CHAPTER VIII. The Nesting-Burrow

THE NESTING-BURROW

The earliest statement concerning the burrowing habits of Ornithorhynchus that I have been able to find is contained in Jamison’s brief sentence (1818, p. 585):--“The female is oviparous, and lives in burrows in the ground, so that it is seldom seen either on shore or in the water.” This would imply that the colonists were familiar with the burrowing habits of the animal, and it seems strange that no account of the nesting-burrow was published prior to that of Maule in 1832.

Hill (1822, p. 623) certainly gives a garbled account, of which we have made mention before:--“On returning, however, we were gratified in finding that a female Ornithorhynchus had been brought in alive, having been found on its nest in a lagoon near Campbell’s River, by Mr. Rawley, who says that he was obliged to tear the nest to pieces before he could get the animal out, the nest being formed of reeds and rushes, with a long tube or entrance into it, out of which the bill of the animal only was visible.” On the following page Hill gives us a second piece of information, obtained from his aboriginal informant, Cookoogong:--“... the female sits a considerable time on her eggs in a nest which is always found among the reeds on the surface of the water.” It seems probable that Hill misunderstood both his informants. Rawley’s description is quite incomprehensible as it stands; that of Cookoogong has no foundation at all in fact.

In the anonymous article in the Antologia di Firenze, quoted in the Annales des Sciences Naturelles for 1827, we find on pp. 193-4:--

“Ornithorhynchus inhabits the marshes of New Holland. It makes among the tufts of reeds bordering the water a nest composed of down and interlaced roots, in which it deposits two white eggs smaller than those of ordinary fowls: it broods on them for a long time, hatches them like a bird, and only abandons them when threatened by a formidable enemy. It seems that during the whole time it eats neither seed nor herb, but contents itself with mud, taken close at hand, which serves to nourish it; at least, that is the only substance found in its stomach.”

With the exception of the reference to the mud, this account seems to be a mixture of that given by Rawley and Cookoogong, with a dash of Lesson, and seasoned to taste by the anonymous author’s imagination. It is possible, however, that the writer had access to some account which has escaped observation, and even that he had had personal experience.

The first actual description of a platypus’ nesting-burrow is that given by Lieutenant the Hon. Lauderdale Maule of the 39th Regiment (P.Z.S., 1832, pp. 145-6):--

“By the care of a soldier of the 39th Regiment who was stationed at a post on the Fish River, a mountain stream abounding with Platypi, several nests of this shy and extraordinary animal were discovered.

“The Platypus burrows in the banks of rivers, choosing generally a spot where the water is deep and sluggish, and the bank precipitous and covered with reeds or overhung by trees. Considerably beneath the level of the stream’s surface is the main entrance to a narrow passage which leads directly into the bank, bearing away from the river (at a right angle to it) and gradually rising above its highest watermark. At a distance of some few yards from the river’s edge this passage branches into two others, which, describing each a circular course to the right and left, unite again in the nest itself, which is a roomy excavation, lined with leaves and moss, and situated seldom more than twelve yards from the water, or less than two feet beneath the surface of the earth. Several of their nests were, with considerable labour and difficulty, discovered.”

Dr. George Bennett (1835, p. 248) published the first figure of a burrow--a woodcut, of which the details were not very true to the facts as we now know them. He also described the burrow, believing himself to be the first European who had opened one. As he does not give the precise date, one cannot speak with certainty; but it is most probable that Maule was before him, as it is hardly likely that Bennett, had he made such important observations during, or prior to, 1831, would have withheld them from publication. Bennett’s description in Gatherings of a Naturalist (1860), is substantially the same as in the paper of 1835. Describing the first burrow discovered, he writes, commencing on page 114:--

“The entrance or vestibule of the burrow was large, particularly when compared with the width of the passage continued from it, measuring 1 foot 3 inches in depth and 1 foot 1 inch in breadth. Instead of laying the burrow entirely open from the entrance to the termination, which would have been a laborious undertaking, holes were opened at certain distances in the direction of its course, according to the method adopted by the natives. Daraga assisted us by digging with a sharp-pointed stick; and he was able to effect his object with much greater rapidity by it than we with our spades. The burrow became narrower as it receded from the entrance, its diameter being about the usual breadth of the animal. We traced it for the distance of 10 feet 4 inches; and having just delved down upon it again, so as to perceive it still continuing its course up the bank, the beak and head of a Water-Mole were seen protruding for an instant from the upper part, as if it had been disturbed from its repose and had come down to see what we were about. It only remained for an instant; for as soon as it beheld us,--imagining, no doubt, that we could not be making such a noise there for any benevolent purpose,--it immediately turned up to take refuge in that part of the burrow which yet remained unexplored. In turning round, however, it was seized by the hind leg and dragged out. The animal appeared very much alarmed and astonished when it was hauled out of its subterranean dwelling--that is, if so paradoxical a creature could look surprised at anything....

“It was a great curiosity to the European residents in the vicinity, who, though often seeing them dead, had never before had an opportunity of observing one alive. Although they were supposed to be burrowing animals, yet I believe this was the first burrow explored, and the first living Ornithorhynchus captured by a European....

“I found by measurement that the distance of the entrance of this burrow from the water’s edge was 5 feet; it was on a moderately steep bank, abounding with long wiry grass and shrubs, among which, and concealed by them, was the opening of the subterranean dwelling. From the judgment which I have been enabled to form from the examination of this, as well as of several other burrows, I do not imagine that the natives have ever seen, or that anyone could see (except in a state of confinement), the mother in the act of suckling her offspring; for in the tedious process of digging the old animal is disturbed, and either endeavours to escape, or succeeds in escaping long before the termination of the burrow is attained. I did not observe any heaps of earth in the vicinity of the holes I examined; nor can I form any opinion how, in the process of excavation, the animal disposes of the loose mould. May we not suppose that the animal carries away the earth collected during the excavation, in order that the heap, which would otherwise be formed, may not point out the situation of its retreat? The burrow we explored ran up the bank in a serpentine course, approaching nearer to the surface of the earth towards its termination, at which part the nest is situated.

“No nest had yet been made in the termination of this burrow; for that appears to be formed about the time of bringing forth the young, and consists merely of dried grass, weeds, etc., strewed over the floor of this part of the habitation. The whole extent of the burrow, from the entrance to the termination, I found by measurement to be 20 feet. The burrows of the Ornithorhynchi are situated above the usual river height, but do not appear to be out of the reach of the extensive floods of the river which frequently take place during the winter season.”

On p. 126 of the same work there is another reference to the burrow:--

“I had previously remarked, that the situations where burrows of these animals were known to exist, had been selected by their instinct where the ponds of the river contained water even during the dry summer season, and when other parts of the river were nearly dry, or formed at best a mere trickling rivulet. Of course, where the water remained the river-weeds flourished, and the flowers now produced by them probably attracted insects, which would furnish these animals with food, in addition to the minute shell-fish which might also be found among the plants. Can they, I asked myself, confine themselves to their holes during the period of gestation? To ascertain this, two burrows were dug up, about the entrance of which tracks had been seen: one was only half completed, the animal having very probably been killed before the habitation had been finished; the second was empty, the owner having probably met with the same fate. The long grass and shrubs were very luxuriant and dense at this, the summer season of the year, rendering the exploration or even discovery of the burrows more difficult than we had before experienced; and the thick grass afforded shelter for venomous reptiles, among which black and brown snakes were numerous, rendering the process not a little dangerous.”

On pp. 130-131 another burrow is described:--

“I left Yas on the 23rd of December, and arrived at Lansdown Park, Goulburn Plains, on the 24th. On the 28th of this month, with a small party of aborigines, we visited a very beautiful part of the Wollondilly River, which passes near this estate, and which has the native name of Koroa. It was a noble sheet of water, extending to some distance, and abounding in wild ducks of various species. We then proceeded to explore the burrow of an Ornithorhynchus which had been discovered. The aborigines used their hard-pointed sticks, called kiar by them (the same name is applied to our spade in their language); and although the ground was firm, they succeeded as quickly as we could have done with our spades. The method of laying open the burrow was by holes dug at about 4 or 5 feet apart, a stick being passed up to ascertain the direction of the excavation.

“As we proceeded in exploring, there were abundant good omens to encourage us; for, besides fresh tracks of the feet of the animal, pieces of grass, weeds, etc. (such as they strew at the bottom of the termination of the burrow to form a warm nest for their young), were seen. On every indication of the presence of the inmate, the older blacks quietly passed either the earth from the under surface of the burrow bearing recent impressions of its feet or tail, or the pieces of grass, reeds, etc., to one another, for the opinion of each, and if in favour of the presence of the occupant, the digging up of the burrow was continued, the indications so well known to them giving fresh hopes and renewed vigour to the diggers. The extent to which this burrow was continued up the bank in a serpentine form was very great; and after a most laborious task in exploring it, in consequence of the hardness of the ground, the termination was attained at a distance of 35 feet from the entrance. Extensive as this may appear, burrows have been found of even 50 feet in length.

“On arriving at the extremity of this very large burrow, a growling was distinctly heard: this I at first thought proceeded from the old one, which I now believed I should have an opportunity of viewing with her young; but, on reconsideration, thinking it more probable that the old one had forsaken them (as I noticed during the course of laying open the burrow that we had not seen her come down, in the usual manner, to ascertain why we destroyed her habitation), I could not account for it, more especially when, on the termination of the burrow being laid a little more open, the fur of the animal or animals was seen. What then surprised me was, that although there was abundance of growling, there was no movement of the animals to escape. On being taken out, they were found to be full-furred young ones, coiled up asleep, and they growled exceedingly at being exposed to the light of day. There were two of them, a male and a female, of the dimensions of 10 inches from the extremity of the beak to that of the tail. They had a most beautiful, sleek and delicate appearance, and seemed never to have left the burrow. The nest, if it may be so termed, consisted of dry river-weeds, the epidermis of reeds, and small dry fibrous roots, strewed over the floor of the cavity, which was of sufficient size to contain the mother and her young.”

The only further reference to the burrow which need concern us here occurs on p. 146:--

“I have no doubt the Water-Moles make their burrows high in the banks--at such a height as to be out of the reach of the floods which occasionally prevail: if they did not adopt some plan of the kind, they would be destroyed, or drowned in their burrows by the floods; for although very amphibious in their habits, they require to repose on the dry land, and also to respire atmospheric air at short intervals of time.”

George Bennett maintained his interest to the very end of his long life. But the pioneer discoverer rarely wins through to complete and adequate knowledge of his discovery; and Bennett, owing to the fact that he adopted the aboriginal method of opening up the burrows, and did not follow them along their full length, missed several important features in the structure of the burrow.

The next description of the burrow is that of Verreaux (1848, pp. 128-9), who observed the platypus in Tasmania. His account is passably accurate. He makes no specific claim to have opened up burrows himself, but mentions (p. 131) that a Dr. Casy found two nests, one with one young one, the other with two; so it may be that his description is founded on Casy’s observation rather than on his own. However that may be, the translation of the account is as follows:--

“The Ornithorhynchus dwells by preference in marshy places, but is nevertheless not so wholly aquatic as one might suppose from its structure. It digs deep burrows, that is to say, of considerable extent, which are at most fifteen to eighteen inches below the surface of the ground. These burrows have two or three outlets, and are usually divided into twelve or fifteen branches; usually one of these outlets is placed beneath, or at the level of, the water, to facilitate retreat in case of danger. Although these burrows, which are dug in clayey banks, have a great number of passages, they ordinarily include but a single nest, placed right at the end farthest from the water, and in an enlarged space; this space seems able to hold three or four of the animals. The nest is composed of débris of reeds and other aquatic plants, and forms a bed thick enough to protect the Ornithorhynchus from the damp produced by the constantly percolating water.... It is ... an excellent digger; and I have watched one, in very gravelly and very hard ground, successfully dig a hole more than two feet deep in less than ten minutes. For the purpose of this operation, the webs of the fore-feet, which are so highly developed for swimming, undergo a curious transformation, disappear, and leave visible only the powerful claws, which are equally useful for climbing when it is a question of surmounting an obstacle. In the attitude which it assumes when burrowing, one might take this animal for a mole rather than a swimmer. I have witnessed the quickness with which they can dig in the muddy places which they prefer. The beak is first used to dig the earth, then the claws are brought into play.

“One observation worthy of note, which shows a resemblance between Ornithorhynchus and the beaver, is that, as it digs, it uses its tail to beat and consolidate the earth. During this work the animal, twisted in the shape of an auger, turns upon itself. The tail, moved by powerful muscles, follows the movement. I have observed this fact with several live individuals, which I placed in a box filled with moist earth, and have been able to study at all times.”

The account of Ornithorhynchus in Gould’s beautiful folio Mammals of Australia (1863) is based very largely upon the observations of Bennett and Verreaux, supplemented by those of the author himself. He writes, on p. 1, of the burrow as--“a retreat to which it resorts during the day or on the approach of danger.” No distinction is made between resting- and nesting-burrows; nor does Gould add anything from personal observation to the sum of knowledge of the burrowing habits.

The first precise description, with measurements, of nesting-burrows is that of Mr. G. F. Bennett (1877, pp. 161-166), son of Dr. George Bennett, who examined three burrows on Lockyer Creek, a tributary of the Bremer River, near Helidon in southern Queensland. The first one opened was twenty feet in total length, with cross-section four inches by three. The entrance is shown as just under water; five feet from it a chamber measuring twelve by eight by six inches opened off the burrow on the right side. Five feet farther on, and also on the right side, a second chamber occurred; ten feet beyond that, the nest. The nesting-chamber measured eighteen inches by ten, with a height of eight inches; in the nest, which was composed of dried grass, reeds, and gum-leaves--all, from their blackened appearance, evidently collected under water--were two young ones, estimated to be a month old. This nest was opened on 27 October, 1876.

Almost a month later a second burrow was found, eleven feet in length, with two offset chambers, one on each side, and with a nesting-chamber excavated but containing no nest. A third contained three offset chambers, and two young in a nest, which was four feet underground--a very unusual depth--and twenty-two feet above the level of the entrance.

Caldwell has nothing to say about the burrows. His only reference (1887, p. 466) is as follows:--

“In September my friend Bloxsome superintended the transfer of the camp to the colder river Mole, further south, where we hoped to dig out the later stages of Ornithorhynchus from their nests. I employed some white navvies, who opened up a large number of burrows, but the renewed exposure in Queensland had brought on my fever again, and this seriously interfered with the completion of the Ornithorhynchus series.

“The later stages of Monotreme development have, therefore, to be worked out, mainly with Echidna material.”

The last sentence might imply that some later platypus material was obtained; but, if this were the case, it is remarkable that Caldwell has made no mention of it. His remarks are so non-committal as to justify a suspicion that he got neither eggs nor young from the burrow.

Semon (1899, p. 42) describes the burrows thus:--

“These burrows have one entrance above and one below the water, uniting in a tube of 20 to 50 feet, which leads slantingly upwards from the river-surface and ends in a little cave. I have never found burrows with a greater number of tubes labyrinthically entwined, and believe that their existence may be regarded as exceptional.”

Semon here repeats the popular belief that there are two entrances, one below, and one above the level of the water. This does happen occasionally, and the reason will be explained later; but it is the exception rather than the rule.

Lucas and Le Souef (1909, p. 145) allow the burrow one paragraph:--

“The home of the Platypus is a long narrow tunnel-like burrow, which begins with an entrance under the surface of the water, and then runs obliquely upwards in the bank for a distance of 20-50 feet. It eventually terminates in a rounded chamber or living apartment, lined with grass and leaves, and situated not very far below the surface of the ground. Very often the main burrow gives off at intervals lateral branches also terminating in chambers. Sometimes the burrow is provided with an additional opening above the level of the water, though this is certainly not always present.”

Despite its brevity, this paragraph contains some minor errors. The burrow does not begin with an entrance below water-level; the nesting-burrow is not a ‘home;’ and the lateral chambers (pug-pits) are excavated in the side walls of the main tunnel, and are not connected with it by branches.

1. Asleep; 2. Awake; 3. Away.

Plate 17]

To Kershaw (1912, pp. 102-105) must be given the credit of the first adequate account of the nesting-burrow, several examples of which he carefully examined in the banks of the Hopkins River near Mortlake in Victoria. He writes:--

“The river, at the spot visited, is a fairly broad, quiet stream, winding about through an extensive grassy flat, margined and shaded with large old red-gum trees. The banks in places are steep; in others they slope rapidly to the water’s edge. At the time of my visit the river was low, but when in flood it rises fully ten or twelve feet.

“After a careful search along the banks several burrows were located, places where the soil is easily worked being usually selected. The entrances to the burrows, when once seen, may always be easily recognised. In every instance these were situated some feet above the level of the water, varying from four to fully twelve feet above the present level. In no case did the burrows open out below the water, though during heavy floods several of those seen would certainly be covered. In nearly every instance there was an opening to the surface higher up the bank, and in one case a double opening was found in the same burrow--one immediately beyond the other--the narrow ridge of earth separating the two holes being worn smooth, as though the animal, when passing up, came out of one and immediately entered the other. These surface-holes are probably for use during flood times, when the entrance is under water.

“Along the steep, muddy bank, close to the water’s edge, the tracks made by the animals when passing to and from the water through the grass tussocks were easily recognised, being worn flat and smooth by the animal’s wet bodies. Claw marks could also be seen here and there. Such places where the projecting roots of the gum-trees run into the water are, apparently, most favoured for landing.

“The entrances to the burrows varied from four to five inches wide and three inches high. They are always arched above and quite flat below, following the shape of the animal’s body. This form is followed throughout the whole length of the burrow, though the latter may be enlarged here and there. The entrance is occasionally blocked up with earth.

“The burrows do not follow a direct line, but wind about, at times very considerably. They always follow up the slope of the bank, parallel to and rarely more than a foot below the surface. In no instance was one found deeper than fifteen inches, while here and there they approach within a few inches of the surface. A short branch, from one to three feet long, is frequently made on either side of the main burrow; in one instance four were seen--two on either side. These end abruptly, without any apparent cause. Although in the same class of soil, the length of the burrows varies, the shortest examined being ten feet, and the longest, carefully measured, thirty-five feet. While usually terminating in the nest-chamber, the burrow is occasionally continued beyond the nest for a foot or two.

“A peculiarity which, I believe, has not been noted before is that when the burrow is occupied it is completely blocked with earth for about a foot. This occurred in three places in one burrow, and is apt to lead one to conclude that the end has been reached. From this it is also assumed that the female does not leave the nest for some time after the eggs are laid or the helpless young are hatched, and during the time the latter are attached to the mother. The object in so securely blocking the burrow is, probably, for protection against flood water in the event of a sudden rise of the river, or from possible enemies. Unless the block is removed from time to time, however, it is difficult to conceive how the animal obtains sufficient air during her lengthened seclusion.

“The nest-chamber is rounded in form, the bottom always being much lower than the floor of the burrow, which enters about its centre. All those examined were of about the same dimensions, the two carefully measured being each twelve inches deep, thirteen inches wide, and eleven inches from the entrance.

“The nest itself is composed of fine grass and gum-leaves, and completely fills the cavity. The bottom of the chamber is first covered with a thin layer of grass, followed by a thick layer of gum-leaves about four inches deep, which is continued up the sides, and--at least in one case--completely encircles the cavity, forming a compact circular nest, with the entrance in the side opposite the tunnel. It was noticeable that many of the leaves were green, and appeared to be quite fresh. The bottom of the chamber in every instance was wet, owing to surface soakage, and it is apparently with the object of avoiding this that the bottom of the chamber is excavated below the level of the burrow and so thickly lined with leaves.

“The first burrow examined had been opened up by Mr. Hood on the 17th October (nine days prior to my visit), and from the nest two fresh eggs were taken. These, when found, were attached to one another, and had been kept in a small box; but in two or three days they collapsed. When I received them on the 26th October they were flattened and dry. Several other burrows close by, though having the appearance of recent occupation, were untenanted, but each contained a nest, more or less complete. From one of these the shrivelled remains of two eggs were obtained.

“Later in the day a burrow was discovered which had every appearance of being occupied. It was situated on the sloping side of the bank, fully twenty-five feet from the water, and was the only one I saw with recently scratched earth at the entrance. The burrow followed up the slope of the bank for about twenty-five feet, terminating in a nest-chamber, some ten or twelve feet above the level of the river. This one was blocked up in three separate places, the last within three feet of the nest. When about a foot from the nest a peculiar grating growl was heard, and the next shovelful of earth exposed the side of the very compact structure, through which the old one hurriedly forced her way. She was permitted to traverse the excavation for a couple of feet to a depression, where she was covered with clods of earth, while a careful examination of the nest was made. This proved to be the most complete of all those examined, the whole cavity being completely filled with leaves. No opening into the interior of the nest could be seen, the entrance being evidently closed by the animal during occupation. As was the case in all those examined, it was composed of gum-leaves, with a small layer of grass at the bottom of the chamber. The lower portion was tightly compressed from the weight of the animal, while those forming the sides and top were loosely interwoven. The interior measured six inches across and four and a half inches deep. The shrunken remains of one egg were found in the nest.”

With the exception of the blocking of entrances, and two or three other minor matters, I can confirm Kershaw’s statements after a long term of practical experience in the field. What this has involved in labour alone may best be judged from the remarks of Professors Wilson and Hill (1908), who, like myself, spent a number of years getting together material for embryological study. They write:--

“The records of Australian biology bear sufficient witness to the fact that it is extremely difficult, even for residents in Australia, to procure the material necessary for an investigation into the development of the eggs of Ornithorhynchus. It is only through the organisation of special expeditions, with ample resources both of time and money, that any large measure of success in this direction can be hoped for.

“The animal itself, though pretty widely distributed, and probably still far from becoming extinct, is to be found, in any one locality, only in comparatively small numbers. It is now much less plentiful than formerly, owing to the demand for its fur--a demand which is still satisfied in spite of the measure of legal protection which the animal has obtained in the various Australian states. The depredations of the fur-hunter are not easily repaired, since the animal breeds only once in the year and produces but two eggs at a time.

“The eggs, when laid, are deposited in a burrow which it is far from easy to locate, and whose opening up involves a considerable amount of labour, since, apart from its great length, the river-bank in which it is situated is commonly enough permeated by tree-roots. And when at length the actual dwelling chamber or nest is successfully opened up, no reward at all may be forthcoming, or the material which is obtained may be unsuitable for the immediate purpose in view....

The earth has been removed, leaving cavity and nest undisturbed.

Plate 18]

“We have only to a very limited extent been able to superintend personally the work of collection, and have found it necessary to rely largely upon the efforts of the scientifically untrained collector in the accumulation of the material for our investigations. Unfortunately, the material result of the work of such a collector during an entire season is so meagre that he requires considerable inducement to carry on the work at all; whilst the product to the investigator may be practically nil. The genuineness of the difficulty in procuring this kind of material is sufficiently indicated by the fact that whilst Semon, in his expedition, was successful in obtaining a very considerable number of eggs and foetal specimens of Echidna, his collection of Ornithorhynchus and embryos would appear to have been limited to a small number of early intra-uterine eggs.

“Our own collection is very far indeed from being complete, though the work of collection has been going on for quite a number of years.”

Plate 18]

Semon (1894, p. 13) tells a similar story:--

“Concerning the development of the eggs after being laid and concerning the young after hatching I can unfortunately make no statements. I had not the good fortune even to find eggs or young in the numerous burrows which I and my white companions opened. My blacks showed the greatest dislike for this work, and were averse to troubling about Ornithorhynchus at all: we ourselves were too absorbed with the several other works in hand ... to be able to give our whole time and zeal to the wearisome and for the most part disappointing work of digging out Ornithorhynchus nests.”

The two great expeditions of Caldwell and Semon failed to secure eggs and young of the platypus from the nest. Wilson and Hill, after many years of endeavour, obtained only a very limited number, most of which have not yet been described. Since the eggs and young, which cannot escape from the nest, are so difficult to get, it may well be imagined that the doings of the elusive female, hidden in the darkness of her burrow, are infinitely more difficult to observe.

The interest attaching to precise observations of the actual burrowing method is great, the chief point being to determine the relative extent to which the highly sensitive muzzle and the powerful claws of the fore-paws are used in the process. I therefore experimented with a single female platypus which I had in captivity at my home.

The creature was placed in a specially contrived enclosure, of which the essential features were a plate-glass front and a wooden back, four inches apart at the bottom, widening to six inches at the top. This enclosure was filled with sifted soil, which was put in a bucketful at a time, each bucket of soil being followed by a bucket of water, until the frame was full, when the whole was tamped and thoroughly flooded with water. The object of the taper was to cause the soil to wedge, and so prevent it from collapsing on the burrowing animal. Water was used to consolidate the soil to something like the consistency of the banks in which the platypus burrows naturally in the wild state.

The enclosure thus prepared was allowed to stand for twenty-four hours, and the animal was then introduced at one side, where a portion of the earth was removed to make room for it, at 2 p.m. No attempt was made to burrow until about 5.30 p.m.; but I am not certain whether this time bears any relation to what normally happens. The observations which follow were made during a period of about an hour, by means of the plate-glass front, and during this time I kept myself concealed as far as possible from the animal.

The platypus, in order to obtain the greatest purchase before beginning to burrow, tucked the tapering end of its pliable tail between its hind legs and simultaneously hooked its out-turned hind-claws into the earth at each side. While in this crouching attitude, with stiffened top lip and splayed fore-claws it proceeded smartly to break away the earth. After burrowing for several inches it rested awhile; then it energetically contorted its neck and body so as to tamp the freshly-loosened earth tightly into the hollow surrounding it.

While stationary, the creature occasionally beat the walls with its trowel-like tail; but whether this action was deliberately intended as part of the tamping process, or was due to muscular reaction as the result of the strenuous exertion, or both, I do not profess to know. Nevertheless, it certainly did not distort or tend to break away the true design of the burrow by its spasmodic action. In fact, the structure and shape of the tail suggest that it is designed as an actual modelling-tool.

After a few minutes’ rest, during which the breathing was laboured and was accompanied by an apparently involuntary gulping, the animal again deliberately shovelled the earth over its head with the end of its snout. In this way it created a crude cavity in which to loll its head to one side, while, with neck shortened, it reached to the utmost with one web-palmed paw, and scratched a hemispherical hollow to one side of the tunnel, contorting its shoulders the while. The energetic digger then placed its head in the recess so made, and, without any hesitation, performed a similar operation on the opposite side with the other splayed paw. Then, to dislodge the partition separating the two recesses, it probed its muzzle vigorously into it, and then proceeded as far as the out-stretched neck permitted, to form another “loll-hole” to one side with its muzzle, in readiness for its head in burrowing a further section. Meanwhile, the well-worked earth trickled round the wriggling creature’s body, principally about the powerful shoulders, whence a portion eventually filtered down as far as the hips.

While the fore-paws and rooting muzzle were working at high tension, the hind legs were alternately keeping the specialized fore-parts well up to their work by clinging tenaciously to the solid earth. In their struggle to do this, the hind-claws actually aided the process of excavation by cutting the lateral angles which complete the arched design of the burrow.

A remarkable feature of the burrow is that no soil is ejected, but the whole of it is reduced to a consistency suitable for packing into the sides of the burrow, and even the soil dislodged when commencing a burrow is consolidated into the river-bank. When the platypus has burrowed about six inches, it packs the loose soil by contorting its body and pressing against the sides, at the same time beating the floor with its tail. The width of the excavation is about double that of the finished burrow, that is to say, the earth excavated is tamped into about half its original bulk. The whole of the earth taken from each section is not always beaten in before a new section is commenced; but the animal appears to work very systematically, and, according to the consistency of the soil, it moves backward from time to time and adds the finishing touches. In the experiment the captive repeated the burrowing and tamping alternately, until it eventually broke through the surface.

The powerful claws of the fore-paws do practically all the burrowing, the only help they receive being a probing or lateral shearing movement of the muzzle (when the earth is not too hard) and the pressure exerted by the hind feet. Although apparently preferring to burrow in the normal position, the platypus will readily lie on its back, or on either side, and at times will work in a spiral fashion, either while excavating or while tamping the tunnel, even in the absence of any serious obstacle; but eventually it reverts to the normal position for the purpose of finally shaping the burrow. The position in which it works does not affect the rate of its progress in the least. When confronted with temporary obstacles, such as impoverished or caked earth between the roots of trees, etc., the platypus will, miner-like, in order to avoid retracing its steps or branching off in another direction, throw itself into the required position and follow, not the line of least resistance, but of favourable soil.

The thickness of the pug or pugs tamped into position with the tail by the female prior to retiring to the nest for the purpose of laying and incubating the eggs, is about six inches, and it is packed so closely as to be indistinguishable from the surrounding soil.

Note rocky obstruction above nesting cavities. The cavity nearer to the entrance has been abandoned.

Plate 19]

During the process of dragging back and packing into the sides of the burrow the soil scratched out by the fore-paws and muzzle, the platypus exhibits its remarkable ability to reverse the action of its hind legs, and actually “advances” backwards. Students of anatomy will have observed, no doubt, that the hind feet of a platypus have a tendency to turn outwards from the flanks, and that the grooved, curved claws continue in that trend towards the tail. This is as it should be for the purpose of gripping and scratching back surplus soil while tunnelling ahead. It is also as it should be when reversing. Especially is this so when burdened with a packing of adhesive pug, and when, incidentally, the fore-paws are practically out of action. The muscular contortions of the body at that time certainly afford great assistance. Nevertheless the hind quarters and cumbersome tail would during one of its essential functions become useless, if not a burden, to the platypus, but for the natural provision of a “two-way” action of the hind limbs. Preparatory to pulling backwards, the trend of the hind feet continues until the sets of claws are facing one another beneath the tail. This contortion enables the animal to take a firm grip of the flattened earth-floor; it then pulls the body backwards step by step, while the fore-paws, when freed, do the shoving. The alternate action of the fore-legs in walking forward is imitated exactly by the hind legs in the reverse direction, and the platypus can thus, at will, reverse the direction of its movements. It is rather amusing to witness this act, for, at the outset, the fore-parts are usually obliterated with earth, and the tail, which in contour and elevation somewhat resembles the head, sometimes puts one at a loss to guess whether the creature is really coming or going.

In addition to the uses of the hind-claws and to the reversible nature of the entire limb, the hind foot of the platypus, especially of the male, is just as versatile in its actions as that of a chimpanzee, even to the power of deliberately gripping. During a backward march it resembles, in action and general appearance, the fore-limb of a grizzly bear, or, more nearly still, that of a long-clawed sloth. Is it possible that this versatility is reptilian in origin? What other mammal in the world is able thus to interchange the functions of its hind- and fore-legs to such advantage? Such ability may be characteristic of moles, etc., but it is certainly a remarkable adaptation.

I have already mentioned the stiffening of the top lip, and, when one realizes its extreme limpness when not employed in burrowing, it will be obvious that stiffening is necessary in order that a groove may be made in earth by lateral oscillation (“lipped-in,” so to speak). While this groove is being formed, and until it is sufficiently deep for the bony prong of the upper jaw to be used for shovelling the earth away, the direction of the stiffened lip is maintained by the earth above and below it.

Given favourable conditions, a platypus can excavate a cavity of approximately 6 × 5 × 5 inches in five minutes. It can tamp loosened earth completely into a six-inch section of a tunnel in fifteen minutes. This makes twenty minutes in all to a shift. Therefore, if the tunneller continued unceasingly in this manner, in sixteen hours it would have completed a tunnel twenty-four feet in length. This is the average length of a breeding-burrow.

Quite possibly the spasmodic gulping observed in the throat of a burrowing platypus is due to involuntary exhalations of spent air, corresponding to the periodical bubbling when the creature blows beneath water. Under open conditions, normal breathing may be reasonably governed at will, but this exhaust spasm appears to be quite beyond control. This is probably so that it may collect its food unhampered beneath water, and that it may also delve freely while tunnelling, without having to relinquish such immediate business for the purpose of replenishing spent air. The breathing observed while the platypus was tunnelling was at the rate of thirty respirations per minute, and the gulps in the throat approximately at intervals of twenty-five seconds. The normal breathing of a sleeping platypus I have found to be fifteen respirations per minute.

During the experiment the animal apparently kept the facial furrows, embracing the orifices of the eyes and ears, tightly closed, as when swimming under water.

Accounts of the bionomics of the platypus in current works on natural history invariably assume that the nesting-burrow is the home of the animal, in the same sense in which a rabbit-warren is the home of the colony that it contains. As has already been stated, this is not so, each burrow being inhabited by a single female, and that only during the nursing period. The usual statement that the burrow has two entrances, one under water and one above, is also erroneous. It is true that such a condition often exists; but this merely indicates that a rise in the river has covered the original entrance, which is always put in above water-level, and that a new entrance has been broken out, or that the secondary under-water “getaway” has been formed later by a washaway at the root of an adjacent tree.

From model by H. Burrell.

Plate 20]

The female excavates her burrow unaided. She selects a suitable site, preferably in a high sloping bank, well held together by the roots of trees; it is all the better if a fringe of reeds adorns its edge, so that she may come and go as safe from observation as possible. The entrance is made above water, at a height varying from nearly water-level to as much as twelve feet. When fresh, it has a definite shape--low-arched above and flat below--and measures from four to six inches in width, and from three to four inches in height, corresponding pretty accurately to the cross-section of the occupant. On account of this definite shape, it is easily distinguishable from the burrows of water-rats (Hydromys) and rabbits found in similar localities. But after a time this entrance may become adventitiously enlarged and altered in shape, so that the distinction is not always completely maintained. Bennett suggests that the entrance is usually concealed among the vegetation of the bank; but, though this may happen where banks are covered with a lush vegetation, my observations agree with those of Kershaw that there is no attempt actually to ‘camouflage’ the entrance, even though ample vegetation may be found growing close by. The entrance is usually quite obvious, and, in any case, attention directed to it by the runs of the animal, worn smooth and hard by the dragging of its wet body over them. Upon these runs the impress of the tail is usually to be seen, and the pricked tracks made by the claws of the hind feet are unmistakable.

The burrows exhibit an infinite variety of form and length, but all show the arched roof and flat floor adapted to the size of the female which has constructed them. This size may therefore be gauged with reasonable accuracy during the process of digging the creature out. Here and there, however, there may be, in the calibre of the tunnel, some irregularity apparently due to the adoption of an unusual position during digging in order to deal with some particular obstruction. The tunnel usually follows a sinuous course, but invariably keeps at about the same distance (from 12 to 18 inches) below the surface. This power of the platypus to judge how far it is below the surface is as remarkable as that sense which warns it when it is approaching another burrow, whether of its own kind, or of some other animal. In constructing its tunnel, it sometimes circles round, so that, if it continued on its course, it would break into an earlier section of its own burrow. This, however, never happens. In one case it was observed that the platypus stopped a foot from this earlier section, and forthwith constructed its nesting-chamber. In a number of others, it avoided breaking into its own burrow by passing under it at a distance of a foot, coming up on the other side to the usual distance below the ground-level (see page 127 and Plate 20). In the same way it avoids the burrows of water-rats and rabbits by tunnelling clear of them. I have known it go down to the very unusual depth of four feet to avoid a rabbit-burrow. In some cases, instead of going under an obstructing burrow, it turns aside, and continues its tunnel in a different direction. One more example of this uncanny sense may be quoted. Plate 19 illustrates a model of a burrow made to scale. In this the nesting-cavity is shown at a vertical depth of about three feet. The animal burrowed in at the usual level below the bank where it sloped, until it came to a place where the bank suddenly rose steeply. Here two alternatives were open to it. Either it must go straight upwards through rocky soil for more than two feet, or it could construct its nest where it found itself, three feet below the level of the upper bank, but still the normal distance from the lower face. It chose the latter alternative.

1. Couch grass stems, and a few gum-leaves; 2. Willow “swishes,” willow rootlets, and reed flags; 3. River reed roots, entirely; 4. Couch grass stems, and gum-leaves; 5. Fuzzed reed stalks, and reed flags.

Plate 21]

A great amount of discrimination is necessarily displayed in selecting a suitable soil in which to burrow. Burrows commenced in soil that ultimately proves unsuitable because of its friable nature, are--as often--promptly deserted. Although a preference for soft soil is indicated, I noticed that in one burrow which I investigated the animal had removed three inches of compacted river-gravel from the face of the bank before reaching the ideal earth behind, which she evidently knew was to be found by perseverance. Sandy banks, unless reinforced with lengthy lucerne or similar roots, are left severely alone, since the burrows tend so easily to collapse.

No general statement can be made as to the length and direction of the nesting-burrows, since both are infinitely variable. The length varies from five feet to upwards of sixty in one season’s burrowing; but all the short lengths recorded are probably abnormal, and due to exigencies brought about by flood-conditions. From fifteen to twenty feet would appear to be a fairly adequate length, though that is under the average, which is raised by a number of extraordinary length.

What it is that impels the animal sometimes to go on burrowing up to the amazing distance of 100 feet, I will here explain. These abnormalities occur only in ground that has been occupied undisturbed by platypus for many years. Indirectly, floods are responsible for the trouble that compels platypus to enlarge and renovate. These shy and timid creatures naturally do not wish to make their ideal breeding-bank more conspicuous to their enemies than is necessary; they therefore prefer to use the same entrance (when convenient) year after year, rather than perforate the face of the bank with tell-tale porches for which there is no necessity. Leave well alone, is their motto. Now suppose that a platypus puts in a burrow in any one season, and that it is, at some later time, covered by flood waters. Being in soft soil, it soon becomes silted up--at least, here and there. The following year, the same or even another female enters the porch and traverses the tunnel, as far as it is habitable, to investigate. If she approves of it up to a certain distance, she will cut a pit in the side wall there and then, and plug up the section which is not to her liking; then she will tunnel anew in another direction to the average distance, as if commencing from the original porch. This may go on year after year, with flood after flood, until the enormous length of 100 feet is recorded by the collector as the longest breeding-burrow discovered. Peter Yates, my able assistant, can vouch for that fact.

The nesting-chamber is generally placed at the extreme end of the burrow, though sometimes a blind branch is carried on beyond it. It varies somewhat in size, but is usually more wide than high, and measures on an average about twelve inches by eleven. In this chamber a nest is constructed, a variety of materials being used, the nature of which depends upon the locality. In the New England district the commonest type of nest is composed almost entirely of eucalyptus leaves and the rhizomes of couch grass, but I have also found willow “swishes,” roots, branchlets, and leaves; roots and stems of the reed Arundo phragmites; and the leafless branches of she-oaks (Casuarina spp.). These materials are arranged as a lining of the cavity, except for the opening where the burrow enters, and are of even thickness, except on the floor, where the lining is distinctly thicker. I have been informed by an aboriginal that he has seen a male carrying nesting-material by means of his spurs, but am not disposed to believe it, although I am quite sure he could do so. All my own observations suggest that the female alone digs the burrow and makes the nest. They also show that the amount of nesting-material never more than half fills the cavity until the babies begin to grow and toss the litter about. For instance, where triplets are concerned one is apt to find the nest-cavity crammed to the dome with such material.

In all the years during which I have been making observations I have never once actually seen the female in the act of carrying in nesting-material. It seems likely, therefore, that nest-building is done at night. The materials used are those closest to hand. As the foundation of one nest I found a willow “swish” more than five feet in length. It would surely be awkward to drag such a thing to the end of a long burrow. It seems likely that the animal collects a great deal of material that is lying loose upon the ground, or even in the water. But there is no doubt that it is also able to cut off reeds and grass rhizomes by means of the elongate horny ridges in the anterior part of the mouth. These ridges are also used to shred the stems of reeds, reducing them to a mass of soft fibres (see Plate 3).

As soon as the nest is completed, and before the eggs are laid, the most remarkable habit of all comes into evidence--that of plugging the burrow. G. F. Bennett (1877) was the first to observe the lateral pits from which the earth to form these plugs is drawn; but he did not observe the plugs, and therefore missed the import of the pits. Kershaw (1912, p. 103) saw the plugs, but did not connect them with the lateral pits. He writes:--

“A peculiarity which, I believe, has not been noted before is that when the burrow is occupied it is completely blocked with earth for about a foot. This occurred in three places in one burrow, and is apt to lead one to conclude that the end has been reached. From this it is also assumed that the female does not leave the nest for some time after the eggs are laid or the helpless young are hatched, and during the time the latter are attached to the mother. The object in so securely blocking the burrow is, probably, for protection against flood water in the event of a sudden rise of the river, or from possible enemies. Unless the block is removed from time to time, however, it is difficult to conceive how the animal obtains sufficient air during her lengthened seclusion.”

The mining term “pug” seems appropriate to the earth composing these plugs, and “pug-pits” to the cavities from which it is derived. I therefore propose to use these terms. What apparently happens is this. When the female has completed the nest to her satisfaction, she goes to the entrance of the burrow, and, turning, re-enters it. At a varying distance from the entrance she excavates a chamber opening off the side of the tunnel and compacts the earth so obtained into a pug, which is always placed close to the first pug-pit, and between it and the entrance. Proceeding towards the nest, she excavates a second pug-pit in the same way, compacting the pug behind her, and so on, without having to turn once from beginning to end. I have found from two to nine of these pugs in different burrows, the usual number being three. The pits would seem to occur indifferently on either side of the burrow, but I have noticed that the latter usually changes direction immediately after a pug. This device has nothing to do with preventing the entry of ordinary flood waters. The nest is almost always placed at a sufficient height above water-level to escape these, and the platypus can easily break out a new entrance anywhere it pleases. It is obviously a cunning device, partly for protection from enemies, but principally for securing ideal brooding conditions. It has doubtless baffled many investigators, human and otherwise. With all my experience, it has frequently taken me some time, after having arrived at a pug, to pick up the further course of a burrow. Consequently it is easy to imagine that a less experienced observer would be entirely misled, believing he had come to the end of an untenanted burrow, and thus retiring in disgust. This probably accounts for the long time it has taken to ascertain fully the animal’s nesting habits. A final pug is always placed near the entrance to the nesting-chamber.

Note bare tip of tail, due to pugging off unnecessary openings in camping burrow.

Plate 22]

Note position of paw when bearing weight.

Plate 22]

Now to explain how the labyrinths mentioned by Verreaux are made. Very much the same thing happens as that described above, except that more breeders make use of the bank at the same time, and, in their endeavours not to clash with one another, they take most erratic courses to avoid neighbouring burrows. Admittedly they do not see these obstacles, any more than Verreaux did. Nevertheless they are quite aware of their proximity, and act accordingly. Hence the maze as seen and wondered at by man.

Another point is that, where an objectionable section of a burrow has been pugged off in the past, the pug, through soakage during floods, becomes solidified like the surrounding earth. When the burrow is followed by man later to that particular partition, this is generally looked upon as a blind or dead end; yet possibly, a foot or two farther on (not necessarily in a direct line), a so-called “beginningless” subway may be found further to bewilder the weary mattock-wielder. Again, every time a flood occurs, fresh strata of silt and débris are left lying on the surface of the bank after the water recedes, and thereon grass and herbage readily take root, thus increasing the depth of earth above the ancient tunnels. These, when hit upon, appear to the student as extraordinarily deep for platypus to work, in comparison to the depths previously described by the more fortunate collector who accidentally happened upon a positively new burrow, which, throughout its entire length, did not lie more than fifteen inches below the surface.

My experience goes to show that the female does not leave the nest between the times when the eggs are laid and the young are able to suck, and that she digs through and replaces the pugs whenever she leaves the nest or returns to it. The constant removal and working of the soil of which the pugs are built makes it so soft and fine that it sometimes becomes quite velvety to the touch, and clings like flour if squeezed in the hand. It is always slightly damp after being worked by the platypus; but on one occasion when I took a nest containing large triplets without their mother, the pug near the nest had dried and shrunk away from the sides of the tunnel so much that I was able to remove it in a mass, though it fell to powder in my hands when slightly squeezed. Possibly, that absent female had met her death in some way, for I am positive that it would require some days for the pug to dry and shrink to such an extent. However, the young, which I presented later to the Australian Museum, appeared none the worse for their term of imprisonment without food.

How does the animal make and replace the pug? Wood Jones states (p. 48):--“... the form of the tail resembles that seen in the Beaver ... It has nothing whatever to do (either in the Platypus or the Beaver) with any supposed habit of puddling clay....” After much experience of observing the habits of platypus in their wild state, I succeeded in 1925 in proving the accuracy of my own notions on this subject. While collecting on the Namoi River for the proposed National Museum which is to be erected at Canberra, I unearthed a female platypus on her nest, which contained twin young. While I was registering the temperature of the nesting-cavity, the mother made off from the nest along an offset tunnel. Knowing that I could secure her later, I leisurely finished my duties with the young and carefully extracted the nest (whole). About fifteen minutes later my assistant started in pursuit of the mother. To our surprise, the offset, which usually measures only a few feet in length, on this occasion measured about seven. After following along for about three feet we discovered a pug sealing the runway. I carefully examined the hurriedly-constructed barrier, and then followed closely along the workings as my assistant tactfully broke the earth away. Presently another pile of pug met my gaze, and beyond it the platypus could be clearly seen up against solid earth scratching for her life, and at the same time deliberately shoving the refined earth behind her with the dorsal end of her tail to pug herself in. Of course, this tunnel was there prior to her leaving the nest, but was evidently pugged off at the first section from the nest before she retired to perform her maternal duties. This pug, then, she had burrowed through and built again behind her, and she had almost succeeded in excavating sufficient solid earth at the end of the runway to pug herself in with the idea of outwitting us.

During an exceptionally wet season the earth, in certain situations, becomes sodden, like clay. At such times the pugging in the tunnels is found to be a conglomerate mass of marble-sized mud balls, more closely compacted than shot in a gun-shell. This confirms my statement that the nesting-cavity is sealed up before incubation begins, and that in building the pug, the tail is used for rolling the earth along the runway into position. Hence the baldness of that trowel-like member. It proves also that, wet or dry, pugging is worked methodically into position a portion at a time.

The hair on the dorsal surface of the tail of the female becomes increasingly abraded during the nursing period, so that by the time the young are in fur she usually exhibits a bald patch of some size. So much is this the case that I have several times accurately guessed the size of the young from the degree of baldness of the mother’s tail. Now this particular hair is exceedingly coarse and harsh, and could only be worn away by some such drastic use as in the tamping of the pugs, which must be removed and replaced so many times during the nursing period. Wood Jones’ suggestion that the flat tail is used to warn neighbours by continuous smacking of the surface of the water is certainly not supported by my observations of the living animal.

The thickness of the pugs varies from three inches to a foot; ordinarily it is from six to eight inches. I have found pug-material beneath the nest itself to a depth of several inches; the animal must therefore dig up and work the earth on the floor of the cavity before she begins to build the actual nest. This is probably a provision for the more rapid carrying away of any water that may reach the nest by seepage; but, on the other hand, it may serve to hold moisture, and thus maintain the damp atmosphere which seems to be necessary for successful incubation.

The platypus would appear to dig a new burrow for each breeding-season, and it certainly does so after being disturbed. Occasionally an old burrow may be refurnished and remodelled, and one main entrance used for several seasons. On two occasions only have I found more than one nest in a burrow; and a diagram of one of these burrows is given here. It contained, in different positions, no fewer than four nests, three of which were in various stages of dilapidation; the fourth, though scanty in material, contained twins. Mr. Hoy examined these four nests with me, and agreed that they bore the appearance of having been built at different times, probably at yearly intervals. The bank in which this burrow was placed was unusually faulty for the purpose, having a sandy subsoil. It was on a beautiful stretch of water about three miles below the New England waterfalls, where the river is in places actually bridged by rocks, so that when it is at a low level, as in periods of drought, the river becomes a chain of ponds, and the platypus is prevented from travelling far. Possibly, therefore, the animal or animals which inhabited the four nests successively had become river-locked, so to speak, and compelled to make shift in the only available quarters. We dug out three other unoccupied burrows in the same bank, each of which had evidently been abandoned on account of the surface sand collapsing while the nesting-chamber was being excavated. The platypus was probably compelled to shift along the bank until, in the only part which could be used for its purpose, it found the old burrow, and so contented itself with burrowing on past the vacant nests.

A second example was opened up on the Macdonald River on September 28, 1920. After a rainy spring the river was eighteen inches above its normal level. The original entrance to the burrow was then twelve inches below water, but a new entrance had been broken out seven inches above. The main tunnel wound gradually up the sloping bank for 36 feet, and then divided into two, one arm, ten feet long, going to the first nest--which contained a female and twin eggs--the second branching off to the left to another nest, which, at fifteen feet from the junction, contained a female and a single egg. The two nests were eleven feet apart in a direct line. The season of 1920 was abnormal in rainfall; but, even so, it was remarkable to find two females occupying self-contained flats in one and the same burrow. The accident that the females and their eggs were duly preserved in the interests of science probably saved them from considerable complications, which might have ensued if they had been allowed to bring out their young.

1 and 3. Eggs, new-laid; 2. Week-old young and egg capsules; 4. Relaxed egg capsules (triplets and twins), showing adhesion, also rents made by young in hatching; 5. Young, about fourteen days old; 6. (above) Dried-up egg, (below) single, twin, and triplet collapsed capsules; 7 and 9. Eggs, partly incubated; 8. Day-old young, and capsules from which hatched.

(About two-thirds natural size)

Plate 23]

It is of some interest to note in passing that on the previous day I had suffered a defeat only fifty feet away from this burrow. Opening up a burrow, I came to a pug. When this was opened it broke into a rabbit-burrow, and was discarded as a ‘duffer.’ The following morning, on examining the excavation again, I found a freshly-dug platypus-shaped hole in the side of the trench, and, following up this clue, came upon a finished but empty nest, its tenant having deserted it during the night. Such a congestion of tenanted burrows is, however, quite unusual, and in this case was due primarily to the fact that the ordinary conditions of the season had been considerably upset by freshes in the river. I have since found several platypus breeding in the same bank side by side, but not from choice nor because of a neighbourly disposition. Scarcity of suitable soil is the main reason for overcrowding, but rabbits and even the European rat are invading these ideal water frontages and are ousting Ornithorhynchus from his birthright. Otherwise, as I have explained before, breeding platypus prefer “solitary confinement.”

To describe a number of burrows in detail would not serve any good purpose, for they are too variable to be sorted into types, and each seems to be constructed according to the vagaries of its builder. One burrow which I examined at Manilla on 22 September 1920 had an entrance four feet above water-level in an almost perpendicular bank. It was opened up and followed for a distance of twenty feet. and came to an end just as it was tending back towards the river. The nesting-chamber was found five feet from the end, but was placed in a very unusual situation, twelve inches above, and just to one side of, the tunnel, so that it was approached by a short lead from below. As this passage was securely pugged, the nest was extremely difficult to locate and might easily have been overlooked. However, since then. I have found a similar burrow in New England, thus proving that the first find was not unique.

The platypus invariably begins its burrow above water-level, and the presence of other entrances, whether above or below water, is due to variations of that level. I at first thought that some of these additional entrances might be of the nature of air-shafts, but found that the pugging of the burrow would prevent their being of any use for such a purpose. Kershaw, too, wonders how the animal and its young obtain sufficient air-supply, plugged in as they are in an almost air-tight chamber. The resting-burrows are often provided with two or more adventitious entrances, which may perhaps serve as apertures for ventilation; but additional entrances to the breeding-burrow could not serve this purpose.

Occasionally the entrance to a breeding-burrow is at a considerable distance from the water; I have found one as much as forty feet from the river, and fifteen feet above its surface--the river being at the time two feet above normal level.

I have also found burrows coming to the surface on the river side of a sheep-track, and disappearing into the ground on the other side of it. Kershaw (1912, p. 103) noted one such, in which the narrow ridge of ground separating the two holes was worn smooth, as if the animal passed up, came out of one, and immediately entered the other. I think the reason is that stock using an accustomed track may annoy the platypus by bringing down her roof, until in exasperation, she abandons the section of burrow underneath.

My opinion is that the vagaries observed in breeding-burrows are due to the nature of the ground. Obstacles such as roots, stones, and rabbit-burrows--or, more important still, patches of friable soil which tends to cave in--force the female to continue her efforts until she finds a suitable position for her nesting-cavity. As I have said before, the faulty nature of the bank often leads to the abandonment of a burrow.

As to the question of a sufficient air-supply, I find that in the case of the very young little air is required up to the age when they begin to take milk. Seeing that the mother is, up to that time and until disturbed, always found in the pugged-up cavity with her eggs or her young clamped to her abdomen, it is evident that the only air obtained is that which percolates through the several pugs arranged here and there throughout the tunnel. From tests made with very young platypus thus taken direct from the mother, I find that, the younger the nestling, the longer it can remain under water.

At Manilla, on 3 September, 1925, I unearthed a female platypus nursing twins. The temperature of the outer air was 70 deg.; of the nesting-cavity (containing young) 68 deg.; the river water 62 deg.; of the mother (cloacal) 82 deg. The young were apparently about three days old and measured 28 mm. from tip to tip. The mother’s cloacal temperature would, I think, correspond approximately with her body heat when coiled up around the young. Immediately after being taken from the nest, the twins were put into a bottle containing river water; yet one of them lived for three hours; the other survived for three and a half hours before drowning; and both would probably have survived much longer but for the shock caused by the sudden change of temperature from 82 to 62 deg. In other cases, when drowning young platypus taken for scientific purposes, I have always tried to be sure that they were dead before preparing them for pickling; but on several occasions, when they have been placed in wadding for the removal of moisture and while I have been engaged on other work, they have been found crawling about and have had to be “redrowned.” This is on all fours with the fact that flies, apparently drowned and then placed in the sunshine, recover their vitality.

On an earlier occasion (3.35 p.m. on 2 October 1920) in bitterly cold weather I took twin young ones measuring 40 mm. from a nest at the Macdonald River and put them into a bottle of about 6 2½ inches, into the bottom half of which I pressed a handkerchief tightly, so that they might not crawl within its folds. The bottle was then securely corked and carried to camp in a billy-can--four miles over a rough bush track on the floor of a stiffly-sprung van. On arrival I was surprised to find them alive, but they were apparently asleep, snugly coiled. On my flicking the bottle with my finger, both uncoiled and pawed the air, only to subside again. This performance was repeated half-hourly for 5½ hours, until, being tired after a hard day in the field, I decided to chance results, wrapped the bottle in a chaff-bag, and retired to my canvas sleeping-bag until morning. At 6.40 a.m. I found them asleep, and when the bottle was uncorked they were as lively as when collected. Three hours later they were “snap-shotted” with the shells of their eggs (see Plate 23, fig. 8), and, as I had another busy day ahead, I concluded the observation by dropping them into a bottle of fixative. During the day I unearthed a set of triplets of the same age as the twins. These, with a portion of the nest, were placed in a loosely-woven calico collecting-bag. After being several times exposed to light and air, they were found dead at the end of six hours.

Tests made with young at various ages prove that, from the time when they begin to consume milk, the time required for drowning is gradually lessened. I have observed that, during the period when the mother has to replenish her food and milk supply, the pugs, through being continually removed, are less compactly re-arranged. Fresh air is, therefore, introduced into the nesting-cavity in graduated supplies, and this would indicate that the young require more air in proportion to their growth.

The banks here are ideal for burrowing, in the shade of the river-oaks. The granite boulders in centre are favourite sun-baking places of the long-necked river tortoise.

Plate 24]

The banks, being rocky, are unsuited for burrowing, but the river at this “bend” is an ideal feeding-place.

Plate 24]