The echidna (or spiny ant-eater) (1969)

[Image of an echidna appears on screen with the text: The Echidna (or Spiny Ant-eater)]

[New text appears: A CSIRO Film]

[New text appears: Produced by the Film Unit collaborating with the Division of Wildlife Research, 1969]

Narrator: The echidna and the platypus belong to a group of egg laying mammals called the prototheria. They are called mammals because they have mammary glands for the sustenance of the young, but at the same time they exhibit reptilian characters, which make them of particular fascination for students of evolution.

[Image changes to a map of Australia and New Guinea, these places here highlighted black]

The prototheria occur only in Australia and New Guinea. There are two genera of echidnas. One, tachyglossus, ranges through the whole of Australia, from the wetter coastal regions to the arid interior, as well as the south-east part of New Guinea.

[Text appears: Tachyglossus, placed on the right hand side at the top of Australia. The camera zooms in on this area and Australia and New Guinea have turned a peach colour]

The other, zaglossus, is now found only in the higher parts of New Guinea, and probably Salawati Island.

[Text appears: Zaglossus, placed underneath New Guinea. The camera zooms in on this area and a strip of yellow appears through the middle of New Guinea]

[Image changes back to the echidna moving through the grass]

This is a mainland Australian echidna, tachyglossus aculeatus, which is a terrestrial warm blooded mammal covered with hair.

[Camera zooms in on the echidna’s hair]

Some of these hairs have been modified into spines, like those of the porcupine.

[Image changes to show the face of the echidna]

The eyes are small and beady, and the retina is composed entirely of rods, probably giving it good vision in poor light. In the long snout is a vermiform tongue used for catching ants and termites on which the animal feeds, hence the name spiny ant-eater.

[Image changes to show two hands parting the echidna’s hair to expose its ear]

The ear has no visible pinna, but it has one buried in the muscles of the head.

[Image changes to show an echidna hanging upside with its belly facing the camera]

Echidnas have only one opening for the passage of reproductive products, and the urine and faeces. For this reason echidnas and platypuses are called monotremes, which means one holed.

[Camera zooms in on the upside down echidna and pans up it slowly]

The spiny ant-eater exhibits a curious collection of reptilian characters. The male animal has no scrotum; the testes are internal, as they are in some eutherian mammals.

[Image changes to a skeleton of an echidna]

Other reptilian characters of the echidna can be seen in the skeleton – an examination of the pectoral girdle shows that it has coracoids and precoracoids extending to the sternum, and in interclavicle.

[Camera pans over to a skeleton of a goanna with three bones labelled. The Precoracoid, Coracoid and Inter-Clavicle.

The goanna pectoral girdle has all these bones. They can also be found in the skeletons of extinct therapsid reptiles.

[Image has changed back to the skeleton of the echidna, in particular its pelvic girdle]

The echidna pelvic girdle however is mammalian in structure, and even has epipubic bones identical to those of marsupials. The wombat pelvic girdle demonstrates this point quite well.

[Camera pans over to a skeleton of a wombat, in particular its pelvic girdle]

[Image changes to show a man walking through the bush looking for an echidna. He finds one buried lightly under some forest litter]

Echidnas are powerful diggers, but they don’t live in burrows like rabbits, they seek out well camouflaged hiding places which gives the impression that echidnas are much rarer than they really are. The claws of the forepaws are spatulate, enabling them to dig in hard earth and stony ground, and even to rip open small logs to get at the ants and termites inside.

[Inage changes to show an echidna scratching at a small log]

This is a subspecies from Kangaroo Island. It is known as tachyglossus aculeatus multiaculeatus. There are two colour polymorphs or varieties of the Kangaroo Island echidna. One is brown, and the other is straw coloured.

[Image changes to a shot of the two varieties of echidna side by side]

When on the move searching for food echidnas are active and inquisitive. Sense of smell plays a most important part in detecting prey, and this is assisted by the tactile functions of the snout. Physiological experiments on the cochlea suggest that the snout also has auditory properties, so that echidnas may even hear moving insects with it.

[Image changes to different shots of the echidna searching for food]

Apart from these specialised functions, the snout is a powerful tool for penetrating wood, to enable the tongue to get at hidden prey. Together with the forepaws, it is used to rip open forest litter to expose ants or termites. On occasions it can be used for bulldozing through light forest soil. Sometimes a more dexterous approach is called for, to lick off very small ants from individual sticks. The ants bite, but the long grooming claw can be used to relieve irritation.

At a single feeding, echidnas do not concentrate on one kind of insect, but range over a wide area, feeding on different kinds of ants and termites in different habitats.

[Image changes to show a large ants nest and then zooms in on the entrance of the nest showing ants coming and going]

They have a great liking for meat ants, iridomyrmex detectus, which live in mounds. In spring many of these mounds are broken open, usually on the north side where the queen ants have come to the surface seeking the warmth of the sun.

[Image changes to show the echidna breaking open the ant mound to get to the ants]

The echidna feeds on the fat filled bodies of the queen ants that can be found in the mounds at this time of year. Echidna attacks leave large holes at the mounds, but after the queens leave on their nuptial flight echidna attacks cease, and are not renewed until the following spring when virgin queens again move to the surface.

[Image changes to a shot of the bush, the camera pans around and then zooms in on the ground]

Not all echidnas live in dense scrub. Some live in sparsely covered grasslands. This is a soft Spinifex ironbark association, growing on sand in western Queensland. In this sort of country echidna sign, in the form of tracks, is easily detected.

[Camera zooms in on the echidna prints on the ground]

Droppings also give a clue to the animals present in the area.

[Camera pans over the ground and then zooms in on the droppings]

We can see that there are both kangaroos and echidnas here.

[Image changes to man with a spade in his hand looking under a log]

If we follow the tracks, not infrequently we find that they lead to the echidnas hideouts in the sand, where they like to hold up to avoid the heat of the day.

[Image changes to show the man clearing away ground with his spade]

Echidnas have to avoid the heat in this way when temperatures rise above 35 degrees centigrade. They are poor thermo regulators at ambient temperatures higher than this. If they cannot escape from hot conditions by such means, they will die of heat apoplexy.

[Camera zooms in on the hole being dug, the man reaches down and pulls out an echidna]

This is a variety known as Collets echidna, which is found in Queensland, Central Australia, and the Northern Territory.

[Image changes to show an echidna hanging upside with its back facing the camera. The camera tilts up the echidna]

Unlike other spiny ant-eaters, it has very large spines and no hair on the back. In the cool of the late afternoon Collets echidna is active and ranges through the Spinifex looking for food.

[Image changes to show the echidna on the move looking for food. Then to a man collecting droppings which are put under a microscope]

There is no need to observe the animal actually feeding to find out what it eats – examination of droppings under the microscope gives this information, since echidnas do not digest the skeletons of the insects that they eat, and these are invariably found in their droppings.

[Image changes to show the hard, undigested sections of the ants and termites under the microscope]

In this instance the echidna has been eating both ants and termites. Very often these remains can be identified to species level.

[Image changes to show a man measuring out food]

When echidnas are kept in captivity they may conveniently be fed on the termite, nasutitermes exitiosus. The mounds in which these insects live are broken up and separation is effected by the simple method of allowing the termites to crawl off a glass plate and fall into a collecting tray.

[Image changes to show the termites leaving the mounds and falling onto the trays where they are scooped up and put into jars]

The termites are then harvested into jars and kept deep frozen until they are needed.

[Image changes to show an echidna in a small cage]

This animal is being held in a metabolism cage. Its rate of growth and the nutritional value of various insects can be studied. A three kilogram echidna eating about 150 grams of termites per day will be a negative nitrogen balance, in other words this quantity of food does not provide enough protein to support growth.

[Image changes to show a graph on screen measuring the weight gain of a diet with termites alone and then when glucose is added to the diet]

However, if glucose is added to this ration, far more nitrogen is retained and more protein is formed. There is then an increase in growth rate. This shows that a natural food like termites is deficient in carbohydrates, and it is therefore a limiting factor for the growth of an echidna living on such a diet.
With the animal captive in a metabolism cage we were able to study the action of the tongue with a high speed camera, slowing the action down ten times.

[Camera zooms in on the echidna’s tongue as it eats termites]

The scientific name tachyglossus means quick tongue. The jaws are toothless and are extended into this long snout which helps to house the long prehensile tongue. The tongue is lubricated with a sticky secretion of the sublingual gland. The insects sticking to the tongue are drawn into the buccal cavity, where they are ground up finely between keratinous ridges on the pallet and a series of keratinous spines situated at the base of the tongue. The rhythmic sound of an echidna feeding is quite unlike the crushing, tearing sound of say a carnivore eating.

[Camera zooms in on the echidna eating, a grinding sound can be heard]

The structures involved in the grinding can be seen in these preserved specimens.

[A man has picked up the specimen with tweezers and holds it up closer to the camera]

This is the palette showing keratinous ridges, and this is the tongue with a knob at its base bearing keratinous spines.

[The camera zooms in on the spines]

[Image changes to show a graph depicting the breeding season of the echidnas]

The breeding season of echidnas is short. In the females uterine eggs are found through July to late September. In the males spermatogenesis begins about April, and reaches a maximum of activity in July and August.

[Image changes to show two images of microscope slides showing the differences between active and inactive testicles]

During the rest of the year the testes are inactive, the tubules being densely packed with undifferentiated cells. During the breeding season the tubules of the active testes contains spermatozoa.

[Image changes to a shot of spermatozoa under a microscope]

They’re thin elongated coiled heads give them a striking resemblance to those of sauropsida, which are represented today by the living reptiles and the birds.

Here are sauropsidan spermatozoa for comparison.

[Image changes to a shot of sauropsidan spermatozoa under a microscope]

These differ from those of echidnas in not having rounded cytoplasmic droplets, but the general structure is the same.

[An animated diagram of what the narrator is saying appears on screen]

The reproductive organs of the female consist of two ovaries, two fallopian tubes, and two uteri opening into the urogenital sinus, which in turn opens to the exterior via the cloaca. After mating the sperm pass up the urogenital sinus, enter the uteri, and ascend to the fallopian tubes. The mature follicle in the ovary bursts and emits a very large egg filled with yoke and resembling that of a reptile.
The egg passes to the fallopian tube, where fertilization takes place. The male and female pronuclei fuse to form the zygote. As it passes down the fallopian tube, a thick layer of albumin, secreted by glands in the oviduct, is deposited around the egg. After ovulation a corpus luteum forms in the collapsed follicle. When the fertilized egg settles in the uterus it is ready for the cleavage process. This occurs in the germinal disc. The cleavage is meroblastic, as in reptiles. The first furrow divides the germinal disc into a larger and a smaller area. The second furrow is laid down at right angles, so that the four cell stage consists of two large and two small blastomeres. Further division of the blastomeres leads to the formation of a blastodisc which grows and finally envelopes the yoke to form a blastocyst.
While it is in the uterus the egg absorbs secretions which cause it to grow. These and the yoke will supply a nutriment to the embryo after the egg is laid. Three distinct shell membranes have been deposited around the egg during the growth period. Eighteen to 27 days after mating the single egg leaves the uterus and passes down the urogenital sinus.

[A microscope slide cross sectional image of the corpus luteum, which has a sponge like appearance, appears on screen]

When the egg is laid the corpus luteum is already degenerate, and shows development of vacuoles.

[Image changes to a man cradling an echidna who is on its back and zooms in on its under body]

A pouch develops on the ventral surface of the echidna at the beginning of the breeding season, and the egg is presumably laid directly into it. This is not on scientific record as ever having been observed, but the thrusting out of the cloaca and these contortions of the body clearly demonstrate that the animal is capable of laying the egg into the pouch directly from its urogenital opening.

The egg is carried in the animal’s pouch, and is held there by apposition of the lips of the pouch.

[The camera zooms in on this area of the echidna]

It has diameters of approximately 16 and 13 millimetres, and has a rubbery shell.

[The camera zooms in again and an egg is visible in the pouch]

After ten, to ten and a half days incubation, a young echidna breaks out of its egg and attaches itself to a milk patch from which it sucks milk.

[The camera zooms in on the tiny echidna]

The four limbs are very well developed, but the hind limbs are rudimentary. In this, and many other ways, it closely resembles marsupial young of the same age. This is a newborn marsupial in the pouch with similar well developed forelimbs and immature hind limbs.

[Image changes to a shot of marsupial young showing the similarities]

How these young find their way to the milk patch or teat unassisted by the mother is not known, but probably sense of smell plays a large part in each case.

[Image changes to show a cross section of the head of a baby echidna]

A section of a one day old echidna’s head shows that the olfactory epithelium is differentiated. Sense cells are well developed, so it could find its way to the milk patch by sense of smell.

[Image changes to show a man picking up an echidna, the pouch and baby become visible]

The female carries the young in the pouch for about six weeks, until it starts to develop spines. When the adult is held up like this the pouch muscles contract and the young one nearly falls out. In this case the young is about 35 days old, and the spines have not yet developed.

[Image changes to show a baby echidna being extracted from its mothers pouch]

Back in the laboratory we were able to extract the young for examination. We can see the pouch young now has a well developed snout, and that his hind limbs are as well developed as his forelimbs.

[The baby echidna is rotated gently around so these sections can be seen]

The ear opening is still occluded, and the eyes not yet open. The milk produced at the aerola or milk patches at this stage of growth of the pouch young is thick and creamy, containing about 50% solids.

[The pouch is opened to reveal the milk patch and milk is squeezed out]

When the young is about 70 days old it develops spines and the parent can then no longer carry it in her pouch. It is left in a burrow.

[Image changes to show a man retrieving a baby echidna from a burrow. The baby echidna is placed on a set of scales]

We weighed the young on frequent occasions, sometimes with considerable difficulty, and found that about 27 grams of milk had been imbibed at each feeding. The spines are well developed, and the eyes are now open.

[Camera zooms in on the baby echidna]

Even at this early stage the anlagen of the pouch can be seen.

[Image changes to show the man putting the baby echidna back into the burrow]

After weighing, the animal was returned to the burrow, where it quickly tried to get away from the light.

[Image changes to show the mother entering the burrow to feed her baby]

The adult returns at one or two day intervals to suckle the young. To feed her young the mother stands over the top of the infant, which hangs on upside down and thrusts its head into the pouch to gain access to the milk patches. After 20 to 30 minutes the young is pushed aside.

A single feeding every one or two days is enough to maintain growth.

[Image changes to a graph showing the weight increase of the baby echidna]

The sharp weight rises occur at the feeding times, and the arrow indicates when the young has left the pouch. As we have seen, the adult produces milk, but has no teats.

[The camera zooms in on the pouch and squeezed to show the aerola and the milk]

The milk is ejected from two milk patches or aerola found within the pouch. This process can be studied under anaesthesia. Ejection of the milk is brought about by the action of the pituitary gland hormone oxytocin. When oxytocin is injected milk flows freely from the aerola about seven minutes later, and as much as ten millilitres can be collected for analysis.

[The milk sample is collected and put into a test tube. Then a microscope slide of a cross-section of the myoepithelium appears on screen]

The action of the oxytocin is to bring about contraction of the myoepithelial cells investing the hundreds of alveoli within the lobules. This raises the intra alveoli pressure, and so the milk is ejected in a way identical to that in other mammals. Milk actually appears at the base of mammary hairs.

[The camera zooms in on milk being ejected]

This has led to the erroneous idea that the milk is licked off the hair. In fact the young sucks its milk like any other mammal. Under normal conditions this sucking is one of the causes of milk letdown.

[Image changes to show the milk samples being tested and then to a graph]

The milk of echidnas is rich in fat, the amount changes with the age of the young. At hatching it contains about 1.3% crude lipid, but mature milk has as high as 35% crude lipid. Dolphin milk has a similar lipid content, but for man the content is much lower. Gas chromotography enables us to separate and estimate the amounts of the component fatty acids of the lipids.
[Image changes to show the milk sample being put into a machine and then to another graph]
This analysis shows that the percentage composition of these acids in milk obtained at hatching consists mainly of palmitic and oleic acids. In mature milk there is proportionately far more oleic acid. Kangaroo milks share the same changes during the lactation period. Echidna milks have almost no short chain fatty acids. In this way they are not unlike the milks of insectivores and marsupials.

[Image changes to show the echidna moving through the bush]

Echidnas are fascinating animals to the student of evolution, partly due to the fact that they have such detailed resemblances to other living mammals – a marsupial type larvae, epipubic bones, mammary glands with alveoli and myoepithelium responsive to oxytocin, milk like that of marsupials and insectivores, a well developed maternal behaviour, which is a characteristic of other mammals, highly evolved and fantastically sophisticated specialisations. Yet at the same time they exhibit so called primitive reptilian characters such as a reptilian pectoral girdle, reptilian spermatozoa, the habit of laying eggs, they even have a reptilian walk

It is true that reptile like animals evolved into mammals in past ages, but we certainly don’t have evidence that echidnas gave rise to other present day mammals, the marsupials and the eutherians, represented here by the kangaroo and the fox.

[Image changes to show a shot of a kangaroo and then a fox]

In fact, the fossil record strongly suggests that echidnas have evolved entirely independently of these.

[Image changes back to an echidna]

Some kinds of reptile like animals evolved mammalian characteristics independently in past ages. This phenomenon is called parallel evolution or convergence. The task facing the zoologist is to find out how this comes about.

[Credits roll: Script & Scientific Direction – Mervyn Griffiths, Photograph – Ederic Slater, Film Direction & Editing – Peter Bruce, Animation – Perce Watson, Aileen Weinberg, Production – Stan Evans. The end]