CHAPTER XI. Breeding Habits and Life History

BREEDING HABITS AND LIFE HISTORY

MATING

Since the platypus is such a shy and secretive creature, rarely seen at all except by those who take the trouble of investigation, it is not surprising to find that, in the past, very little was gathered as to its mating and nest-building habits. One platypus-hunter quoted by Anderson Stuart makes the statement that the males fight furiously for possession of the females. Although I have never seen such combat between platypus at large, from what I have seen in this direction with specimens in captivity I feel justified in agreeing fully with that statement.

Early in the breeding-season there is in the male a correlated rapid growth and enlargement of three sets of glands--the testes, the crural glands, and the scent-glands. The enlarging of the testes (which from quite a small size become as big as pigeon’s eggs) needs no explanation. The scent-glands are more prominent in the male than in the female, and the fact that they become so remarkably enlarged at mating-time implies that the female seeks the male. This is rendered more probable by the fact that, when once impregnation has been effected, the female (so far as I have been able to ascertain) carries out the remaining duties of reproduction entirely unaided--quite a mammalian trait.

At breeding-time platypus usually leave their accustomed feeding-grounds and select a stretch of bank suitable for nesting. Under normal conditions nesting-burrows are not found close together. For the upper Namoi, the Macdonald, and the Manilla rivers, upon which most of my observations have been made, the average is one tenanted burrow to six miles of bank--that is, to three miles of river. Under abnormal conditions (in times of flood, etc.,) a different state of affairs often occurs, as will be evident from what follows, but normally the female does not seem to care about neighbours at this season. She requires a stretch of undisturbed water to herself while hatching her eggs and nursing her young, so that she may be assured of a near and easily accessible food supply.

Verreaux claims to have witnessed the actual act of copulation. According to his own account he spent whole days and nights hidden in a specially constructed hut, and was completely successful. I give his account for what it is worth (1848, p. 130):--

“The male, after chasing the female for nearly an hour, ended always by taking her into the middle of the rushes. There, clinging tightly to her with the help of his bill, he clutched the skin of her neck, while his hind-spurs grabbed her hinder end. The female, struggling violently against him, swam along uttering plaintive cries that were rather like the squeaks of a young porker and grew louder as they went on. The act took five or six minutes, and afterwards the two animals played together for more than an hour.”

If Verreaux meant that the male grasped the skin on the female’s neck, as a drake does, he manifestly did not understand that the extent to which the upper mandible overhangs the lower would render that impossible, quite apart from the pliable nature of the lips and the fact that both jawbones are divided at their extremities and are pliable as far back as the secateuring ridges. With regard to the squeak, I have never heard any sound from a platypus which might be so described, and I am convinced that either the whole account is imaginary or Verreaux was misled by a sound from some other source, possibly overhead.

Two observations which I was fortunate enough to make were widely separated. The first was made at 7.30 a.m. on August 27, 1909, in the Namoi River at Manilla. A platypus appeared on the surface, and was immediately followed by a second. As neither exhibited any surprise or alarm, it is probable that they had been together for some time. After a short interval one animal began to swim in a circle round the other, which, however, soon followed, so that the two were swimming in a circle one behind the other. They were about equal in size--probably an old female and an adult, but young, male--and it was on this account difficult to distinguish which had been the one to start the proceedings. After about a minute of this circling, one of the animals (which proved to be the female) submerged its body and tail, and floated perfectly still with its head alone above water. The male then came slowly up, and mounted in a leisurely fashion. The whole process offered a very close resemblance to the early stages in copulation of a drake and duck, with the exception that the male platypus did not take a grip with his ‘bill.’ The male then threw himself back into a sitting posture, partly out of water, but at this moment there was a great splash, and both animals disappeared.

The second observation was made twelve years later, on September 23, 1921, in the same river at a spot three miles from the first, and at 7 o’clock in the morning. There I saw what looked at first like a large male platypus floundering on the surface of the river as if sick or hurt; this, after a moment or so, dived. Immediately another object followed in its wake, partly-submerged and upside down. In a couple of minutes a second platypus (this time apparently of smaller size) rose to the surface; more floundering, or wallowing, took place at the surface, and I could now perceive quite distinctly that what I had seen was a pair of platypus coupled in an extraordinary position. The tail of each was laid flat along the belly of the other, completely hiding the bright gold and silver of the ventral fur. The precise position of the hind limbs could not be made out, as no movement thereabouts was discernible; but it must have been the grip of these that kept the animals together. So closely were they apposed that they appeared at times like a single giant platypus. The processes of diving, blowing, and floundering at the surface were repeated several times at short intervals. On each occasion the first to rise for a blow would be the only one visible above water, the second being clearly visible only as it trailed upside down in the wake of the first on diving. During the floundering and rolling which took place at the surface the under animal could be seen dimly. The animals rose alternately, apparently for breathing purposes, and on each occasion the second animal seemed to follow, helpless and rudder-like, upon the movements of the first. The whole of the manoeuvring was carried out in a calm, slow, deliberate manner, and almost noiselessly. How long they had been coupled together before I observed them I do not know; but my observation lasted fully three minutes; then the couple separated beneath water, and simultaneously appeared again on the surface, finally diving and disappearing.

I made an attempt to follow the actual process of copulation, making use of a recently killed male and female which, though dead, were still limp and pliable. I found that, when the male rises into the sitting position which I have already described, the tips of his spurs may easily be inserted into the sockets which occupy the corresponding position on the hind legs of the female. If the male then throws himself right back, and at the same time the female draws her pliable tail between his legs, the position observed in the final stages is reached, and in this position the cloacal apertures may easily be opposed. This use of the spur would supply the reason for its being movable, a matter for discussion later.

To insert the spur into the socket to its full extent, as stated by Home, would certainly involve gymnastic ability. My firm belief as to what really happens, is this: When in the sitting position, the male straddles the hips, keeping his feet at right angles with his legs. In this attitude the spur can be erected to its fullest extent (in a manner similar to the action of one’s thumb when the clenched hand is opened suddenly). In an adult male the space between the foot and the tip of the spur, at that time, measures over an inch. This, then, is more than sufficient to embrace the female ankle comfortably. Presumably the female does not resist the application of the grip, and, when it is securely taken, she flicks her pliable tail into position. It is with such simplicity then that the grip is secured and steadfastly retained to the end, though, possibly, aided with two pairs of clutching feet.

Now if we take into consideration the shape and length of the curved spur, which follow befittingly the depth and ovalness of the female ankle, it will not be difficult to ascertain the reason why it is impossible for the male to insert more than the tip of the spur into the socket, even were he desirous of doing so.

As regards Home’s statement that the male ejects the secretion of the spur into the female socket to bring about release, to me this also appears wide of the mark. On examining the spur, which averages ¾ in. long and is shaped somewhat like that of a cock, it will be found that the aperture is not at the tip, but back from it, and, furthermore, is situated on the convex surface farthest away from the socket during the embracing position. I therefore fail to see how a fluid ejected beneath water could enter the socket. As a matter of fact, the poison cannot be ejected unless the spur is pressed against the male’s own leg. How, considering the shape of the weapon, can this possibly be done while the thickness of the female’s ankle, or lower leg, intervenes? The purpose of the aperture’s being set back from the ripping tip of the spur is obviously to prevent clogging while fleshing prior to administering the poison. This strengthens my belief that the secretion is intended to irritate a wound.

BREEDING-SEASON

The breeding-season of the platypus in northern New South Wales may be estimated with fair accuracy from my long series of observations and records. I have observed copulation, as I have already stated, on two dates only, 27 August 1909, and 23 September 1921; but I had a report from a competent observer, Mr. J. Maclean, that platypus were “courting” in the Macdonald River in the middle of July, 1920. The earliest date on which I have actually taken eggs from the nest is 24 August (1925). On the same day I unearthed twin young ones which I estimated to be three days old. This would carry back the date of laying to, perhaps, the first week of August. I have no guide as to the time which elapses between the laying of the egg and the hatching of the young, but I think it must be less than that necessary for the incubation of birds’ eggs, owing to the partial development of the foetus before the egg is laid.

The latest date upon which I have taken eggs is 22 October (in 1918); Kershaw (1912) has recorded the taking of eggs upon the same day of October in 1912 in Victoria. Consequently the period during which oviposition has actually been observed ranges from about the middle of August to a little after the middle of October. There are certain conditions, however, which affect the breeding-season, and I do not believe that normally it has so great a length. In the first place there is, as one would expect, a general tendency towards earlier breeding in the north; as one proceeds south littering becomes progressively later. In central Queensland July and August are the most active months; in New South Wales, August and September; in Victoria, September and October. I have no information as to the breeding-season in Tasmania. But the normality of the breeding-season may be seriously interfered with by conditions of weather, and particularly rainfall. Heavy rains occurring in August and September flood the burrows, which are consequently deserted. This flooding is brought about chiefly by seepage through the soil, as the upper parts of the burrow are almost always above ordinary flood-level of the river. It is thus clear that the pugging of the tunnel is not a precaution against the entrance of water from the river, as has sometimes been suggested, but is for the purpose of providing conditions necessary for incubation. The effect of floods upon the breeding-season in the New England district was observed during September and early October of 1920. In July, the rivers were in flood. Platypus were reported plentiful after the waters had receded in the early part of August. A second flush occurred late in August, when the waters rose to six feet above their normal level; and sporadic rain continued until September 18. Under these conditions a number of burrows which I opened up were found to be deserted and the contained nests sodden with water. More remarkable still, several tenanted burrows were found in close proximity to one another, which is unusual; and in one case two females were found occupying nests in burrows which had a common entrance, an occurrence without parallel in my observations. Furthermore, several tenanted burrows showed signs of extremely hasty construction, being only a few feet in length, and with fewer pug-pits than usual. Again, in 1923 an eight-foot rise in the river had washed out all the breeding females, and no tenanted burrows at all were found on the plateau. A female killed on October 9, when examined, was found to have in her left uterus eggs in the early stages of segmentation. In a normal season eggs are laid during the later part of August and early September, and from late September on to the end of October large young are found in the nests.

THE INTRA-UTERINE EGG

The ovum of Ornithorhynchus (that portion of the egg which corresponds to the yolk of a fowl’s egg) is, at the time of fertilization, a small yellow sphere about 3 millimetres in diameter. After fertilization it becomes surrounded by an exceedingly thin layer of albumen, and, outside this, by a thin, transparent, horny shell. In birds there is a thick layer of albumen (the white of the egg) deposited in several distinct layers, the innermost having spirally twisted prolongations which serve to hold the yolk suspended in the middle of the egg. Outside this thick albumen are two delicate membranes, closely adherent everywhere except at the larger end of the egg, where they separate to enclose an air-chamber; these membranes are closely applied to the rigid, calcareous shell. The most remarkable difference between the egg of a bird and that of a monotreme is that in the former the shell is deposited round the fully-formed egg, and there is no increase in its size during the growth of the embryo, whereas in the monotreme the eggshell increases in size and alters in structure during the intra-uterine development of the embryo. The thin shell, which is at first only 4 millimetres in diameter and spherical in shape, stretches and thickens until a diameter of 10 millimetres has been attained. At this stage the egg is still practically spherical, but the shell has become thicker, and has been rendered opaque by the deposition of lime-salts. As it goes on growing, it becomes ellipsoidal in shape, the longer axis corresponding to that of the contained embryo (Wilson and Hill, 1908), and finally attains its maximum size (16 to 18 millimetres long by 14 to 15 wide) just prior to being laid. There can be no doubt that this curious growth of the egg, which has no parallel in nature outside the monotremes, is due to the imbibition of fluid from the uterus. There is not sufficient nutrient material in the yolk-mass of the fertilized ovum to produce the young platypus which hatches from the egg; consequently the intra-oval foetus is nourished in a considerable measure by secretions from the maternal uterine wall. Since secretions can pass into the egg through the mesh-work of horny fibres of which the shell is at first composed, it is also possible that foetal secretions may pass out; there is at present, however, no evidence that such a process takes place. The point is of interest, for, in the higher mammals, a secretion from the foetus--of the nature of a hormone, or chemical messenger--which passes into the maternal blood-stream, is conveyed to the mammary glands, and stimulates them to the active secretion of milk; but, in the platypus, neither at the time of oviposition, nor later, when the young actually hatch out, are the milk glands of the mother in active condition. The stimulus to lactation would seem to be derived in some external and mechanical way from the presence of the young. This question will be further considered below.

It is thus obvious that, though the monotreme egg is usually looked upon as similar to those of birds and reptiles, it is in reality very remarkably different. The nourishment of the embryo within the egg by secretions from the uterine wall is distinctly a mammalian feature. Such a process does not occur among birds and reptiles; in these, when once the egg-shell is formed, the embryo has to depend altogether upon the nutrient material packed within the egg. It must be admitted, however, that what occurs in the monotremes appears to afford very little clue as to the evolution of the placental habit in the higher mammals.

Plate 29]

Plate 29]

EGG-LAYING

The platypus lays one, two, or three eggs, two being the usual number. There is no record of four eggs, or four young, ever having been discovered, and the statements of Bennett and by Lucas and Le Souef that the number of eggs is one to four are probably based on Geoffroy St. Hilaire’s unfortunate experience (see p. 36). When two eggs are laid, these are invariably found joined together side by side. So closely do the apposed walls adhere that considerable force, comparable to that required to pull apart two fairly dry ‘tangle-foot’ fly-papers, must be exerted to separate them. The only records of triplet eggs are of two sets taken after hatching and one intra-uterine set. Each of the former sets of shells was in the form of a cluster, with the length of all three eggs lying practically parallel, but the ends free for emergence of the young. In one of the sets the three were all joined to each other; in the second set there was a small space separating two of the shells, but both of these were joined to the third. This difference is evidently unimportant, and occurs when the eggs are being clamped together immediately after they are laid.

There is no longer any mystery as to the way in which the eggs are laid and afterwards fastened together. When the eggs are ready to be laid, they are well apart from one another in the uterus, with their longer axes in the direction of the length of that organ. The capsule of each separate egg is at that time completely covered with a sticky fluid. When about to lay, the female squats on her rump in the nest, with her tail between her legs, and her fore-paws in readiness at each side of the aperture of the protruding cloaca. Presumably, the eggs are deposited singly into her soft rubber-like hands and held there firmly until each egg capsule, where in immediate contact with its fellow, becomes affixed thereto; then, with the help of the under surface of her fat, pliable tail, they are clamped to her warm abdomen. Meanwhile the solution still covering the remaining portions of the capsules soon loses its stickiness through either evaporation or absorption. It is improbable that the eggs are held for long in the fore-paws, or that their adhesion and the drying of the exterior portion of the capsules is a lengthy process.

It may interest my readers to know how I have come to these conclusions, seeing that the eggs are laid in a pugged cavity beneath the solid earth. While collecting embryological material for the Canberra National Museum on September 2, 1925, at Manilla, I took a female platypus with mature triplet eggs in her left uterus, and on extracting these I found that their capsules were conspicuously glossed with a thick coating of sticky liquid. I immersed them separately in a powerful drying fixative called “Bles;” but notwithstanding the bleaching and hardening properties of this mixture of formalin, spirit, and acetic acid, the eggs not only stuck firmly together on coming into contact with each other, but also adhered to the glass and to a paper label attached to another specimen in the bottle. This, I think, proves that the eggs adhere to each other only after being laid, and not even in the cloaca, as has been hitherto supposed. Drying off after adhesion and exposure to the atmosphere would appear to be quite natural; this could not occur prior to deposition. But one may ask why, if the eggs will stick to paper and glass, they do not stick to the hands of the mother during the act of manipulation. The probable explanation is that the rubber-like membrane of the fore-paws of a platypus is always cold and moist, however dry and warm the rest of the animal’s body may be. This is as it should be, because of the various functions that the versatile fore-paws are called upon to perform at a moment’s notice, such as walking and swimming, involving rapid action similar to the alternate opening and closing of an umbrella. It will be obvious that the very existence of the platypus depends on the versatile manipulation of this membrane. Nature would not permit any temporary clogging of this intricate member. In 1924, while Mr. Ramsay and I were making a moving picture of the natural habits of monotremes, I tried with strong fish glue to fix the staff of an Australian flag in the closed palm of a living platypus, but found that it was impossible.

My observations of intra-uterine eggs apply to two pairs in the early stages of segmentation. Each pair was found in the gravid left uterus, which alone is functional; in each case the eggs, one behind the other, were separated by an appreciable interval. Wilson and Hill obtained a number of intra-uterine eggs, some in a fairly advanced state of development; but they give no information as to how the eggs were situated in the uterus. They do, however, figure (1908, Pl. 4, fig. 6) a section of a uterus containing a fairly advanced single egg, which shows that the egg lies in the position one would expect, namely, with its longer axis in the direction of the length of the uterus. Caldwell (1887, p. 464) states that he shot a platypus which had laid her first egg, while the second lay in the mouth of the uterus. He does not, however, state that he actually found the first egg, and it may be that he went on the assumption that the animal invariably laid two eggs, which we know not to be the case. Gold-receiver Rumby’s platypus is reported to have laid two separate eggs in a gin-case (see pp. 41-43); but there is so much that is dubious about this particular story that it cannot be accepted as definite proof. If the evidence could be relied upon, it would show that the eggs are laid separately, and the fact that they were not found cemented together might be attributed to the unnatural circumstances in which the mother found herself. As, however, the two eggs follow each other down the uterus lengthwise, it is obvious that they cannot in the process become joined together side by side. Moreover, when there are three eggs it would not be possible for them to be laid if they became cemented together within the body of the mother. It is therefore certain that this cementing takes place after they are laid.

My observations, together with those of Wilson and Hill, show that any pair of eggs contains embryos in an identical state of development, even in the earliest stages of segmentation. This would imply uniform conditions for both eggs, and from this it may be presumed that the ova are extruded from the ovary and are fertilized simultaneously, that they pass down the oviduct (including its uterine portion) close to one another, but end to end, and that they are laid practically at the same moment. At the moment of extrusion they are coated with a sticky secretion from the oviduct walls, and it is a simple enough matter for them to become adherent. But the fact that their long axes are always parallel after laying would seem to necessitate definite action by the platypus. There might be one other suggestion--that the eggs, having passed through the pelvis separately, are retained in the cloaca long enough to become attached to one another before their final extrusion; but this I consider impossible. A further curious feature is that the embryo lies along the long axis of the egg, parallel to that portion of the shell which is cemented to its fellow, and, after the egg is bleached, may be seen through the shell with the naked eye.

My observations of the right ovary and oviduct go to show that, though not functioning in the true sense, they appear to be affected in a certain proportion to the activities of those on the left. For instance, while the left is maturing triplet eggs, the right becomes enlarged to about the size that the organs on the left would have when carrying a single egg. In cases of twin and single eggs, the enlargement is proportionately less.

Of seventy tenanted nests examined by me, eleven contained either one egg or one young one; fifty-four contained two; only five contained three. Of six nests recorded by Kershaw, one contained a single egg and five either twin eggs or twin young. Hill’s record of stages of eggs taken from nests comprises one single egg and four twins. Wilson and Hill’s paper (1907) on the intra-uterine stages does not state clearly what was the exact proportion of singles to twins in the material examined. There are a number of older records by Maule, the Bennetts (father and son), and others, but the figures given above are sufficient to indicate the approximate frequency of each egg-number. Triplets are rare, the only record apart from my five (four times of young in nest, and once of intra-uterine eggs) being a single record of young by Dr. George Bennett. Ignoring triplets altogether, singles are in a proportion of roughly twenty per cent, and twins of eighty per cent.

The position is same as shown on Plate 28. The earth has been removed to render photography possible.

Plate 30]

THE LAID EGG

Kershaw (1912, p. 106) gives the measurements of a pair of eggs as 18 × 15 mm. and 16 × 14 mm., and of a single egg as 18 × 15 mm. My observations give the following measurements (in millimetres):--Shrivelled egg (unhatched), 16 × 10; single egg, 18 × 14; twin intra-uterine eggs, 3 × 3; twin eggs, 16 × 12; twin eggs, 17 × 14, 16 × 14; single egg, 17 × 13; twin eggs, 15 × 26 and 17 × 26; twin eggs, 15 × 25 and 16 × 25; twin intra-uterine eggs, 5 × 5; twin eggs, 18 × 14, 15 × 14; twin eggs, 18 × 25 and 16 × 25; single egg, 15 × 13; twin intra-uterine eggs, 6 × 6; triplet intra-uterine eggs 17 × 15, 18 × 15, third smashed (abortion). No triplet eggs have yet been taken from a nest. It would appear from the measurements given above that the average of the single egg is usually larger than that of either of twin eggs, the measurements being 17.25 × 14 mm. against 17.5 × 13.8 mm. It will also be noticed that in only one of five pairs of twin eggs are the eggs of equal size. In the others one is usually distinctly smaller than the other.

Of the intra-uterine specimens listed above, the 1920 pair measured 3 mm. in diameter; they were spherical in shape, yellow in colour, and were situated in the oviduct close to the left ovary. The 1923 intra-uterine pair were 5 mm. in diameter, spherical in shape, yellow in colour, and were situated in left uterus. The 1925 pair measured 6 mm. in diameter, were spherical in shape, “orange” in colour, and surrounded by albumen.

The 1925 triplets measured (1) 17 × 15, (2) 18 × 15, the third being smashed through abortion while in the uterus but uniform in size with the others. They were ellipsoidal in shape, glossy white in colour, and were situated in an abnormally enlarged left uterus, two adjoining nearest to the oviduct, the other in a separate compartment close to the cloaca. All three were lying lengthwise with the trend of the uterus, while the ovary connected therewith was partly covered with a crystal jelly encased in a thin transparent film. This jelly, or “hydatid-like” sac, was almost as large as the swollen ovary, and, except where it was attached, somewhat similar in shape to its host ovary. Whether it was for the purpose of supplying the compound which envelops the eggs when capsuled in the uterus, I cannot say; but it was preserved intact for later investigation.

When first deposited, the eggs have a full outline, but as incubation proceeds they frequently become dented, as well as discoloured. Kershaw (1912, p. 106) notes that mere exposure to dry air produces denting in a few minutes. I think, however, that the denting is due to the age of the egg, because on two occasions I have exposed apparently new-laid eggs for thirty minutes in the open while photographing them, and they retained their full outline perfectly throughout, although handled considerably. It may be that the necessity for a moist atmosphere is part of the reason for the pugging of the burrow.

The shell is thin, and easily compressible. It is composed of a mesh-work of keratin fibres, impregnated with calcareous salts; except in shape, it resembles that of a lizard. The shape is broadly ellipsoidal, the ends being shortly and evenly rounded. The colour of the newly laid egg is a glossy white, which, when the egg is dry, turns to a flat chalky white, and later becomes stained brownish. The shells from which the young have been hatched have a more polished and translucent brownish-white appearance, which may best be compared with that of the celluloid balls used in “ping-pong,” when they have become crushed (see Plate 23).

INCUBATION

Incubation is carried out by the female alone. During many years of observation no male has been found in a breeding-burrow, and Joseph’s account of platypus in captivity (p. 205) indicates that females resent the intrusion of the male even into their resting-places. On one occasion I was fortunate enough to catch a mother-platypus napping, and she did not wake until an attempt was made to insert a thermometer into the middle of her curled-up form. Her tail was turned up over her abdomen, holding against it two 65 mm. young. I am convinced that the eggs are held during incubation in a somewhat similar manner, and that their cohesion supports this view (see Plate 30). Kershaw (1912, p. 105) records the finding of newly hatched young, one of which was attached firmly to the skin of the mother. At this stage the young have not begun to suck, but they are very helpless and might become entangled in the fur of the mother’s abdomen, where they invariably nestle. Kershaw has also expressed this view, and he mentions, incidentally, that the other young one fell off as the mother left the nesting-chamber. Of two observations by Kershaw (loc. cit.), and six by myself, of nests in which eggs were found, in no instance was the mother absent from the burrow. This would surely indicate that the female incubates her eggs in the way described.

The point is of importance, because there is a considerable difference between the temperature of the nest and the body temperature of the mother. I have found that the average temperature of the nesting-chamber (taken by thrusting in a thermometer the moment it was opened up, and blocking the hole with a sack) was 64.5° Fahrenheit (19.2° centigrade), whereas the average cloacal temperature of seven brooding females was 90° Fahrenheit (32.2° centigrade). Such a difference in temperature would have a considerable influence upon the duration of incubation. The length of the incubation period is unlikely to be ascertained by observation in the field, but I think it should be possible to discover it artificially. At present it can only be conjectured from comparison with other oviparous animals. Passerine birds which lay eggs of the same size as those of the platypus have an incubation period of from twelve to fourteen days; they also have a higher blood-temperature. But, as against this, Caldwell has made the observation (1887, p. 464) that the egg of Ornithorhynchus, when ready to be laid, contains an embryo already in approximately the same stage of development as a thirty-six-hour chick. It may not be very wide of the mark to suggest that the incubation period of the platypus is about fourteen days, though possibly less.

I am convinced that, during the three weeks or more which elapse between the laying of the eggs and the onset of lactation, the female not only does not leave the nest, but also passes into a condition of partial aestivation. I think the eggs are laid in the fore-paws and placed, in the manner described above, between the upturned tail and the abdomen, and that they remain there throughout the incubation period and possibly beyond, until mammary secretion is excited by the mechanical stimulus of the movements of the young. These convictions pass beyond the observed facts, but are, nevertheless, consistent with them.

The protrusion near centre of upper edge shows where the gland was connected with perforation in abdominal skin.

(From fresh carcass)

Plate 31]

HATCHING

The hatching process has not been observed completely. The shells from which the young have hatched are always found in the nest in a flattened condition. On relaxing them in warm water, months later, I have found that they can be made to resume their original shape. It is worth noting here that the warm water has no effect upon the substance which caused the eggs to stick together when laid. The young are seen to have emerged through ragged rents, and seldom is any portion of the shell missing. The twin eggs examined all show that both young emerged at adjoining ends. In the only two sets of three shells recorded, a somewhat different state of affairs was found (see Plate 23, fig. 4). In two of the eggs the rents were at opposite ends; in the third the rent opened at right angles to the plane of the other two; the result being three openings as far removed from one another as the surfaces of the eggs would allow. It is impossible to credit the mother with an instinct enabling her to arrange the eggs with regard to the planes of embryos which are only very little formed at the time of laying. It is likewise difficult to conceive that the embryos themselves can possess any instinct which leads them to break their way out in a given direction in which their passage is assured. The probability is that the eggs are invariably attached to each other by their sides, and that the young always break out at the ends, and so no complication can arise.

When young from the nest were first described by Owen, it was thought that the caruncle on the muzzle, which is hard and sharp a couple of weeks after hatching, corresponded to the ‘egg-tooth’ found in many birds and some reptiles, and was the means by which the young one cut its way out of the egg. The discovery of earlier mammary foetus stages has shown, however, that at the time of hatching the caruncle is merely a fleshy knob, which could not possibly serve as a cutting instrument. Professor J. T. Wilson states that in the newly hatched young there is a small egg-tooth anterior to the caruncle; but this must be a very evanescent structure, for no sign of it can be seen in young ones 18 mm. long, the earliest mammary foetus (a term to be explained later) collected by me. These, from the soft, moist condition of the shells, and the umbilical protrusions exhibited by the young, I considered to be newly hatched, but Professor Wilson’s specimens are smaller. Whether helped by an egg-tooth or not, it is probably an easy matter for a restless, muscular creature like the platypus embryo to break out through the thin shell, using the caruncle as a point of resistance against the pull of the fore-paws, with an action similar to that adopted by the young when pummelling the mother’s abdomen and nuzzling for milk. I have observed a similar action on the part of overturned naked young regaining their feet.

NURSING HABITS

The young, when first hatched, show few of the characteristic features of the adult platypus. There is no sign of the curious muzzle, which Geoffroy St.-Hilaire protested would be such a stumbling-block to the imbibing of milk from mammary glands. Indeed, except for the flattening of the rudimentary tail, and the shortness of the limbs, with their even fingers and toes, the young platypus has little to distinguish it upon casual examination from any early mammalian foetus. It grows into characteristic platypus form at a later stage.

The most remarkable and mysterious feature about the baby platypus is that it is not suckled at all by the mother for some days after hatching, for the very good reason that the maternal mammary glands are not yet actively functional. Investigations of this extraordinary phenomenon have advanced far enough to place the matter beyond doubt. On October 9, 1923, I took three female platypus, two with the mammary glands almost dry, the third with them quite so. I noted the inflamed condition of the uterus of the third individual, and it proved on examination to contain two eggs in the early stages of segmentation. On September 27, 1921, I took a female with a young one 45 mm. long. Here the mammary glands had begun to enlarge, but had not reached half the maximum size, and would seem not to have been actively functional, though four or five days must have elapsed since the time when the young one was hatched. Against this it must be recorded that Kershaw (1912, p. 105) took a 30-mm. young one, “so securely attached to the skin as to require a little force to detach it.” Kershaw does not state the method of attachment; but, as the limbs are very feebly developed at this stage, it is certain that the little creature must have been sticking to the fur accidentally. In all my experience I have never found the young ‘attached’ to the fur. Whether the individual which Kershaw observed was really obtaining nourishment cannot now be determined. Had he dissected the mother, he would have discovered that at that stage the milk glands were dry. I have made a careful examination of the glands of a mother with 20-mm. young, and could not induce milk to flow by external pressure--a process which can easily be carried out when the young are older--nor, on dissection, could I find any trace of milk, the glands appearing quite dry. An examination of a considerable series of mammary glands from nursing mothers which have been collected with their young has convinced me that during the first week, at least, after hatching there cannot be more than a very slight milk-secretion, if any at all, and I think this characteristic applies to the echidna also.

I have made the experiment of placing young apparently two weeks old upon the abdominal fur of the mother, who was laid upon her back and held in that position. The young ones crawled aimlessly about, and passed and repassed over the mammary area without appearing to be aware of its presence. They made no attempt whatever to cling to the fur with either mouth or feet. Older ones, however, clung tightly with their fore-paws, using their hind limbs as additional supports when the mother was held up vertically (see Plate 10). While observing these latter young, I noticed a peculiar action which appears to lend some support to a rather speculative conclusion previously reached as to the function of the caruncle. The young were observed to bend their heads under their bodies, and to withdraw them in such a way that the dorsal surface of the muzzle, towards the end of which the caruncle is borne, was dragged forward over the mammary area so that the sharp point of the caruncle scored its surface. It had occurred to me previously that, since the caruncle could not, for reasons given above, have the function of an egg-tooth, it might well serve the purpose of exciting mammary secretion. It is at least a significant coincidence that the onset of obvious lactation occurs simultaneously with the completed growth of the caruncle. It may be that the stimulus afforded by this ‘milk-spur’ is necessary to induce the very primitive milk glands of Ornithorhynchus to act; and the delayed lactation which I have observed may be due to the incomplete development of the necessary stimulus in the early young. How the young platypus is nourished in the meantime I do not know.

On one occasion only have I found the mother to be absent from a nest containing young less than two weeks old. It must be remembered, however, that burrows are always opened up by daylight, and, as the female is certain to be more than usually wary during the nursing period, it is safe to assume that she would generally feed under cover of darkness. When the young are older, they are frequently found by themselves, presumably because the mother has to feed long and often in order to maintain her milk supply. When the female leaves the burrow, all the pugs are closed behind her, which means that she digs her way through each of them, passing the earth under her body and compacting it with her tail as she goes. Since the same process has to be repeated on her return, and since as many as nine pugs have been observed in a burrow, feeding excursions must be a laborious business and are not likely to be indulged in more often than is necessary.

During the early nursing period, then, the mother spends most of her time curled up about her babies, holding them snugly against her abdomen by means of her tail. The young at this stage are generally referred to by the term ‘mammary foetus,’ which is the name given to the pouch-young of marsupials during the time they remain on the teat. There is, however, no close correspondence between the two. The marsupial, coming to active life for a brief space after birth--long enough, it is said, to reach the pouch of the mother by its own voluntary effort--relapses into a foetal condition in which it is apparently incapable of voluntary movement. It cannot even suck, the milk being pumped into it by the mother. This act is obviously impossible for the teatless monotremes, hence their fasting. The young platypus, although its bodily form undergoes a fair degree of modification during the first fortnight, and though it is not hatched entirely after the image of its parents, is continuously capable of voluntary movement from the time it leaves the egg. In the newly hatched young the movements are comparatively feeble; but after the first week the young are in a state of perpetual motion, at least when exposed to daylight, which they appear to dislike. In the darkness of the nest, however, they probably spend most of their time in sleep.

In the echidna the mammary areas open into the pouch, which is formed as a temporary structure during the breeding-season, and the young one laps its nourishment with its slender projecting tongue. The platypus has no pouch, and the milk oozes out through numerous fine apertures upon two mammary areas of the abdomen, each about half an inch in diameter. These areas are covered with fur, and are not in any way distinguishable from their surroundings, so that they are difficult to locate by external examination. Pressure upon the mammary glands, however, causes the milk to flow gently out, and their precise position can then be seen. The hair covering the areas serves, apparently, in place of a teat, and the young pluck at this and suck the milk from it, much as a little aboriginal eats his honey by sucking it from a piece of shredded bark. The “lips” of the young, owing to the shortness and undeveloped form of the bill at this early stage, are adapted for sucking in conjunction with the tongue (see plate 33).

An unsuccessful attempt was made to keep alive two young ones (about a fortnight old), which had been sent to the Department of Zoology at the University of Sydney, by inducing them to suck cow’s milk from pads of cotton wool. Even when the milk was squeezed out upon their tiny muzzles, they did not appear to take the slightest interest in it. I had much the same experience in the field with a pair of well-grown young, which were in fur and had their eyes open. I placed drops of milk upon the hairy portion of my forearm, but they merely brushed it aside. After crawling inside my sleeve (evidently to avoid daylight), they did, however, pluck at the skin of my arm with their jaws, with a sucking action sufficiently strong to bring a flush of blood to the spot. The opening of the jaws was accompanied by a feeble kiss-like explosive sound, resembling that made by a man releasing smoke from the side of his mouth when lighting a pipe.

The naked young range from bright red to rose-pink in colour when alive, and their delicate skin has a silken sheen, and is minutely wrinkled. In preserved specimens these wrinkles become much exaggerated, owing to the contraction of the underlying muscle-layer. When the hair-follicles begin to develop, the skin of the back assumes a bluish appearance, like that of a shaven chin, but the under surface of the body still keeps its pink colour (see Plate 33). Finally, with the growth of the hair, which is at first fine and silky, the adult appearance is reached.

A feature of interest in connection with the nursing habits is that the nest never shows any sign of having been fouled with excreta. Young which I have had in temporary confinement defaecate freely; the faeces take the form of an odourless, greenish, oily slime, which may be compared with the contents of the gall-bladder of an ox. (Adults, in captivity or at large, always defaecate in water.) As the nest is always clean, this inoffensive matter would readily filter through the nesting-material without causing more discomfort than that endured by nestling Owlet Nightjars--in fact, not so much, since it would not form hard masses like the bird droppings. But it is reasonable to suppose that very little faecal matter passes from either the mother or the young during the period of incubation and the nursing of little ones. While suckling, the mother leaves the nest occasionally to feed, and, presumably, to defaecate also. The nest is then probably relined from time to time, as is the custom of the Owlet Nightjar.

It is for these reasons that I maintain that platypus, when brooding, fall into a lethargic state and, thus requiring little food, do not evacuate in the nest unless abruptly disturbed.

Note cautious attitude and rubber-like muzzle.

Plate 32]

ADOLESCENCE

About six weeks after hatching, the young will have reached a length of twelve inches. By this time their eyes are open, their fur is a quarter of an inch in length, and they are able to crawl freely about the burrow. (See Plate 12, fig. 2). This may seem a rapid rate of growth, but such rapidity is explained by the fact that when once the young commence to suck, their appetites increase rapidly. The quantities of food found in their stomachs on dissection are surprising. (Because of this, the stomachs must be tapped and the contents drained before fixing in “Bles,” otherwise the milk putrefies and is apt to spoil the entrails.)

Just prior to the appearance of pelage on a large nestling, the intestines and stomach are found actually bloated with milk. When fully extended, the skin of the abdominal area becomes remarkably tight-fitting; but elsewhere the trunk does not nearly fill out the skin. By holding such a specimen, back down, on one’s palm, and slightly tilting the hand from side to side, the entire contents of this out-size skin slop about as a quantity of mercury would if placed in a pliable bladder.

The largest young that I have found in the nesting-burrow measured thirteen inches in length. The smallest known by me to have been found at large were fourteen inches long, captured in shallow water among weeds. My own youngest capture at large measured fifteen inches. It would thus appear that the young are generally from thirteen to fourteen inches long when they leave the burrow. For some time prior to this, however, they would seem to run about the burrow, and, possibly, to enter the water in company with their mother. When once their eyes are open, the young are able to swim. The mother then no longer replaces the pugs in her tunnel, and one which contains large furred young may be recognized during excavation by the fact that nesting-material is strewn along its course. This is mentioned casually by Bennett (1860, p. 130), and it seems probable that this material is scattered about by the young in running to and fro. At this stage the breeding-burrow is about to be deserted.

Bennett (l.c., p. 131) records the capturing of twin young, ten inches in length, which had a most beautiful, sleek, and delicate appearance, and seemed never to have left the burrow. These were kept alive for some little time, and enjoyed playing in a dish of shallow water, in which, however, they never remained longer than fifteen minutes at a time. It would thus seem likely that platypus of that age have already begun to make short excursions into the water. Bennett captured a female which he assumed to be the mother of these, and found her to be in a very poor condition, with the mammary glands practically dry. Two healthy young, ten inches in length, would certainly need more nourishment than could be supplied by glands in this condition; but possibly they had just previously sucked the mother dry. But we may assume that the young are by this time learning to eat the food of the adults, and that, as this is always eaten in the water (unless taken from the mother’s mouth, pigeon-fashion), they go to the water to be fed. Verreaux’s observations are certainly interesting, but I cannot entirely agree with him. He records (1848. p. 131, Owen’s translation):--

“I redoubled my attention and care, and by dint of perseverance, having at my disposal (always on the banks of the New Norfolk) a pretty considerable number of adults and young, I saw the latter accompany their mothers, with which they played, especially when they were too far from the bank to take their nourishment. I distinguished very well that when they wished to procure it they profited by the moment when the mother was among the aquatic plants, near the land, and where there was no current. The female having her back exposed, one can easily conceive that on the exercise of a strong pressure, the milk would float to a little distance, and that the young might suck it up with facility; this it does, turning about so as to lose as little as possible. The manoeuvre is the more easy to be distinguished, since one can see the beak move with rapidity. I cannot better compare the greasy liquid of the female than to the iridescent colours produced by the solar rays upon stagnant water. I have witnessed the same fact repeated daily and nightly. I have also remarked that the young, when it was fatigued, climbed upon the mother’s back, who brought it to land, where it caressed her.”

It is improbable that the female discharges milk into the water. Most likely she catches and crunches food for her young, and releases it into the water where they can pick it up; this would be good schooling for both swimming and diving. It is possible that Verreaux witnessed some such process, though he mistook its nature.

I have observed a young captive platypus feeding upon an aquatic plant, stripping the leaves one after another from the stem with its mouth, and using its fore-paws to assist in the process. The adult may disdain a vegetable diet; but, if any significance can be attached to this single observation, it may be that the young devour a certain amount of vegetable food, for which they can forage for themselves in the shallows. Bennett’s aborigines, who were not altogether reliable concerning what took place in the burrows, informed him that the animals ate water-weeds, and that the mother fed the young first with milk, and then with comminuted insects and molluscs (1860, p. 131).

The furred young are more vocal than the adults, and, when disturbed or hungry, indulge in a puppy-like growling, accompanied by a kiss-like popping sound. Bennett records a squeaking call, which was answered; this I have never heard. In a passage quoted on p. 160 he describes the careful toilet which they make. They habitually sleep in a curious position, sitting up upon their hind quarters, with the muzzle laid flat against the chest, and the tail drawn up over it (see Plate 28).

SEXUAL MATURITY

Little is known of the age at which the platypus reaches sexual maturity. Judging from the size which it attains in a couple of months, one would imagine that it could breed by the following season. Against this, however, is Semon’s opinion that only those two years old, or older, take part in breeding. Semon’s observations are based upon the examination of a long series of individuals; but I collected a female in “full milk,” on October 27, 1922, measuring 407 mm. (16 inches), only four inches longer than a 12-inch nestling of my collecting. To me this is rather perplexing, seeing that the nestling would grow another inch or two more before deserting the nest. And who can say that that was her first breeding-season? Therefore I cannot agree altogether with Semon’s statement.

All that can be said at present is that the female begins to breed when sixteen inches in length, and finally reaches a maximum size of eighteen inches; but there is no precise information as to the length of time involved.

LONGEVITY

The length of life of the platypus is not known. It is my intention to ring-mark some fully-furred young as opportunity offers, and it may be that we shall gain some information on this point at a later date, if these marked individuals are captured.