A wild ass foal on the plains of Northern lndia
is bonded to its mother.
But it has no idea who its father is
even the stallion whose sperm conceived it isn't certain.
Stallions have to battle with each other to decide who mates.
lt's the winners of these duals who appear to father the foals.
A mare in season gives the victorious stallion... a powerful libido
The mare will eventually accept him,
but a single act of sex doesn't necessarily guarantee conception,
for no male can be sure that the female hasn't already been mated
if so, his sperm will be competing with those of rival males.
and other stallions wait their chance to follow him
The sexual act starts the battle for the control of conception,
where sperm from different males race for the egg.
Sex began in the sea,
where bringing sperm and egg together was by far the biggest problem
These are eggs of reef building corals, jettisoned at night into the open ocean,
and free to drift and be diluted in this vast space.
lndividual, sedentary polyps synchronise
the release of billions of sperm and eggs
with the cycle of the moon when the tides are especially high.
To enhance the chances of conception,
coral sex is a game of astronomical numbers.
Even so, there are huge odds against the eggs being fertilised,
only one in 10 million will be successful.
ln the morning, all that remains on the surface
is a scum of dead sex cells and developing embryos.
But where animals can move,
the sophistication of their mating behaviour increases.
The male spotted box fish follows every move the female makes.
He times the release of his sperms to the moment
when she castes her eggs into the sea.
The female boxfish are coy,
another one waits to be picked up and escorted to the surface.
By spawning in pairs,
the male boxfish can be sure that the offspring will be his.
But female blue-headed wrasse force an entourage
of males to compete for their eggs.
When ready to spawn, individual females race to the surface,
hotly pursued by eager males,
sperm and eggs are simultaneously released in an explosive spectacle.
ln this form of broadcast spawning,
no male can be sure how successful he's been.
Such female-driven competition is taken
to unusual extremes in the shallows bays of Bermuda.
For three days after a full moon, about an hour after sunset,
mysterious lights appear on the sea.
They're made by female Bermuda fire worms
which emerge from the sea-bed, wriggle to the surface, and release
their eggs in coiled trails of luminous mucus.
The briefly glowing trails act as beacons for male fireworms,
they race each other to the spawning females
and fertilise their floating eggs.
ln fifteen minutes, the show reaches its peak.
Female fire worms have no control over whether their eggs
are fertilised or not, but the sea does harbour females who do.
This is a sexual orgy of squid off the coast of California.
ln these super-molluscs,
the eggs of each female are fertilised inside her body.
This way she can be sure of complete success.
Now the males compete to take possession
of her body and her concealed eggs,
and force their packets of sperm into her.
Once mated, possibly by many partners,
the females deposit their egg sacs in piles on the seabed.
But by retaining the fertilisation process inside her,
she has also presented the males with a challenge,
how to increase the chances of their own sperm meeting with success.
The male cuttle fish is very wise to this.
His assumption is that all females have been previously inseminated.
They mate head on.
But before doing so, he pumps water
vigorously through her body cavity to flush out
any parcels of sperm deposited by other males.
lnternal fertilisation gave animals the means to breed on land,
because their delicate sex cells could be safe from desiccation.
And on terra firma,
the battle of the sexes escalated.
Suspended on a thread of mucus a pair of land-lovers
slugs in a sexual embrace.
Being hermaphrodites, they are potentially self fertile.
But in their quest to perpetuate themselves,
each one tries to inseminate the other,
without being inseminated itself.
So they try to bite each other's penis off.
On this occasion, neither slug succeeds,
and their penises eventually entwine for an exchange of sperm.
But the veterans in the battle for fertilisation
are to be found among the insects and their kin.
To raise the odds on their paternity,
the males have taken measures
which reach new heights of sophistication.
These bush crickets live in the Sierra mountains of Spain.
While mating, the male,
the one underneath, deposits a sticky parcel
of sperm on the end of the female's abdomen,
it's called a spermatophor.
This is capped by a cache of nutritious jelly.
After the male has dropped away, she gladly eats it.
Besides helping her lay a larger brood of eggs,
it keeps her mind off mating again with another partner,
and gives the male's sperm time to swim into her body.
Not many males play for time like crickets,
instead, they have penises to inject sperm
as close to the eggs as possible.
The design of some beggars belief...
This belongs to a minuscule fruit fly
The male millipede's is perhaps the most intricate in the animal world
A jumping spider's sexual palp
why should males arm themselves with such strange structures?
Damsel flies provide a clue,
a female deposits her eggs in underwater vegetation.
Above the surface, a battle for paternity rages
among the sooty-winged males.
They face two problems.
Every female damsel is naturally promiscuous,
and she can store sperm from several males.
This is not a situation that any male will tolerate.
To attract a female, a male damsel must demonstrate
his ownership of a stretch of fast flowing water
which includes a bed of her favourite sub-aquatic plants
where she can lay her eggs.
The strength of the current is vital
because the water delivers the oxygen to nurture her developing eggs.
She flies in to inspect The male has a trick up his sleeve.
He shows off the quality of his patch by floating down stream.
So the female can asses the speed of the current.
Once the female shows willing, he grasps her neck with his claspers
Then he transfers sperm from the tip of his tail to
his penis which is located just behind his legs.
Now the pair make the sexual connection.
But first, the male has a crucial job to do.
He scrapes out the female's sperm storage chamber
with his specially shaped penis.
This lashing, spine-covered weapon
removes all sperm from previous trysts,
only when the chamber is empty does the male deposit his own sperm.
A male counter attack on female promiscuity.
but it's not fool-proof.
She might mate again before laying.
Some male butterflies have taken the sexual arms race one step further.
This Latin American Zebra wing can smell out
females even before they hatch.
There's a female inside this chrysalis and he wants her.
While he waits for her to hatch,
the male sprays an aerosol of obnoxious chemicals
which puts other males off her scent.
But just in case, he guards her,
to make sure that no-one else wins this prize.
He'll not have long to wait.
As she struggles free,
even before she inflates her wings,
he forces her to mate with him.
Like the damsel fly,
the male holds onto the female with special claspers.
And his penis is uniquely shaped to fit only females of his own kind.
Safe in the knowledge that
his chemical suppressant will ward off rival males,
he takes his time to transfer his sperm,
so ensuring that he is the sole father of her offspring.
Perhaps the ultimate fool-proof strategy is practised by
an Australian swallow tailed butterfly called the Big Greasy.
This is a virgin female drying her wings.
She exudes an alluring scent
and doesn't have long to wait for a sexy male to find her.
The males have a cunning ploy to make sure
that the females cannot play fast and loose.
Mating is a lengthy affair the couple are in tandem for
about eighteen hours, but during that time,
more than sperm is being transferred.
When they separate, the male flies off to find another virgin,
but the female is now fitted with a chastity belt,
secreted by the male and which blocks her sexual orifice for good.
ln this battle, the male triumphs over female promiscuity.
But some females force males into taking desperate measures.
ln the Australian bush lives one of the world's most venomous spiders.
The male red-back is minuscule compared to the fearsome female.
She's invested heavily in reproduction,
and become a monster egg factory.
When courting, the male walks a tight rope between life and death,
one foot wrong and she'll eat him.
Whether he becomes a father or
dies celibate depends upon the delicacy of his touch.
Carefully massaging her,
he cautiously gets into position.
He brandishes a pair of mating palps charged with sperm.
After a period of foreplay,
one of them penetrates her genital opening.
He then proceeds to pumps its contents into her.
Then he does something very odd indeed!
He vaults into her open jaws,
a deliberate act of suicide which occupies her
while his sperm are still being delivered.
Mission accomplished,
the male red-back has no further role to play.
The female eventually devours him,
but only after he has successfully fathered her next brood.
By contrast, 'higher' land animals, like these garter snakes appear
uncomplicated, it's back to the orgy,
where many males race to mate with mature females.
Within a few weeks,
every female is destined to produce
as many as seventy young, so no male is going to
miss the chance of fathering them.
One male eventually succeeds
- his barbed penis grips the inside of the female.
Whether by choice or force,
she'll probably mate with several partners.
Like insects, the different shapes
of vertebrate penises suggest that they do more than merely transfer sperm.
The rattlesnake's organ is forked.
The spiny ant-eater's is four headed.
Others, like the Tarsier's, are spiny
Many hoofed mammals have folding rods of gristle for rapid deployment.
Some are coiled as in the wild boar.
Some penises are stiffened by bone.
Other kinds are mobilised by blood pressure like the Tapir's.
No matter what the shape is,
the penis is designed to enhance the male's success at conception.
lt delivers sperm to the neck of the female's cervix,
as close to her eggs as possible.
But sometimes special techniques are called for.
After ejaculating, the Jackal's penis expands,
binding the male to the female for as long as thirty minutes.
Like all dogs, the male buys time
to make sure that his sperm reach her eggs.
But one mammal uses timing in a very bizarre way.
A male stoat raids a nest of nearly weaned juveniles.
The mother is out feeding,
and while she's away, he rampages round the babies and rapes the females.
Surprisingly,
such young females are not too young to conceive.
Once their eggs are fertilised,
their pregnancies are simply delayed for several months.
To us, such behaviour is repugnant - but,
for stoats, it's the natural outcome of the male's race for the egg.
The quest for paternity equally drives that great icon of all wildlife,
the African elephant.
Weighing as much as a double-decker bus,
a dominant bull elephant would have fought hard for his rank.
Between forty and fifty years old,
he's a force to be reckoned with, and roams amongst the cows,
testing them for their sexual state.
This female is ready to mate.
This colossal bull has a penis to match.
lt's highly mobile to compensate for his huge, inflexible body.
With bulls out-weighing females by as much as 4 tons,
mating needs a lot of care.
The female calls to attract other bulls,
she wants to make sure he's the best.
But other males respect his superior status and hold back...
His motorised penis does the work.
After copulating the bull must guard his mate for some time,
with other males loitering nearby, her fidelity cannot be assured.
She's still interested in sex.
Now pregnant, she has 22 months of gestation to endure,
and then she's shackled to her calf for 5 years.
The bull provides no care for his offspring-
an extraordinary inequality.
And the story continues with that crucial question,
''who rears the kids?''.