[Music Plays]
In one form or another the unusual way in which marsupials are born
has been the subject of strange myths and legends for hundreds of years.
The early navigators made many observations
on kangaroos and wallabies
as they explored the coast of the Great South Land.
Perhaps one of the earliest was the Dutch merchant François Pelsaert
who ran aground on the Abrolhos Islands
off the coast of Western Australia in 1628.
Despite the fact that his ship was wrecked
and some of the crew had mutinied,
Pelsaert was sufficiently observant to record some interesting notes
on the tammar wallabies, which live on these remote islands.
“Their manner of generation is exceedingly strange
and highly worth observing.
Below the belly the female carries a pouch
inside which are the teats
and we have found that the young ones grow up in this pouch.
We have seen young in the pouch which were only the size of a bean,
so that it seems certain that they grow there,
out of the teats from which they draw their food.
The young occupy the pouch
until they are grown up and able to walk
and there after keep creeping in to the pouch when they are hunted”.
By suggesting that marsupial young grew from the teat,
Pelsaert and others
created a myth which persists to the present day.
The facts of birth in kangaroos and other marsupials
were not finally known until scientific studies
were made of all aspects of their reproduction.
Most of these investigations have been carried out with red kangaroos
in captivity and this film has been complied during these observations.
In the wild red kangaroos live and breed throughout most of
inland Australia where the annual rainfall is less than 20 inches.
The red kangaroo is well adapted to these semi-arid conditions
and unlike most other marsupials it breeds all through the year.
This seems to be a very efficient form of adaptation to the environment
for if they had a regular breeding season
there could be frequent failures of whole generations of young
because they were born at times unfavourable for their survival.
The red kangaroo seems to avoid these catastrophes
by having young enter the population at all times of the year.
Those that leave the pouch at time when they can find
sufficient food and water survive.
Those that do not, perish.
In the red kangaroo only the males, like the one in the background,
have the characteristic rusty red fur.
The females, in the foreground, are usually blue grey in colour,
often referred to as blue flyers or blue does.
In some areas variations of colour can occur in either sex.
As in many other mammals,
female red kangaroos have a cyclic reproductive pattern.
The unmated female has a period of oestrus,
or sexual receptivity, every 35-days.
The male is continuously fertile
and will mate with a female whenever she is in oestrus.
The male becomes aware of the approach of sexual receptivity
and begins to follow the female.
Such behaviour may start two or more days before mating
and at times the chase may be energetic.
He smells the pouch and the external genital opening
and quite often grasps the tail of the female.
Once full sexual receptivity is attained
the female stands for the male and copulation occurs.
This may last for up to 20-minutes.
If mating is not observed it may be diagnosed from the vaginal smear,
a collection of cells from the vaginal tract of the female.
The sample is then placed on a slide
and under the microscope sperms can be seen
in the vaginal smears of kangaroos which have recently mated.
The reproductive organs of the female red kangaroo
lie in the body cavity
just behind the pouch and consist of two ovaries,
two fallopian tubes and two uteri.
The two uteri open separately into the anterior vaginal sinus.
The walls of this canal are thin
and capable of great extension after mating
as they become filled with seminal fluid.
The two lateral vaginae are much narrower canals
with a thick muscular wall.
The median vagina forms a direct connection
between the anterior vaginal sinus and the urogenital sinus;
it is down this median vagina that the young kangaroo passes at birth.
If we section this drawing we will be able to follow more
easily the various stages of fertilisation and pregnancy.
After mating the sperms passes up the two lateral vaginae
filling out the anterior vaginal sinus and moving up into the two uteri.
The mature follicle which has developed in one ovary
bursts just after oestrus and a single egg is shed.
This egg is fertilised in the fallopian tube
and moves down into the uterus.
The follicle collapses and is replaced by a corpus luteum,
an endocrine organ which produces the hormone of pregnancy, progesterone.
Now, the embryo starts the development which will last for about 33-days.
The embryo is surrounded by the yolk sack and a number of membranes.
Throughout this period there is no way of telling
whether a female kangaroo is pregnant or not.
This can only be diagnosed by opening up the animal
and finding a developing embryo in one of the two uteri.
However, the pregnant female is immediately distinguished
from the non-pregnant one
during the several hours just before birth.
At this time she begins an intensive period of pouch cleaning,
putting her muzzle right into the pouch, licking the lining.
She also licks around the opening of the pouch,
but rarely the fur between the pouch and the urogenital opening.
The female red kangaroo about to give birth
takes up a normal grooming position.
The back is usually supported
and the tail passed forwards between the hind legs.
Now the young has completed development
and starts to leave the uterus.
It goes down the median vagina,
a short cut between the uterus and the urogenital sinus.
Birth is preceded by the appearance of fluid from the ruptured yolk sack
and just before the young is born
the female vigorously licks the urogenital opening.
If you watch carefully you will be able to see one of the embryonic membranes,
the allantois sac fall to the ground just as the animal moves.
Now, the young is born, enclosed in another embryonic membrane,
the amnion
and quickly moves itself into a position where
it’s head is towards the pouch opening.
The young may be released from the amnion by the mother's licking
or by the use of the sharp claws which are present on the front limbs at birth.
During the early part of its climb to the pouch
the young is attached by umbilical cord to the yolk sack,
eventually this cord is broken,
sometimes by the licking action of the female
and the young is attached only by its grip on the fur.
The time taken for the young to reach the pouch
is about three minutes - ranging from one and a half minutes
to about four and a half minutes.
During birth and the journey to the pouch
the young seems to be ignored by the female
who concentrates on licking blood and birth fluids from the fur.
The female does not lick a track for the young to follow,
but licks mainly behind the young removing all traces of its passage.
After the young has reached the pouch
the female remains in the birth position licking the fur.
Fluid will continue to ooze from the urogenital opening for up to
40-mins after birth and all of it is carefully licked up by the female.
In the film we’ve just seen it was difficult to observe the moment of birth
because of the movement of the female as she continually licked the fur.
So to see this more closely
it is necessary to anaesthetise a female with Nembutal
a few minutes before the young is born.
This female is anaesthetised, again, a few millilitres
of fluid from the ruptured yolk sack appear just before birth.
The young is born head first enclosed in an embryonic membrane, the amnion.
Once released from this membrane the young starts
to make its way over the fur to the pouch.
The young, at birth, and for some time after
is attached to the yolk sack by the umbilical cord.
This cord stretches and eventually breaks
as the young makes its way to the pouch.
Sometimes the cord is broken by the licking of the female,
but with this anaesthetised animal the cord had to be broken artificially.
The umbilical cord is attached to the yolk sack
which is still in the birth canal.
The newborn red kangaroo, as with the newborn of other marsupials,
is entirely without fur. It is blind and
lacks even the development of organs
giving a sense of balance.
However, those centres of the brain concerned in the sense of smell
are well developed and the nostrils are open and large.
During its climb to the pouch only the front limbs are used
since the hind limbs which become so large later in life
are mere buds at this stage.
The fur below the pouch lies in such a way that the ends
of the hairs point towards the mid-line
so that the young kangaroo crawls in the general direction of
the ends of the hairs over the fur rather than through it.
Now the young has reached the edge of the pouch
after its remarkable journey over the fur.
Once inside it will stay there for about six months
before it starts to leave the pouch for short periods.
As the young crawls down into the pouch,
notice the lack of development in the hind limbs
and that they are not being used for locomotion at all.
On this female the pouch is being held open so that the young can be filmed.
With the pouch held open like this it’s unlikely that the
young will attach to a teat because it seems to need
the ventral wall of the pouch as support while drawing
the teat into its circular and terminal mouth opening.
Later it finds an attaches itself to one of the four teats.
Its tongue is large and muscular and capable of a suction pump like action.
This sucking stimulus on the teat
causes the release of clear fluid on which the newborn young lives.
As it grows this fluid will become more like the usual milk of mammals.
Once it has attached the young maintains a strong grip on the teat
which forms a bulbous swelling inside the mouth.
It is because of this that many people have been led to believe
that the young kangaroo was born on the teat,
presumably by some process of buddying off.
However, the young can be removed from the teat
provided that it is less than one day old.
If it is any older than this
it will probably be unable to draw the teat into its mouth again.
Newborn young removed from a teat
can be placed on another teat in the same pouch.
The sucking stimulus will cause the release of fluid from the new teat.
The newborn young may also be removed from the pouch of one female
and transferred to a teat in the pouch of another female.
The clear fluid on which the newborn young is suckled
is present 33-days after estrus
whether the female has mated and become pregnant or not.
Females which have never been mated will raise young transferred to their pouches
provided that the transfer takes place 33-days after oestrus.
In most marsupials the oestrous cycle is longer than the gestation period.
In the red kangaroo the oestrous cycle lasts for an average of 35-days,
while the gestation period is 33-days.
Pregnancy does not prevent a return to oestrus
just after the young is born,
however, the sucking stimulus of the new born young
terminates further oestrous cycling
for as long as the young is suckled in the pouch.
Because a female kangaroo returns to oestrus a few days after giving birth,
mating and fertilisation can again take place.
This postpartum mating is similar in many respects
to the mating earlier in the film,
this time however, the developing follicle is in the left hand ovary.
The recently vacated and partly collapsed uterus is in on the right.
Again, sperms enter the uterus.
The follicle bursts shedding a single egg
which is fertilised in the fallopian tube.
The fertilised egg descends into the uterus
but only develops to a blastocyst stage
with little or no change in actual size.
This is because the sucking stimulus of the young already in the pouch
prevents the complete development of the embryo.
This dormant blastocyst now remains in a partly developed state
while the first young grows up in the pouch, usually about 200-days.
As this first young grows the composition of the milk
gradually changes to become thick and white,
quite different from the clear fluid on which the newborn young was fed.
It is a remarkable feature of kangaroo reproduction,
but when two young of different ages are being fed at the same time
the mother is producing milk of two different compositions.
In this pouch the four month old young
is feeding from a teat which has become greatly elongated.
On the right is a second elongated teat
on which the previous young, now out of the pouch, is feeding.
Not until it is about 150-days old
does the young first put its head out of the pouch.
Soon afterwards it begins to eat grass from a position in the pouch.
By the time it’s about 200-hundred days old
it makes the first short journey from its mother.
When the 200-day old young starts to leave the pouch
the sucking stimulus is lessened and the blastocyst resumes development.
This also happens if the young is lost prematurely.
After about 31-days the new embryo is ready to be born.
The young out of the pouch
stills feeds from the elongated teat which it occupied throughout pouch life
and which it now grasps from the outside.
Under ideal circumstances
adult female red kangaroos produce one young every eight months.
Since each young is dependent on its mother until
it’s about 12-months old,
females are frequently accompanied by a juvenile
while suckling a young in the pouch.
Although François Pelsaert’s observations in 1628 were inaccurate
his interest in the details of marsupial reproduction
is a tribute to his enquiring spirit.
“Their manner of generation is exceedingly strange
and highly worth observing.
Below the belly the female carries a pouch inside which are the teats”.
The facts of marsupial birth have been more accurately observed
in recent years and are still highly worth observing.
The production of a very small young
which climbs virtually unaided into the pouch
and develops as an externalised foetus is common to all marsupials.
In red kangaroos the offspring rarely fails to find the teat
and fertilisation immediately after birth
ensures that the female has a reserve embryo
should the first fail to complete pouch development.
Although this mechanism is remarkably different from that in other mammals
it is highly efficient
and seems well adapted to the environment in which these marsupials live.
[Music Plays]