Popular Science Monthly/Volume 8/December 1875/Opossums and their Young
By Prof. W. S. BARNARD.
IN the "Perfect Description of Virginia," 1649, the opossum was noticed as "a beast that hath a bagge under her belly, into which she takes her young ones, if at any time affrighted, and carries them away." Lawson says: "She is the wonder of all animals. The female doubtless breeds her young at her teals, for I have seen them stick fast thereto, when they have been no bigger than a small raspberry, and seemingly inanimate. She has a pouch or false belly wherein she carries her young, after they are from those teats, till they can shift for themselves. ... If a cat has nine lives, this creature surely has nineteen; for if you break every bone in their skin, and mash their skull, leaving them for dead, you may come an hour after, and they will be gone quite away. ... Their fur is not esteemed nor used, save that the Indians spin it into girdles and garters." Aside from its curious appearance and habits, the opossum (Fig. 1) possesses an unusual interest from being our typical, and the only North American representative of that large order of peculiar mammals known as marsupials. Its mode of reproduction long remained a mystery, and even at
Fig. 1.—Common Virginia Opossum (Didelphys Vlrginiana)
this day almost nothing is known of its development, which, when thoroughly understood, must explain the origin of the pouch and other parts characterizing marsupials, and their relationship to allied groups. Having had some experience with these animals, and examined seven sets of young ones, at important stages of development, I think it may be worth while to record some of the observations made.
With the general proportions of (but a longer nose than) the common rat, almost the size of a domestic cat, it presents a rather disagreeable appearance and odor. A dense coat of light-gray wool, with scattered long hairs interspersed, covers the body, while the short ears, the eyes, the long pointed nose, the feet and tail, are colored quite dark. The strong, round, slender tail is destitute of hair, but covered, like the beaver's, with scales. But the most peculiar feature of this animal is the mammary pocket, or marsupium, formed by a folding-in of the skin on the abdomen. Its character is marked by wonderful cunning and stupidity combined. The daytime it spends in slothful idleness, but prowls about nocturnally seeking for food. Walking or slowly ambling at an awkward gait, it proceeds from place to place, usually following the borders of streams and ponds, often wading where the water is shallow. But its limbs seem best adapted to climbing; the plantigrade, hand-like feet, with thumbs opposable to the fingers, and the long, prehensile tail, strongly indicate scansorial habits and arboreal life. Among the trees it manifests astonishing agility, climbing or swinging from branch to branch with perfect safety, and may be seen hanging by one or more of its feet, or by its tail alone, while busily engaged gathering and eating the wild-grapes, or haw, or persimmon, of which it is peculiarly fond, or robbing birds'-nests of their eggs or young. A varied diet suits its omnivorous appetite, and it fares promiscuously on fruits, vegetables, eggs, insects, worms, reptiles, small quadrupeds, and birds, often stealing domestic fowls. It commonly hides among vines and branches, in hollow trees or logs, or in holes in the ground. In these places also its nests of grass and leaves are found. In autumn, the opossums become excessively fat, and are then prized for food in the Southern States, especially by the negroes, whose fondness for hunting them and eating their flesh has already exterminated them from many localities where they abounded plentifully before. Their flesh, when cooked, resembles roast-pig. The animal is usually sullen, stupid, and slow, but if attacked assumes a terribly fierce attitude, snarls, utters a kind of hiss and low growl, and will often bite ferociously, though at the first blow wall usually feign death, and no amount of torture will make it revive or show a sign of suffering, but when beaten and left for dead it will often crawl away as soon as its enemy is gone. Its great endurance is also shown by the fact that when fat it can live for three or four weeks without food or water.
The female is very fond of her young, enjoying with them that domestic felicity portrayed by Florian in his happy table, "La Sarigue
Fig. 2.—Merian's Opossum (Didelphys Dorsigera) with Young.
et ses petits," and she will offer every resistance, and suffer greatly, to prevent any one looking into her pouch to examine her offspring.
In Europe, Asia, and Africa, not a single marsupial exists. Our only species, Didelphys Virginiana the opossum, is found from the Great Lakes to the Gulf, and from ocean to ocean; but it has several relatives in South America, where about twenty species exist, such as the sarigue, shupati, and carigueya, of Brazil. In some of these the pouch is rudimentary, affording little protection to the young, which hang fast to the nipples until able to jump about, and then are carried on the back of the female, where they cling to her wool and gain additional support by coiling their tails around hers. Perhaps the most cunning of this sort is the so-called Merian's oppossum (Didelphys dorsigera), of Surinam, represented in Fig. 2. Also, the yopock (Cheironectes palmatus) is peculiarly interesting on account of its aquatic habits and webbed feet, adapted to swimming. Its foot also has a long-tubercle, which has been mistaken for a sixth toe, and the mouth is furnished with large cheek-pouches. It inhabits holes along the streams of Brazil, and lives on small aquatic animals, spawn of fish, etc. Its mode of life reminds one of the ornithorhynchus and the otter. A specimen of this species was caught alive near Pará, in a fish-trap similar to the kind of basket with a funnel-shaped opening used for catching eels. Although marsupial animals are so exceedingly rare in other parts of the world, the kangaroos and almost all of the great variety of animals of Australia belong to this group. Thus it appears they are mostly tropical.
The earliest fossil mammals known appear to be marsupials allied to the opossum. In the bone-caverns of Brazil quantities of bones of opossums, such as live in that country now or similar, are found. One species of Didelphys was found fossil in the Paris Basin, of Eocene formation. Other relatives of the opossum have been found in a fossil state, associated with the palæotherium, anoplotherium, and. other extinct pachydermous quadrupeds; but the most remarkable are found in Jurassic rocks, as the earliest fossil mammals known. Their discovery in this ancient reptilian age in the limestone of Stonesfield was so extraordinary that attempts were made, on the one hand, to prove that their remains were reptilian; on the other, to prove that the rocks were of Tertiary origin; but it has been established, beyond all doubt, that these animals originated in this early reptilian age, and, probably, by descent, either directly or indirectly, from not very remote reptilian ancestry. This relationship is indicated, not only by the fossil remains of marsupials, but also by the anatomical and embryonic characters of marsupials and monotremes, so far as known. The organization of marsupials seems to be a kind of reptilian and mammalian combination, as has been shown by the valuable investigations of Prof. Owen, Dr. Coues, and others.
The monotremes present the lowest grade of mammalian organization, in many respects approaching closely to the oviparous classes of birds and reptiles. It is probably through these that the marsupials have gained some reptilian characters. The opossum, for example, has "a genuine reptilian skull," as Dr. Coues has remarked in his estimable memoir on the anatomy of this animal.
The main difficulty in tracing out the genealogy of marsupials is that our knowledge of them is confined chiefly to the living forms, while these must be but a small remnant of the whole group as it existed in ancient times, when its members inhabited every land on the face of our globe. Even in the imagination we cannot resurrect the manifold varieties of the past. But, in all probability. Prof. Haeckel is right in believing that this group affords a series of forms connecting the lower apes or lemuroids above them with the monotremes below. This would bring some of the marsupials within the lineage of human ancestry, and, before all others, the opossums seem most closely allied to the lemuroid apes. Indeed, they have already been grouped with man and the apes, although their structure hardly warrants such a classification. Storr congregated into one group all mammals with an opposable thumb. Also, Ogilby adopted the name cheiropeds for the same group, and subdivided it into Bimana (men), Quadramana (monkeys), and Pedimana (Semiadæ and opossums).
The characters of groups are generally arranged into categories intended to show how groups are distinct from each other; but, if it is equally fair to arrange those characters in such a way as to show the affinities of groups with each other, and what they have in common, we may say briefly that the placental mammals are connected with the marsupials by having—1. Nipples; 2, Free clavicles; 3. An embryonal cloaca, and by these characters both groups are distinguished from the monotremes below them; the marsupials and monotremes are united by having in common—1. Marsupial bones; 2. Undeveloped bigeminal bodies; 3. No placenta, and by these characters
Fig. 3.—Young Opossum. Natural Size.
are distinguished from the placental mammals above; while the monotremes join with the reptiles in possessing—1. United clavicles; 2. A permanent cloaca; 3. No nipples, and by these characters are distinguished from the marsupials above. A great many more characters and facts from the comparative anatomy, embryology, and palæology, could have been used in this comparison; but those given are enough to show how characters usually regarded as distinctive only may also at the same time be viewed as connective.
The order of living marsupials presents remarkable diversity of structure and habits, containing herbivorous, insectivorous, and carnivorous species; yet we find all these traits combined in one and the same species, the opossum. It is probable that, by adaptation to similar modes of life, the marsupials have developed groups parallel to those of the placental mammals. However, it is certain the Quadrumana seem represented by the Phalangers, the Carnivora by the Dasyuri, Insectivora by the Phascogales, Ruminantia by the kangaroos, and Edentata by the Monotremes. Rodents and bats are numerous in Australia, but only one of the former is marsupial, and none of the latter. The subdivisions of the order are indicated by the modifications of the extremities and digestive system. A gradual transition is found passing from the Phalangers through the Paramelidæ to the kangaroos. All arboreal species have an opposable thumb. This thumb is rudimentary or wanting in the terrestrial species, but in both the carnivorous and herbivorous groups we find a gradual transition to the species possessing a well-developed thumb; thus the Didelphidæ (opossums) have a well-developed thumb; in some of the Dasyuridæ it becomes very small, while a tolerably distinct thumb characterizes the Phascogales; a rudimentary thumb in Dasyurus; no external thumb in D. Maujei, but its metatarsal exists, while in Thylacinus even its metatarsal is gone.
Fig. 4.—A Young Female Opossum (Didelphys Virginiana). Natural Size. B. Marsupium, clitoris, and vent of the same, enlarged; C. Marsupium, penis, and vent of a male of the same litter, enlarged.
Below the marsupials stands the group of Monotremes, including the remarkable Australian Ornithorhynehus and Echidna. In the former the openings of the milk-glands on the abdomen are not marked by any elevation or depression; but in Echidna we find a similar pair of glands, the opening of each becoming depressed at maturity, so as to form a small pit, into which the nose of the young is inserted and attached, where it remains pendant and nourished while its development advances. This pair of little pits may be regarded as the beginning of the bilateral pocket so largely developed in some marsupials. If we can imagine that these depressions have become so deep as to envelop not only the nose of the young, but also its whole body, we can understand the evolution of a marsupial from something lower. At the same time we should notice that these depressions are just the opposite of what we find in the higher mammalia, where the mammary glands form larger or smaller abdominal or pectoral prominences. The milk-glands of Ornithorhynehus seem primitive, while the depressed glands of Echidna and the marsupials, and the elevated glands of higher mammals, may be viewed as differentiations of the same.
The opossum is the animal on which the first observations of marsupial reproduction were made. At first the young, found in an imperfect condition within the pouch, were not examined closely enough to disclose their real nature. They were regarded as formless and inanimate. Even in the "Natural History of New York," Part I., the young is spoken of as "a small gelatinous body, not weighing-more than a grain." But these ideas of the early observers still exist in the popular mind, and are as imperfect as their explanations as to how the young originated. The peculiar character of the young led to the belief that they must have developed from the parents' teats, by a kind of metamorphosis or budding process. This gemmiparous theory existed already in Tyson's time, and was discussed by him. But to-day we have a more correct knowledge of their mode of reproduction, which so long remained clouded with mystery.
An animal born so premature as the little opossum must necessarily perish from exposure, were it not for the curious provision for its protection and the constant supply of milk afforded in the pouch of the female. The internal cavity of the adult female marsupium seems to be formed by an infolding of the external skin. From its opening on the median line of the abdomen the pouch extends backward and laterally, forming a kind of bilateral pocket. From the posterior wall of this about thirteen teats project. To these the young are attached after birth. The two so-called marsupial bones are found in both the male and female Virginian opossums, as well as in some of the South American opossums, which have only a rudimentary pouch, and the monotremes, which have no pouch at all. The investigations of Prof. Owen have shown that these bones are no essential part of the marsupium, although formerly regarded as such; they attach to the anterior border of the pelvis and lie against the mammary glands, where the cremaster muscle winds around them, and makes them act to compress the glands and force out the milk into the throats of the young, which at first seem too feeble to suck.
The young opossums are born as almost helpless little bodies, with mouth and fore-limbs well developed. The transfer of the embryo from the uterus to the pouch has not been observed, but this must be done as with the kangaroo, where it is believed that the mother takes each new-born embryo between her lips and places it upon one of the nipples, which it grasps firm with its mouth and the claws of its fore-feet. Immediately after birth, the young opossums are found hanging upon the mammary glands fixed in the above manner, each with the hind part of its body free and pendant. At first, the mouth is a transverse, gaping fissure; but, when attached to the nipple, its corners soon grow up, leaving only a small, round pore surrounding the neck of the teat, which enlarges, so that the suckling cannot let go nor fall off, but hangs on without any exertion. Each of the largest fœtal specimens (Fig. 3) I have examined was covered with scattered hairs. The nose was large and blunt, unlike that of the adult. These measured, from the tip of the nose to the ear, 17 millimetres; from the ear to the base of the tail, 39 millimetres; length of the tail, 20 millimetres. Those of the second size (Fig. 4, A) were much smaller, and, in general appearance, looked more like opossums than the next larger size. Perhaps they were of a different species. These were, from the tip of the nose to the ear, 8 millimetres; from the ear to the base of the tail, 27 millimetres; length of the tail, 10 millimetres. The other specimens formed a very good series down to those of the smallest size, which were taken from the uterus. These smallest specimens (Figs. 5, 6) measured, from the tip of the nose to the ear, 3.1 millimetres; from the ear to the tail, 8.0 millimetres; the tail, 3.2 millimetres. Thus the total length of the smallest was 14.3 millimetres, or about one-half an inch. These smaller ones resemble the hippopotamus more than the opossum. Although found within the parent, they were, apparently, nearly ready to be born. A set of sixteen of these was taken from the uterus by Prof. Wilder. As the mother had but thirteen nipples, it is evident that improvidence would allow three embryos to perish. Sometimes as many as eighteen are brought forth, and often only twelve nipples exist. No attachment of the embryos to the uterine walls has been discovered, hence no true placenta is known. Still a kind of umbilicus is formed,
Figs. 5, 6.—Front and Side Views of Smallest Embryo Opossum (D. Virginiana). Enlarged; entire length when straightened out, one-half inch.
and its cicatrix marks the embryo as it did in Prof. Owen's kangaroo, where it wrongly led to the supposition that a placenta might have been attached. At birth, the hind-limbs appear as short stumps, with their flattened ends presenting slight marginal elevations, the beginnings of toes. These toes and legs gradually elongate. Soon each toe has one joint, and the inner toe becomes set off from the rest. Later, the two longer fingers show two joints, and, finally, the inner toe becomes a thumb with two joints, while each finger has three; and now the hind-foot closely resembles the hand of the higher quadrumana and man, while its fore-feet, much earlier developed, remain more animal-like, the great-toe being set off not so far from the others, but the fingers quite long. The hind-limbs are primarily much shorter than the front, but, developing faster, soon equal and afterward outgrow the others. The same is true of the young kangaroo, where the hind-limbs, shortest at first, finally become many times longer than those in front. Thus we see that what is smallest in the embryo may become largest in the adult. At birth, the nostrils are large, with a high rim; but the eyes are covered beneath the skin, and the ears are represented by small elevations on the sides of the head, while the lips have a remarkable development and peculiar covering, which reminds us of the first embryonic traces of the duck-like bill of ornithorhynchus. The tongue has a peculiar papillated groove above, to fit the nipple, and three very large papillæ on its base. The larynx and epiglottis project so high into the broad pharynx that the milk swallowed passes in two currents, one on either side. A very large three-lobed thymus gland lies above the heart. Only a rudiment of this exists in the adult. The heart is large, and situated on the median line. Its position changes somewhat as it grows older. The lungs are equal in size. Curiously, the œsophagus enters the stomach near its pyloric end. A very large gland lies on the cardiac end of the stomach. Prof. Owen, speaking of the character of the stomach in marsupials, says: "The stomach is simple in the genera Didelphys, Myrmecobius, and Parameles, and likewise simple in Dasyurus and Phalangista; also in the kaola and wombat, but in these two animals it is provided with a glandular apparatus situated to the left of the cardiac orifice." This is so large in the young Didelphys, that it is curious it does not exist when the animal is fully developed. In the possession of this organ, the young opossum agrees with the old kaola and wombat, but the old opossum has developed a stage further, so that the organ becomes rudimentary, or disappears. The cæcum is relatively twice as large as in the adult. The optic lobes of the brain were relatively larger, and the cerebral lobes somewhat smaller than when full grown. When first born, the male and female are, externally, exactly alike; clitoris and penis are large external organs, just in front of the vent, and so much alike, that it is impossible to distinguish the female from the male by these parts, so markedly different at maturity. Even in the oldest specimens studied, the same similarity of size and form of these parts exists, but the female organ stands nearer to the margin of the vent. Some time after birth, the testes descend into a large scrotum, which has a peculiar position, being at some distance in front of the penis. This is the first external sexual difference, for, although the marsupium begins to appear about the same time, it is remarkable that the male at first has as good a pouch as the female. This is first seen as a cluster of very low papillæ on the abdomen, nearly surrounded by a slight ridge. Slowly this ridge rises higher, and the depression extends itself deeper and more laterally, while the outer edge becomes a fold of skin growing inward toward the median line, until, finally, only a narrow opening is left. The marsupium of the male never becomes fully developed, but gradually diminishes in size; still it was well marked in the largest specimens studied.
To the embryologist every one of these curious facts has great significance. We have seen how organs exactly alike in the beginning may differentiate before our eyes into parts altogether dissimilar, just as individual animals of a like kind may have their progeny gradually modified from generation to generation, until, finally, different races are produced from a common ancestry. The adult opossum has rather slender and delicate limbs and fingers, and a long, slender, pointed nose; hence it may naturally be wondered that her offspring, even at such an early period of development, should have the parts of the body of an opposite character, they being, as is shown in Fig. 3, wonderfully bulky and clumsy, more like those of the hippopotamus than any thing else. But, if we look to its possible ancestry, and find something similar, we can discover a tolerably satisfactory reason for this by regarding it as inherited. Going back to the Diluvial formation, we find the remains of huge fossil marsupials with similar coarse, bulky proportions. Such were the Diprotodon and Nototherium of New Holland. The skull of the former is three feet long, really surpassing that of the hippopotamus in clumsiness, while its body and limbs were built in the same bulky style, and it is probable that numerous smaller marsupials of the same pattern existed in those remote ages. The embryo opossums show resemblance to lower animals in the general shape of the body, in the early form of the brain, the peculiarities of the lips, the thymus gland, the glandular apparatus of the stomach, the early conditions of the reproductive and urinary organs, and the primitive condition of the mammary glands. Peculiar embryonic resemblances are found in the young of every animal of which the embryology is known, and these facts have no meaning at all to us unless they mean inheritance and descent.