When a peacock mantis shrimp strikes, it generates the heat of a star. The crustacean retracts a spring-loaded mouthpart known as a dactyl club and puts it on a latch, like a cowboy cocking the hammer of a gun.
After pulling the trigger to release the stored elastic potential energy, the club moves like a bullet to reach speeds of up to 31m per second (around 111kph).
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The punch is so fast that it lowers the surrounding pressure, causing water to boil and form steam-filled bubbles that suddenly implode by ‘cavitation’ – a pop of sound, flash of light and temperatures approaching 5,000ºC, as hot as the surface of the sun.
“Cavitation bubbles collapse and emit an intense burst of energy,” explains Sheila Patek, a biologist at Duke University in North Carolina.
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The bubbles can damage metal if they form around boat propellers, but ‘smasher’ species of mantis shrimp exploit this force (more than 100 times their bodyweight) to crack open snails or other packaged food.
“They evolved hammers and spring-latch mechanical systems to fracture hard-shelled prey with high accelerations,” says Patek, whose research focuses on fast and powerful animal movements. “They can produce up to 1,500 Newtons (N) at impact.”
A peacock mantis shrimp’s punch holds the record for fastest strike in the aquatic world but, on land, the title of fastest gun in the west goes to the Dracula ant.
The ant’s ultrafast mandibles can close at 90m per second (321kph) in 23 microseconds (more than 4,300 times faster than the blink of an eye) so the vampire can bite through an insect’s exoskeleton and drink its blood. But while these tiny jaws are certainly terrific, they wouldn’t terrify larger creatures.
Are teeth a weapon?
Nature’s most fearsome weapons are arguably the jaws of the great white shark. From computer models, scientists predict that its bite force can reach 18,216N, the highest estimate of any living species (the highest ever measured, at 16,414N, was recorded from a saltwater crocodile).
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Each great white tooth also has a triangular shape, which concentrates that force over a small surface area so the predator doesn’t need to press hard to pierce prey.
Shark and human teeth are made of slightly different materials but have similar hardness and strength, which is enough to withstand massive forces.
So why do great whites continuously grow new teeth?
“We don’t know how many bites each tooth experiences or how often most sharks lose their teeth, but it’s certainly not breakage,” says Lisa Whitenack, a biologist at Allegheny College in Pennsylvania, who did her PhD on the biomechanics and evolution of shark teeth.
Instead, great whites may need to regularly replace teeth because they get blunt and lose their sharp edge.
“It only takes 10 to 20 bites for the shark tooth to noticeably dull.”
The serrated structure of great white teeth also helps tear through flesh: once the predator has clamped down, it produces a sawing action by shaking its head.
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“A butter knife doesn’t really catch fibres in the meat, but a steak knife has a heavier serration that can trap the fibres,” says Whitenack. “The harder you push down, the less force you have to use to pull and cut your steak.”
What can be classed as a weapon?
Features such as the great white’s jaws and a mantis shrimp’s club have evolved over millions of years into forms that seem perfectly adapted for their functions.
They match how the Oxford English Dictionary defines the word ‘weapon’: “An instrument of any kind used in warfare or in combat to attack and overcome an enemy.”
But such physical instruments don’t qualify as weapons under the traditional definition in biology: “Conspicuous morphological structures used by males during contests over access to females.”
This means well-known examples such as deer antlers and beetle horns. Defining weapons so strictly is problematic, however.
It ignores features exclusive to females, including the stingers of wasps, bees and ants, which evolved from ovipositors that were once used for laying eggs, for instance. More importantly, focusing on conspicuous male structure can overlook weaponry in living things besides animals.
In the plant kingdom, carnivorous species such as the Venus flytrap can capture small arthropods. However, some scientists would argue that the plant’s jaws are no more than a mechanical bear-trap, not a weapon for combat.
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Similarly, spiky structures such as thorns, spines or prickles are analogous to barbed wire and evolved to discourage plant-eaters, which is why they’re known as plant defences against herbivory.
But despite the fact that flowering plants can’t move or attack, the group has some clear cases of traditional weapons used by males.
In milkweed, male pollen is contained in sacs that attach to insects and are deposited when a pollinator visits the female parts of another flower.
A 2014 study showed that some species have sacs with horns to block the sacs of other males from attaching – equivalent to when a dominant male mammal, such as a walrus with big tusks, monopolises mating opportunities by occupying the prime spot on a beach.
“Our study was the first to demonstrate that plants can engage in male-to-male physical fights,” says Andrea Cocucci, director of the Botanical Museum at the National University of Córdoba, Argentina.
How have animal weapons evolved?
Weapons have evolved through natural selection because they enabled a species’ ancestors to survive or reproduce.
This survival of the fittest can by driven by ‘ecological selection’, when changes in the environment (such as availability of prey) favour individuals who carry beneficial adaptations such as claws or talons.
But features used in competition between males – such as the microscopic horns of milkweed pollen, majestic antlers of male deer and mandibles of stag beetles – are usually selected by females.
Such structures require energy to grow, are cumbersome to carry and are conspicuous to predators, meaning they exist because the potential costs to survival were outweighed by benefits to reproductive success.
This is the theory of sexual selection first proposed by Charles Darwin.
“The widespread occurrence of horns in vertebrates and insects is explained by Darwin’s seminal work as a product of evolution through sexual selection,” says Cocucci. “Triumphant males in a fight get access to females for reproduction.”
Does chemical warfare exist in the animal world?
Habitats are battlefields where organisms wage chemical warfare, too. But while some species make chemical poisons that cause sickness or death, these toxins have evolved mainly as deterrents.
They make an organism unpalatable and passively enter the body. By contrast, when a toxin is actively delivered by injection, via bite, sting or spray, it’s a venom – a weapon for attack or self-defence.
“Predatory venoms typically act by quickly immobilising prey, while defensive venoms generally cause rapid pain, which deters an attacker,” says Ronald Jenner from the Comparative Venomics Group at the Natural History Museum in London, co-author of Venom: The Secrets of Nature’s Deadliest Weapon.
But while many venoms aren’t designed to kill, death is often the end result. Their range of devastating effects ranges from causing internal bleeding, by preventing blood from clotting, to manipulating behaviour or liquifying a host.
A venom can be a mixture of toxins, each affecting a specific molecule in the intended victim. Those molecules can vary among species, however, meaning that a given venom may be harmful in one species yet harmless in another. This makes it difficult to determine which is the worst, says Jenner.
“The effects of venoms are diverse, so the power of venom cannot be captured by any single measure.”
Nonetheless, scientists often study potency through a metric called LD50, the lethal dose that would kill 50 per cent of a population of lab mice, as a model for how a venom might affect another mammal, humans.
But LD50 also shows why it’s hard to identify the worst venom, as illustrated by Brazilian frogs that use the head as a weapon: while the venom of a Bruno’s casque-headed frog is 25 times more lethal than the poison of a Bothrops pit viper, the snake’s fangs are more dangerous as they deliver more venom in a single bite.
One of the deadliest delivery methods belongs to cone snails, whose arsenal is typically a cocktail of 100–200 ingredients.
Like many molluscs, snails have a tongue-like organ known as the radula, which is covered in tiny teeth (denticles) and is used to scrape and lick the surface of food.
In fish-hunting cone snails that inhabit tropical coral reefs, this organ has been partly modified into the radula tooth, a harpoon that shoots from a snail’s proboscis to pierce a prey’s skin before its hypodermic needle injects chemical agents called conotoxins.
The harpoon’s backwards-pointing barbs then hold the fish in place while the slow-moving mollusc reels in its catch. This is the ‘taser-and-tether’ strategy.
“The fish becomes immobilised in a few seconds,” says Baldomero Olivera, a neurobiologist and conotoxin expert at the University of Utah.
“So the snail just retracts its proboscis and the paralysed fish is engulfed, then the snail regurgitates the scales and bones and the harpoon, and the soft parts go into the gut and are digested.”
Snails that live on muddy or sandy bottoms of the sea are exposed to danger and employ a different strategy: hunting as ambush predators. They use the radula tooth as a stabbing or throwing spear then track down prey, mainly from smell.
The deadliest species to people, killing dozens thanks to a 70 per cent fatality rate (without medical treatment), is the geography cone snail. Its conotoxins have numerous effects on a victim’s nervous system, including disorientation and drowsiness, convulsions and coma.
“In about half-an-hour, the fish becomes immobilised,” says Baldomero. “When the fish is finally crumpled in a heap where it can’t move, then a snail will approach it, engulf it and quickly hide.
What about psychological weapons?
There’s one other kind of weapon after physical and chemical: mental ability. While most living things are limited to the body parts they’ve inherited through biological evolution, the mind can come up with new technologies and strategies within a lifetime, and then pass them on to the next generation via culture.
The most effective mental instrument is the human mind, which allowed us to do everything from inventing farming methods that turned prey into livestock to making nuclear bombs and other weapons of mass destruction.
We’re not the only animals that have harnessed the mind, however. Chimpanzees carve spears out of branches and use military tactics such as taking the high ground to do reconnaissance on an enemy, or
leaving dead bodies as an act of terror – psychological warfare.
Clearly, evolution has created an array of features for fighting and killing. Some, like the mind, are hidden. Others, like the bright bubbles produced by mantis shrimp, are visible.
They all highlight the diversity of life, what Darwin described as “endless forms most beautiful”. But as nature’s greatest weapons reveal, even beauty can be brutal.
