Chapter VIII: The Nesting-Burrow (2)

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,[14] 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_]