Extreme Animal Weapons

Discover how a secret biological code has shaped nature’s battleground. Airing May 23, 2018 at 10 pm on PBS Airing May 23, 2018 at 10 pm on PBS

Program Description

From lobster claws and dog teeth to bee stings and snake fangs, every creature depends on a weapon. But some are armed to extremes that make no practical sense—whether it’s bull elks with giant 40-pound antler racks or tiny rhinoceros beetles with horns bigger than their body. What explains giant tusks, horns, and claws that can slow an animal down and even impair health and nutrition? NOVA investigates the riddle of outsize weaponry and uncovers a bold new theory about what triggers an animal arms race. In creatures as varied as dung beetles and saber-toothed tigers, shrimp and elephants, the same hidden factors trigger the race and, once started, these arms races unfold in exactly the same pattern. Join scientists as they crack the secret biological code that underlies nature’s battleground. 



Extreme Animal Weapons

PBS Airdate: November 22, 2017

NARRATOR: Of the millions of species on Earth, only a few thousand are armed with something strange and special: huge weapons, growing from their bodies.

DOUG EMLEN (University of Montana): Look at that set of antlers. Wow.

NARRATOR: Antlers, horns and tusks, wielded in titanic battles against one another.

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 gets sucked into an arms race and ends up with these huge weapons, and this species, which is otherwise incredibly biologically similar, does not?

NARRATOR: What forces have driven animal weapons to evolve over millions of years? Are some weapons just for show? And can cheaters ever win at this evolutionary game?

What are the secrets of Extreme Animal Weapons? Right now on NOVA.

In Southwest Montana is a hall of wonders.

DOUG EMLEN: 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.

NARRATOR: Antlers are just one form of animal weapon. There are many others: horns, tusks, stings; nature's armory is diverse.

Weapons can be used in attack or defense, and sometimes their purpose is a mystery.

DOUG EMLEN: 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, while others do not?

Right on his doorstep, in Montana, Doug can find one of the best examples of extreme animal weapons.

DOUG EMLEN: Look at that guy.

NARRATOR: Rancher Doug Averill keeps elk on his land. They are tagged so he can monitor them.

DOUG AVERILL (Ranch Owner): 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."

DOUG AVERILL: 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.

DOUG AVERILL: This is pretty rare. I haven't seen this all summer.

DOUG EMLEN: You can see they're totally locked together.

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…

DOUG AVERILL: Got the temperament to be an aggressive bull down the road.

NARRATOR: …and he charges Doug.

DOUG EMLEN: Ha ha, that got my attention.

It's okay.

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, another 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. The weakest are the most likely to become prey.

DOUG EMLEN: So, imagine the predators of these guys, something like a wolf.

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.

But now, if you turn around and you look at the elk, well, it doesn't make sense there either. Considering what we know about the costs of these weapons, why would you ever want one? These structures are not helping the bulls survive.

Across the animal kingdom there's a high price to being armed, and these armaments don't usually help animals live longer.

NARRATOR: So what's the point of these extreme weapons?

DOUG EMLEN: Survivorship isn't the only game in town, and in fact, when it comes to evolution, the thing that matters the most is reproduction.

NARRATOR: Animals have a multitude of weird and wonderful traits that make them attractive to the opposite sex. The same evolutionary pressure that produces these characteristics is also behind many extreme weapons.

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…it 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.

DOUG EMLEN: 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 in 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.

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.

DOUG EMLEN: So, imagine things that whirl around in the water; sometimes animals are fighting over resources that can't be defended. Think about things like raptors, fighting in the air; they'll get into these big frenzied acrobatic mid-air snarls. In fights like that, things like agility or speed are likely to matter more than bulk and strength. 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.

DOUG EMLEN: The males that are able to win these battles or to hold onto that territory, I mean, the ultimate prize is reproduction.

NARRATOR: This male has fought off all his rivals and now he's earned the opportunity to mate with a female.

DOUG EMLEN: So, when we start to look at these animals and say, "What sets these species apart? Why do 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.

NARRATOR: Most animals with extreme weapons are keeping females away from male rivals. Male hippos use their enormous teeth to do this. 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.

DOUG EMLEN: Each of those tusks can be, like, a hundred pounds of ivory. These are huge teeth.

Aaahh, his tusk shattered. Okay, there you go. That's the impact that we're talking about here. It shattered the tusk.

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. 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.

NARRATOR: 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. 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.

DOUG EMLEN: Competition is absolutely critical. In a sense, it's the fuel that drives the arms race. And all of these animals, where you get these massive weapons, the males, the bulls, the bucks, face fierce competition over access to females.

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 (Cornell University): 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, 'cause 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, timewise, the female, she's able to reproduce, in theory, about every 10 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.

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. They use their spurs in battles to control access to breeding males.

DOUG EMLEN: 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.

STEPHEN EMLEN: And this is, sort of, a horrendous thing to think about. Basically, she is now destroying his eggs.

DOUG EMLEN: She wipes the slate clean.


DOUG EMLEN: So, how much faster?

STEPHEN EMLEN: If he's on eggs like that one, where she 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.

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.

But is it the only force at work? Many animals compete intensely for territory, food or a mate. Their fights can be vicious, but most of these species don't have giant weapons.

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 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 the 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.

DOUG EMLEN: But then there's this other type of dung beetle that had been less well studied, and those are what we call the tunnelers.

NARRATOR: The tunnelers 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 tunnelers were doing below the surface. He had to use red lights to avoid disrupting their natural behavior.

DOUG EMLEN: 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, not even bump each other, they couldn't care less, but you put them in a tunnel, and just like that, you've got a war.

Any rival males have to enter one at a time, and they'd have to face their opponent face to face.

NARRATOR: The difference between the tunnelers and the ball rollers is that the ball rollers fight in group scrambles, while the tunnelers, 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.

DOUG EMLEN: Chameleons have to be one of my favorite animals of all time. 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!

NARRATOR: A chameleon's tongue is able to extend to twice the length of its body.

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.

DOUG EMLEN: Think of this like Jurassic Park jousting, as these males push and pry and try to twist each other off of the branch.

NARRATOR: The branch itself is the key.

DOUG EMLEN: 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.

DOUG EMLEN: So, for me, the, the penny dropping, the moment, the epiphany when everything clicked and came together was the, 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. The battles almost always include ritualized head to head combat. When this happens, the evolution of extreme weapons seems to outweigh the costs.

DOUG EMLEN: 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. The rare species that possess them coming from a diverse range of animal families.

DOUG EMLEN: 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. Fossils bearing extreme weapons have been found around the globe. Doug heads to Salt Lake City to examine the fossilized remains of a famous dinosaur family.

Mark Loewen studies a group of dinosaurs called the ceratopsids.

MARK LOEWEN (Natural History Museum of Utah): Welcome to the horned dinosaurs.

DOUG EMLEN: They're amazing.

MARK LOEWEN: I mean we have triceratops, the iconic horned dinosaur.

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. 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.

MARK LOEWEN: 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 re-healed during its life.

NARRATOR: Mark thinks that holes like these are puncture wounds from violent face-offs with other ceratopsids.

MARK LOEWEN: So, this is totally consistent with head locking behavior and being punctured by the horn of another adult animal.

DOUG EMLEN: So, that's just like what other animals do?

MARK LOEWEN: Yeah. And, 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. 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. But there are exceptions. 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. 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.

DOUG EMLEN: Saber-tooths are special because they're ambush predators. 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. But for ambush predators that strike quickly, the evolution of big weaponry makes sense.

DOUG EMLEN: And that logic works under the ocean for things like mantis shrimp and pistol shrimp. It works for antlions. It works even for those crazy deep-sea anglerfish 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.

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.

DOUG EMLEN: 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?

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?

BROOK SWANSON (Gonzaga University): Sure. Here it is.

DOUG EMLEN: That's it? That's not very big.

BROOK SWANSON: So, these are fiddler crabs.

DOUG EMLEN: Ow! Okay that was…maybe it's a little bit bigger than I thought. So, this is it?

BROOK SWANSON: Grab one, too.

DOUG EMLEN: The record holder.

BROOK SWANSON: 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.

DOUG EMLEN: That's incredible.

NARRATOR: Fiddler crab claws are proportionally bigger than any other known animal weapon on Earth.

BROOK 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.

BROOK 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.

DOUG EMLEN: All right, so he's bitten me three times already. Can you show me what these guys can do?

BROOK SWANSON: We can use this force meter to measure how strong their claws are.

DOUG EMLEN: This just measures how hard they squeeze?

BROOK SWANSON: Exactly. We 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.


DOUG EMLEN: 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.

BROOK SWANSON: That could take about five newtons of force.

DOUG EMLEN: 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.

DOUG EMLEN: So, this is technically a lethal weapon?


NARRATOR: Crab claws are powerful enough to kill a rival. But there's a paradox.

BROOK 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.

BROOK SWANSON: So, here we have two male fiddler crabs, and their body size is about, about the same. But if you look at their claws—the claw's twice as big as the other one.

DOUG EMLEN: Wow. That is so obvious.

BROOK SWANSON: 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.

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. 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.

DOUG EMLEN: 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.

BROOK SWANSON: 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.

DOUG EMLEN: So, these are the ultimate signals. Both males and females are paying attention. 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.

DOUG EMLEN: 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, come in 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.

DOUG EMLEN: Big males have the weapons; the big males fight the conventional battles. The little guys break the rules.

NARRATOR: And that means that the biggest animal with the largest weapon doesn't always win.

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.

DOUG EMLEN: 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. 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.

Bighorn sheep are famous for engaging in epic ritualized battles. During the rut, rival males size each other up. Occasionally, when it's an even match, they fight.

Through a series of horn-to-horn clashes, the rams establish a hierarchy, with the dominant males at the top. During the breeding season, they will guard fertile females.

JACK HOGG (Montana Conservation Science Institute): Largest horned, largest bodied rams search for and defend ewes during their fertile period.

DOUG EMLEN: 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.

JACK HOGG: 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.

NARRATOR: The dominant male here is called Crud Horn. He's guarding a fertile female.

JACK 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.

JACK 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. She's, 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.

JACK 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.

DOUG EMLEN: That's all it takes, is a few seconds?

JACK HOGG: A few seconds.

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.

JACK HOGG: 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.

DOUG EMLEN: 'Cause 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?

JACK HOGG: They do better. 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.

DOUG EMLEN: To the victor go the spoils. We see this in every one of these animal systems, the biggest males are the ones that can afford to produce the biggest weapons, and these males win, in every sense of the word.

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.

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.

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.

DOUG EMLEN: So, this has been quite a journey for me, a ride more wild than I ever could have 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.

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.

Broadcast Credits

Peter Fison
Mark Robertson
Stuart Dunn
Paul Hartnoll
Hello Charlie
Eric Meyers
Helena Berglund
Pauline Gates
Sarah Janalli
Mary Melville
Alan Neal
Mark Richard Adams
Matt Coster
Harry Hills
Ben Wood
Sophie Meyjes
Guthrie O’Brien
Monterey Bay Aquarium Research Institute
Getty Images
Oxford Scientific Films
Andrew Farke
Flathead Lake Biological Station
Jim Phillips
Laura Denitto
Montana Conservation Science Institute
Montana State Parks
Natural History Museum of Utah
Stephen and Natalia Emlen
US Dept of Defense & Malmstrom AFB
University of California Museum of Paleontology
Gonzaga University
University of Montana
Philip Wright Zoological Museum
Emlen Laboratory
‘Animal Weapons’ by Douglas J. Emlen
Roger Webb
yU + co.
Walter Werzowa
John Luker
Musikvergnuegen, Inc.
Ray Loring
Rob Morsberger
The Caption Center
David Bigelow
Jennifer Welsh
Eileen Campion
Eddie Ward
Brian Kantor
Linda Callahan
Cory Allen
Sarah Erlandson
Janice Flood
Susan Rosen
Kristine Allington
Tim De Chant
Lauren Miller
Vanessa Ly
Kevin Young
Michael H. Amundson
Nathan Gunner
Ariam McCrary
Caitlin Saks
David Condon
Pamela Rosenstein
Elizabeth Benjes
Evan Hadingham
Chris Schmidt
Melanie Wallace
Laurie Cahalane
Julia Cort
Paula S. Apsell

A BBC Studios and NOVA/WGBH Boston Co-Production

© 2017 BBC

Additional Material © 2017 WGBH Educational Foundation

All rights reserved

This program was produced by WGBH, which is solely responsible for its content.

Original funding for this program was provided by Draper, 23andMe, the David H. Koch Fund for Science, and the Corporation for Public Broadcasting.


Image credit: (Jackson's Chameleon)
© Aernout Zevenbergen/Alamy Stock Photo


Doug Averill
Ranch Owner
Doug Emlen
University of Montana
Stephen Emlen
Cornell University
Jack Hogg
Montana Conservation Science Institute
Mark Loewen
Natural History Museum of Utah
Brook Swanson
Gonzaga University

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