Original subtitles

(water roaring)

♪ ♪

(flamingos squawking)

NARRATOR: Of the millions of animal species on Earth,

only a few thousand are armed

with something strange and special-- huge weapons,

growing from their bodies. (moose roaring)

Look at that set of antlers, wow.

(grunting)

NARRATOR: Antlers, horns, and tusks

wielded in titanic battles against one another.

♪ ♪

(trumpeting)

What gives rise to these animal arms races?

DOUG EMLEN: Wow, look at that. Out of nowhere.

NARRATOR: How do some creatures develop these massive weapons?

DOUG EMLEN: Why is it that this species get sucked into an arms race

and ends up with these huge weapons,

and this species, which is otherwise

incredibly biologically similar, does not?

(grunting)

NARRATOR: What forces have driven animal weapons to evolve

over millions of years?

(roaring)

Are some weapons just for show?

And can cheaters...

(antlers crashing)

...ever win at this evolutionary game?

(clacking)

What are the secrets

of "Extreme Animal Weapons"?

Right now on "NOVA."

(barking)

♪ ♪

Major funding for "NOVA" is provided by the following:

♪ ♪

NARRATOR: In southwest Montana is a hall of wonders.

♪ ♪

(door creaking)

(whispering): Oh, my god.

Wow.

Unbelievable!

NARRATOR: It's a temple of bone,

adorned with some of the most impressive antlers

in the animal kingdom.

DOUG EMLEN: Look at this.

This is an antler from an elk, a bull elk from here in Montana.

This is 20 pounds of bone

and they of course produce two of them,

coming off the top of these animals' heads every year.

Antlers are the fastest growing bones described

from any living vertebrate.

(barking)

NARRATOR: Antlers are just one form of animal weapon.

There are many others.

(crashing)

Horns.

Tusks.

Stings.

(hissing)

Nature's armory is diverse.

♪ ♪

Weapons can be used in attack.

(grunting)

Or defense.

And sometimes their purpose is a mystery.

(splashing)

Almost any animal has a weapon of one sort or another.

I mean, cats have claws,

eagles have talons,

even dogs have a respectable set of teeth,

but those weapons stay small.

NARRATOR: But sprinkled through the tree of life,

there are species whose weapons are taken to an extreme.

These have long fascinated biologist Doug Emlen.

DOUG EMLEN: For me, I'm interested

in the weapons of offense,

weapons that are used in fighting,

and, in particular, the weapons that are big.

NARRATOR: Why do some animals get drawn into an evolutionary arms race,

with weapons getting bigger and bigger?

(grunting)

While others do not?

♪ ♪

Right on his doorstep in Montana,

Doug can find one of the best examples

of extreme animal weapons.

♪ ♪

Look at that guy.

NARRATOR: Rancher Doug Averill keeps elk on his land.

They are tagged so he can monitor them.

DOUG AVERILL: Here comes another one.

NARRATOR: Male elk use their antlers to control harems of females

and fight off rivals every fall.

The successful males get the most mating opportunities

in the breeding season, known as the rut.

They're all going to size each other up here

when you get this many bulls together.

This might get interesting.

♪ ♪

NARRATOR: Two bulls are approaching one another.

(clacking)

♪ ♪

This is pretty rare.

I haven't seen this all summer.

(clacking)

You can see they're totally locked together.

(clacking)

NARRATOR: Elk rarely fight to the death.

Because the male with smaller antlers usually backs down.

In this case, the bigger bull on the right has won.

The loser still wants a fight.

That's a temperament to be an aggressive bull

down the road.

NARRATOR: And he charges Doug.

(chuckling) That got my attention.

It's okay.

(bird calling)

NARRATOR: There is a downside to animal weapons like elk antlers.

They require a huge biological investment.

DOUG EMLEN: This is almost 20 pounds of bone.

I mean that would be like me producing another leg

and wearing it around on my head.

They'll shed this.

They'll throw this away at the end of the season,

and then they have to turn right around

and start growing a whole nother one again.

The only way they can grow a bone this big,

this fast, is to shunt the calcium,

shunt the minerals away

from the rest of the bones in their body,

so they're literally pulling these things

out of the rest of their skeleton

and allocating it to the weapons.

The biggest bulls and bucks have brittle bones

at exactly the time of year when they're hurling themselves

against each other in all-out battles.

NARRATOR: Growing and maintaining a rack of antlers each year

uses enormous amounts of energy.

DOUG EMLEN: Often by the end of the rut,

bulls will have lost as much as a quarter of their body weight.

They come out of that season starved and damaged

and they've only got a few short weeks

to make up the calories that they've lost before winter

or they're not going to survive.

NARRATOR: Elk need to stay strong.

(bugling)

The weakest are the most likely to become prey.

So imagine the predators of these guys,

something like a wolf.

(rapid footsteps)

Wolves have to be fast; they have to be agile.

Think about what would happen to a wolf

if it had a set of antlers on the top of its head.

A wolf that awkward wouldn't be fast enough to catch their prey.

They wouldn't be able to turn quickly enough

to catch their prey.

(wolves whimpering)

But now if you turn around and you look at the elk,

well, it doesn't make sense there either.

(elk crying)

Considering what we know about the costs of these weapons,

why would you ever want one?

These structures are not helping the bulls survive.

NARRATOR: Across the animal kingdom,

there's a high price to being armed,

and these armaments don't usually

help animals live longer.

(bird calling)

So what's the point of these extreme weapons?

Survivorship isn't the only game in town,

and, in fact, when it comes to evolution,

the thing that matters the most is reproduction.

♪ ♪

(liquid sloshing)

NARRATOR: Animals have a multitude of weird and wonderful traits

that make them attractive to the opposite sex.

(feathers rustling)

The same evolutionary pressure

that produces these characteristics

is also behind many extreme weapons.

(cooing)

It's known as sexual selection.

And elk bulls are a prime example.

Bigger antlers help them win more battles

and mate with more females.

So the genes for the largest weapons are passed

to the next generation.

Research has shown that the biggest bulls

sire the most offspring over 80 percent of the time.

This happens throughout the animal kingdom.

♪ ♪

Doug observed it while studying a creature

much smaller than elk.

But one that still wields an impressive armory.

DOUG EMLEN: So this is an example

of a Japanese rhinoceros beetle.

And in this particular species the males have a horn

that's like a pitchfork that sticks forward

from the front of their heads.

And in some of the specimens

these pitchforks can be almost as long

as the whole rest of the body.

This might look small relative to an elk,

but I assure you, as far as insects are concerned,

this is big,

every bit as impressive.

They can be 30 percent of their body weight.

That is literally like you or me wearing a coffee table

around on the tops of our heads.

NARRATOR: There are hundreds of thousands of different beetle species.

But only a small fraction carry extreme weapons.

Why do these few enter into an arms race?

DOUG EMLEN: So there's an animal

with an incredible weapon.

NARRATOR: The male Darwin's beetle from Chile

has an enormous set of pincers for its size.

These specially adapted jaws can make up half their body length.

That is one of the biggest weapons

of any living animal ever.

They've got mouthparts or mandibles

that have been elongated

so that they have these curved, arcing pincers,

and the pincers on these males can be even longer

than the rest of the body of the animal.

So the males are fighting battles with rival males,

so these are two males sparring and facing off

against each other, trying to fling each other from the tree.

♪ ♪

For the loser, it's a long way down.

(clattering)

NARRATOR: But what's the point of this epic battle?

DOUG EMLEN: Beetles like this fight over wounds,

sort of marks or nicks on the sides of a tree

where sap will ooze out and drip down the side.

Females feed on that sap.

NARRATOR: By guarding the sap, a male beetle gains access to females.

DOUG EMLEN: So that means that if you're a male,

and you can hold onto that real estate,

you have opportunities to mate with the females

when they come in to feed.

NARRATOR: Darwin's beetles use their weapons

to keep other males away.

But not all conflicts are tied

to such a specific place or food source,

and those battles have a different set of requirements.

(water rushing)

So imagine things that whirl around in the water.

Sometimes animals are fighting over resources

that can't be defended.

(underwater movement echoing)

Think about things like raptors fighting in the air.

(flapping loudly)

(crying, calling)

They'll get into these big, frenzied,

acrobatic, mid-air snarls.

(calling)

In fights like that, things like agility or speed

are likely to matter more than bulk and strength.

(insects buzzing)

For these kinds of fights,

big weapons just aren't worth the price.

NARRATOR: But male Darwin's beetles do have a specific resource

to defend.

The males that are able to win these battles

or to hold onto that territory...

I mean, the ultimate prize is reproduction.

(crackling)

NARRATOR: This male has fought off all his rivals

and now he's earned the opportunity

to mate with a female.

So when we start to look at these animals

and say what sets these species apart,

why did these particular species have such incredible weapons?

The first clue, the first piece to the puzzle

is a defendable resource against which the fights can take place.

(stretching)

(loudly flapping)

NARRATOR: Most animals with extreme weapons

are keeping females away from male rivals.

Male hippos use their enormous teeth to do this.

(water roaring)

Male white rhinos try to defend territories

that females travel through.

And male elephants use their tusks in combat

to keep fertile females away from adversaries.

(trumpeting)

(growling)

(grunting)

Each of those tusks can be like 100 pounds of ivory.

These are huge teeth.

(grunting)

Ah, his tusk shattered.

Okay, there you go.

That is the impact we're talking about here:

it shattered the tusk!

(thudding, clashing)

(roaring)

NARRATOR: But across the animal kingdom,

why do the largest weapons almost always belong

to the males?

DOUG EMLEN: Bull elephants have these massive tusks

but if we want to understand the tusks,

we actually have to look at the females.

NARRATOR: Female elephants also have tusks,

but theirs are smaller and not used for fighting

like the males'.

(adult elephants calling, juveniles squeaking)

Unlike the males, females spend most of their time

caring for offspring.

Pregnancy lasts for 22 months and after females give birth,

they take care of their young for another two years.

DOUG EMLEN: A female will only be fertile for five days

out of every four years.

That's an incredibly brief window of time.

It's less than one half of one percent of a female's lifetime.

So fertile females are extremely rare.

DOUG EMLEN: Every now and then, a female will become receptive

and when she goes into that window of fertility,

every male in the landscape enters into the fray.

♪ ♪

NARRATOR: It's common throughout the natural world

that females are often unavailable to breed

for long periods of time.

♪ ♪

After giving birth, most female mammals provide milk

and care for their offspring

until they can survive on their own.

Only then can the mothers reproduce again.

After laying their eggs, female birds usually incubate

and raise their chicks before they can breed once more.

(birds calling, insects buzzing)

Reptiles, fish, insects.

The same trend is repeated in many classes of animals.

Males are almost always ready to breed.

But reproductive females are rare.

And this imbalance sets the stage

for huge competition among males.

Competition is absolutely critical.

In a sense, it's the fuel that drives the arms race.

In all of these animals where you get these massive weapons--

the males, the bulls, the bucks--

face fierce competition over access to females.

(elephants grunting)

(antlers clacking)

(rustling)

♪ ♪

NARRATOR: While males are usually the ones that compete,

occasionally, the roles can be reversed.

Thirty years ago behavioral ecologist Stephen Emlen,

Doug's father, helped capture some remarkable footage

to illustrate this.

STEPHEN EMLEN: Man, I haven't seen this in a long time.

NARRATOR: The film features the unusual tropical bird

that Stephen studies, called the jacana.

STEPHEN EMLEN: We conducted all this study

basically in Panama.

Sometimes these are called Jesus Christ birds

because they seemingly walk on water

because their toes are so long they spread out their weight.

NARRATOR: The smaller male jacanas incubate the eggs

and raise the chicks

while the bigger females patrol large territories

where males are usually found.

STEPHEN EMLEN: So time-wise the female,

she's able to reproduce in theory about every ten days.

She could lay another clutch.

He's stuck for almost three months

tending the eggs and the chicks.

DOUG EMLEN: So the tables are turned

and that means that females have to compete

with each other for access to the males.

STEPHEN EMLEN: Absolutely.

Smart son. (laughing)

NARRATOR: Stephen Emlen is an expert in animal breeding behavior.

STEPHEN EMLEN: So, not only are the males doing

the parental care but it means

the females are fighting

over access to the males and the care they provide,

and so the expectation is they should have

larger weapons than males.

NARRATOR: These Jacanas have a single sharp yellow spur on each wing.

They are made of a tough fingernail-like material

called keratin.

The spurs of the females can be

25 percent larger than the males.

(jacana calling)

They use their spurs in battles

to control access to breeding males.

So jacanas actually teach us a lot about the evolution

of animal weapons: they show us

that when the roles are reversed,

then the weapons are backwards too.

Competition in this case is stronger in females;

they're the ones with the bigger weapons.

NARRATOR: And the winner presses her advantage.

Claiming the territory,

the lone male, and obliterating

the previous female's as-yet-unhatched chicks.

(chirping)

(squawking)

And this is sort of

a horrendous thing to think about.

Basically, she is now destroying his eggs.

(chirping)

She wipes the slate clean.

Exactly. So how much faster?

STEPHEN EMLEN: If he's on eggs like that one

where she just destroyed the eggs,

she's basically saved herself two months.

She gains a reproductive male.

NARRATOR: Instead of spending the next two months

taking care of his eggs and chicks,

the male is now available,

and mates with the female who killed his offspring.

(calling)

With their small but sharp weapons,

female jacanas show how competition

is a powerful evolutionary force,

even when the roles of the sexes are reversed.

(insects buzzing)

(birds chirping)

But is it the only force at work?

Many animals compete intensely for territory, food, or a mate.

(birds squawking)

Their fights can be vicious.

But most of these species don't have giant weapons.

(squeaking)

Doug began to study dung beetles,

looking for what else might be needed for extreme weapons

to evolve.

DOUG EMLEN: When you look at something like the dung beetles.

They're literally competing for the same piles of dung.

And yet some of those beetles have these huge,

I mean spectacular weapons and others have nothing at all.

There's two kinds of dung beetles.

There's a kind of dung beetle that carves the balls

and rolls them away--

class one, the ball-rollers.

♪ ♪

NARRATOR: The ball-rollers are unarmed.

They collect dung and move it to a safe place

to raise their families.

Sometimes they have free-for-all brawls

for control of a dung ball.

But they don't have large weapons.

But then there's this other type of dung beetle

that had been less well studied,

and those are what we call the tunnellers.

NARRATOR: The tunnellers have big horns.

They don't move the dung.

They dig straight underneath it.

By comparing the two types of beetle,

Doug believes he can find the crucial factor

that launches an arms race.

♪ ♪

He set up a viewing system

to see what the tunnellers were doing below the surface.

He had to use red lights to avoid disrupting

their natural behavior.

The missing piece to the story was what happened underground.

Females dig tunnels beneath the piles of dung,

stashing it into these little brood balls, they're called,

and then she'd lay an egg very carefully

at the end of each one of these.

What you find is that the males

plant themselves at the entrance to one of these tunnels,

they brace themselves there.

They've got hooks and spines on their legs

that they can wedge into the soil,

and they use their horns in fights with rival males.

NARRATOR: Until Doug captured this unique scientific footage 20 years ago,

nobody even knew these beetles fought.

Doug revealed that males use

their shovel-like heads and sharp horns

as weapons.

Rivals engage in brutal underground battles

to control the females' brooding tunnels.

DOUG EMLEN: These beetles will walk right by each other on the surface

and not even bump each other.

They couldn't care less, but you put them in a tunnel...

(snaps): And just like that, you've got a war.

Any rival males have to enter one at a time

and they have to face their opponent face to face.

NARRATOR: The difference between the tunnellers and the ball-rollers

is that the ball-rollers fight in group scrambles,

while the tunnellers, stuck in a confined space,

engage in one-on-one battles.

And these are the beetles with the weapons.

Doug wondered if this kind of fight

could be found in other types of animals.

Chameleons have to be one of my favorite animals of all time.

♪ ♪

(birds chirping)

They are the quintessential ambush predator.

So they sit tight,

their eyes can swivel in different directions

so they move independently.

And then their ultimate weapon: thwap.

(gurgling)

NARRATOR: A chameleon's tongue is able to extend to twice the length

of its body.

(crunches)

♪ ♪

DOUG EMLEN: Wow.

Look at that, out of nowhere.

NARRATOR: It accelerates from zero to 60 miles per hour

in one-hundredth of a second.

But another type of chameleon

has an additional set of weapons:

big horns on their faces.

DOUG EMLEN: So there is a set of weapons.

These guys look like little dinosaurs, like a triceratops,

with the horns coming forward from the head.

NARRATOR: Male Jackson's chameleons use their horns

to fight one another over access to females.

Think of this like "Jurassic Park" jousting,

as these males push and pry

and try to twist each other off of the branch.

(hissing)

NARRATOR: The branch itself is the key.

♪ ♪

So it turns out it's the details of the fight that matters.

♪ ♪

If you look at this fight,

they're approaching each other face to face.

They're locking horns.

They're pushing they're straining.

NARRATOR: It almost looks like they're dueling.

DOUG EMLEN: When males face off one on one in a duel,

males with bigger weapons win.

NARRATOR: Chameleons use their horns to help win face-to-face battles.

♪ ♪

So for me, the... the penny dropping, the moment,

the epiphany when everything clicked and came together

was that the nature of the fight mattered,

and, in particular, duels were the crucial ingredient

that could spark an arms race.

♪ ♪

NARRATOR: So competition is not enough to set the stage

for extreme weapons.

(birds squawking)

The battles almost always include

ritualized head-to-head combat.

(blowing)

When this happens, the evolution of extreme weapons

seems to outweigh the costs.

♪ ♪

(rain pounding)

♪ ♪

Selection for big weapons becomes so strong

that it eclipses any costs

associated with these structures.

Nothing else matters.

♪ ♪

Launching their populations

onto trajectories of explosive weapon evolution.

♪ ♪

NARRATOR: Extreme weapons have evolved many times over

in the history of life...

(snorts)

...the rare species

that possess them

coming from a diverse range of animal families.

♪ ♪

So in each of these cases,

the conditions had to be just right

in order for the populations

to launch on to this type of a trajectory,

leading to these bigger and bigger and bigger weapon sizes.

♪ ♪

NARRATOR: This evolutionary mechanism dates back

hundreds of millions of years.

(bellowing)

Fossils bearing extreme weapons...

(feet clawing)

...have been found around the globe.

(grunting)

♪ ♪

(grunting and pounding)

♪ ♪

Doug heads to Salt Lake City

to examine the fossilized remains

of a famous dinosaur family.

(lift humming)

♪ ♪

Mark Loewen studies a group of dinosaurs

called the ceratopsids.

Welcome to the horned dinosaurs.

They're amazing.

We have triceratops, the iconic horned dinosaur. I recognize that one.

LOEWEN: So these lived during the Cretaceous,

between 80 million years ago and 66 million years ago,

when most of the dinosaurs went extinct.

Here we have a mega herbivorous animal.

This is, like, the rhino of the Cretaceous, or the elephant.

♪ ♪

NARRATOR: Scientists estimate that some ceratopsids

could weigh seven tons.

(snorting)

They were probably preyed on by Tyrannosaurus rex.

But did the ceratopsid dinosaurs use their horns

in fights with each other

like male animals with weapons do today?

In this collection, there are hundreds of different specimens

of ceratopsid dinosaurs.

Right here, we have actually the oldest

of the ceratopsid dinosaurs.

This is diabloceratops,

a really cool animal, 80 million years old.

If we look in places where the horns would hit

when these two animals locked together,

here we have a hole.

This is a puncture wound into this bone

that's been rehealed during its life.

NARRATOR: Mark thinks that holes like these are puncture wounds

from violent face-offs with other ceratopsids.

(dinosaurs grunting)

LOEWEN: So this is totally consistent with head-locking behavior

and being punctured by the horn of another adult animal.

That's just like what other animals do.

Yeah.

And this is something we can show quite conclusively

in triceratops.

NARRATOR: These scarred fossils suggest that horned dinosaurs

were using their weapons in face-to-face contests,

just like animals with extreme weapons do today.

(barking)

And like today's elephants and elk,

these dinosaurs were herbivores.

Their giant weapons were used in battles for dominance,

not for dinner.

♪ ♪

It turns out that predators,

from Tyrannosaurus rex to today's wolves and lions,

don't usually wield extreme weapons.

Their teeth and claws are relatively small.

(yelp)

But there are exceptions.

♪ ♪

(water gurgling)

♪ ♪

The most iconic example is another ancient extinct beast.

♪ ♪

DOUG EMLEN: We're looking at a fossil of a saber-tooth cat.

And this is the actual fossil-- it's not a cast--

and the animal probably lived in California

about a million years ago.

Saber-tooths actually teach us

an awful lot about animal weapons.

For one thing, you can't miss the teeth, right?

I mean, the teeth are huge.

But that's actually an interesting problem,

because this is a predator.

♪ ♪

(grunting and growling)

This is why saber-tooths are so exciting.

They're the exception in this case.

NARRATOR: While the enormous fangs may have helped attract females,

their main use seems to have been to hunt for prey.

Saber-tooths are special because they're ambush predators.

(horse whinnying)

Imagine what it would be like to get chomped on

by something like that.

♪ ♪

They sit and wait and then lunge out with a quick strike,

to grab unsuspecting prey.

♪ ♪

NARRATOR: For most predators,

heavy armories are simply too bulky.

They'd slow them down.

(roaring)

(neighing)

But for ambush predators that strike quickly...

(growling)

(horse moans)

...the evolution of big weaponry makes sense.

DOUG EMLEN: And that logic works under the ocean

for things like mantis shrimps and pistol shrimp.

(bubbles echoing)

It works for ant lions.

(scrabbling)

It works even for those crazy deep sea angler fish

that are essentially a big jaw with a tail.

These guys had lures that they would dangle in front of them

that would pull the prey into them.

(water rushing)

(splashing)

NARRATOR: Ambush predators

use their extreme weapons to capture and kill their prey.

But most animals that are in an arms race aren't hunters.

All of the rest of the species with big weapons,

species with the biggest and the craziest things

sticking off of their bodies,

those animals are using their weapons for reproduction.

NARRATOR: So is mortality among these heavily armed males

exceptionally high?

(grunting)

(antlers clacking)

How do they avoid lethal injuries

when using their extreme weapons?

♪ ♪

A clue may come from a tiny animal

with the largest known weapon proportionate to body size.

♪ ♪

DOUG EMLEN: All right, Brook, can I see the biggest weapon

in the animal world?

Sure.

♪ ♪

Here it is.

That's it? That's not very big.

BROOK SWANSON: So these are fiddler crabs.

Ow! Okay, that was...

Maybe it's a little bit bigger than I thought.

(laughing)

So this is it, the recordholder.

So it may be a small crab,

but this claw, the weapon,

can be half of its body mass,

so the claw can weigh as much as the whole rest of the crab.

It'd be like you walking around,

carrying my whole bodyweight as one of your arms.

That's incredible.

NARRATOR: Fiddler crab claws

are proportionally bigger

than any other known animal weapon on Earth.

SWANSON: There's about 103 different species of fiddler crabs.

They live all over the world in the tropics,

and they eat algae off the surface of the mud.

NARRATOR: Brook Swanson studies the costs and benefits

of this surprisingly large claw.

SWANSON: So females have two little claws,

and they can actually eat twice as fast as the males.

The males, only, have these giant claws

and can't use their weapon claw to eat.

NARRATOR: The huge claw comes at a great cost.

Males must eat extra food to fuel their muscles,

but they are only able to gather it

half as fast as the females.

All right, so he's bitten me three times already.

Can you show me what these guys can do?

SWANSON: We can use this force meter

to measure how strong their claws are.

This just measures how hard they squeeze?

Exactly.

SWANSON: Put their claw right there.

They squeeze.

That's about 20 newtons.

So the crab is producing

about 20 newtons with its claw.

Five pounds of force,

so like having a bag of sugar on a pin pushing on you.

DOUG EMLEN: So it's not just five pounds;

it's five pounds concentrated

on a very sharp point. Exactly.

That's why it hurt so much.

NARRATOR: So is this strong enough to pierce another crab's shell?

A machine in the lab can measure that

using the shell of a dead crab.

SWANSON: That could take

about five newtons of force.

Five newtons.

So way less than the squeezing force.

NARRATOR: The big claw's pinching strength is four times more

than what is needed to pierce another crab's shell.

So this is technically a lethal weapon.

Exactly.

NARRATOR: Crab claws are powerful enough to kill a rival.

But there's a paradox.

SWANSON: So they're plenty strong enough to kill each other

but when we keep them in the lab,

we hardly ever see them fighting

and I've never seen one kill another one in the lab,

and when you study these in the field,

it's very rare to see them fighting there, as well.

NARRATOR: Male fiddler crabs sometimes duel with their weapons

on the beach.

But they spend much more time using their giant claws

in another way.

So here we have two male fiddler crabs

and their body size is about the same.

But if you look at their claws...

It's huge.

The claw's twice as big as the other one.

Wow, that is so obvious.

What they spend most of their time doing

is not fighting with these claws,

but waving them in the air.

So they walk around on the sand and they wave the claw,

and they're signaling to the other crabs

how big and how strong they are.

SWANSON: So what makes the claw a good signal

is that it's hypervariable.

There's a lot of variation between crabs,

and so if you're looking at crabs by their claws,

you can easily tell the difference.

You can easily tell that this crab is bigger and stronger

and a better fighter,

and that's what makes the claw a good signal.

DOUG EMLEN: This is awesome.

You'd think that the species with the really big weapons

would use them to fight all the time,

and yet what we see is the reverse,

that the species with the biggest weapons

are actually the most peaceful.

♪ ♪

NARRATOR: It turns out that what's true for fiddler crabs

is also true for a variety of animals with extreme weapons.

Display and ritual are often more important

than brute-force combat.

♪ ♪

When rival male elk meet,

they measure the prowess of their opponent

with ritualized behaviors.

(elk barking)

They do this by strutting in parallel lines

to assess the competition.

Most of these encounters end with one animal backing off

without a single blow.

♪ ♪

The fiddler crab's deadly claw is the ultimate example

of a weaponized deterrent.

So we've gone from what was essentially

a blunt-force weapon

to something that we now realize

is a whole lot more than that, right?

These biggest weapons of all are acting as a signal.

They're behaving like a deterrent,

settling dangerous contests without actual battle.

NARRATOR: And the destructive claw still has another important purpose.

It's not just the other males that are seeing

this giant signal, this huge claw,

but it's the females, too.

The females will walk through groups of males

that are waving their giant claws

and she'll make a choice.

So she'll choose the males with the largest claws

or the males that wave the claws the best,

and so it's not good enough for a male to be able

to just win fights.

He also has to be able to attract a female

and he does that with his claw.

So these are the ultimate signals.

Both males and females are paying attention.

(laughs): I know what she would choose.

♪ ♪

NARRATOR: But what about individuals with less impressive weapons?

Is there any chance for them to find a mate?

Doug uncovered some sneaky tactics

while he was studying dung beetles.

DOUG EMLEN: So one of the things that we were able

to learn from these beetles

is that the little beetles cheat.

NARRATOR: Female dung beetles dig burrows

and big males guard the entrances.

This female is dragging dung down to her nest

before she lays her eggs.

You've got this main tunnel.

You've got the big beetle guarding that tunnel.

If you're another big beetle, you can challenge him

in outright, open battle.

But if you're tiny, you don't stand a chance.

So instead of fighting a losing battle,

they go right next to a tunnel

and they start to dig their own tunnel.

They mine their way into the tunnel,

coming beneath the guarding male,

go straight down to the female,

find the female, mate with the female,

turn around and leave.

NARRATOR: This small, sneaky male has found a way

to evade the guard and mate with the prized female.

Then he can escape up his own secret tunnel.

Big males have the weapons;

the big males fight the conventional battles.

The little guys break the rules.

(waves roaring)

NARRATOR: And that means that the biggest animal

with the largest weapon

doesn't always win.

(water bubbling)

DOUG EMLEN: So consider the cuttlefish.

In cuttlefish, you've got all these little tiny males.

I mean, these wimpy tiny runts in these populations.

There's no way they would win

if they tried to fight by the rules,

so they don't.

NARRATOR: Big dominant males guard fertile females.

If an evenly matched contender challenges him,

sometimes they will battle one another

with sharp beaks and tentacles.

I would not want to get caught up in a tangle

with one of these guys.

♪ ♪

NARRATOR: The winners hover above the females,

often smaller with paler coloration.

DOUG EMLEN: So now we've got another cuttlefish coming up.

This one looks like a female,

but it's not a female.

If you look closely, this is actually a tiny male,

but he's cloaked himself in colors

that make him look like a female.

So he can come right up to the guarding male unmolested.

♪ ♪

The sneaky male works his way right on in there,

and by looking and acting like a female,

he's able to get into a position

where he can breed with the female, too.

NARRATOR: When this female lays her eggs, her offspring will be a mix

of some macho males and some tiny tricksters.

♪ ♪

(boat engine running)

These strategies appear in a variety of species.

Back in Montana, bighorn sheep biologist Jack Hogg

is taking Doug to an island on Flathead Lake

to search for another underdog tactic.

(insects and birds chirping)

♪ ♪

Bighorn sheep are famous for engaging

in epic ritualized battles.

(grunting)

During the rut, rival males size each other up.

(birds calling, sheep huffing)

Occasionally, when it's an even match,

they fight.

♪ ♪

(crashing)

♪ ♪

Through a series of horn-to-horn clashes,

the rams establish a hierarchy

with the dominant males at the top.

(crashing)

♪ ♪

During breeding season,

they will guard fertile females.

The largest-horned, largest-bodied rams

search for and defend ewes during their fertile period.

So the biggest males with the biggest weapons,

their strategy is to guard access to the females

for that one day when they're fertile.

NARRATOR: In order to mate, the subordinate males

have their own cunning strategy.

Jack calls it coursing.

The coursing strategy in essence is to do whatever it takes

to evade the defense of a socially dominant ram

who's defending a female during her fertile period,

but it's whatever needs to be done, whatever dirty trick

to force a mating.

(thunder rolling in distance)

NARRATOR: The dominant male here is called Crud Horn.

He's guarding a fertile female.

HOGG: The fertile female has a white spot,

so we're calling her White Neck.

♪ ♪

NARRATOR: A large group of subordinate males

are watching White Neck's every move.

They want to mate with her,

but Crud Horn is guarding her closely.

HOGG: So Crud Horn has two important tasks in front of him.

One is to mate with her during her fertile period,

but he also has to defend the female against any male

who wishes to breed with her.

♪ ♪

Typically, females run away from this group of large bodies

that are, you know, interacting.

It's a dangerous place to be, so they run away,

and that's what creates the chase.

♪ ♪

♪ ♪

NARRATOR: White Neck attempts to escape the rush of coursing males.

♪ ♪

Crud Horn is trying to keep up with her

and deflect the subordinate males during the chase.

♪ ♪

HOGG: Crud Horn's defense actually is extraordinary.

It's very good.

He's very physical in terms of clashing

and pushing and shoving the other rams.

But every once in a while,

one of these coursers will succeed in forcing a breeding.

♪ ♪

NARRATOR: One coursing male attempts to mate with White Neck

while she's separated from Crud Horn by the melee.

(hoofs beating ground)

That's all it takes is a few seconds.

A few seconds.

(bird calls)

NARRATOR: He now has a chance of fathering White Neck's offspring.

♪ ♪

In this case, the smaller size of his weapons

may have had some advantage.

The coursing ram has succeeded

in breeding the female.

He has a ticket in the lottery.

He has a chance of fathering the lamb

that she produces next spring.

It's clever in a way.

It's almost like they're able to use the bulk and the weight

and the size of the weapons of the alpha males

against them,

by getting them to lunge in a particular direction

and then taking advantage of being smaller and agile

by zipping around them and getting access to the female.

So if all these males are using these alternative strategies,

is it even worth being the big alpha male?

They do better. They still do better?

HOGG: They have more babies, basically.

A high-ranking male, he would be the father

of 60 percent of the lambs produced

by the females he defends.

NARRATOR: White Neck's offspring is far more likely to be fathered

by Crud Horn, the dominant male with the biggest horns,

than all his rivals combined.

To the victor go the spoils.

We see this in every one of these animal systems,

that the biggest males are the ones that can afford to produce

the biggest weapons,

and these males win in every sense of the word.

(birds chirping)

NARRATOR: In nearly every species with weapons,

cheating is the best strategy for the underdogs,

but bigger weapons still provide the best opportunities

for males to produce more offspring.

(hoofs shifting sand, animals grunting softly)

Yet the balance of the animal arms race may be tipping.

♪ ♪

Weapon sizes in some populations,

including bighorn sheep, elephants, and caribou,

are decreasing.

♪ ♪

And the trigger for this change is us.

(gun fires)

Human trophy hunters prize

the biggest tusks, horns, and antlers.

But animals shot down can't have offspring.

Trophy hunting removes the genes for the biggest weapons.

In one elephant population, the average tusk size

was reduced by 40 percent in just 25 years.

And in another, the number of tuskless individuals

increased by over 20 percent,

threatening an evolutionary trend that has existed

for millions of years.

♪ ♪

(calling)

So this has been quite a journey for me,

a ride more wild than I ever could've imagined.

Who'd have thought that the battles of beetles held lessons

for weapons everywhere?

♪ ♪

NARRATOR: Whether in beetles or fiddler crabs,

elk or elephants,

extreme weapons have arisen independently

many different times.

(growling)

Despite the enormous cost, in the right conditions,

head-to-head combat with extreme weapons

is still the best strategy evolution offers

to help pass genes to the next generation.

(trumpeting, growling)

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