NERVOUS ORGANIZATION AND SENSORY PERCEPTIONS
A primitive kind of mammal, which is in some respects intermediate
between the higher mammals and the reptiles, might be expected to show
a low grade of nervous organization and limited powers of sensory
perception, as well as something of the characteristic reptilian
cold-bloodedness. Strangely enough, these characters are not found in
the platypus.
The brain is surprisingly large--much larger in proportion to the
body-weight than that of any reptile. It is, moreover, definitely
mammalian in its structure, except that it lacks the definite band of
fibres, connecting one hemisphere of the brain with the other, which
is known as the _corpus callosum_. This deficiency is not, however,
confined to monotremes, but also characterizes marsupials. Wood Jones
(1923, p. 32) writes:--
“The corpus callosum is a cerebral commissure, or a nerve fibre
bundle, which keeps the higher centres of the right and left central
hemispheres in communication across the middle line of the brain. It
cannot be said that the living monotremes are deficient in the extent
of their cerebral hemispheres--they are indeed mysteriously well
endowed with cerebral cortex. But it may be asserted--borrowing an
expression from the electrician--that their brains are underwired.”
In the size and structure of its brain, then, the platypus proves to be
an animal with a considerable degree of intelligence, with a cerebrum
better organized than that of the lower marsupials and even of some of
the lower Monodelphia. A well-organized brain and a large surface of
cerebral cortex indicate a degree of intelligence far removed from
that of the reptiles.
The psychology and sensory perceptions of animals other than ourselves
present a very difficult field for investigation. Philosophers have
never tired of telling us that the world about us is, though not
exactly a figment of our imaginations, yet an illusion due to the
limitations of our sensory perceptions. The physicist further confuses
our simple minds by supporting the philosopher with a statement that
the trees and grass and bricks and mortar which we imagine we see
are mere buzzings of electrons. We have, it is true, a proper scorn
for these attempts to befog us; yet, when we come to inquire into
the mental attributes or sense-reactions of any animal other than
ourselves, we are brought up sharply by an uneasy suspicion that the
philosopher may be right. Fabre, describing the behaviour of a wasp,
may record all its visible actions with scrupulous accuracy; when
he comes to interpretation of them, he can interpret only in terms
of Henri Fabre. We cannot project our minds into the mind of any
other animal; nor can we be sure that any other animal sees, smells,
tastes, hears, or feels just what we see, smell, taste, hear, and
feel. In fact, we know that there are differences, that our eyesight
is much inferior to that of the vultures, our sense of smell to that
of predatory carnivorous mammals in general, and so on. There is also
plenty of evidence that other animals possess sensory perceptions
denied to us. This being a matter which seriously affects human
prestige, we hasten to reassure ourselves by means of a dogmatic
statement that _we_ reason, while the rest of the animal kingdom,
unable to indulge in this majestic process, is at the mercy of tropisms
and instincts.
This digression is merely a warning that what we conclude about the
mentality and sensory processes of the platypus must be derived chiefly
from what we know of these processes in ourselves. We say ‘chiefly,’
because we have, after all, an evolutionary history of the development
of brains and sense-organs, and a surety (since our own have been
derived through those of the lower Mammalia) that the two must have a
great deal in common. Bearing in mind the limitations of the method, we
may proceed to an examination, first of the sensory perceptions, and
later of the intelligence, of the platypus, so far as we know anything
about them.
The eye of the platypus is small; on this account it has generally
been assumed that the animal’s vision is poor. The earlier observers,
obsessed by the small size of the organ and the burrowing habits of its
proprietor, were led to compare its powers of vision with those of the
mole. As a matter of fact, though small, the eye is extraordinarily
bright, and is during life not deep sunk (as is commonly reported
from examinations of spirit specimens) but fairly prominent. The eyes
are certainly placed high towards the dorsal surface of the head;
but, since they are used only for sub-aerial vision, this position
would seem to be a direct adaptation to the particular purpose.
Wood Jones has certainly written, and that quite recently (1923, p.
52):--“Although the eye of the living animal is said to be brilliant,
it is small and deep-set, and so situated that its range of vision
must be very limited. In any case, one would not expect the sense of
vision to be remarkably acute in an animal which spends so much of its
time in a dark burrow, or at the bottom of the water.” I know that the
animal does not use its eyes under water--an observation which was not
available to Wood Jones--and I believe, though I have no proof of this,
that they are not kept open in the burrow. Their whole purpose is that
of sub-aerial vision, and they are so situated as to have the widest
possible range of vision when the animal is floating at the surface
with just the top of the flat head exposed.
Bennett (1860, p. 135) observes of his captive animals:--“When running,
they were exceedingly animated, their little eyes glistened, and the
orifices of their ears contracted and dilated with rapidity.... Their
eyes being placed so high on the head, they do not see objects well in
a straight line, and consequently run against everything in the room
during their perambulations, spreading confusion among all the light
and easily overturnable articles. I have occasionally seen them elevate
the head, as if to regard objects above or around them.”
When the animal is swimming in the water, it often raises its head,
so that the short neck is vertical to, and the plane of the head
horizontal with, the surface. Anyone who has ever attempted to shoot a
platypus is soon made to realize the sharpness of its vision. Bennett,
Semon, and many others have shown how useless it is to move towards the
animal when it is at the surface. The slightest movement is enough to
send it down; and, when it dives in alarm, it rarely reappears during
the same feeding period. It will also dive at the flash of a gun.
My own observations go to show that, while the vision is acute in
diffused daylight (e.g., at the usual crepuscular feeding-times),
bright sunlight seems to affect it very considerably. When the sun
is shining, I find that there is no need to take precautions against
abrupt movements, as these do not seem to be observed. A gun may be
raised and pointed at the animal without the slightest notice being
taken. I have on many occasions released, during the daytime, platypus
collected for purposes of photography, and noted that, while the
animal dashes hurriedly for the water, when once there it displays no
further alarm. Waving the arms does not frighten it. The softest clap
of the hands, however, is sufficient to cause it to dive instantly. I
therefore believe that the auditory sense is much more acute than the
visual, at least during bright sunlight. This observation may account
for the apparent tameness of the animals observed by Geoffrey Smith.
[Illustration: EXPERIMENT ON SENSORY PERCEPTIONS OF PLATYPUS BENEATH
WATER.
1. Touching top lip; 2. Retreat from danger; 3. Returning from the
surface; 4, 5, 6. Hands held still for testing sense of smell.
_Cinematograph by J. S. P. Ramsay._
_Plate 13_]
I cannot say what kind of image is formed upon its retina; but we
are not entitled to assume that its power of sight is limited to the
detection of objects in movement, though that would appear to be the
chief purpose subserved. My own experience tells me that moving objects
are more easily perceived than stationary ones; and anyone who has had
successful experience of approaching or taming wild animals well knows
that the thing to be avoided at all costs is abrupt movement. The
capacity for detecting this kind of movement is as fully developed in
the platypus as in any other mammal, at least over the short range
which is necessary for the animal’s safety and well-being. Apart from
this, we have no criteria for comparing its powers of sight with those
of other animals, and certainly no justification, apart from the small
size of the eye, for assuming that these are limited.
The auditory sense is undoubtedly acute. The creature dives on hearing
an unaccustomed noise just as rapidly as on seeing an unaccustomed
movement. Bennett, in the quotation given above, remarks upon the
dilatation and contraction of the aural orifice, which, in spite of the
absence of a definite auricle, remind one irresistibly of the nervous
twitchings and twistings of the ear made by a spirited and fidgety
horse. No observer of the living animal could have any doubt that it
depends very largely upon auditory sensations. Semon (1894, p. 11)
writes:--“Every doubtful noise causes it to disappear. I saw one dive
immediately on the discharge of a gun a mile away. It appeared again
rather soon, which decidedly it would not have done had it been alarmed
by a sound at close quarters.” In this instance, however, I think
the diving and distant report were merely a coincidence. As regards
Bennett’s statement concerning the twitching of the facial furrow
at the region of the ear-hole, I find that the platypus is actually
capable at will, of “cocking” it to act as a temporary auricle to pick
up sounds (see Plate 6, fig. 2).
The olfactory organs are more definitely reptilian in character than
the rest of the sense-organs. To determine the measure of this sense, a
careful experimental enquiry would be necessary, and the platypus has
not shown any inclination to become a tractable laboratory animal. The
large size of the olfactory nerves, however, and the development of
scent-glands, are sufficient proof that the platypus has the sense of
smell.
The optic nerves are small; the olfactory relatively large, though
not so enormously developed as in the echidna. Those branches of the
trigeminal nerve which are distributed to the muzzle are, however,
relatively enormous, a mass of nerve-fibres passing out through the
infra-orbital foramen above, and the inferior dental and mental
foramina below, and dividing up to go to every part of the sensitive
naked skin. These nerves end in special tactile corpuscles, the sites
of which are marked by innumerable pores (or pits) thickly scattered
over the naked integument. The whole of this area forms, as has been
indicated before, a remarkably sensitive tactile apparatus without
parallel among the Mammalia.
The most sensitive portion of this remarkable muzzle is undoubtedly the
anterior border of the upper lip. As Bennett has remarked, the animal
exhibits signs of acute discomfort when this lip is touched or pressed,
and struggles violently to withdraw. At the date of this statement
Bennett was evidently unaware of the fact that the sensitive lip of
a platypus is used vigorously as a shovel when burrowing. Of course,
this portion of the muzzle would be the first to come into contact with
those animals which serve as food. Verreaux (1848, p. 129) states that
in burrowing the beak is first used to dig the soil.
The extension of the naked integument into dorsal and ventral flaps is
very remarkable, and there has been much surmise as to the function of
these. Bennett first suggested (1860, p. 100):--
“In the base of both the lower and upper mandibles is a transverse
loose fold or flap of the integument, always similar in colour to
the skin covering the mandibles, that is to say, of a dull dirty
greyish-black in the upper, and white or mottled in the lower. In
the upper mandible this is continued very nearly to the eyes, and
may perhaps afford some protection to those organs when the animal
is engaged in burrowing, or seeking its food in the mud. The upper
fold or flap is continuous with another portion arising from the
lower mandible, also at its base. Some consider the use of these
folds to be to prevent the beak from being pushed into the soft
mud beyond this part, which is so broad as completely to stop its
further progress. From careful observation of the actions of living
specimens, I can assign no other use to this part than that which I
have just mentioned.”
Oldfield Thomas (1888, p. 388) states:--
“Beak smooth, evenly rounded, its junction with the head marked, both
above and below, by a projecting leathery flap, evidently developed
to save the face from injury when the head is plunged in mud or
gravel.”
Since the platypus has been observed to bury itself up to half its
body-length in mud and gravel, it becomes obvious that the purpose of
these flaps cannot be that of preventing it from burying more than
its ‘beak.’ If the function of the flaps be to protect the eyes, it
is a remarkable fact that the dorsal flap is emarginated at each side
just in front of the eye, where the protection would be most needed.
Moreover, the ventral flap could take no part in such protection,
and the eyes and ear are already quite sufficiently protected by the
apposable lids of the facial furrow. I am thus forced to the conclusion
that the flaps subserve a function wholly tactile, a conclusion
supported by the fact that their anterior faces are studded with
touch-corpuscles. In the platypus, therefore, these flaps may be said
to take the place, and serve the purpose, of the facial vibrissae of
higher mammals (see Plate 12, fig. 1).
Passing to the sense of taste, we again find ourselves without means
for determining the precise degree of gustatory perception possessed
by the platypus. Poulton (1883) has investigated the structure of the
tongue. The anterior portion, which is free from the floor of the
mouth for only a third of its length, and is therefore capable of
only very limited movement, is covered with rough papillae directed
backwards, between the bases of which lie many mucous glands. This part
of the tongue contains only tactile terminal organs, somewhat like the
Pacinian corpuscles of ducks, and is concerned with the passing of
food back to the grinders. The taste-areas occur upon the posterior,
swollen portion of the tongue, and are four in number. The anterior
pair are situated in deep antero-lateral grooves, the posterior in much
shallower postero-lateral depressions. All four grooves are crammed
with taste-buds, approximately of the same type as those occurring in
mammals in general. The platypus is a dainty feeder, and it would seem
that its taste-sense is an important feature in its general economy. It
prefers its food alive, although in captivity hunger will induce it to
accept dead food. This must, however, be fresh.
In addition to the five commonly-accepted senses, which are provided
for by special sense-organs, I have referred elsewhere to the
remarkable development of a sense of direction which the platypus
displays in locating the nearest water, in detecting obstacles
beneath the earth (thus inducing it to alter the course of its
burrow), and in procuring its food. The first of these powers is the
common possession of many animals, but is not, so far as I am aware,
understood or accounted for in any of them. The second and third are
quite as remarkable. Even in man there are still traces of a sense of
perception of something solid in the immediate neighbourhood. This
so-called “sixth sense” is probably due to the intense development of
some extremely sensitive organ not yet accurately defined. The ability
to “sense” the proximity of certain objects may be exemplified in the
bat, which flits unharmed about the darkest caverns, aided probably
by highly-sensitized skin-folds situated on its snout, and in certain
fishes, in which the well-known lateral-line organs help to serve a
similar purpose.
My observations of the platypus under water support the view that, of
the five senses ordinarily possessed by animals, the only one operative
while the platypus is gathering its food at the bottom of a river or
water-hole is that of touch. My opinion is that this animal must have
developed some extraordinary means of finding its prey, apart from the
sense of touch, and that the sensory apparatus through which this acts
is connected in some way with the fleshy nature of the bill. If this
“sixth sense” is not responsible, then we must fall back upon that
makeshift word “instinct,” which is referred to by Hornaday (1922, p.
9) when he says “Instinct often functions as a sixth sense.”
In a recent controversy concerning its use of the senses of smell and
touch while searching for food at the bottom of a river, the question
arose as to whether it relied on either of these exclusively, or on
both. No attempt had previously been made to solve the problem, so I
there and then decided to investigate systematically, and had a glass
tank constructed, with an observation chamber (see Plate 13). I was
well aware at the outset that the intended captive would be nervous
and excited, but concluded that such unavoidable conditions might be
to some extent counterbalanced by liberating it suddenly into fifty
gallons of cool, fresh water. At least, I imagined that a sudden
stimulant of that nature would somewhat, if only temporarily, lessen
the shock, since my presence would be unsuspected.
When all was in readiness for the try-out, I obtained an uninjured
adult female from a burrow near by, and, so as to give her ample
experience of the smell of human hands (to say nothing of the sight and
feel of them), I handled her repeatedly and deliberately, occasionally
rubbing my hands, back and front, over her sniffing nostrils, besides
frequently fingering her sensitive lips. I reasoned that, if her sense
of smell was only half as efficient below water as on top, she might be
expected to shy clear of the submerged hands that had just previously
overhauled her. But such was not the case; in fact, quite the reverse
occurred, and to the marked astonishment of several onlookers she
instantly sounded, and, after probing her way along the sandy bottom of
the test-tank, reclined leisurely beneath the unscrupulous hands which,
only a few minutes earlier, were beyond doubt the dread of her precious
life. Several times she swam to the surface for a timely blow and
repeated the first performance, notwithstanding that I had deliberately
interfered with her several times below, with the specific object of
forcing her into action (see Plate 13, fig. 3).
While she was practically stationary on the sand beneath my hands, I
placed my finger directly above, and within an eighth of an inch of
her nostrils. But obviously she was quite unaware of my presence or
behaviour until my finger came in contact with the tip of her bill.
“Cat-like,” then, she arched her back, raised her tail, turned a kind
of twisting somersault, and swam away (see Plate 13, fig. 1). Again
she returned and casually nuzzled my hands as though nothing unusual
had happened. Possibly the tap on the snout did not seem to her to
be any more than the sudden flip of a startled shrimp. Nevertheless,
I am quite convinced that she did not return solely to satisfy her
curiosity, or purposely for shelter, as she had already taken advantage
of that elsewhere in the tank.
Judging from all that occurred, I ultimately came to the conclusion
that, if the creature had any sense of smell at all beneath the water,
she certainly did not display it during my investigations. The supposed
abnormal sense of touch was also not nearly so keen as I had surmised,
at any rate in the detection of immediate danger (at least of an
unfamiliar nature), while she was wholly submerged. Still, the bill may
be extremely sensitive and quite indispensable while sifting mud and
selecting suitable foodstuffs below. In that case (which I think may be
assumed) it is probably safe to assert that an adult platypus has no
enemies of consequence to fear beneath the water. Otherwise the docile
_Ornithorhynchus_ would surely not have fared so well during the many
evolutionary phases since prehistoric times.
What is it, then, that is responsible for the prolonged existence of
this primeval creature? Is it a sixth sense, or simply because such
creatures fare well--as they appear to do--on so-called mud? As regards
the five ordinary senses of a platypus when on land, or afloat, I have
nothing to add except direct confirmation, if necessary, of their
existence; but I suggest that, beneath the water, where mud is possibly
consumed regardless of taste, the sense of taste is without function,
and that it remains so until the animal swims to the surface and
leisurely masticates the food taken while below, at least in the case
of large prey.
Of the senses of hearing and seeing, so far as I could observe,
both were rendered inoperative, while the creature remained
submerged, principally by means of a water-tight facial furrow which
simultaneously envelops the orifice of both ear and eye directly the
animal decides to sound. I endeavoured to attract its attention by
shouting at the top of my voice, while it groped about my submerged
hands and around the glass helmet enveloping my head, without
demonstrating the slightest symptoms of agitation. Testing the sense of
sight proved rather perplexing. However, I came to the conclusion that,
strictly speaking, the animal could not then see, although, whenever
it came to a part of the tank where the light was at all strong, it
immediately turned and sought the most shaded parts.
As a guide to those interested in this subject, let me state that
it is practically impossible, owing to its timid nature, to test
accurately the sense of smell of a freshly-captured platypus beneath
water by introducing natural foods. Hence the omission of such an
attempt in this instance. Of course, a test could be made, and no doubt
successfully, with a contented and undisturbed captive, but I have
intentionally left that opening to other enthusiasts perchance intent
on checking my slender theoretical views.
In a previous test case[3] I proved beyond doubt that a platypus in
captivity will devour in assorted foodstuffs (not including mud) half
its own weight nightly. Now, if similar quantities are consumed when
the creature is at large, then, as a platypus providore, it is beyond
my imagination to comprehend how it obtains such an enormous supply,
unless mud be included in the bill of fare as a kind of “fill-up”
necessity, if not as a staple food.
Of the many platypus that I have had occasion to dissect, the
intestines and stomachs of most, especially the robust and
healthy-looking specimens, contained much mud, and apparently little
else, while the cheek-pouches of all bulged, crammed with grit, mud,
and crunched aquatic creatures, seemingly of minute forms.
On one occasion, in the presence of the late Charles M. Hoy, who was
collecting in Australia for the Smithsonian Institution, I extracted a
small shrimp (entire) from the stomach of a foraging female platypus,
captured at noon. Possibly, the crustacean slipped down accidentally.
Nevertheless, that query does not upset my theory that platypus are
indiscriminate mud-suckers, but it tends rather to strengthen my
contention that the sense of taste is apt to cease functioning beneath
water.
Live shrimps collected directly from the feeding-grounds of platypus
are readily devoured by _Ornithorhynchus_ in captivity. In fact most
specimens prefer them to indigenous river-bank worms or other varieties
of food supplied to them. From personal observation, I can vouch for
one male platypus in captivity that lived entirely on dead raw prawns
(salt-water variety) for eight months, and was then, apparently, in
splendid condition. Furthermore, it is said to have lived four months
longer, when unfortunately for the experimenters, it escaped from its
neglected enclosure. I have no hesitation in saying that platypus are
occasional mud-suckers, and that at least some part of their living
food is both discovered and secured by that sucking process.
While under water, the sense of touch is apparently the only one of
the five ordinary senses functioning fully, and, in the procuring of
food, even that sense appears to be less effective than the animal’s
mysterious ability to track and secure living prey.
Again, as mud is seemingly a necessity, if only to appease an enormous
appetite, why has the platypus such an abnormal nervous system of
touch? Surely the presence of mud could be readily determined by a
puddling platypus, without such special equipment as super-sensitized
abnormally-formed lips, etc.
If a platypus must actually touch before being aware of the presence of
shrimps or other active prey that it so relishes and thrives upon in
captivity, then, to my mind, such wary creatures would be rarely taken
alive when at large. On the other hand, if a sixth sense functions to
assure direction, then it would be quite possible for a platypus to
collect half of its own weight in live animal foodstuffs nightly, but
not otherwise.
The ease with which the platypus can be killed indicates that there is
something delicate in its organization. The ordinary method in shooting
is to fire a heavy rifle-bullet beneath the animal as it floats at the
surface of the water, and the concussion is usually sufficient to cause
its death.
What I have written is scanty enough; but I think it shows, despite
our lack of more precise knowledge, that the platypus is an
animal possessed of acute sensory perceptions, a delicate nervous
organization, an active metabolism, and a degree of cunning which
must be based upon considerable intelligence and which is far from
justifying Professor W. K. Parker’s epithet of ‘frog-witted duckbill.’
A final point of distinction is that the platypus possesses a voice.
Vocal powers are somewhat curiously distributed among vertebrate
animals. Frogs are notorious for vocal ability, and the Australian
species can hold their own with those of other parts of the world both
in the variety and in the volume of their choruses. Reptiles, however,
are practically voiceless. Snakes and some lizards hiss, and some
geckoes can scream when disturbed or handled, but the vast majority are
silent under all circumstances. The songs of birds have called forth
many tributes. The lower orders of mammals are not famously vocal. The
echidna, so far as I am aware, has no voice at all, the only sound it
makes being a sniff through its nostrils. Marsupials are in general
silent animals, the cough of the wallaby and staccato snort of the
bandicoot affording examples of the sounds they do make. Phalangers can
scream, but do so only exceptionally. The lower Monodelphia, too, have
remarkably little in the way of voice. That the platypus should possess
a voice was unexpected, and certainly few have recorded it. The furred
young, when disturbed, keep up a continuous growling noise, which it is
usual to compare to that of a growling puppy. The adults make a noise
which can best be imitated by a tremulous snoring.