Sex is a selfish business.
The king of beasts wants to populate the world with his own offspring.
And so does she.
But sex compels them to join forces and donate their sperm
and eggs to create new life.
For this they must co-operate.
But on everything else to do with breeding,
the sexes are at loggerheads.
The recurring theme of this series is that the burden
of reproduction falls mostly on the female,
and this inequality riddles the relationship between
the sexes with tension and mistrust.
Dissent between males and females
sometimes erupts into terrifying violence.
But sex isn't just a source of strife.
lt's responsible for creating the
tremendous diversity of life on Earth-
ln the struggle to breed, sex generates great drama and beauty
as contestants in the mate market
aggressively sell themselves at the expense of their rivals.
ln nature, infidelity is widespread, as both males and females cheat.
to obtain the best mates and produce the most offspring.
They even differ over who should look after the babies.
But the first of many battles is waged
between fiercely competitive males.
ln North American bison, it's possible to see how sex provides
males and females with quite different opportunities.
By nature, every cow is a lone parent
Although each raises only a single calf a year,
mothering involves a tremendous effort.
After nurturing the baby inside her womb,
she alone provides the milk to sustain its growth.
As the calf is her only chance of breeding during the year
she takes good care of it.
With so much of her time devoted to being a mother
she and the other cows are only fertile for a brief
period during the summer after they've weaned their calves.
Only now do the bulls take an interest in the opposite sex.
Although the bulls shirk all parental
duties - each one has the capacity to father numerous calves.
lts an ambition that every rutting bull tries
to achieve but few succeed.
Aroused by the smell of the cow's urine,
the hot tempered males fight for sexual supremacy.
The intense rivalry favours the heftiest bulls.
The fights are serious because it pays
in terms of offspring to be one of the victors.
Promiscuity has no attraction for a female bison-
each simply wants a champion bull to sire her single offspring,
and so acquire his winning qualities for her own calf.
But a stud bull benefits enormously the more cows he can cover,
the more calves he'll father in a single season.
The male's huge capacity for fatherhood and the female's forced
commitment to her offspring,
holds true across the whole of the Animal Kingdom.
This asymmetry between the sexes encourages males and females to
pursue their own selfish interests, and drives them apart.
For males, competing for as many partners
as possible turns them into well-armed warriors.
Macho males risk life and limb to prove
that they are the best of their kind.
Fighting can be lethal
...some bull musk ox die from shattered skulls.
Being battered is not the only danger faced by males.
Some face very different risks when they compete for sex.
ln Western Australia,
male phascogales emerge at night with sex on their minds.
But each one has a difficulty.
The females all come into season within a day or two.
So, to score well, these little marsupials have
to be super-charged and super-sexed.
For several days, the male is in a turmoil,
luring females from the tree tops.
With just a brief period to mate with as many females
as possible before rival males get to them
there's no time for feeding.
Several females follow him into his snug boudoir.
Each night, he impregnates up to a dozen.
Once serviced, the females return to their normal routine in the canopy.
But the male pays for his highly frenzied sex life.
Starved and over-stressed, he is wrecked by his affairs
While the females feed and fatten all of the males die
killed by their intense burst of sex.
Four months later, his sacrifice pays off as the females emerge,
each with a single youngster.
A really fast male may have fathered as many as sixty!
But there are other strategies such as conducting an arms race.
Where success is a death sentence.
This spider is 'armed to the teeth'.
The mature males possess huge 'jaws' incorporating
a pair of folding fangs which are used like sabres.
The weapons are relatively the largest in the animal kingdom.
While enabling a male to fight for females,
the fangs prevent him from feeding.
So the victor must mate before he dies from starvation.
But competing for sex doesn't have to involve such lethal tactics.
These extraordinary flies are masters assessing
the odds of winning a dual.
A male's rank depends upon the length of his eye-stalks.
eyes set farther apart are a sign of strength
When squared up to an opponent a fly quickly eyes him up,
and judges whether to flee or fight.
Males with the longest eye stalks
win the battles and claim the females.
These Grants Gazelles are also measuring each other up
and trying to decide whether to risk a dangerous duel - or not.
Competing by brute force is one technique adopted by males.
But there's another way by being glamorous.
Threatening rivals with striking plumage is the domain of birds.
these are mandarin drakes
beautiful but displaying for sexual dominance.
Showing off as a way of winning
a power struggle can be seen in black grouse.
A Swedish dawn,
before the hens appear, the cock birds declare choral warfare and
challenge each other to ritualised fights.
Flashing feathers is no soft option to armed combat
the rivalry between these males is just as
serious as it is among those which fight.
The best performer reaps his reward by mating
with the majority of the hens.
ln Costa Rica lives one of the natural
world's greatest glamour boys red-capped manakin.
They're born showmen, designed to dazzle,
with each trying to outshine the others with their brilliant plumage,
fancy footwork, and startling wing snap.
Egged on by the presence of a female, the males intensify their acts.
When really worked up, they reveal their eye-catching trousers!
Although driven by sex, this is also a display
of aggression warning rival males to keep their distance.
There are eleven males in this jungle theatre
all vying with each other to become the most desirable performer.
Whereas every female raises a family,
competition between male manakins ensures that most never mate.
Young and less assertive males stand little chance of success.
But some creatures have solved this problem they're all born female.
Amongst the coral reefs of the
lndo Pacific an amazing thing happens gender jumping.
Many kinds of fish such as these small Anthias begin life as females,
and switch sex when the time is ripe.
Only a few of the biggest females transform into males like
this one with a yellow chin.
They get the best of both worlds.
As young females, they avoid competition and regularly spawn.
Those that change into males have the added bonus
of fathering the offspring of a coterie of wives.
This is the born-again male of another kind of Anthias.
like the others, he originally had a set of ovaries.
Now he sports a pair of testes and a harem of yellow-eyed females.
He constantly chivvies them,
and somehow his bullying tactics prevent them from jumping gender
Only when he disappears,
will the largest of his wives change sex and take his place.
Why more animals don't do this is a mystery.
So many extra-ordinary things happen in the sea,
especially in the abyss beyond the reach of sunlight
Down here, fish are few and far between.
This is the realm of weird angler fish,
the females carry luminous lures to attract their prey.
The males are mere dwarfs with a huge problem.
The females are so scattered that a male might never find one.
But should the male encounter
a female he is equipped never to let her go.
With special teeth, he hooks onto her body.
Then, remarkably, his flesh merges
with hers and he becomes a life long sexual slave.
When she spawns, she chemically commands him to fertilise her eggs.
The rock gardens of the Mojave desert in southern California,
are a battle ground in which colour coded males
use different tactics in their quest for sex.
Male side-blotched lizards come in three distinct forms,
distinguished by their behaviour and the colour of their throats.
This male has a blue throat.
Like all blue-throats, he is long lived, and born faithful.
Once he has established a territory,
he guards a single female for as long as she is fertile.
But blue-throated males like him live in constant danger
of being cuckolded by another kind
of male that roams around these rocks.
These sport orange throats like this one.
He is a Lothario among lizards a belligerent, over sexed, rake.
Scurrying between territories,
he conducts a vigorous campaign to challenge
as many blue throats as possible,
thrash them and seduce their fickle females.
The relatively placid blue throats put up a reasonable resistance...
....but they're no match for the lecherous orange throated males.
However, this battle takes a surprising turn.
There's yet another sort of male.
Displaying a yellow throat, this one passes for a female
and goes unchallenged by other males.
While the blue and orange throated males are locked in combat
the transvestite reptile sneaks into the territory
and courts the resident female.
She accepts him.
Although the blue male has already mated
with her there's a chance that one or two
of her eggs remain to be fertilised.
Sneakiness can be a very successful ploy
when there are warriors around.
ln Lake Tanganyika in East Africa,
there are many opportunities for clandestine sex.
These are young males of the shell-carrying cichlid
they're frustrated and on the rampage.
Their target a group of females guarded by a colossal male.
He needs to be big and powerful to gather shells.
He can be a record thirty times the size of one of his mates.
The females are tiny so they can curl up inside the shells where they
The warrior constantly checks his harem,
and should he detect any newly spawned eggs,
he fertilises them by shedding
his sperms over the entrances to the shells.
But for this mighty male, there's trouble on the way.
The only chance that young males have of breeding is to band together,
overwhelm the territorial males, and fertilise their females.
On this occasion, the warrior male sees them off.
But a cheeky young bachelor remains.
He's tolerated because he cunningly assumes female coloration
and behaves as though he's a she prospecting for a desirable shell.
The female mimicry works until he transvestite starts to
shed his sperms over an occupied shell.
His cover is blown but he managed to fertilise some eggs.
And yet the big male's problems are not over
because another sneaker is scouting for sex.
He's a permanent dwarf and an extremely clever seducer.
Unseen and unsuspected, he chooses his moment...
...and slips in with one of the females.
By cohabiting, the miniature male is bound to father some of her fry.
But for most males, being big and pugnacious is the only way.
On a beach in California bull elephant seals
are spoiling for a fight where size is everything and biggest is best.
With three tons of quivering flesh
the heaviest males have sex appeal and females galore.
However, there's constant pressure from bachelor bulls
like this one to take over their harems.
Bulls are ten years old before they become serious contenders for sex.
As males can only expect to live for twelve,
this is a crucial contest for the challenger.
lt's perhaps his only chance to breed.
Only one in ten males ever succeeds in fathering some pups
The champions emerge through combat.
On this occasion, its the challenger who is thrashed.
After his violent demonstration of brute force the victorious
beach-master returns to claim his spoils multiple matings
the prize which sets all males apart from females.
But females are not simply passive recipients of male lust.
Although it looks as though competitive males
have sex all their own way they don't !