especially squids and cuttlefishes forming their princ.i.p.al food. Most of the species are gregarious, the individuals swimming together in "schools." The dolphins and porpoises compose a family (Delphinidae) including the smaller and many of the most active and voracious of the Cete. The whales compose two families, the sperm-whales (Physeteridae) with numerous teeth (in the lower jaw only) and the whalebone whales (Balaenidae) without teeth, their place being taken in the upper jaw by an array of parallel plates with fringed edges known as "whalebone."

The great sperm-whales or cachalots (_Physeter macrocephalus_) found in southern oceans reach a length (males) of eighty feet, of which the head forms nearly one-third. Of the whalebone whales, the sulphur-bottom (_Balaenoptera sulfurea_) of the Pacific Ocean, reaching a length of nearly one hundred feet, is the largest, and hence the largest of all living animals. The common large whale of the Eastern coast and North Atlantic is the right whale (_Balaena glacialis_); a near relative is the great bowhead (_B. mysticetus_) of the Arctic seas, the most valuable of all whales to man. Whales are hunted for their whalebone and the oil yielded by their fat or blubber. The story of whale-fishing is an extremely interesting one, the great size and strength of the "game" making the "fishing" a hazardous business.

[Ill.u.s.tration: FIG. 154.--Male elk or wapiti, _Cervus canadensis_.

(Photograph by E. Willis from specimen mounted by Prof. L. L. Dyche, University of Kansas.)]

[Ill.u.s.tration: FIG. 155.--Antelope, male, female, and young, _Antilocapra americana_. (Photograph by E. Willis from specimens mounted by Prof. L. L. Dyche, University of Kansas.)]

=The hoofed mammals (Ungulata).=--The order Ungulata includes some of the most familiar mammal forms. Most of the domestic animals, as the horse, cow, hog, sheep, and goat, belong to this order, as well as the familiar deer, antelope, and buffalo of our own land and the elephant, rhinoceros, hippopotamus, giraffe, camel, zebra, etc., familiar in zoological gardens and menageries. The order is a large one, its members being characterized by the presence of from one to four hooves, which are the enlarged and thickened claws of the toes.

The Ungulates are all herbivorous, and have their molar teeth fitted for grinding, the canines being absent or small. The order is divided into the Perissodactyla or odd-toed forms, like the horse, zebra, tapir, and rhinocerus, and the Artiodactyla or even-toed forms, like the oxen, sheep, deer, camels, pigs, and hippopotami. The Artiodactyls comprise two groups, the Ruminants and Non-ruminants. All of the native Ungulata of our Northern States belong to the Ruminants, so called because of their habit of chewing a cud. A ruminant first presses its food into a ball, swallows it into a particular one of the divisions of its four-chambered stomach, and later regurgitates it into the mouth, thoroughly masticates it, and swallows it again, but into another stomach-chamber. From this it pa.s.ses through the other two into the intestine.

The deer family (Cervidae) comprises the familiar Virginia or red deer (_Odocoileus america.n.u.s_) of the Eastern and Central States and the white-tailed, black-tailed, and mule deers of the West, the great-antlered elk or wapiti (_Cervus canadensis_) (fig. 154), the great moose (_Alce americana_) (fig. 152), largest of the deer family, and the American reindeer or caribou (_Rangifer caribou_). All species of the Cervidae have solid horns, more or less branched, which are shed annually. Only the males (except with the reindeer) have horns. The antelope (_Antilocapra americana_) (fig. 155) common on the Western plains also sheds its horns, which, however, are not solid and do not break off at the base as in the deer, but are composed of an inner bony core and an outer h.o.r.n.y sheath, the outer sheath only being shed.

The family Bovidae includes the once abundant buffalo or bison (_Bison bison_) (frontispiece), the big-horn or Rocky Mountain sheep (_Ovis canadensis_) (fig. 151), and the strange pure-white Rocky Mountain goat (_Oreamnos monta.n.u.s_). The buffalo was once abundant on the Western plains, travelling in enormous herds. But so relentlessly has this fine animal been hunted for its skin and flesh that it is now practically exterminated (fig. 156). A small herd is still to be found in Yellowstone Park, and a few individuals live in parks and zoological gardens. In all of the Bovidae the horns are simple, hollow, and permanent, each enclosing a bony core.

=The carnivorous mammals (Ferae).=--The order Ferae includes all those mammals usually called the carnivora, such as the lions, tigers, cats, wolves, dogs, bears, panthers, foxes, weasels, seals, etc. All of them feed chiefly on animal substance and are predatory, pursuing and killing their prey. They are mostly fur-covered and many are hunted for their skin. They have never less than four toes, which are provided with strong claws that are frequently more or less retractile. The canine teeth are usually large, curved, and pointed.

[Ill.u.s.tration: FIG. 156.--A buffalo, _Bison bison_, killed for its skin and tongue, on the plains of Western Kansas thirty years ago.

(Photograph by J. Lee Knight.)]

While most of the Ferae live on land, some are strictly aquatic. The true seals, fur-seals, sea-lions, and walruses comprise the aquatic forms, all being inhabitants of the ocean. The true seals, of which the common harbor seal (_Phoca vitulina_) is our most familiar representative, have the limbs so thoroughly modified for swimming that they are useless on land. The fur-seals, sea-lions, and walruses use the hind legs to scramble about on the rocks or beaches of the sh.o.r.e. The fur-seals (fig. 157) live gregariously in great rookeries on the Pribilof or Fur Seal Islands, and the Commander Islands in Bering Sea.

The bears are represented in our country by the widespread brown, black, or cinnamon bear (_Ursus america.n.u.s_) and the huge grizzly bear (_U. horribilis_) of the West. The great polar bear (_Thalarctos maritimus_) lives in arctic regions. The otters, skunks, badgers, wolverines, sables, minks, and weasels compose the family Mustelidae, which includes most of the valuable fur-bearing animals. Some of the members of this family lead a semi-aquatic or even strictly aquatic life and have webbed feet. The wolves, foxes, and dogs belong to the family Canidae. The coyote (_Canis latrans_), the gray wolf (_C.

nubilus_), and the red fox (_Vulpes pennsylvanicus_) are the most familiar representatives of this family, in addition to the dog (_C.

familiaris_), which is closely allied to the wolf. "Most carnivorous of the carnivora, formed to devour, with every offensive weapon specialized to its utmost, the Felidae, whether large or small, are, relatively to their size, the fiercest, strongest, and most terrible of beasts." The Felidae or cat family includes the lions, tigers, hyenas, leopards, jaguars, panthers, wildcats, and lynxes. In this country the most formidable of the Felidae is the American panther or puma (_Felis concolor_). It reaches a length from nose to root of tail of over four feet. Its tail is long. The wildcat (_Lynx rufus_) is much smaller and has a short tail.

[Ill.u.s.tration: FIG. 157.--The Lukanin rookery of fur seals, _Callorhinus alasca.n.u.s_, on St. Paul Island, Pribilof Group, Bering Sea. (Photograph from life by the Fur Seal Commission.)]

=The man-like mammals (Primates).=--The Primates, the highest order of mammals, includes the lemurs, monkeys, baboons, apes, and men. Man (_h.o.m.o sapiens_) is the only native representative of this order in our country. All the races and kinds of men known, although really showing much variety in appearance and body structure, are commonly included in one species. The chief structural characteristics which distinguish man from the other members of this order are the great development of his brain and the non-opposability of his great toe.

Despite the similarity in general structure between him and the anthropoid apes of the Old World, in particular the chimpanzee and orang-outang, the disparity in size of brain is enormous.

[Ill.u.s.tration: FIG. 158.--"Bob Jordan," a monkey of the genus _Cercopithecus_. (Photograph from life by D. S. Jordan.)]