CHAPTER VI. Nervous Organization and Sensory Perceptions

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.

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 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.