BREEDING HABITS AND LIFE HISTORY
MATING
Since the platypus is such a shy and secretive creature, rarely seen
at all except by those who take the trouble of investigation, it is
not surprising to find that, in the past, very little was gathered as
to its mating and nest-building habits. One platypus-hunter quoted by
Anderson Stuart makes the statement that the males fight furiously
for possession of the females. Although I have never seen such combat
between platypus at large, from what I have seen in this direction with
specimens in captivity I feel justified in agreeing fully with that
statement.
Early in the breeding-season there is in the male a correlated rapid
growth and enlargement of three sets of glands--the testes, the crural
glands, and the scent-glands. The enlarging of the testes (which
from quite a small size become as big as pigeon’s eggs) needs no
explanation. The scent-glands are more prominent in the male than in
the female, and the fact that they become so remarkably enlarged at
mating-time implies that the female seeks the male. This is rendered
more probable by the fact that, when once impregnation has been
effected, the female (so far as I have been able to ascertain) carries
out the remaining duties of reproduction entirely unaided--quite a
mammalian trait.
At breeding-time platypus usually leave their accustomed
feeding-grounds and select a stretch of bank suitable for nesting.
Under normal conditions nesting-burrows are not found close together.
For the upper Namoi, the Macdonald, and the Manilla rivers, upon which
most of my observations have been made, the average is one tenanted
burrow to six miles of bank--that is, to three miles of river. Under
abnormal conditions (in times of flood, etc.,) a different state
of affairs often occurs, as will be evident from what follows, but
normally the female does not seem to care about neighbours at this
season. She requires a stretch of undisturbed water to herself while
hatching her eggs and nursing her young, so that she may be assured of
a near and easily accessible food supply.
Verreaux claims to have witnessed the actual act of copulation.
According to his own account he spent whole days and nights hidden in
a specially constructed hut, and was completely successful. I give his
account for what it is worth (1848, p. 130):--
“The male, after chasing the female for nearly an hour, ended always
by taking her into the middle of the rushes. There, clinging tightly
to her with the help of his bill, he clutched the skin of her neck,
while his hind-spurs grabbed her hinder end. The female, struggling
violently against him, swam along uttering plaintive cries that were
rather like the squeaks of a young porker and grew louder as they
went on. The act took five or six minutes, and afterwards the two
animals played together for more than an hour.”
If Verreaux meant that the male grasped the skin on the female’s
neck, as a drake does, he manifestly did not understand that the
extent to which the upper mandible overhangs the lower would render
that impossible, quite apart from the pliable nature of the lips and
the fact that both jawbones are divided at their extremities and are
pliable as far back as the secateuring ridges. With regard to the
squeak, I have never heard any sound from a platypus which might be
so described, and I am convinced that either the whole account is
imaginary or Verreaux was misled by a sound from some other source,
possibly overhead.
Two observations which I was fortunate enough to make were widely
separated. The first was made at 7.30 a.m. on August 27, 1909, in the
Namoi River at Manilla. A platypus appeared on the surface, and was
immediately followed by a second. As neither exhibited any surprise
or alarm, it is probable that they had been together for some time.
After a short interval one animal began to swim in a circle round the
other, which, however, soon followed, so that the two were swimming in
a circle one behind the other. They were about equal in size--probably
an old female and an adult, but young, male--and it was on this
account difficult to distinguish which had been the one to start the
proceedings. After about a minute of this circling, one of the animals
(which proved to be the female) submerged its body and tail, and
floated perfectly still with its head alone above water. The male then
came slowly up, and mounted in a leisurely fashion. The whole process
offered a very close resemblance to the early stages in copulation of
a drake and duck, with the exception that the male platypus did _not_
take a grip with his ‘bill.’ The male then threw himself back into a
sitting posture, partly out of water, but at this moment there was a
great splash, and both animals disappeared.
The second observation was made twelve years later, on September 23,
1921, in the same river at a spot three miles from the first, and at 7
o’clock in the morning. There I saw what looked at first like a large
male platypus floundering on the surface of the river as if sick or
hurt; this, after a moment or so, dived. Immediately another object
followed in its wake, partly-submerged and upside down. In a couple
of minutes a second platypus (this time apparently of smaller size)
rose to the surface; more floundering, or wallowing, took place at the
surface, and I could now perceive quite distinctly that what I had seen
was a pair of platypus coupled in an extraordinary position. The tail
of each was laid flat along the belly of the other, completely hiding
the bright gold and silver of the ventral fur. The precise position
of the hind limbs could not be made out, as no movement thereabouts
was discernible; but it must have been the grip of these that kept the
animals together. So closely were they apposed that they appeared at
times like a single giant platypus. The processes of diving, blowing,
and floundering at the surface were repeated several times at short
intervals. On each occasion the first to rise for a blow would be
the only one visible above water, the second being clearly visible
only as it trailed upside down in the wake of the first on diving.
During the floundering and rolling which took place at the surface
the under animal could be seen dimly. The animals rose alternately,
apparently for breathing purposes, and on each occasion the second
animal seemed to follow, helpless and rudder-like, upon the movements
of the first. The whole of the manoeuvring was carried out in a calm,
slow, deliberate manner, and almost noiselessly. How long they had
been coupled together before I observed them I do not know; but my
observation lasted fully three minutes; then the couple separated
beneath water, and simultaneously appeared again on the surface,
finally diving and disappearing.
I made an attempt to follow the actual process of copulation, making
use of a recently killed male and female which, though dead, were still
limp and pliable. I found that, when the male rises into the sitting
position which I have already described, the tips of his spurs may
easily be inserted into the sockets which occupy the corresponding
position on the hind legs of the female. If the male then throws
himself right back, and at the same time the female draws her pliable
tail between his legs, the position observed in the final stages is
reached, and in this position the cloacal apertures may easily be
opposed. This use of the spur would supply the reason for its being
movable, a matter for discussion later.
To insert the spur into the socket to its full extent, as stated by
Home, would certainly involve gymnastic ability. My firm belief as
to what really happens, is this: When in the sitting position, the
male straddles the hips, keeping his feet at right angles with his
legs. In this attitude the spur can be erected to its fullest extent
(in a manner similar to the action of one’s thumb when the clenched
hand is opened suddenly). In an adult male the space between the foot
and the tip of the spur, at that time, measures over an inch. This,
then, is more than sufficient to embrace the female ankle comfortably.
Presumably the female does not resist the application of the grip,
and, when it is securely taken, she flicks her pliable tail into
position. It is with such simplicity then that the grip is secured and
steadfastly retained to the end, though, possibly, aided with two pairs
of clutching feet.
Now if we take into consideration the shape and length of the curved
spur, which follow befittingly the depth and ovalness of the female
ankle, it will not be difficult to ascertain the reason why it is
impossible for the male to insert more than the tip of the spur into
the socket, even were he desirous of doing so.
As regards Home’s statement that the male ejects the secretion of the
spur into the female socket to bring about release, to me this also
appears wide of the mark. On examining the spur, which averages ¾ in.
long and is shaped somewhat like that of a cock, it will be found that
the aperture is not at the tip, but back from it, and, furthermore, is
situated on the convex surface farthest away from the socket during the
embracing position. I therefore fail to see how a fluid ejected beneath
water could enter the socket. As a matter of fact, the poison cannot
be ejected unless the spur is pressed against the male’s own leg.
How, considering the shape of the weapon, can this possibly be done
while the thickness of the female’s ankle, or lower leg, intervenes?
The purpose of the aperture’s being set back from the ripping tip of
the spur is obviously to prevent clogging while fleshing prior to
administering the poison. This strengthens my belief that the secretion
is intended to irritate a wound.
BREEDING-SEASON
The breeding-season of the platypus in northern New South Wales may
be estimated with fair accuracy from my long series of observations
and records. I have observed copulation, as I have already stated, on
two dates only, 27 August 1909, and 23 September 1921; but I had a
report from a competent observer, Mr. J. Maclean, that platypus were
“courting” in the Macdonald River in the middle of July, 1920. The
earliest date on which I have actually taken eggs from the nest is
24 August (1925). On the same day I unearthed twin young ones which
I estimated to be three days old. This would carry back the date of
laying to, perhaps, the first week of August. I have no guide as to
the time which elapses between the laying of the egg and the hatching
of the young, but I think it must be less than that necessary for the
incubation of birds’ eggs, owing to the partial development of the
foetus before the egg is laid.
The latest date upon which I have taken eggs is 22 October (in 1918);
Kershaw (1912) has recorded the taking of eggs upon the same day of
October in 1912 in Victoria. Consequently the period during which
oviposition has actually been observed ranges from about the middle
of August to a little after the middle of October. There are certain
conditions, however, which affect the breeding-season, and I do not
believe that normally it has so great a length. In the first place
there is, as one would expect, a general tendency towards earlier
breeding in the north; as one proceeds south littering becomes
progressively later. In central Queensland July and August are the most
active months; in New South Wales, August and September; in Victoria,
September and October. I have no information as to the breeding-season
in Tasmania. But the normality of the breeding-season may be seriously
interfered with by conditions of weather, and particularly rainfall.
Heavy rains occurring in August and September flood the burrows, which
are consequently deserted. This flooding is brought about chiefly by
seepage through the soil, as the upper parts of the burrow are almost
always above ordinary flood-level of the river. It is thus clear that
the pugging of the tunnel is not a precaution against the entrance of
water from the river, as has sometimes been suggested, but is for the
purpose of providing conditions necessary for incubation. The effect
of floods upon the breeding-season in the New England district was
observed during September and early October of 1920. In July, the
rivers were in flood. Platypus were reported plentiful after the waters
had receded in the early part of August. A second flush occurred late
in August, when the waters rose to six feet above their normal level;
and sporadic rain continued until September 18. Under these conditions
a number of burrows which I opened up were found to be deserted and
the contained nests sodden with water. More remarkable still, several
tenanted burrows were found in close proximity to one another, which
is unusual; and in one case two females were found occupying nests in
burrows which had a common entrance, an occurrence without parallel in
my observations. Furthermore, several tenanted burrows showed signs of
extremely hasty construction, being only a few feet in length, and with
fewer pug-pits than usual. Again, in 1923 an eight-foot rise in the
river had washed out all the breeding females, and no tenanted burrows
at all were found on the plateau. A female killed on October 9, when
examined, was found to have in her left uterus eggs in the early stages
of segmentation. In a normal season eggs are laid during the later part
of August and early September, and from late September on to the end of
October large young are found in the nests.
THE INTRA-UTERINE EGG
The ovum of _Ornithorhynchus_ (that portion of the egg which
corresponds to the yolk of a fowl’s egg) is, at the time of
fertilization, a small yellow sphere about 3 millimetres in diameter.
After fertilization it becomes surrounded by an exceedingly thin layer
of albumen, and, outside this, by a thin, transparent, horny shell.
In birds there is a thick layer of albumen (the white of the egg)
deposited in several distinct layers, the innermost having spirally
twisted prolongations which serve to hold the yolk suspended in
the middle of the egg. Outside this thick albumen are two delicate
membranes, closely adherent everywhere except at the larger end of the
egg, where they separate to enclose an air-chamber; these membranes are
closely applied to the rigid, calcareous shell. The most remarkable
difference between the egg of a bird and that of a monotreme is that
in the former the shell is deposited round the fully-formed egg, and
there is no increase in its size during the growth of the embryo,
whereas in the monotreme the eggshell increases in size and alters in
structure during the intra-uterine development of the embryo. The thin
shell, which is at first only 4 millimetres in diameter and spherical
in shape, stretches and thickens until a diameter of 10 millimetres has
been attained. At this stage the egg is still practically spherical,
but the shell has become thicker, and has been rendered opaque by the
deposition of lime-salts. As it goes on growing, it becomes ellipsoidal
in shape, the longer axis corresponding to that of the contained embryo
(Wilson and Hill, 1908), and finally attains its maximum size (16 to
18 millimetres long by 14 to 15 wide) just prior to being laid. There
can be no doubt that this curious growth of the egg, which has no
parallel in nature outside the monotremes, is due to the imbibition of
fluid from the uterus. There is not sufficient nutrient material in the
yolk-mass of the fertilized ovum to produce the young platypus which
hatches from the egg; consequently the intra-oval foetus is nourished
in a considerable measure by secretions from the maternal uterine wall.
Since secretions can pass into the egg through the mesh-work of horny
fibres of which the shell is at first composed, it is also possible
that foetal secretions may pass out; there is at present, however, no
evidence that such a process takes place. The point is of interest,
for, in the higher mammals, a secretion from the foetus--of the nature
of a hormone, or chemical messenger--which passes into the maternal
blood-stream, is conveyed to the mammary glands, and stimulates them
to the active secretion of milk; but, in the platypus, neither at the
time of oviposition, nor later, when the young actually hatch out,
are the milk glands of the mother in active condition. The stimulus
to lactation would seem to be derived in some external and mechanical
way from the presence of the young. This question will be further
considered below.
It is thus obvious that, though the monotreme egg is usually looked
upon as similar to those of birds and reptiles, it is in reality very
remarkably different. The nourishment of the embryo within the egg by
secretions from the uterine wall is distinctly a mammalian feature.
Such a process does not occur among birds and reptiles;[16] in
these, when once the egg-shell is formed, the embryo has to depend
altogether upon the nutrient material packed within the egg. It must be
admitted, however, that what occurs in the monotremes appears to afford
very little clue as to the evolution of the placental habit in the
higher mammals.
[Illustration: (1) TWIN EGGS IN NEST COMPOSED ENTIRELY OF REED ROOTS.
_Plate 29_]
[Illustration: (2) TRIPLET NESTLINGS, ABOUT FOUR DAYS OLD. NEST
COMPOSED OF GUM-LEAVES AND GRASS.
_Plate 29_]
EGG-LAYING
The platypus lays one, two, or three eggs, two being the usual number.
There is no record of four eggs, or four young, ever having been
discovered, and the statements of Bennett and by Lucas and Le Souef
that the number of eggs is one to four are probably based on Geoffroy
St. Hilaire’s unfortunate experience (see p. 36). When two eggs are
laid, these are invariably found joined together side by side. So
closely do the apposed walls adhere that considerable force, comparable
to that required to pull apart two fairly dry ‘tangle-foot’ fly-papers,
must be exerted to separate them. The only records of triplet eggs are
of two sets taken after hatching and one intra-uterine set. Each of the
former sets of shells was in the form of a cluster, with the length
of all three eggs lying practically parallel, but the ends free for
emergence of the young. In one of the sets the three were all joined to
each other; in the second set there was a small space separating two of
the shells, but both of these were joined to the third. This difference
is evidently unimportant, and occurs when the eggs are being clamped
together immediately after they are laid.
There is no longer any mystery as to the way in which the eggs are laid
and afterwards fastened together. When the eggs are ready to be laid,
they are well apart from one another in the uterus, with their longer
axes in the direction of the length of that organ. The capsule of each
separate egg is at that time completely covered with a sticky fluid.
When about to lay, the female squats on her rump in the nest, with her
tail between her legs, and her fore-paws in readiness at each side
of the aperture of the protruding cloaca. Presumably, the eggs are
deposited singly into her soft rubber-like hands and held there firmly
until each egg capsule, where in immediate contact with its fellow,
becomes affixed thereto; then, with the help of the under surface of
her fat, pliable tail, they are clamped to her warm abdomen. Meanwhile
the solution still covering the remaining portions of the capsules soon
loses its stickiness through either evaporation or absorption. It is
improbable that the eggs are held for long in the fore-paws, or that
their adhesion and the drying of the exterior portion of the capsules
is a lengthy process.
It may interest my readers to know how I have come to these
conclusions, seeing that the eggs are laid in a pugged cavity beneath
the solid earth. While collecting embryological material for the
Canberra National Museum on September 2, 1925, at Manilla, I took a
female platypus with mature triplet eggs in her left uterus, and on
extracting these I found that their capsules were conspicuously glossed
with a thick coating of sticky liquid. I immersed them separately in
a powerful drying fixative called “Bles;” but notwithstanding the
bleaching and hardening properties of this mixture of formalin, spirit,
and acetic acid, the eggs not only stuck firmly together on coming into
contact with each other, but also adhered to the glass and to a paper
label attached to another specimen in the bottle. This, I think, proves
that the eggs adhere to each other only after being laid, and not even
in the cloaca, as has been hitherto supposed. Drying off after adhesion
and exposure to the atmosphere would appear to be quite natural; this
could not occur prior to deposition. But one may ask why, if the eggs
will stick to paper and glass, they do not stick to the hands of the
mother during the act of manipulation. The probable explanation is
that the rubber-like membrane of the fore-paws of a platypus is always
cold and moist, however dry and warm the rest of the animal’s body may
be. This is as it should be, because of the various functions that the
versatile fore-paws are called upon to perform at a moment’s notice,
such as walking and swimming, involving rapid action similar to the
alternate opening and closing of an umbrella. It will be obvious
that the very existence of the platypus depends on the versatile
manipulation of this membrane. Nature would not permit any temporary
clogging of this intricate member. In 1924, while Mr. Ramsay and I were
making a moving picture of the natural habits of monotremes, I tried
with strong fish glue to fix the staff of an Australian flag in the
closed palm of a living platypus, but found that it was impossible.
My observations of intra-uterine eggs apply to two pairs in the early
stages of segmentation. Each pair was found in the gravid left uterus,
which alone is functional; in each case the eggs, one behind the other,
were separated by an appreciable interval. Wilson and Hill obtained
a number of intra-uterine eggs, some in a fairly advanced state of
development; but they give no information as to how the eggs were
situated in the uterus. They do, however, figure (1908, Pl. 4, fig. 6)
a section of a uterus containing a fairly advanced single egg, which
shows that the egg lies in the position one would expect, namely, with
its longer axis in the direction of the length of the uterus. Caldwell
(1887, p. 464) states that he shot a platypus which had laid her first
egg, while the second lay in the mouth of the uterus. He does not,
however, state that he actually found the first egg, and it may be that
he went on the assumption that the animal invariably laid two eggs,
which we know not to be the case. Gold-receiver Rumby’s platypus is
reported to have laid two separate eggs in a gin-case (see pp. 41-43);
but there is so much that is dubious about this particular story that
it cannot be accepted as definite proof. If the evidence could be
relied upon, it would show that the eggs are laid separately, and the
fact that they were not found cemented together might be attributed
to the unnatural circumstances in which the mother found herself. As,
however, the two eggs follow each other down the uterus lengthwise, it
is obvious that they cannot in the process become joined together side
by side. Moreover, when there are three eggs it would not be possible
for them to be laid if they became cemented together within the body of
the mother. It is therefore certain that this cementing takes place
after they are laid.
My observations, together with those of Wilson and Hill, show that any
pair of eggs contains embryos in an identical state of development,
even in the earliest stages of segmentation. This would imply uniform
conditions for both eggs, and from this it may be presumed that the ova
are extruded from the ovary and are fertilized simultaneously, that
they pass down the oviduct (including its uterine portion) close to
one another, but end to end, and that they are laid practically at the
same moment. At the moment of extrusion they are coated with a sticky
secretion from the oviduct walls, and it is a simple enough matter for
them to become adherent. But the fact that their long axes are always
parallel after laying would seem to necessitate definite action by the
platypus. There might be one other suggestion--that the eggs, having
passed through the pelvis separately, are retained in the cloaca long
enough to become attached to one another before their final extrusion;
but this I consider impossible. A further curious feature is that the
embryo lies along the long axis of the egg, parallel to that portion
of the shell which is cemented to its fellow, and, after the egg is
bleached, may be seen through the shell with the naked eye.
My observations of the right ovary and oviduct go to show that,
though not functioning in the true sense, they appear to be affected
in a certain proportion to the activities of those on the left. For
instance, while the left is maturing triplet eggs, the right becomes
enlarged to about the size that the organs on the left would have
when carrying a single egg. In cases of twin and single eggs, the
enlargement is proportionately less.
Of seventy tenanted nests examined by me, eleven contained either one
egg or one young one; fifty-four contained two; only five contained
three. Of six nests recorded by Kershaw, one contained a single egg
and five either twin eggs or twin young. Hill’s record of stages of
eggs taken from nests comprises one single egg and four twins. Wilson
and Hill’s paper (1907) on the intra-uterine stages does not state
clearly what was the exact proportion of singles to twins in the
material examined. There are a number of older records by Maule, the
Bennetts (father and son), and others, but the figures given above are
sufficient to indicate the approximate frequency of each egg-number.
Triplets are rare, the only record apart from my five (four times of
young in nest, and once of intra-uterine eggs) being a single record
of young by Dr. George Bennett. Ignoring triplets altogether, singles
are in a proportion of roughly twenty per cent, and twins of eighty per
cent.
[Illustration: FEMALE PLATYPUS IN NEST OF GUM-LEAVES AND GRASS.
The position is same as shown on Plate 28. The earth has been removed
to render photography possible.
_Plate 30_]
THE LAID EGG
Kershaw (1912, p. 106) gives the measurements of a pair of
eggs as 18 × 15 mm. and 16 × 14 mm., and of a single egg as 18
× 15 mm. My observations give the following measurements (in
millimetres):--Shrivelled egg (unhatched), 16 × 10; single egg, 18 ×
14; twin intra-uterine eggs, 3 × 3; twin eggs, 16 × 12; twin eggs, 17 ×
14, 16 × 14; single egg, 17 × 13; twin eggs, 15 × 26 and 17 × 26; twin
eggs, 15 × 25 and 16 × 25; twin intra-uterine eggs, 5 × 5; twin eggs,
18 × 14, 15 × 14; twin eggs, 18 × 25 and 16 × 25; single egg, 15 × 13;
twin intra-uterine eggs, 6 × 6; triplet intra-uterine eggs 17 × 15, 18
× 15, third smashed (abortion). No triplet eggs have yet been taken
from a nest. It would appear from the measurements given above that the
average of the single egg is usually larger than that of either of twin
eggs, the measurements being 17.25 × 14 mm. against 17.5 × 13.8 mm. It
will also be noticed that in only one of five pairs of twin eggs are
the eggs of equal size. In the others one is usually distinctly smaller
than the other.
Of the intra-uterine specimens listed above, the 1920 pair measured
3 mm. in diameter; they were spherical in shape, yellow in colour,
and were situated in the oviduct close to the left ovary. The 1923
intra-uterine pair were 5 mm. in diameter, spherical in shape, yellow
in colour, and were situated in left uterus. The 1925 pair measured
6 mm. in diameter, were spherical in shape, “orange” in colour, and
surrounded by albumen.
The 1925 triplets measured (1) 17 × 15, (2) 18 × 15, the third being
smashed through abortion while in the uterus but uniform in size with
the others. They were ellipsoidal in shape, glossy white in colour,
and were situated in an abnormally enlarged left uterus, two adjoining
nearest to the oviduct, the other in a separate compartment close to
the cloaca. All three were lying lengthwise with the trend of the
uterus, while the ovary connected therewith was partly covered with
a crystal jelly encased in a thin transparent film. This jelly, or
“hydatid-like” sac, was almost as large as the swollen ovary, and,
except where it was attached, somewhat similar in shape to its host
ovary. Whether it was for the purpose of supplying the compound which
envelops the eggs when capsuled in the uterus, I cannot say; but it was
preserved intact for later investigation.
When first deposited, the eggs have a full outline, but as incubation
proceeds they frequently become dented, as well as discoloured. Kershaw
(1912, p. 106) notes that mere exposure to dry air produces denting in
a few minutes. I think, however, that the denting is due to the age of
the egg, because on two occasions I have exposed apparently new-laid
eggs for thirty minutes in the open while photographing them, and they
retained their full outline perfectly throughout, although handled
considerably. It may be that the necessity for a moist atmosphere is
part of the reason for the pugging of the burrow.
The shell is thin, and easily compressible. It is composed of a
mesh-work of keratin fibres, impregnated with calcareous salts;
except in shape, it resembles that of a lizard. The shape is broadly
ellipsoidal, the ends being shortly and evenly rounded. The colour
of the newly laid egg is a glossy white, which, when the egg is dry,
turns to a flat chalky white, and later becomes stained brownish. The
shells from which the young have been hatched have a more polished and
translucent brownish-white appearance, which may best be compared with
that of the celluloid balls used in “ping-pong,” when they have become
crushed (see Plate 23).
INCUBATION
Incubation is carried out by the female alone. During many years of
observation no male has been found in a breeding-burrow, and Joseph’s
account of platypus in captivity (p. 205) indicates that females resent
the intrusion of the male even into their resting-places. On one
occasion I was fortunate enough to catch a mother-platypus napping,
and she did not wake until an attempt was made to insert a thermometer
into the middle of her curled-up form. Her tail was turned up over
her abdomen, holding against it two 65 mm. young. I am convinced that
the eggs are held during incubation in a somewhat similar manner, and
that their cohesion supports this view (see Plate 30). Kershaw (1912,
p. 105) records the finding of newly hatched young, one of which was
attached firmly to the skin of the mother. At this stage the young
have not begun to suck, but they are very helpless and might become
entangled in the fur of the mother’s abdomen, where they invariably
nestle. Kershaw has also expressed this view, and he mentions,
incidentally, that the other young one fell off as the mother left the
nesting-chamber. Of two observations by Kershaw (_loc. cit._), and six
by myself, of nests in which eggs were found, in no instance was the
mother absent from the burrow. This would surely indicate that the
female incubates her eggs in the way described.
The point is of importance, because there is a considerable difference
between the temperature of the nest and the body temperature of
the mother. I have found that the average temperature of the
nesting-chamber (taken by thrusting in a thermometer the moment it was
opened up, and blocking the hole with a sack) was 64.5° Fahrenheit
(19.2° centigrade), whereas the average cloacal temperature of seven
brooding females was 90° Fahrenheit (32.2° centigrade). Such a
difference in temperature would have a considerable influence upon the
duration of incubation. The length of the incubation period is unlikely
to be ascertained by observation in the field, but I think it should
be possible to discover it artificially. At present it can only be
conjectured from comparison with other oviparous animals. Passerine
birds which lay eggs of the same size as those of the platypus have
an incubation period of from twelve to fourteen days; they also have
a higher blood-temperature. But, as against this, Caldwell has made
the observation (1887, p. 464) that the egg of _Ornithorhynchus_, when
ready to be laid, contains an embryo already in approximately the same
stage of development as a thirty-six-hour chick. It may not be very
wide of the mark to suggest that the incubation period of the platypus
is about fourteen days, though possibly less.
I am convinced that, during the three weeks or more which elapse
between the laying of the eggs and the onset of lactation, the female
not only does not leave the nest, but also passes into a condition
of partial aestivation. I think the eggs are laid in the fore-paws
and placed, in the manner described above, between the upturned tail
and the abdomen, and that they remain there throughout the incubation
period and possibly beyond, until mammary secretion is excited by the
mechanical stimulus of the movements of the young. These convictions
pass beyond the observed facts, but are, nevertheless, consistent with
them.
[Illustration: MILK GLAND FROM LEFT SIDE.
The protrusion near centre of upper edge shows where the gland was
connected with perforation in abdominal skin.
(_From fresh carcass_)
_Plate 31_]
HATCHING
The hatching process has not been observed completely. The shells
from which the young have hatched are always found in the nest in a
flattened condition. On relaxing them in warm water, months later, I
have found that they can be made to resume their original shape. It is
worth noting here that the warm water has no effect upon the substance
which caused the eggs to stick together when laid. The young are seen
to have emerged through ragged rents, and seldom is any portion of the
shell missing. The twin eggs examined all show that both young emerged
at adjoining ends. In the only two sets of three shells recorded, a
somewhat different state of affairs was found (see Plate 23, fig. 4).
In two of the eggs the rents were at opposite ends; in the third the
rent opened at right angles to the plane of the other two; the result
being three openings as far removed from one another as the surfaces
of the eggs would allow. It is impossible to credit the mother with an
instinct enabling her to arrange the eggs with regard to the planes
of embryos which are only very little formed at the time of laying.
It is likewise difficult to conceive that the embryos themselves can
possess any instinct which leads them to break their way out in a given
direction in which their passage is assured. The probability is that
the eggs are invariably attached to each other by their sides, and that
the young always break out at the ends, and so no complication can
arise.
When young from the nest were first described by Owen, it was thought
that the caruncle on the muzzle, which is hard and sharp a couple of
weeks after hatching, corresponded to the ‘egg-tooth’ found in many
birds and some reptiles, and was the means by which the young one cut
its way out of the egg. The discovery of earlier mammary foetus stages
has shown, however, that at the time of hatching the caruncle is merely
a fleshy knob, which could not possibly serve as a cutting instrument.
Professor J. T. Wilson states that in the newly hatched young there is
a small egg-tooth anterior to the caruncle; but this must be a very
evanescent structure, for no sign of it can be seen in young ones 18
mm. long, the earliest mammary foetus (a term to be explained later)
collected by me. These, from the soft, moist condition of the shells,
and the umbilical protrusions exhibited by the young, I considered
to be newly hatched, but Professor Wilson’s specimens are smaller.
Whether helped by an egg-tooth or not, it is probably an easy matter
for a restless, muscular creature like the platypus embryo to break out
through the thin shell, using the caruncle as a point of resistance
against the pull of the fore-paws, with an action similar to that
adopted by the young when pummelling the mother’s abdomen and nuzzling
for milk. I have observed a similar action on the part of overturned
naked young regaining their feet.
NURSING HABITS
The young, when first hatched, show few of the characteristic features
of the adult platypus. There is no sign of the curious muzzle, which
Geoffroy St.-Hilaire protested would be such a stumbling-block to the
imbibing of milk from mammary glands. Indeed, except for the flattening
of the rudimentary tail, and the shortness of the limbs, with their
even fingers and toes, the young platypus has little to distinguish it
upon casual examination from any early mammalian foetus. It grows into
characteristic platypus form at a later stage.
The most remarkable and mysterious feature about the baby platypus
is that it is not suckled at all by the mother for some days after
hatching, for the very good reason that the maternal mammary glands
are not yet actively functional. Investigations of this extraordinary
phenomenon have advanced far enough to place the matter beyond doubt.
On October 9, 1923, I took three female platypus, two with the mammary
glands almost dry, the third with them quite so. I noted the inflamed
condition of the uterus of the third individual, and it proved on
examination to contain two eggs in the early stages of segmentation.
On September 27, 1921, I took a female with a young one 45 mm. long.
Here the mammary glands had begun to enlarge, but had not reached half
the maximum size, and would seem not to have been actively functional,
though four or five days must have elapsed since the time when the
young one was hatched. Against this it must be recorded that Kershaw
(1912, p. 105) took a 30-mm. young one, “so securely attached to the
skin as to require a little force to detach it.” Kershaw does not
state the method of attachment; but, as the limbs are very feebly
developed at this stage, it is certain that the little creature must
have been sticking to the fur accidentally. In all my experience I have
never found the young ‘attached’ to the fur. Whether the individual
which Kershaw observed was really obtaining nourishment cannot now
be determined. Had he dissected the mother, he would have discovered
that at that stage the milk glands were dry. I have made a careful
examination of the glands of a mother with 20-mm. young, and could not
induce milk to flow by external pressure--a process which can easily be
carried out when the young are older--nor, on dissection, could I find
any trace of milk, the glands appearing quite dry. An examination of a
considerable series of mammary glands from nursing mothers which have
been collected with their young has convinced me that during the first
week, at least, after hatching there cannot be more than a very slight
milk-secretion, if any at all, and I think this characteristic applies
to the echidna also.
I have made the experiment of placing young apparently two weeks old
upon the abdominal fur of the mother, who was laid upon her back and
held in that position. The young ones crawled aimlessly about, and
passed and repassed over the mammary area without appearing to be
aware of its presence. They made no attempt whatever to cling to the
fur with either mouth or feet. Older ones, however, clung tightly with
their fore-paws, using their hind limbs as additional supports when the
mother was held up vertically (see Plate 10). While observing these
latter young, I noticed a peculiar action which appears to lend some
support to a rather speculative conclusion previously reached as to the
function of the caruncle. The young were observed to bend their heads
under their bodies, and to withdraw them in such a way that the dorsal
surface of the muzzle, towards the end of which the caruncle is borne,
was dragged forward over the mammary area so that the sharp point of
the caruncle scored its surface. It had occurred to me previously
that, since the caruncle could not, for reasons given above, have the
function of an egg-tooth, it might well serve the purpose of exciting
mammary secretion. It is at least a significant coincidence that the
onset of obvious lactation occurs simultaneously with the completed
growth of the caruncle. It may be that the stimulus afforded by this
‘milk-spur’ is necessary to induce the very primitive milk glands
of _Ornithorhynchus_ to act; and the delayed lactation which I have
observed may be due to the incomplete development of the necessary
stimulus in the early young. How the young platypus is nourished in
the meantime I do not know.
On one occasion only have I found the mother to be absent from a nest
containing young less than two weeks old. It must be remembered,
however, that burrows are always opened up by daylight, and, as the
female is certain to be more than usually wary during the nursing
period, it is safe to assume that she would generally feed under cover
of darkness. When the young are older, they are frequently found by
themselves, presumably because the mother has to feed long and often in
order to maintain her milk supply. When the female leaves the burrow,
all the pugs are closed behind her, which means that she digs her way
through each of them, passing the earth under her body and compacting
it with her tail as she goes. Since the same process has to be repeated
on her return, and since as many as nine pugs have been observed in a
burrow, feeding excursions must be a laborious business and are not
likely to be indulged in more often than is necessary.
During the early nursing period, then, the mother spends most of her
time curled up about her babies, holding them snugly against her
abdomen by means of her tail. The young at this stage are generally
referred to by the term ‘mammary foetus,’ which is the name given
to the pouch-young of marsupials during the time they remain on the
teat. There is, however, no close correspondence between the two. The
marsupial, coming to active life for a brief space after birth--long
enough, it is said, to reach the pouch of the mother by its own
voluntary effort--relapses into a foetal condition in which it is
apparently incapable of voluntary movement. It cannot even suck,
the milk being pumped into it by the mother. This act is obviously
impossible for the teatless monotremes, hence their fasting. The
young platypus, although its bodily form undergoes a fair degree of
modification during the first fortnight, and though it is not hatched
entirely after the image of its parents, is continuously capable of
voluntary movement from the time it leaves the egg. In the newly
hatched young the movements are comparatively feeble; but after the
first week the young are in a state of perpetual motion, at least
when exposed to daylight, which they appear to dislike. In the darkness
of the nest, however, they probably spend most of their time in sleep.
In the echidna the mammary areas open into the pouch, which is formed
as a temporary structure during the breeding-season, and the young one
laps its nourishment with its slender projecting tongue. The platypus
has no pouch, and the milk oozes out through numerous fine apertures
upon two mammary areas of the abdomen, each about half an inch in
diameter. These areas are covered with fur, and are not in any way
distinguishable from their surroundings, so that they are difficult
to locate by external examination. Pressure upon the mammary glands,
however, causes the milk to flow gently out, and their precise position
can then be seen. The hair covering the areas serves, apparently, in
place of a teat, and the young pluck at this and suck the milk from
it, much as a little aboriginal eats his honey by sucking it from a
piece of shredded bark. The “lips” of the young, owing to the shortness
and undeveloped form of the bill at this early stage, are adapted for
sucking in conjunction with the tongue (see plate 33).
An unsuccessful attempt was made to keep alive two young ones (about
a fortnight old), which had been sent to the Department of Zoology at
the University of Sydney, by inducing them to suck cow’s milk from pads
of cotton wool. Even when the milk was squeezed out upon their tiny
muzzles, they did not appear to take the slightest interest in it. I
had much the same experience in the field with a pair of well-grown
young, which were in fur and had their eyes open. I placed drops of
milk upon the hairy portion of my forearm, but they merely brushed it
aside. After crawling inside my sleeve (evidently to avoid daylight),
they did, however, pluck at the skin of my arm with their jaws, with
a sucking action sufficiently strong to bring a flush of blood to the
spot. The opening of the jaws was accompanied by a feeble kiss-like
explosive sound, resembling that made by a man releasing smoke from the
side of his mouth when lighting a pipe.
The naked young range from bright red to rose-pink in colour when
alive, and their delicate skin has a silken sheen, and is minutely
wrinkled. In preserved specimens these wrinkles become much
exaggerated, owing to the contraction of the underlying muscle-layer.
When the hair-follicles begin to develop, the skin of the back assumes
a bluish appearance, like that of a shaven chin, but the under surface
of the body still keeps its pink colour (see Plate 33). Finally, with
the growth of the hair, which is at first fine and silky, the adult
appearance is reached.
A feature of interest in connection with the nursing habits is that
the nest never shows any sign of having been fouled with excreta.
Young which I have had in temporary confinement defaecate freely; the
faeces take the form of an odourless, greenish, oily slime, which may
be compared with the contents of the gall-bladder of an ox. (Adults,
in captivity or at large, always defaecate in water.) As the nest is
always clean, this inoffensive matter would readily filter through the
nesting-material without causing more discomfort than that endured by
nestling Owlet Nightjars--in fact, not so much, since it would not form
hard masses like the bird droppings. But it is reasonable to suppose
that very little faecal matter passes from either the mother or the
young during the period of incubation and the nursing of little ones.
While suckling, the mother leaves the nest occasionally to feed, and,
presumably, to defaecate also. The nest is then probably relined from
time to time, as is the custom of the Owlet Nightjar.
It is for these reasons that I maintain that platypus, when brooding,
fall into a lethargic state and, thus requiring little food, do not
evacuate in the nest unless abruptly disturbed.
[Illustration: BROODING FEMALE STEALTHILY GROVELLING HER WAY FROM HER
NEST WHICH HAS BEEN EXPOSED
Note cautious attitude and rubber-like muzzle.
_Plate 32_]
ADOLESCENCE
About six weeks after hatching, the young will have reached a length of
twelve inches. By this time their eyes are open, their fur is a quarter
of an inch in length, and they are able to crawl freely about the
burrow. (See Plate 12, fig. 2). This may seem a rapid rate of growth,
but such rapidity is explained by the fact that when once the young
commence to suck, their appetites increase rapidly. The quantities of
food found in their stomachs on dissection are surprising. (Because
of this, the stomachs must be tapped and the contents drained before
fixing in “Bles,” otherwise the milk putrefies and is apt to spoil the
entrails.)
Just prior to the appearance of pelage on a large nestling, the
intestines and stomach are found actually bloated with milk. When
fully extended, the skin of the abdominal area becomes remarkably
tight-fitting; but elsewhere the trunk does not nearly fill out the
skin. By holding such a specimen, back down, on one’s palm, and
slightly tilting the hand from side to side, the entire contents of
this out-size skin slop about as a quantity of mercury would if placed
in a pliable bladder.
The largest young that I have found in the nesting-burrow measured
thirteen inches in length. The smallest known by me to have been found
at large were fourteen inches long, captured in shallow water among
weeds. My own youngest capture at large measured fifteen inches.
It would thus appear that the young are generally from thirteen to
fourteen inches long when they leave the burrow. For some time prior
to this, however, they would seem to run about the burrow, and,
possibly, to enter the water in company with their mother. When once
their eyes are open, the young are able to swim. The mother then no
longer replaces the pugs in her tunnel, and one which contains large
furred young may be recognized during excavation by the fact that
nesting-material is strewn along its course. This is mentioned casually
by Bennett (1860, p. 130), and it seems probable that this material is
scattered about by the young in running to and fro. At this stage the
breeding-burrow is about to be deserted.
Bennett (_l.c._, p. 131) records the capturing of twin young, ten
inches in length, which had a most beautiful, sleek, and delicate
appearance, and seemed never to have left the burrow. These were kept
alive for some little time, and enjoyed playing in a dish of shallow
water, in which, however, they never remained longer than fifteen
minutes at a time. It would thus seem likely that platypus of that age
have already begun to make short excursions into the water. Bennett
captured a female which he assumed to be the mother of these, and found
her to be in a very poor condition, with the mammary glands practically
dry. Two healthy young, ten inches in length, would certainly need
more nourishment than could be supplied by glands in this condition;
but possibly they had just previously sucked the mother dry. But we
may assume that the young are by this time learning to eat the food
of the adults, and that, as this is always eaten in the water (unless
taken from the mother’s mouth, pigeon-fashion), they go to the water
to be fed. Verreaux’s observations are certainly interesting, but
I cannot entirely agree with him. He records (1848. p. 131, Owen’s
translation):--
“I redoubled my attention and care, and by dint of perseverance,
having at my disposal (always on the banks of the New Norfolk) a
pretty considerable number of adults and young, I saw the latter
accompany their mothers, with which they played, especially when they
were too far from the bank to take their nourishment. I distinguished
very well that when they wished to procure it they profited by
the moment when the mother was among the aquatic plants, near the
land, and where there was no current. The female having her back
exposed, one can easily conceive that on the exercise of a strong
pressure, the milk would float to a little distance, and that the
young might suck it up with facility; this it does, turning about
so as to lose as little as possible. The manoeuvre is the more easy
to be distinguished, since one can see the beak move with rapidity.
I cannot better compare the greasy liquid of the female than to the
iridescent colours produced by the solar rays upon stagnant water.
I have witnessed the same fact repeated daily and nightly. I have
also remarked that the young, when it was fatigued, climbed upon the
mother’s back, who brought it to land, where it caressed her.”
It is improbable that the female discharges milk into the water. Most
likely she catches and crunches food for her young, and releases it
into the water where they can pick it up; this would be good schooling
for both swimming and diving. It is possible that Verreaux witnessed
some such process, though he mistook its nature.
I have observed a young captive platypus feeding upon an aquatic
plant, stripping the leaves one after another from the stem with its
mouth, and using its fore-paws to assist in the process. The adult may
disdain a vegetable diet; but, if any significance can be attached to
this single observation, it may be that the young devour a certain
amount of vegetable food, for which they can forage for themselves in
the shallows. Bennett’s aborigines, who were not altogether reliable
concerning what took place in the burrows, informed him that the
animals ate water-weeds, and that the mother fed the young first with
milk, and then with comminuted insects and molluscs (1860, p. 131).
The furred young are more vocal than the adults, and, when disturbed or
hungry, indulge in a puppy-like growling, accompanied by a kiss-like
popping sound. Bennett records a squeaking call, which was answered;
this I have never heard. In a passage quoted on p. 160 he describes
the careful toilet which they make. They habitually sleep in a curious
position, sitting up upon their hind quarters, with the muzzle laid
flat against the chest, and the tail drawn up over it (see Plate 28).
SEXUAL MATURITY
Little is known of the age at which the platypus reaches sexual
maturity. Judging from the size which it attains in a couple of months,
one would imagine that it could breed by the following season. Against
this, however, is Semon’s opinion that only those two years old, or
older, take part in breeding. Semon’s observations are based upon the
examination of a long series of individuals; but I collected a female
in “full milk,” on October 27, 1922, measuring 407 mm. (16 inches),
only four inches longer than a 12-inch nestling of my collecting. To me
this is rather perplexing, seeing that the nestling would grow another
inch or two more before deserting the nest. And who can say that that
was her first breeding-season? Therefore I cannot agree altogether with
Semon’s statement.
All that can be said at present is that the female begins to breed
when sixteen inches in length, and finally reaches a maximum size of
eighteen inches; but there is no precise information as to the length
of time involved.
LONGEVITY
The length of life of the platypus is not known. It is my intention to
ring-mark some fully-furred young as opportunity offers, and it may be
that we shall gain some information on this point at a later date, if
these marked individuals are captured.