Tip:
Highlight text to annotate it
X
[Birds chirping]
(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.
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.
The other, zaglossus,
is now found only in the higher parts of New Guinea,
and probably Salawati Island.
This is a mainland Australian echidna,
tachyglossus aculeatus,
which is a terrestrial warm blooded mammal covered with hair.
Some of these hairs have been modified into spines,
like those of the porcupine.
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.
The ear has no visible pinna,
but it has one buried in the muscles of the head.
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. The spiny ant-eater exhibits a
curious collection of reptilian characters.
The male animal has no ***;
The *** are internal, as they are in some eutherian mammals.
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.
The goanna pectoral girdle has all these bones.
They can also be found in the skeletons
of extinct therapsid reptiles.
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.
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.
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.
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.
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.
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.
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 *** queens again move to the surface.
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.
Droppings also give a clue to the animals present in the area.
We can see that there are both kangaroos and echidnas here.
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.
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.
This is a variety known as Collets echidna,
which is found in Queensland, Central Australia,
and the Northern Territory.
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.
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.
In this instance the echidna has been eating both ants and termites.
Very often these remains can be identified to species level.
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.
The termites are then harvested into jars
and kept deep frozen until they are needed.
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.
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.
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.
[Crunching noise]
The structures involved in the grinding
can be seen in these preserved specimens.
This is the palette showing keratinous ridges,
and this is the tongue with a *** at its base bearing keratinous spines.
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.
During the rest of the year the *** are inactive,
the tubules being densely packed with undifferentiated cells.
During the breeding season
the tubules of the active *** contains spermatozoa.
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.
These differ from those of echidnas
in not having rounded cytoplasmic droplets,
but the general structure is the same.
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 *** 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.
When the egg is laid the corpus luteum is already degenerate,
and shows development of vacuoles.
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.
It has diameters of approximately 16 and 13 millimetres,
and has a rubbery shell.
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 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.
How these young find their way to the milk patch or
*** unassisted by the mother is not known,
but probably sense of smell plays a large part in each case.
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.
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.
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 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.
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.
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.
Even at this early stage the anlagen of the pouch can be seen.
After weighing, the animal was returned to the burrow,
where it quickly tried to get away from the light.
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.
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 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 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.
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.
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.
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.
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.
In fact, the fossil record strongly suggests that echidnas
have evolved entirely independently of these.
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.
[birds chirping]