Chapter VIII: The Nesting-Burrow (1)

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.[13]

“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.